Body Fat: Energy Source
and So Much More ...
How do we store fat?
Although many tissue in our body have the ability to house fat as an energy store, their capabilities are very limited. Adipose or fat cells appear to be uninhibited in their ability to store fat. Fat cells are comprised of 80-90% triglyceride which exists mainly as a single large droplet. A body fat level for a lean person will be in the mid to upper teens and under 25% is considered fairly normal. Meanwhile many athletes can be below 15% body fat while figure and physique competitors can be 4-8% body fat. Being 0% body fat is impossible based on fat present in cell membranes, organs and in tissue like muscle and the heart.
How many fat cells do we have?
It would appear that we are not born with a full complement of fat cells. The number of fat cells in our body increases at various stages throughout our growth, but, by the time adulthood is reached the total number is mostly fixed. There is some evidence, however, to suggest that even in adulthood we can increase our fat cell numbers by chronic overeating.
Is all of the fat stored in fat cells derived from fat in our diet?
When we eat a meal containing fat, it is absorbed and ultimately circulates within chylomicrons. As it circulates, fat is slowly transferred from chylomicrons to fat cells, primarily, and skeletal muscle cells and the heart, secondarily. The fat cells will store the fat. Skeletal muscle cells and the heart, however, have a limited ability to store fat and will mostly use it as an energy source. Also, during a meal, absorbed carbohydrate and amino acids in excess of our immediate energy needs can be used to make fat. This process takes place in our liver and in fat cells. Insulin promotes this activity in both locations. The fat made in fat cells is stored within those cells while the fat made in our liver is packaged up and relocated mostly to our fat cells for storage.
How and when do we remove fat from fat cells?
The fat stored in fat cells is available to us when we exercise and/or when food is not being absorbed. The process of mobilizing fat from fat cells is promoted by the hormones circulating in our blood when we are fasting and/or exercising. The hormones glucagon, epinephrine, and cortisol will all promote the release of fat from fat stores. In order for fat molecules to be released from our fat cells they are first dismantled to fatty acids and glycerol. Both fatty acids and glycerol will then enter our blood and circulate. However, because of their general water-insolubility, free fatty acids will hitch a ride aboard a protein in our blood called albumin. Contrarily, glycerol is fairly water-soluble and can dissolve into our blood.
The circulating fatty acids are removed by most cells in our body, especially skeletal muscle, the heart, the liver and other organs, to be used for energy. Contrarily, cells of the CNS and RBCs cannot use fatty acids for energy. During exercise, fatty acids become a major fuel source for working muscles. Glycerol, on the other hand, can be removed from our blood by our liver. It will then be used to make glucose, which certainly will be important during exercise and fasting to maintain a normal blood glucose concentration.
What does our liver do with cholesterol?
Not only will our liver make a fair amount of cholesterol on a daily basis, but, it will also receive cholesterol from diet-derived chylomicrons as well. Like fat, most of the cholesterol transiently found in our liver is destined for other tissue throughout our body. Once cholesterol reaches other tissue, it may be used to make some of the substances listed above or become part of cell membranes. Some cholesterol in our liver is also used to make bile salts, a key component of bile.