Fat cells provide excellent sources of energy (nine calories per gram) and are stored in your adipose and muscle tissues. Unlike carbohydrate metabolism, fat follows a different metabolic path and requires oxygen, water and some glucose to be used for energy. However, having too much fat in your body can lead to many diseases and disorders associated with obesity.
Function
In addition to providing energy, your body fat provides insulation to keep you warm and cushions organs from damage such as blunt trauma and shock. It also stores excess fat-soluble vitamins (A, D, E, K) so that they can use released into the bloodstream and provide cellular functions if you don't consume enough of these vitamins.
Digestion
Fat digestion begins in the stomach, where the enzymes lingual and gastric lipase break triglycerides into smaller chains of fatty acids. In the small intestines, a hormone called cholecystokinin (CCK)--secreted in the duodenum, the first segment of the small intestine--stimulates bile release from the gall bladder and lipase from the pancreas, which break down fat into its basic components: glycerol and simple fatty acid chain.
Absorption
Because fat is hydrophobic, it needs a medium to transport through the bloodstream. Bile acids emulsify fat and carry it to the villi of the small intestines for absorption. Once absorbed, the fatty acid chain and glycerol are repackaged into triglycerides in the absorptive cells (enterocytes) of the small intestines.
Transport
From the enterocytes, triglycerides are packaged into lipoproteins, which are surrounded by a thin membrane of cholesterol and protein. These compounds are called chylomicrons. Then they travel through the lympathic system and into the bloodstream. Once they travel in the bloodstream, the chylomicrons are broken down again into glycerol and fatty acids. Muscle, adipose and other cells in the vicinity absorb most of the fatty acids to be used for fuel (muscles) or stored as energy (adipose tissues).
Energy Use
Aerobic exercise, such as jogging and cycling, expends calories slower than anaerobic exercise but releases more energy. Fifty percent to 60 percent of the energy comes from fat metabolism; the remainder comes from carbohydrates and protein.
Anaerobic exercise (weight-lifting, sprinting) requires carbohydrates as the main fuel source (80 percent to 90 percent during exercise). Although very little or no fat is used during exercise, your body switches to aerobic respiration after the exercise, in which fat is used as the primary source of fuel.
References
- "Perspectives in Nutrition;" Gordon Wardlaw and Margaret Kessel; 2002
- "Revive Your Metabolism": John Berardi; 2006
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