Fat or lipid, is categorized as fattay acids (FA), triglycerides (TG), phospholipids, and sterols. From a human perspective the three most common fattay acids are palmitric acid, oleic acid, and stearic acid. There are essential fatty acids (EFAs) that humans must ingest for good health because the body requires them but can’t make them from other nutrients.
The functions of fats are to: make up part of the structure of cells, form a protective cushion and heat insulation around vital organs, carry fat soluble vitamins, and provide a reserve storage for energy.
Figure 1 – Shows an adipocyte used for fat storage. Triglyceride (TG) molecules that are hydrolized have 2 major fates, either reesterfication to a TG or leave the fat cell to become a free fatty acid in the blood stream.
Fat is stored as triglyceride in fat cells. They are formed in the intesntinal cells and released into the lymphatic system as lipoprotein particles known as chylomicrons. Fat provides energy for the body wen broekn down to release glycerol and free fatty acids (9 calories/gram). This is the process of lipolysis where trigylcerides are hydrolyzed and broken down to yield glycerol and 3 FA molecules. The hormone insulin is the most potent inhibitor of lipolysis. Thyroid and growth hormone have been shown to have a positive effect on lipolysis. In addition to decreasing the rate of lipolysis, insulin also promotes lipogenesis, or glucose uptake into fat cells.
During light to moderate intensity exercise, most FA produced by lipolysis leave the adipocyte. When fatty acids leave a fat cell and enter the blood they become attached to the blood protein albumin since FA are not soluble in the aqueous plasma. When energy sources are low or a stress such as exercise is applied, FA synthesiszed stops, and glycolysis begins. If the activity is done at a lower rate, measured by oxygen consumption (VO2), then fat will be the primary source of energy used for the body.
Figure 2 – Shows that as intenesity increases, the primary fuel source goes from fat to carbohydrate. This is why zone 1 or zone 2 are typically called “aerobic’ or ‘fat burning zones’ . As the intensity is increasted the body is forced to use glucose for energy (stored in the liver) because it can be generated faster and at a higher rate.
Sources of fat.
