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.

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.

What Does Our Body Do with the Glucose from a Meal?

Insulin directs muscle, fat tissue, and the liver to use glucose, fructose and galactose as the primary fuel. This allows for a lot of carbohydrate entering the body from a meal to be used for energy immediately. In

addition, insulin directs muscle and liver, and to a lesser extent other tissue, to store extra carbohydrate as glycogen. Glycogen is composed of large branching links of glucose and is very similar to plant starch. However, only so much glycogen can be made and stored, since it is meant to be a short-term not a long-term energy reserve.

How Does Our Body Respond to the Rise in Blood Glucose?

The concentration of glucose in the blood is very tightly regulated. When the level of circulating glucose climbs above the normal fasting level, the pancreas releases the hormone insulin (see Figures 4.2 and 4.3). Insulin will interact with receptors on muscle cells and fat cells and promote the movement of glucose into these cells. Because skeletal muscle and fat cells together tend to make up more than half of our total body mass, the net effect is a fairly rapid lowering of the glucose concentration. Insulin increases the movement of glucose in these cells by increasing the number of glucose transport proteins on their plasma membranes. As the level of glucose returns to the normal fasting level, the pancreas responds by releasing less insulin into circulation.
 

All cells in our body will continuously take glucose from our blood throughout the day to help meet their need for energy. However, after a meal, the liver, muscle, and fat cells will take a lot more glucose out of the blood than they immediately need. This allows blood glucose levels to quickly return to a normal fasting concentration.
 

Increased blood glucose levels causes the release of insulin to process, use, and store carbohydrate.

What Are Oligosaccharides and Starches?

Monosaccharides not only serve as building blocks for disaccharides but also for some larger forms of carbohydrates as well. The most recognizable larger carbohydrate is starch. Starch is found in varying degrees in plants and their products (for example, legumes, vegetables, fruits, and grains). It consists of large, straight and branching chains of the

monosaccharide glucose (Figure 4.1). Some shorter, branching chains of glucose can be found as well, and food manufacturers will also use these in the production of foods. The short, branching chains used by food manufacturers are often called maltodextrins and is typically derived from the partial digestion of corn starch.
 

In the human diet, we can also find a small amount of carbohydrates, called oligosaccharides, constructed from just a few monosaccharides (three to ten) linked together. Since these are found in relatively small amounts, they are not as essential to discuss. However, a few of these carbohydrates (for example, raffinose and stachyose) will require mention later on, not only for their nutritional value but for their effects within the digestive tract.
 

Plants make starch to store energy kind of like mammals store fat. Plant fibers, on the other hand, are not necessarily stored energy but serve more structural roles for plants. Like starch, fiber is also composed of straight and branching chains of monosaccharides, but their monosaccharides building block are not limited only to glucose. Fibers are discussed later in this chapter.

What Are Disaccharides?

Looking at Table 4.1 we see that glucose is one-half of the disaccharides lactose and sucrose and both halves of maltose. Maltose, or malt sugar,

may be part of our diet naturally in seeds or alcoholic beverages. Sucrose is derived from the sugar cane plant and the beet, and the sucrose-rich product is called “sugar.” Lactose is the primary carbohydrate found in milk and dairy products. Nutrition scientists often refer to monosaccharides and disaccharides as “simple sugars” because of their relatively small carbohydrate size and their sweet taste. Table 4.2 presents the relative s eetness of simple sugars and compares them with sugar alcohols and artificial sweeteners.
 

Monosaccharides such as glucose and fructose are the smallest carbohydrate and are used to build more complex carbohydrates.