Does Fat Play a Structural Role in Our Body?

Our cell membranes contain molecules, called phospholipids, that seem to have structural similarities to triglycerides (see Figure 5.1). Like triglycerides, phospholipids contain a glycerol backbone to which fatty acids are attached. However, phospholipids contain only two fatty acids, not three as in triglycerides. The third fatty acid is replaced by phosphate combined with another molecule, such as choline, serine or inostiol. This helps make the phospholipids special and appropriate to be part of the

membrane.

Phospholipids provide the basis for the water-insoluble properties of our cell membranes. In turn, then, the barrier-like properties of membranes allow each cell to regulate the movement of water-soluble substances into and out of cells and their internal organelles. In addition, the attached fatty acids can be removed and used to make other molecules that help regulate bodily function.

What Is Brown Adipose Tissue?

While most of the fat tissue in an adult’s body is somewhat pale (white adipose tissue), infants tend to have a fair amount of brown adipose tissue (BAT). This type of fat tissue is a little different from white adipose tissue as it contains a lot more blood vessels. This is one reason why it appears darker in color. BAT is especially important for infants to help them maintain their body temperature. When infants are born, they are fairly lean and it is easy for heat to leave their bodies. BAT has the ability to increase some of its metabolic events, which results in the generation of extra heat. BAT is able to uncouple the process of ATP formation via the breakdown of energy nutrients. Although this may seem somewhat “futile” when it comes to making ATP, the molecule that cells use to power most operations, it does allow for the generation of heat which will help maintain the body temperature of the baby. For adults, this may seem like a great way of burning unwanted fat, but this isn’t to be, because as babies become children and then teens, the amount of BAT is reduced and becomes almost nonexistent by adulthood.

Does Body Fat Help Our Body Conserve Body Heat?

Subcutaneous fat not only helps protect skeletal muscle from trauma but it also helps conserve our body heat. This is because fat tissue is a relatively good insulating tissue. Maintaining our body temperature allows cell operations to function optimally. Interestingly, too little subcutaneous body fat might allow for greater heat losses daily. This might partly explain why a leaner person may have a higher energy expenditure than another person having the same body weight but who is less lean. Following this line of thinking it would be easier for a leaner person to maintain their body weight than a heavier person. We’ll take a closer look at this in Chapter 7.

Body fat is important to maintain body temperature and to protect organs and muscle.

Can Fat Help Protect the Body?

Fat tissue provides some protection to various tissues in the body. For instance, fat tissue around our internal organs provides some cushioning. This helps protect the organs against external trauma. Furthermore, the subcutaneous layer of fat storage also provides some cushioning, which protects muscle. Subcutaneous fat is not well vasculated, meaning that there aren’t a lot of blood vessels in that tissue relative to other tissue. Meanwhile, skeletal muscle is heavily endowed with blood vessels which provide oxygen and energy nutrients during activity and exercise. In the absence of subcutaneous fat it would be easier to rupture smaller blood vessels in skeletal muscle, which then would be evident in bruises. As an example, prior to competition, bodybuilders will be very cautious not to bang into things or play contact sports (rugby, football, roller hockey, etc.). As they attempt to “lean out” for the competition, they reduce their subcutaneous fat to nadir levels, which would allow them to bruise more easily. This then would impact their aesthetic presentation during the bodybuilding competition.

Do Fat Cells Do More Than Store Energy?

For a long time fat tissue and their cells were viewed as somewhat inert containers of energy storage. However, today we know that adipose tissue functions as a gland with the capability to release a variety of factors relative to its size and endowed energy. As mentioned previously, some of these factors may promote the formation of more fat cells. Perhaps some of the most interesting released factors are those that circulate to the brain and provide insight to our energy storage status. One of the most important factors seems to be the hormone leptin. Fat cells release more and more leptin into our circulation when fat cells accumulate more fat. Leptin then signals the brain to reduce appetite. In addition, as fat cells swell due to excessive calorie consumption, some of the chemicals they release can promote the development and worsening of diabetes, high blood pressure and other medical conditions.

Are We Born with All of the Fat Cells We Will Ever Have?

We are not born with a full complement of fat cells as some scientists once thought. The number of fat cells in the body increases at various stages throughout growth, but by the time adulthood is reached the total number of these cells can become fixed. This means that if our body fat mass does not change, we probably would not produce new fat cells as adults. However, if we consume excessive calories, the number of fat cells can increase. In adipose tissue there is a small number of so-called pre-adipocytes or fat stem cells. When these cells are signaled, they will produce new fat cells. As you may have guessed, the signals are chemicals, many of which are released by existing fat cells when they become swollen with an increased bounty of stored fat.

Are There Advantages to Storing Energy as Fat?

Storing excess energy as fat rather than as protein or carbohydrate has great advantages. First, we are able to store more than twice the amount of energy in 1 gram of fat (9 calories) as we can in 1 gram of carbohydrate or protein (4 calories). Second, stored fat will have a lot less water associated with it than would be stored in carbohydrate and protein. The net effect of storing excess diet energy as fat versus carbohydrate or protein is that our body weight and volume are minimized. Said differently, it allows the human body to be lighter and smaller despite significant energy stores.

Storing energy as fat, versus carbohydrate or protein, allows our body to remain smaller and lighter.

Where Is Body Fat Stored?

Fat (triglyceride) is an energy source for many of our cells (in particular muscle and liver) and is our primary means of storing the excessive energy

from the foods we eat. Although some fat can be found in several cell types in our body (such as skeletal and cardiac muscle cells), by and large most of the fat stored in our body is housed in fat cells. Collections of fat cells or adipocytes are commonly referred to as fat tissue or adipose tissue. Because a larger percentage of the fatty acids stored in adipose tissue are monounsaturated and saturated, the fat tissue is more semisolid than liquid. This can contribute to the dimpling appearance in the layer of fat found beneath our skin (subcutaneous fat) that is often referred to as cellulite.

What Information Can We Derive from a Blood Cholesterol Test?

When a health professional refers to our blood cholesterol level it is usually total cholesterol. Total cholesterol is the sum of the cholesterol in all of the lipoproteins circulating in our blood at the time of the blood draw. Since chylomicrons will circulate only for a couple of hours after a meal, they should be absent from blood drawn after an overnight fast. If there are chylomicrons in a fasting blood sample it could indicate a medical condition whereby chylomicrons are not rapidly and efficiently processed.

The fractions of total cholesterol are the amount of cholesterol found in each type or class of lipoproteins. Thus LDL-cholesterol is the cholesterol only found in LDL. And likewise HDL-cholesterol is the cholesterol found only in HDL. With regard to heart attacks and strokes, having a total cholesterol level greater than 200 milligrams per 100 milliliters of blood and elevated LDL- and low HDL-cholesterol levels increase the risk (Table 5.6 has a sample lipid profile).

A total cholesterol level is the sum of all the cholesterol in lipoproteins primarily LDLs, HDLs and VLDLs.

Where Do High Density Lipoproteins Come From and What Do They Do?

The last type of lipoprotein is HDL. HDL is made in our liver and to a lesser extent in our intestines. It is HDL’s job to circulate and pick up excess cholesterol from tissues throughout our body and return it to the liver. The whole process is very interesting because in order for circulating HDL to return the cholesterol to our liver, some of the cholesterol is first passed to circulating LDLs. The LDL is then subject to removal from our circulation by the liver and broken down. HDL delivers the rest of its cholesterol directly to the liver. In regard to heart disease, if LDL wears the villain’s black hat, as higher levels are linked to increased risk of a heart attack and stroke, then HDL wears the hero’s white hat, as higher levels are linked to lower risk. We will spend more time talking about blood lipids and cardiovascular disease in Chapter 13.

What Are Low Density Lipoproteins and How Do They Function?

As mentioned earlier, not only will the liver receive cholesterol and some fat from chylomicrons, but it is also a primary cholesterol- and triglyceride-producing organ in the body. Fat and cholesterol in excess of the liver’s needs are packaged up into VLDLs and released into our circulation. As VLDLs circulate throughout our body, they unload a lot of their fat, mostly in fat cells. As a result their lipid to protein ratio decreases, which renders them denser, and they become LDLs (Figure 5.10). Therefore, LDL is derived from circulating VLDL.

LDL has two fates. One fate is to continue to circulate throughout the body and deposit cholesterol in various tissues. The second fate is to be recognized by tissue, removed from the blood, and broken down. Many tissues throughout our body can do this, but the liver handles more than half of the task. The longer LDLs circulate, the more opportunity there is for cholesterol to be deposited throughout our body.

LDLs contain mostly cholesterol and serve to deliver it throughout the body.

What Is the General Activity of Chylomicrons?

As summarized in Table 5.5, chylomicrons are made by the cells lining our small intestine and transport diet-derived lipids throughout the body. Chylomicron composition reflects our dietary lipid intake; therefore, they contain mostly fat. As chylomicrons circulate they unload most of their fat in fat tissue and other tissues such as muscle, as described previously. Once most of the fat has been removed the chylomicron is much smaller and is recognized and removed from the blood by the liver where it is broken down. Any cholesterol and leftover fat becomes the property of the liver.

How Are Lipids Shuttled Around in Our Blood?

Not only will our liver make a fair amount of cholesterol and fat on a daily basis, but it will also receive these nutrients from diet-derived chylomicrons. Like fat, most cholesterol is housed in the liver for only a short period of time as it is destined for other tissues throughout the body. Once cholesterol reaches other tissues, it can be used to make some of the substances listed previously or to become part of cell membranes. Some of the cholesterol in our liver is also used to make bile salts, a key component of bile.

Whether they are coming from the digestive tract or the liver special transportation vehicles or lipoproteins are needed to circulate lipids. Generally speaking, lipoproteins are a protein-containing shell encasing the lipid substances in need of transportation (Figure 5.9). Lipoproteins can be divided into four general classes based upon their densities (see Figure 5.9). In order of increasing density lipoproteins are chylomicrons, very low density lipoproteins (VLDLs), low density lipoproteins (LDLs), and high density lipoproteins (HDLs). Looking at the composition of these lipoproteins in Figure 5.9, we see that the greater the lipid to protein ratio, the lower the density. This makes perfect sense because lipids are less dense than proteins.

The proteins that help make up the lipoprotein shell are called apoproteins. Not only do they make the lipoprotein more soluble in water, but they will also function in helping the lipoprotein be recognized by specific tissues throughout our body. This allows a lipoprotein either to unload some of its lipid cargo or to be removed from the blood and broken down. For instance, the receptor for LDLs is located in the liver tissue and also in other tissue throughout the body. When a specific apoprotein on an LDL docks on the LDL receptor, this allows the LDL to be removed from the blood.

How and When Do We Remove Fat from Our Fat Cells?

The fat stored in fat cells is available to us when food energy is not being absorbed (fasting) and when we exercise. Just as the hormone insulin promoted the storage of fat when energy was coming into our body, the process of mobilizing fat from fat cells is promoted by the hormones released into our blood when we are fasting and/or exercising (Figure 5.8). These hormones are glucagon, epinephrine, and cortisol, and all promote the release of fat from fat stores.

In order for fat to be released from fat cells, fat is first broken down to fatty acids and glycerol, which then enter our blood and circulate. However, because of their general water insolubility, the fatty acids will hitch a ride aboard a protein in the blood called albumin. On the contrary, glycerol is fairly water soluble and can dissolve into blood. In fact, researchers will measure the level of glycerol in the blood to estimate how much fat is being broken down.

Body fat is broken down to serve as energy in-between meals and during exercise.

Circulating fatty acids are removed by cells, especially skeletal muscle and our heart, liver, and other organs and then used by those tissues primarily for energy. However, keep in mind that cells of the brain and red blood cells (RBC) cannot use fatty acids for energy and will continue to use glucose. Conveniently the glycerol released from fat tissue can be used to make glucose in the liver and released into circulation to help maintain a desirable level of circulating glucose during prolonged exercise and fasting.

Can We Make Fat?

While diet-derived fat is being deposited in tissue throughout the body, if a lot of carbohydrate and/or protein were consumed, some can be converted to fat. This takes place in the liver and fat cells, with the latter only able to use glucose to make fat. Insulin promotes this activity, which makes sense since diet-derived carbohydrate and some amino acids raise insulin levels. The principle fatty acid products are palmitic acid (16:0) and oleic acid (18:1 ω-9) and palmoleic acid (16:1 ω-9). The fat made in fat cells is stored within those cells, while the fat made in the liver is packaged up and relocated mostly to fat cells for storage.

Contrary to popular belief the ability of the body to make fat from excessive dietary carbohydrate and protein is not as strong as once thought. However it does occur and for some people and situations, such as long-term excessive calorie intake, the involved processes are stronger. On the other hand, deriving more fat from polyunsaturated fat sources such as plant and fish oils can reduce these processes.

While fat manufacturing from diet-derived energy building blocks such as carbohydrates (glucose and fructose) and protein (some amino acids) does occur, it only explains a portion of the accumulated body fat during weight gain. The majority of the fat accumulated is from the diet. Since fat is mostly consumed with carbohydrate and protein, both of which raise insulin levels, more dietary fat is directed to storage. Since too many total calories are being consumed more fat will be directed into storage than broken down for use as fuel. Thus there is a net gain of body fat which in turn increases body weight.

Fat Storage, Mobilization, and Use

What Happens to Dietary Fat in Our Body?

When we eat a meal containing fat, it is absorbed and circulates within chylomicrons. As it circulates, fat is slowly transferred from chylomicrons to fat cells as well as skeletal muscle, heart, and other organs (breast tissue, for example) (see Figure 5.7). In order to transfer diet-derived fat to our tissue, an enzyme must be present in that tissue. The enzyme is called lipoprotein lipase (LPL) and just like lingual lipase and pancreatic lipase, LPL also removes fatty acids from glycerol. The fatty acids liberated by LPL move out of the chylomicrons and enter the nearby cells. Scientists have studied LPL for years and it now seems that differing levels of LPL activity in different locations of adipose tissue may partly explain why people seem to accumulate more fat stores in some regions of their bodies and not as much in other areas.

While a little bit of dietary fat can be used for energy very early during a meal as the body shifts from a fasting to a fed state, by and large dietary fat is destined for storage or put to use in other ways. By design, fat cells will store loads of fat and insulin promotes this activity. On the contrary, skeletal muscle cells and the heart have a limited ability to store fat. However, the amount of fat that skeletal muscle can store can be increased by aerobic training (such as running and biking). The importance

of this fat is related to performance, as during exercise this fat is readily available to the muscle cells in which it is stored. In addition, aerobic exercise training also promotes adaptations in muscle cells, making them better fat burners during and after exercise. More on the relationship between exercise and fat burning and storage will be discussed in later chapters.

Body fat is primarily derived from food fat and secondarily from fat production in fat tissue and the liver.

How Are Triglycerides and Cholesterol Absorbed?

Absorbing lipids into the body requires special consideration. Since the blood is water-based, how can these water-insoluble substances circulate? Cells lining the wall of the small intestine reassemble triglycerides and package them up along with cholesterol into shuttles called chylomicrons. Chylomicrons can leave these cells and enter the lymphatic circulation before enter the blood. Chylomicrons are very large and are unable to squeeze through the entry holes to the blood stream. Instead they drain into the larger openings to the lymphatic circulation. Within minutes, chylomicrons will circulate to a duct in the chest that gives them access to the blood (Figure 5.7). Once in the blood, a chylomicron will circulate for about a half hour, delivering its lipid bounty to tissue throughout the body.

Will Gall Bladder Removal Stop Fat Digestion?

Bile is made in the liver and stored in the gallbladder in-between meals. Disorders involving the liver or gallbladder can lead to reduced bile production and/or delivery to the small intestine. When fat-containing food particles arrive in the small intestine, bile is squeezed out of the gallbladder and travels to the small intestine through a duct. Some people have their gallbladder removed for medical reasons. Since bile is made in the liver and the gallbladder merely functions as a temporary storage depot for bile, this is not a serious concern. In many cases, the liver sends adequate amounts of bile directly to the small intestine to support adequate digestion of a reasonably sized meal. However, if fat is not efficiently digested and absorbed, a lower-fat diet might be prescribed by a physician. The presence of increased amounts of fat in feces can be used to gauge the efficiency of fat digestion and absorption. Feces will become more pale and greasy in appearance when proper absorption does not occur. In addition, bacterial metabolism of some of the fat may result in some discomforting symptoms as well.

How Efficient Is Fat Digestion and Can We Decrease Fat Absorption?

The efficiency of the digestion and absorption of the fat and cholesterol we eat is greater than 90 percent. Certain drugs and dietary supplements have been marketed to reduce the absorption of fat from the diet. For instance, the supplement chitosan is fiber-like substance derived from chitin. Chitin is a polysaccharide-like structure made up of amino sugars (sugars with nitrogen) which helps harden the shells of shellfish (shrimp, lobster, crab), insects (beetles), and is also found in some other animals and the cell walls of some fungi. Chitosan is a processed form of chitin

and it is used in the food and drug industry and in supplements. Chitosan is more water soluble than chitin and is often marketed as a fat binder in the digestive tract.

In addition the drug xenical (Orlistat) hinders the actions of pancreatic lipase, the principal fat-digesting enzyme. This results in less absorption of diet-derived fat and more fat in the feces. Orlistat has been shown in research studies to be an effective therapy for weight loss and is recommended in conjunction with a healthy, reduced calorie and fat diet and exercise program. Because Orlistat can increase the amount of fat in the lower digestive tract there is the potential for side effects such as loose, oily stools and flatulence. Furthermore, because there is the possibility of reduced absorption of fat-soluble vitamins, manufacturers recommend the use of a supplement at least 2 hours before the use of Orlistat.

Which Enzymes Digest Fat and Cholesterol?

Although a triglyceride-digesting enzyme called lingual lipase is present in saliva, the job of digesting triglycerides is mostly handled by another lipase enzyme delivered by the pancreas. Pancreatic lipase detaches two fatty acids from glycerol, which results in a monoglyceride and two fatty 

acids (Figure 5.6). In turn, the remaining fatty acid may be detached by yet another enzyme from some of the monoglycerides. This would then produce glycerol and a fatty acid. Thus, the products of triglyceride digestion are fatty acids, monoglycerides, and glycerol, which are now small enough to move into the cells lining the small intestine. Meanwhile, some of the cholesterol in our diet is actually linked to other molecules, with the most prevalent attachments being fatty acids. These are often referred to as cholesterol esters. Other digestive enzymes (cholesterol esterase) will liberate cholesterol so that it can be absorbed.

The digestion of fat and cholesterol requires bile and lipase enzymes and the assistance of the lymphatic circulation.

How Are Lipids Digested in the Watery Digestive Tract?

Digestion is a watery affair and has been loosely compared to whitewater rafting. In addition to the water-based fluids we drink, liters of water-based fluid enter the digestive tract daily as part of saliva and other digestive juices. Dissolved in those fluids that our body provides are digestive enzymes. This means that our digestive enzymes are water soluble, while their task is to interact with and break down water-insoluble lipids for absorption. This presents an interesting yet readily solved problem.

When lipids are present in the small intestine the natural course would be for these substances to clump together. This is analogous to oil clumping together in the kitchen sink when we wash dishes, or to the separation of oil from the watery portion of traditional salad dressings. If lipids remain clumped together in the small intestine, surely the efficient digestion of these substances would be hindered? To solve this potential problem, bile is delivered to the small intestine and serves as an emulsifier or detergent during lipid digestion. Here, components of bile coat smaller droplets of lipid, rendering them water soluble, as depicted in Figure 5.5. Bile activity keeps larger lipid droplets from reforming. So instead of having a few very large droplets of lipids, the result is many tiny droplets. When lipids are present as tiny droplets, digestive enzymes have no problem attacking them and efficiently doing their job.

What Is Margarine?

Margarine was first developed in the nineteenth century as an alternative for butter. Early on it was a popular butter substitute for people who could not afford butter or to whom butter was not available. Margarine can be made from animal fats and/or vegetable oils; however, the bulk of margarine containing products today use vegetable oil based margarines. This is partly attributable to the relationship between a diet high in saturated fat in animal fat and the risk of heart disease. Plant oils tend to have fewer saturated fatty acids and do not contain cholesterol. More specifically, plant oils have much lower amounts of three types of saturated fatty acids (that is 16:0, 14:0, and 12:0), which are the SFAs that seem to be most associated with raised blood cholesterol levels.

Today, margarine from plant oils is made by adding hydrogen to unsaturated fatty acids in plant oils. Scientists called this process hydrogenation, during which some PUFAs are converted to MUFAs and some of the MUFAs are converted to SFAs (Table 5.4). This converts the liquid oil to semisolid or to solid fat. Hydrogenation occurs when the oils are heated up in a container and hydrogen gas is applied. The degree of change depends upon how much hydrogenation is allowed to take place. For instance, margarines that come in stick form are typically more hydrogenated than softer tub margarine.

Margarine is typically made by solidifying plant oils in a process called hydrogenation.

The most popular plant oil used for hydrogenation is soybean oil. Because of their relatively high content of MUFA and PUFA, margarines made from soybean, sunflower, safflower, olive, and cottonseed oils are perceived to be healthier than butter. However, when energy (heat) is applied to plant oils during hydrogenation, a small number of the cis double bonds can be converted to trans double bonds, which helps solidify the oil. In fact, conventional margarines have a higher trans fatty acid content than butter and typically the harder the margarine the higher the trans fatty acid level. Food companies have been working successfully over the past decade to alter their process for forming margarine to lower and eliminate the trans fat content, which is reflected on the food labels.

What Foods Contain Trans Fatty Acids?

Trans fatty acids can be found in many fat sources although its prevalence is very low. Bovine (cows, steer, oxen, etc) food sources are probably the greatest natural contributors of trans fatty acids to the human diet. For instance, beef, butter, and milk triglycerides may contain 2 to 8 percent of their fatty acids as trans fatty acids. Interestingly, cattle are not solely responsible for generating this trans fatty acid content. It is actually the bacteria in their unique stomachs that produce the trans fatty acid. These fatty acids are then absorbed by the cow and make their way into the tissues and milk of these animals.

In addition, trans fatty acids can be created during the processing of oils (that is, margarine and other hydrogenated oils), which will be described later, and when cooking oils are re-used over long periods, such as in fast-food restaurants and diners. In more recent decades, more than half of the trans fatty acids in the human diet were derived from processed oils either consumed plain or used in recipes (for example, fried foods, baked snack foods). Cookies, crackers, and other snack foods that utilize hydrogenated vegetable oil may contain up to 9 to 10 percent of their fatty acids as trans fatty acids.

Because the consumption of higher amounts of trans fatty acids is linked to increased risk of heart disease and stroke, the American Heart Association, and the most recent Dietary Reference Intakes (DRIs) in the United States and Canada, recommend limiting the trans fat level of the diet. In addition, food manufacturers in many countries, including the United States and Canada, are required to list the trans fat levels in the Nutrition Facts on food labels. Because of this, snack-food manufacturers are choosing hydrogenated oils with lower trans fat content to produce snack foods. Furthermore, in 2006 New York City placed a ban on trans fat in restaurants, a public health initiative that is being followed by other cities.

What Foods Are Good Sources of Essential Fatty Acids?

Good sources of linoleic acid are safflower oil, sunflower seeds (oil roasted), pine nuts, sunflower oil, corn oil, soybean oil, pecans (oil roasted), Brazil nuts, cottonseed oil, and sesame seed oil. Dietary surveys in the United States suggest that the intake of linoleic acid is about 12 to 17 grams for men and 9 to 11 grams for women.

Good plant sources of α-linolenic acid are flaxseed and walnuts—their oils are among the best sources of α-linolenic acid—as are soybean, canola, and linseed oil as well as some leafy vegetables. Diet surveys in the United States suggest that typical intakes of α-linolenic acid are about 1.2 to 1.6 grams daily for men and 0.9 to 1.1 grams daily for women. Therefore the ratio of linoleic acid to α-linolenic acid is about 10 to 1, a  point that will become more important later in this chapter and in Chapters 12 and 13.

Marine mammals (for example, whale, seal, and walrus) and the oil derived from cold-water fish (cod liver, herring, menhaden, and salmon oils) provide eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). EPA and DHA are fatty acids that are made from linolenic acid in marine animals. A lot of interest in the ω-3 PUFA was created when researchers reported that there is a lower incidence of heart disease in some populations, such as Greenlanders. Diet patterns showed high fish consumption in these people, which leads to greater ω-3 PUFA intake and a reduced incidence of heart disease. In addition, there are links between the consumption of fish and cognitive development as well as reducing age-related losses in memory and cognition.

Fish and fish oil supplements are good sources of the omega-3 fatty acids DHA and EPA.

Are There Essential Fatty Acids?

The need for dietary fat is not necessarily for energy purposes. Fat is needed in our diet as a means of providing two essential fatty acids, linoleic acid, an ω-6 PUFA, and α-linolenic acid, an ω-3 PUFA. Since the amount of these fatty acids in fat storage (adipose tissue) is limited, this suggests that their role in our body isn’t really to provide calories, although they will be used for energy. Linoleic and α-linolenic acid are used to make longer, more complex fatty acids that have special functions.

Linoleic acid is used to make a longer ω-6 fatty acid called arachidonic acid (ARA) while α-linolenic acid is used to produce longer ω-3 fatty acids, namely eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Both ARA and DHA are found in higher concentration in the brain and are vital for the development of the central nervous system and eyes. Meanwhile, EPA and ARA can be used to make factors called eicosanoids (for example, prostaglandins, thromboxanes, and leukotrienes) that help regulate many bodily functions as discussed below and in later chapters.

How Much Fat Do We Need in Our Diet?

At this time there is not a Recommended Dietary Allowance (RDA) (or Adequate Intake (AI)) for total fat. Meanwhile an Acceptable Macronutrient Distribution Range (AMDR) has been declared as 20 to 35 percent of energy, which would be practical for most people based on today’s food supply. It is important to realize that the AMDR is not a requirement level and many nutrition scientists believe that the absolute lowest requirement for fat in our diet could be as little as 5 percent of calories (for weight maintenance) as long as it is derived from healthier sources including seeds, plant oils, as well as fish and other marine life.

Some fat is needed in the diet to provide essential fatty acids, which are important regulatory factors.

How Are Vegetable Oils Produced?

Vegetable oils are the edible oils extracted from seeds, nuts, kernels and other plant tissue. Edible vegetable oils are extracted from plants using solvents such a hexane and/or through mechanical processes such as cold pressing and expelling. Mechanical processing does not involve solvents

and the major difference is the temperature of the extraction processes. Cold pressing involves a hydraulic press between two plates and the temperature tends to stay below 120°F. Meanwhile, expelling involves a screwing mechanism which results in more frictional heat allowing the temperature to reach as high as 185°F.

What Foods Provide Us with Triglycerides and Cholesterol?

As displayed in Table 5.3, fats and oils, and thus triglycerides, are present in both animals and plants. Oil is a natural component of many plant tissues including leaves, stem, roots, kernels, nuts, and seeds. Common edible oils include sunflower, safflower, corn, olive, coconut, canola, and palm oil. Contrarily, butter is made from the fat in milk, while lard is hog fat, and tallow is the fat of cattle or sheep. Other animal flesh will contain fat, including poultry and their eggs.

Cholesterol is not a necessary substance for plants; therefore they do not need to make it. Contrarily, mammals will make cholesterol to help meet their body needs. As a result, cholesterol intake in the diet is attributed only to consumption of animal foods or foods that have animal products in their recipe. It should be mentioned though that plants do create molecules that are similar to cholesterol called phytosterols which we will discuss in Chapters 12 and 13.

Can Different Kinds of Fatty Acids Be Part of the Same Triglyceride Molecule?

There are probably no definite rules as to the selection of fatty acids that make up a triglyceride molecule. One triglyceride molecule may be composed of one saturated, one monounsaturated, and one polyunsaturated fatty acid, all of the same or varying lengths. However, the types of fatty acids found within triglyceride molecules will depend on the plant or the animal source. For instance, the triglycerides in olive oil largely contain the MUFA oleic acid (18:1 ω-9) (about 82 percent), while about two-thirds of the fatty acids in butter are SFAs of varying length.

The presence of certain types of fatty acids in either a plant or an animal largely depends upon the nature of the plant or animal and the purpose of the fat for that life-form. For instance, fish that live in deeper water tend to be better sources of ω-3 PUFA because these fatty acids are found in the cell membranes of these fish and play a protective role against the increased pressure and decreased temperatures at greater depths as well as help regulate their buoyancy. Land animals create storage fat that is largely composed of saturated fatty acid. Since these fat molecules can pack tightly in fat cells it minimizes the necessary space.

Why Are Oils Liquid at Room Temperature While Fats Are Solid?

In general, if the majority of fatty acids in a triglyceride source are saturated, then it most likely will be solid at room temperature. Contrarily, if a triglyceride source contains a greater percentage of unsaturated fatty acids, especially PUFA, then this source will most likely be liquid at room temperature. Saturated fatty acids are straighter than unsaturated fatty acid. This allows them to pack closer together and to be more solid. Take a look at Table 5.2 and notice how the oils have a higher percentage of unsaturated fatty acids while the more solid fats (lard, tallow, etc.) have a high percentage of saturated fatty acids. Despite their names, palm oil and palm kernel oil are more solid at room temperature.

What Do We Mean by Saturated and Unsaturated Fats?

Regardless of the origin of a triglyceride source (plant or animal), the triglycerides will contain a mixture of fatty acids. When we say that a fat source is saturated, we are indicating that the majority of the fatty acids within the source are saturated. For instance, we often refer to butter and beef fat as saturated fats. This is because the majority of their fatty acids are saturated. Table 5.2 lists the approximate percentages of fatty acids for each food source.

What Are “Trans” Fatty Acids?

Taking a closer look at double bonds in Figure 5.4, we see that there can be some variation in the position of the hydrogen atoms. If the hydrogen atoms attached to the carbon atoms of a double bond are positioned on the same side of the double bond, it is a cis bond that is the predominant way they are found in nature. If the hydrogen atoms bonded to the carbon atoms are on opposite sides of the double bond, it is referred to as a trans fatty acid.

Interest has been growing regarding the presence of trans fatty acids in our diet and their potential impact upon health. Although cis versus trans may seem like a very minor point in regard to fatty acid design, these contrasting forms can impart different properties to a fatty acid. Cis double bonds cause a kinking or bending of the fatty acid, while trans double bonds do not. This makes unsaturated fatty acids with trans double bonds similar to saturated fatty acids in that they do not bend or kink. We will discuss trans fatty acids in more detail below as well as in Chapter 13.

Trans fats are like saturated fats in that they don’t bend, and increase the risk of cardiovascular disease.

What Does “Omega” Mean with Regards to Fatty Acids?

Because fatty acids can vary greatly, scientists will indicate the number of carbons and double bonds in a fatty acid. For instance a 18:3 fatty acid will be 18 carbons long and have three double bonds. Scientists also use omega system to indicate where double bonds are in a fatty acid. It works like this. If a fatty acid is linked to glycerol, the second carbon closest to the link is referred to as the alpha (α) carbon (see Figure 5.3). Meanwhile, the carbon furthest from the linkage with glycerol is called the omega (ω) carbon.

The omega system is based on the Greek alphabet. Alpha is the first letter of the alphabet and omega is the last. No matter how many carbons are in your fatty acid chain, these carbon atoms will always be addressed in this manner. Looking at a fatty acid not linked to glycerol, the alpha carbon would be the first carbon atom adjacent to the carbon bonded to two atoms of oxygen. Table 5.1 lists common fatty acids and their abbreviations.

To indicate position of the first double bond we count the number of carbons to the first carbon of the first double bond from the omega end. For instance, if the first double bond starts at the third carbon atom in, it is an omega-3 (ω-3) fatty acid (see Figure 5.3). Likewise, if the first double bond appears at the sixth or the ninth carbon atom in, these would be ω-6 and ω-9 fatty acids, respectively. For the most part, when addressing polyunsaturated fatty acids, we indicate only the position of the first double bond because subsequent double bonds seem to occur in series after one saturated carbon atom.

What Are Saturated and Unsaturated Fatty Acids?

Fatty acids can differ in their degree of saturation. Saturation refers to whether all of the carbon atoms between the end carbons are linked to two atoms of hydrogen. If this is the case, then the carbons are saturated with hydrogen and that particular fatty acid would be called a saturated fatty acid (SFA) (Figure 5.3). However, if, at one or more points, adjacent carbon atoms are bonded to only a single hydrogen atom each, the fatty acid would then be an unsaturated fatty acid (see Figure 5.3).

By nature, when two adjacent carbon atoms in a fatty acid are linked to only one hydrogen atom each, the carbon atoms must bond to each other twice. Chemists call this a double bond and if a fatty acid has only one double bond, it is referred to as a monounsaturated fatty acid (MUFA). Meanwhile, if there is more than one double bond, then it is a polyunsaturated fatty acid (PUFA).

Can Fatty Acids Vary in Length?

For the most part, the length of fatty acids can vary by as much as twenty carbon atoms or so. If a fatty acid has four carbon atoms or fewer, it is referred to as a short-chain fatty acid. On the other hand, if a fatty acid chain has six to twelve or greater than twelve carbon atoms, it would be referred to as a medium-chain fatty acid or a long-chain fatty acid, respectively. Often, fatty acids with twenty or more carbon atoms are referred to as very-long-chain fatty acids. Most fatty acids in nature have

an even number of carbons, yet some fatty acids do indeed have an odd number of carbons.

What Is Cholesterol and Can We Make It in Our Body?

Cholesterol has received its share of negative press over the years, however it is important to realize that cholesterol is absolutely vital to our

existence. Cholesterol can be made in many cells, and under normal situations we seem to make all that we need. In fact, we will make about 1 gram of cholesterol each day depending on how much cholesterol is in the diet. The liver is by far the most productive organ when it comes to making cholesterol and one of its jobs is to share with the rest of the body. Cholesterol is a necessary component of cell membranes and many vital substances in the body are made from cholesterol (Figure 5.2). These substances include bile components, vitamin D, testosterone, estrogens, aldosterone, progesterone, and cortisol.

Cholesterol is needed for cell membranes and to make certain hormones, digestive factors, and vitamin D.

What Is the Difference Between Fat, Oils, and Triglycerides?

Fats and oils are terms commonly used to refer to food sources of triglycerides. Often fat and oil are considered to be different based on appearance: fat is solid at room temperature and oil is liquid. However, they are really two of the same thing, generally speaking. They are both collections of triglycerides. For simplicity, we will use “fat” to include all sources of triglycerides.

A triglyceride molecule is a combination of three fatty acids linked to a glycerol molecule backbone (Figure 5.1). Although a triglyceride molecule will always have this general design, there can be great variability in the type and combinations of fatty acids that link to glycerol. Only one glycerol molecule exists, but like monosaccharides there are numerous different types of fatty acids in nature. Furthermore, if a triglyceride involves three fatty acids then monoglycerides and diglycerides will have one and two fatty acids attached to glycerol, respectively. Technically, they can be considered fat as well.

What Are Lipids?

Fats and cholesterol belong to a special group of molecules called lipids. The members of this club have something pretty significant in common: they are relatively insoluble in water. This might not seem like a big deal, but keep in mind that most of our planet’s surface is water and, more important to our topic, most of our body is water as well. Because of their inability to dissolve into water, we must make special concessions to accommodate lipids both during digestion and also inside of the body.

Fat and cholesterol are lipids, which are a group of molecules that don’t dissolve well into water.

During digestion, an emulsifying substance called bile is called to action to facilitate lipid digestion and absorption. As for fat and cholesterol inside of the body, they require special transport shuttles to circulate. Fat also has its own cell type specifically designed for storage. These cells are called adipocytes, or more commonly “fat cells,” and large collections of adipocytes are called adipose tissue. Adipose tissue is found under the skin (subcutaneous fat) and in deeper deposits (visceral fat) such as in the abdomen, around vital organs, and throughout skeletal muscle.

What Are Noncalorie and Low Calorie Sweeteners and Are They Safe?

Monosaccharides and disaccharides make foods like fruits and honey sweet. They can be used by food manufacturers to make recipe foodshave filed complaints with the FDA about aspartame. Some scientiststhink that these people may be more sensitive to one of the componentsof aspartame or to the small amount of formaldehyde and formate produced. Both formaldehyde and formate are considered toxic at higher intake levels, however the FDA believes the risk to be extremely low under typical circumstances. It is important to point out that since aspartame contains phenylalanine, people with a genetic condition called phenylketonuria (PKU) should avoid aspartame. Aspartame is sold under the trade name NutraSweet and Equal.

Sucralose—Sucralose was discovered in 1976 and the FDA approved it for use in food and beverages in 1998. Sucralose is six hundred times sweeter than sugar and unlike aspartame it is appropriate for most home cooking and baking recipes because it won’t breakdown when heated. Sucralose is made by exchanging three chlorine atoms for hydroxyl (OH) groups on the sucrose molecule. Sucralose is not digested and therefore doesn’t provide calories. However some of it is absorbed into the body. By and large sucralose is urinated out of the body within a few days. Some concern has been expressed by the public regarding the safety of sucralose. Despite several research studies suggesting that sucralose is safe for general use, some argue that not enough is known about longterm consumption of sucralose and whether or not some of the chlorine can be released and be problematic like other chlorine-based molecules.

Acesulfame K—Approved for use by the FDA in 1988 and has an intensity of sweetness about two hundred and fifty times that of sucrose. Acesulfame is used as a sweetener in many countries other than the United States and it appears to be usable with cold and hot food preparation. It is considered safe sweetener and is marketed under the name Sunette.

Stevia—Stevia is not an artificial sweetener as it is derived from a South and Central American shrub. Stevia is approximately three hundred times as sweet as sucrose. Recently Stevia has been approved for use in foods and beverages in Australia and New Zealand, and there is growing pressure for the FDA to approve its use in the US. At this time, Stevia is only available in the US as a dietary supplement.

Sugar Alcohols—Since these substances can be found in plants, sugar alcohols such as sorbitol, xylitol, lactitol, mannitol, and maltitol are recognized as artificial sweeteners. Sugar alcohols are used mainly to sweeten sugar-free candies, cookies, and chewing gums as they do not promote the formation of cavities in the same way as sugars.

Are There Other Dietary Considerations When Eating a High-Fiber Diet?

Perhaps the most obvious consideration is the production of gases, which may lead to bloating and cramping and the possibility of diarrhea. These symptoms seem to be most common when people who are not fiber consumers increase their fiber intake dramatically. It is recommended that people who are sensitive to fiber and these effects ramp up their intake slowly. Because fiber binds water, which is used to soften stool, there might be an additional need for water. This is easily solved by consuming fiber foods and supplements with water or other fluid.

Can Fiber Support Immune Function?

In addition to supporting heart and gut health as well as enhancing the absorption of key minerals, dietary fiber can also enhance the immune system. When soluble fibers are broken down by bacteria in the colon the by-products seem to increase the production of T helper cells and antibodies, as well enhance key immune system operations that provide immune protection.

Can Fibers Enhance Mineral Absorption?

Soluble fibers such as inulin and FOS enhance the absorption of some minerals in the colon, namely calcium and magnesium. While researchers are trying to better understand how this occurs, it would seem that there are a couple of possibilities. First, minerals such as calcium and magnesium can bind to fibers further up in the digestive tract. Then when soluble fibers are broken down in the colon they are released and available for absorption. The creation of acids (short-chain fatty acids and lactate) when soluble fiber is broken down by bacteria decreases the pH of the colon, which in turn enhances the absorption of calcium and magnesium.

Is Fiber Good for Diabetics?

Fiber is important to people who have diabetes for two reasons. First, fiber lowers the glycemic index and load of a food by adding bulk. In addition, soluble fibers promote the formation of gels in the stomach which slows the digestion and absorption of carbohydrates. These effects lower the glycemic response of a food and contribute to better blood glucose management. Fiber consumption, particularly whole grains, seems to increase insulin sensitivity. This means that the level of circulating insulin will be lower throughout the day, which can lower the risk of heart disease (see Chapter 13). Lastly, fiber promotes satiety and can reduce total food consumption at a meal leading to less carbohydrate and fewer calories consumed. In turn, reducing the number of calories consumed

can promote weight loss in overweight people with diabetes, which is important as most are overweight, primarily those with type 2.

Are Certain Types of Fiber Good for Lowering Blood Cholesterol Levels?

Soluble fibers include beta-glucans, mucilages, pectins, gums, and some hemicelluloses and are purported to reduce blood cholesterol. Soluble fibers may bind to cholesterol in the digestive tract rendering them unavailable for absorption. Psyllium, oat, and barley fiber are among the most advantageous providers of soluble fiber and the Food and Drug Administration (FDA) allows claims on food packages linking the consumption of these fibers to a reduction in cholesterol. Look for the following health claim on a food containing psyllium fiber: “The soluble fiber from psyllium seed husk in this product, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease.” A product must contain at least 1.7 grams of soluble fiber from psyllium seed husk per serving in order to have the health claim on its label.

Additionally, there is evidence to suggest that the short-chain fatty acids (acetic, butyric, propionic, and valeric acids) and lactate produced in the colon by bacterial breakdown of soluble dietary fibers may reduce cholesterol formation in the liver. Thus, soluble fibers can inhibit cholesterol absorption from the digestive tract as well as cholesterol production in the liver. These two factors may lead to reductions in the level of cholesterol in blood; this will be explored more thoroughly in Chapter 13.

What Is Diverticulosis and Can Fiber Help?

Diverticulosis is a situation in which there is an out-pouching of the inner wall of the colon. This disorder is believed to be the result of increased pressure within the colon. In turn, this increased pressure is most likely the result of the highly refined diet that people choose to eat in the United States. A refined diet results in less fiber or “roughage” and thus less digestive leftovers or “residue” making its way into the colon. Less content in the colon results in a smaller diameter and greater pressure exerted upon its walls from within. It is a matter of physics, as there is an inverse relationship between the radius (r) of a collapsible tube and pressure (P) as follows:

So you see, if the radius of the colon increases due to increased content then the internal pressure decreases, and vice versa. Researchers have clearly shown that those populations in the world that eat more fiber have a lower incidence of diverticulosis. Diverticulosis can lead to a medical concern called diverticulitis. Here the out-pouchings become impacted with bacteria and debris, leading to irritation, inflammation, pain, and sometimes bleeding.

Insoluble fibers like cellulose and hemicellulose appear to have a beneficial effect upon the formation of feces and their evacuation. Bran is an excellent source of these insoluble fibers and explains the popularity of bran breakfast cereals, muffins, and other products among individuals experiencing constipation and diverticulosis. Soluble fibers can contribute to mass and moistness of feces but not to the same extent as insoluble fiber. However, it is important to recognize that both types of fibers are beneficial and should be sought out for general digestive health.

What Happens to Fiber in the Digestive Tract?

Contrary to starch, fiber is not broken down well by our digestive enzymes. This is partly explained by the manner in which the monosaccharides are linked together. Whereas digestive enzymes (amylases) produced by people are very efficient in breaking the links between monosaccharides in starch, these enzymes are generally ineffective at breaking the links between monosaccharides in fiber. Plants build these bonds in a special way.

In the stomach, soluble fibers attract and bind to water and in turn form a gel-like material. This gel entraps food components such as sugars, cholesterol and fats and slowly carries them through the remaining

digestive tract. Insoluble fibers, on the other hand, tend not to contribute to the formation of gels. Because soluble fibers dissolve in water, psyllium husk, inulin, FOS and others are used in supplemental fiber drinks as discussed below and in Chapter 13.

As fiber reaches the colon, bacteria begin to breakdown (ferment) some of the fibers for energy and in the process produce gases such as carbon dioxide, methane gas, and hydrogen gas. These gases often lead to uncomfortable bloating and flatulence associated with higher fiber intakes. Soluble fibers are more fermentable than insoluble ones. In addition other molecules, such as short-chain fatty acids, are produced by bacteria, which can be absorbed into the body. These fatty acids yield a small amount of energy and health benefits. Therefore, foods or supplements providing psyllium, beta glucan (oats or barley), inulin, FOS, cellulose, guar gum, xanthan gum, and oligosaccharides will be

fermented and you can expect gas production.

About How Much Fiber Do We Eat and What Are the Recommendations?

It is likely that we evolved on a high-fiber diet because of the unavailability of processing techniques. Some have estimated that our fiber consumption may have been as high as 50 grams daily when fiber-rich foods were more bountiful in our diet. Some current populations in Africa have been noted to retain high-fiber intakes. On the other hand, it is estimated the average American woman and man eats about 12 grams and 18 grams of dietary fiber daily, respectively. 

The Adequate Intake (AI) recommendation for total fiber intake for adults who are 50 years of age and younger is 38 grams per day for men and 25 grams for women daily. For adults over 50 years of age, the recommendation is 30 grams per day for men and 21 grams for women. Or 14 grams per 1,000 calories consumed. Table 4.7 provides an overview of fibers commonly used in nutrition supplements and as a food additive.

What Is Soluble and Insoluble Fiber?

Fibers are often classified as being either soluble or insoluble; however, plants tend to contain a mixture of both. When a food is said to be a soluble or insoluble fiber it means that the majority of the fiber found within it is of that kind. For instance, prunes and plums contain both fiber types, with the skin providing more insoluble fiber and the fleshy pulp providing more soluble fiber. Psyllium fiber is referred to as a soluble fiber food source although roughly a third of its fiber is insoluble.

Dietary fiber is important for heart and gut health, immunity, and mineral absorption.

Soluble fiber sources include psyllium husk, oats, barley and legumes as well as many fruits and vegetables, particularly apples and pears. Soluble fibers used as food ingredients include inulin, FOS, guar gum, and xanthan gum.

Insoluble fiber sources include wheat bran, whole-grain cereals and breads, corn bran, flax and other seeds, as well as many fruits and vegetables, such as berries, carrots, celery, green beans, and potato skins. As discussed in Chapter 1, solubility refers to how well a substance will interact with and dissolve in water. With regard to fiber, “soluble” refers the ability to form a gel in the digestive tract in which water is trapped. Soluble fiber supplement drinks can be used as a visual example of the gel-forming (sponge-like) properties of soluble fibers.

What Is Fiber?

Fiber isn’t a single nutrient but a family of plant-based nutrients that are generally resistant to human digestion. Since plants lack the bony skeletal design that provides much of an animal’s shape and form, fibers provide much of the structural support to plant cell walls and the plant in general. Plants also use certain fiber as the foundation for their scar tissue. It is important to remember that while humans and other mammals prefer to produce proteins like collagen as the structural basis of their bodies, plants use carbohydrates.

Fiber consists of non-starch polysaccharides such as cellulose, hemicellulose, gums, mucilages, pectin, and oligosaccharides along with other plants components such as lignin. Chitin is often considered a fiber because it is a polysaccharide. Chitin is found in the exoskeletons of shellfish such as lobster, shrimp and crab as well as some insects such as beetles and ants as well as in the cell walls of some yeast and fungi. Table 4.6 lists fiber content of certain foods.

Fructooligosaccharides (FOS), which are sometimes called oligofructose or oligofructan, are short links of fructose terminating in glucose. Inulin is similar to FOS, however the number of fructose molecules linked together can exceed 100. Both inulin and FOS are found in many plants including Jerusalem artichoke, burdock, chicory, leeks, onions, and

asparagus. FOS and inulin are often used as food additives as they addbulk and mild sweetness to foods while having health promoting properties.

What Happens to Stored Carbohydrate (Glycogen) During Exercise?

The hormone picture that develops during exercise is similar to the one discussed regarding a fasting period; however, there are relative differences. Epinephrine is released from our adrenal glands as a direct effect of exercise.

Quite simply, the greater the exercise intensity, the greater the epinephrine release. Epinephrine stimulates the breakdown of muscle cell glycogen (see Table 4.5 and Figure 4.3). This makes glucose available for the muscle cells hard at work. Epinephrine also promotes the breakdown of glycogen to glucose in the liver. Some of this glucose will then circulate to working muscle to provide support. Cortisol may also be released in response to moderate to intense exercise, particularly as the exercise becomes prolonged (for example, endurance cycling and running). Cortisol will also support the breakdown of glycogen as well as gluconeogenesis in our liver.

What Does Cortisol Do to Help Maintain Blood Glucose Levels During Fasting?

Cortisol is often regarded as the “stress hormone.” It is important to realize that fasting, especially prolonged fasting, is a form of stress—and stress results in the release of cortisol from the adrenal glands along with epinephrine mentioned in the previous question. Cortisol also supports the breakdown of glycogen and the conversion of amino acids, lactate,

and glycerol to glucose in our liver. Because cortisol also promotes the breakdown of our body protein, especially skeletal muscle protein, it ensures a supply of amino acids for conversion to glucose in our liver (Figure 4.5).

Exercise promotes the breakdown of carbohydrate stores in muscle.

How Does Epinephrine (Adrenaline) Help Maintain Blood Glucose Levels During Fasting?

During a fasting period, a little epinephrine (adrenaline) is released into circulation from our adrenal glands (see Figure 4.3 and Table 4.5). Among epinephrine’s many roles will be its influence upon the liver and skeletal muscle. It will support the effects of glucagon in the liver that were just mentioned. In skeletal muscle, the slightly elevated epinephrine will lightly promote the breakdown of glycogen to glucose. Contrary to the glucose produced from the breakdown of liver glycogen, this glucose is not released into the blood. Rather, this glucose becomes a supportive energy source for those muscle cells while fat is the major energy source. However, when this glucose is used for energy in those cells, a little bit of lactate may be produced. This lactate can enter circulation, reach the liver, and be converted to glucose. This glucose can then be released into the blood. Therefore, our skeletal muscle can modestly contribute to maintaining our blood glucose concentration during fasting.

How Does Glucagon Help Maintain Blood Glucose Levels In-Between Meals?

Glucagon works in a manner that is generally opposite to insulin. It will labor to increase blood glucose concentration, thereby returning it toward normal levels. To accomplish this, glucagon promotes the breakdown of liver glycogen to glucose, which is released into circulation.
 

Glucagon will also promote another activity in our liver that will generate glucose. The process is called gluconeogenesis, which literally means to create new glucose if you read its root words right to left. In this process, certain amino acids, lactate (lactic acid), and glycerol from our circulation will be taken up by our liver and used to make glucose. Like the glucose generated from glycogen breakdown, this glucose can also be released into our blood to maintain blood glucose levels.

How Is Blood Glucose Maintained In-Between Meals and Overnight?

The complete digestion and absorption of a meal can take several hours, depending upon its size and composition. Therefore, carbohydrate or more specifically glucose from that meal may be available for several hours as well. However, once this ends, a new blood glucose scenario begins to take shape. Cells throughout the body will continue to help themselves to glucose in the blood to help meet their energy needs. The net effect is that our blood glucose concentration will begin to decrease. When this happens the pancreas responds again. However, this time it responds by releasing the hormone glucagon into our blood (see Figure 4.3). In addition, epinephrine (adrenaline) and cortisol will promote efforts in different tissue that will help maintain blood glucose levels inbetween meals.

Does Sugar Cause Diabetes?

Over the years, many theories have evolved about the relationship between higher consumptions of sugar and various diseases and conditions. However, dietary sugar does not appear to promote the development of diabetes, at least not directly. As discussed above, diabetes can be largely categorized into two groups: those individuals that have a reduction in ability to make insulin (type 1 diabetes) and those individuals that appear to make insulin, but whose muscle and fat cells appear to be less sensitive to its presence (type 2 diabetes). In most cases of type 2 diabetes mellitus, one of the most significant underlying factors is an excessive body weight in the form of fat. So, if a person eats excessive amounts of sugary foods, which by simple excess of energy intake will lead to fat accumulation, obesity, and subsequent diabetes, then perhaps an argument can be made. However, sugar would then be an indirect factor, not a direct factor. On the other hand, high sugar foods such as soda, cookies, cakes, and pies can make it more difficult to manage diabetes because of their glycemic effect described above.

What Causes Type 2 Diabetes?

In type 2 diabetes mellitus, muscle and fat cells become less sensitive to insulin. What has become very clear to researchers, physicians, and nutritionists is that there is a strong relationship between obesity and this form of diabetes mellitus. In fact, nearly 90 percent of all individuals diagnosed with type 2 diabetes mellitus are also recognized as obese. In support of this relationship, most obese type 2 diabetics regain the ability to regulate their blood glucose as they reduce their body fat through weight loss and exercise. Although the relationship seems clear enough, the mechanism has been somewhat elusive to scientists. However, today, some evidence suggests that swollen fat cells themselves may release (and/or not release) chemicals that contribute to decreased sensitivity to insulin.

What Is Diabetes?

For many people, the fine regulation of the level of blood glucose becomes impaired. This results in chronic high blood glucose concentrations medically known as hyperglycemia. The impairment may be due to a decreased ability of the pancreas to produce insulin, which is the case in type 1 diabetes. The lack of insulin allows glucose levels to remain elevated even in a fasting state. Furthermore, after a meal blood glucose levels can climb exceptionally high (see Figure 4.4). For most people diagnosed with diabetes, blood glucose regulation is impaired despite their ability to produce insulin. In fact, many of these individuals produce more insulin than what seems normal, at least initially. This type of diabetes is referred to as type 2 diabetes.
 

In the past, type 1 diabetes has also been called insulin-dependent diabetes because medical treatment involves insulin therapy via needle injections or automated subcutaneous pumps. Insulin nasal sprays seem to be promising to simplify diabetes management. Type 1 diabetes has also been referred to as juvenile (or child-onset) diabetes because diagnosis is much more common in children. However, since type 1 diabetes can develop at any age, type 1 diabetes is the most correct terminology. Type 2 diabetes has also been called non-insulin-dependent diabetes mellitus, as medical treatment does not absolutely require insulin injections. However, because insulin injections may be prescribed from time to time this terminology is confusing. Furthermore, type 2 diabetes has been referred to as adult-onset diabetes mellitus since it is more commonly diagnosed in adults. Again, this is confusing as more children are being diagnosed with type 2 diabetes. While type 2 diabetes occurs in people of all ages and races, it is more common in US population among African Americans, Latinos, Native Americans, and Asian Americans/ Pacific Islanders, as well as the aged population.
 

Diabetes is an impairment to the processes that regulates blood glucose.

• Type 1 is caused by a lack of insulin production.
• Type 2 is caused by a failure of insulin to effectively regulate glucose levels.

Are Glycemic Index and Glycemic Load Important to Health?

Foods with lower glycemic responses are more desirable for people who are actively managing their blood glucose levels. This includes people with prediabetes and diabetes. The lower glycemic response could mean
less medication necessary to keep blood glucose levels in check. Furthermore, lower glycemic diets are often positioned as ideal to help people lose weight. Whether or not this is true remains to be conclusively determined, however, lower glycemic foods are associated with better satiety (fullness) and hunger control, which can be helpful to people trying to shed a few pounds. Lastly, lower glycemic foods are associated with a reduced risk of heart disease. We will discuss the application of glycemic index and load to weight loss and improved fitness in Chapter 8.

What Is Glycemic Load?

While the concept of glycemic index is pretty straight forward, it is not always easy to apply to how people eat. One issue with glycemic index is that the amount of food used to determine its glycemic index is not typically the amount of food consumed. A good example is boiled carrots which will have a glycemic index of about 90. Since one cup serving of carrots only has about 4 grams of available carbohydrate, rarely would a person eat enough carrots to achieve the level used to determine its glycemic index, which would be about 12 times that amount. That’s why researchers developed a second glycemic measure more appropriate for the “real world”, called glycemic load.
 

A food’s glycemic load is derived by taking the glycemic index and then multiplying it by the amount of digestible carbohydrate and then dividing by one hundred. For instance, carrots have a glycemic index of 90, which multiplied by 4 (grams of digestible carbohydrate) and divided by 100 gives you a glycemic load of roughly 4. See Table 4.4 for a listing of glycemic loads of common foods relative to glycemic index.

How Is Glycemic Index Determined?

Glycemic index is determined in a research lab. Fasting people are fed 50 grams of either pure glucose or enough white bread to provide 50 grams of digestible (non-fiber) carbohydrate, and blood glucose is measured over the next 2 hours. On a different day, the same people would be provided a food in an amount to allow for 50 grams of digestible carbohydrate and again blood glucose is measured over the next 2 hours. If a food raises blood glucose to 50 percent of the rise caused by glucose then the glycemic index is 50.
 

Because of the difference between white bread and pure glucose, glycemic indexes determined for foods using these different standards can vary. The glycemic index scale when using pure glucose is 0 to 100 and is
more common because it is a little easier for the public to use. Meanwhile, when white bread is used as the standard for determining glycemic index, several foods, such as a baked potato, rice cakes, jelly beans, and Cheerios® have a value greater than 100. When this book discusses the glycemic index of foods we will use glucose as the standard as per the values of the Human Nutrition Unit at the University of Sydney (www.glycemicindex.com).

Why Does Glycemic Index Vary Among Foods?

To understand why different carbohydrate-containing foods have a different glycemic index, we can start with the type of monosaccharide derived from a food. This is important because fructose and galactose do not raise blood glucose to the same extent that glucose does. For instance, the digestible carbohydrate in breads and potatoes is starch, which is made up of glucose. Meanwhile, milk and milk products contain lactose which is made up of glucose and galactose. Based on the difference in glucose content between starch and milk products, it is predictable that milk would have a lower glycemic index than bread.
 

Ripened fruits contain mostly fructose and glucose as well as some sucrose. For example, a medium apple contains about 8 grams of fructose and 3 grams of both glucose and sucrose. Meanwhile a medium banana contains between 5 to 6 grams of both fructose and glucose and 2 grams of sucrose. One tablespoon of honey contains 8 grams of fructose and 7 grams of glucose and less than 1 gram of sucrose, galactose, and maltose combined. So even though fruits and honey are very sweet, they will have a moderate glycemic index and load (see Table 4.4).

Glycemic load is a glycemic index adjusted for a standard serving size.
 

In addition to monosaccharide type, protein, fiber, and fat, as well as the processing of a food can influence its glycemic index. Fiber and 

fat seem to be able to slow the digestion process and thus can lower glycemic index. Certain types of fiber, often referred to as viscous fibers, can thicken the digestive contents in the stomach and small intestine, sort of like thickening up gravy with starch. This slows the digestion of carbohydrate and absorption of monosaccharides, which in turn reduces the rise in glucose.
 

Some amino acids in protein can increase the level of insulin released in response to carbohydrate and thus decrease glycemic index. Meanwhile, pasta has a lower glycemic index than what might be expected of such a high starch food. That’s because starch molecules become trapped within gluten protein networks within the dough. Thus, wheat-based pastas have a relatively lower glycemic index value than expected and relatively lower than pastas made from other grains (for example, rice or corn) which don’t contain gluten.

What Is Glycemic Index?

As expected, the level of circulating glucose increases after eating a carbohydrate-containing meal. But to what level, and will different foods having the same amount of carbohydrate result in the same increase in blood glucose? This kind of information surely would be of interest to many people, especially those managing their blood glucose levels (such as in diabetes).
 

As shown in Figure 4.4, the level of glucose circulating in the blood increases after eating or drinking a carbohydrate-containing food or beverage and then is reduced back toward the normal fasting level. This response is often referred to as a glucose tolerance curve and it can be used to assess how well a person’s body is able to take glucose out of the blood and use it for energy and to build stores.
 

Since different foods will produce different glucose tolerance curve patterns, scientists developed the glycemic index. Simply put, glycemic index is a measure of the power of carbohydrate-containing foods to raise blood glucose levels after being eaten or drunk. In addition to people managing their blood glucose levels, glycemic index has become popular for many people trying to lose weight which will be discussed this in more detail in Chapter 11. See Table 4.4 for standard levels for glycemic index and load.

Glycemic index is a measure of a food’s ability to raise the level of blood glucose.
 

For a long time it was assumed that because starch was more structurally complex than simpler sugars, starchy foods would be digested more slowly and therefore absorbed more slowly and evenly after a meal. On the other hand, foods containing simpler sugars (for example, soda and candy) would be digested and absorbed more rapidly, leading to a faster and greater rise in blood glucose. However, the relationship between different foods and blood glucose turned out to be more complex, which is why the determination of glycemic index for individual foods has been helpful.

Can Eating a Low Carbohydrate Diet Make Us Fatter?

As will become clearer in Chapter 8, eating too much energy makes us fat, not too much of any one energy nutrient such as carbohydrate. Without question eating a high carbohydrate diet in conjunction with eating excessive energy will certainly support weight (fat) gain; so too will eating excessive fat and/or protein.
 

One of the reasons that carbohydrates have been bashed as of late is because of the effects of insulin upon stored fat. Insulin hinders the release of fat from adipose tissue. Therefore many dieters believe that carbohydrates, or more specifically insulin, are working against them. However, this function of insulin is very important in the normal scheme of things. By design, insulin keeps the fat tissue from breaking down and releasing fat during and for a couple of hours after a meal. At this time absorbed food energy nutrients are circulating in our blood so there would be no need to break down our fat stores. Insulin will also promote the formation of fat from excess diet energy. So, the combination of decreased fat breakdown and increased fat production may lead people to believe that insulin makes them fat!
 

Before we dismiss the notion that insulin is working against people in their quest to lose body fat, we should recognize that many people have elevated insulin and glucose levels during fasting. More times than not this occurs in people who have a higher level of body fat and low levels of activity. Thus eating a higher carbohydrate diet may indeed work against them to some degree. And eating a lower carbohydrate diet would allow for a higher proportion of fat to be used for energy. We discuss this more in Chapter 8.
 

High carbohydrate diets can increase body fat when too many calories are consumed by decreasing the burning of stored fat and forming new fat.