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.