Metabolism refers to the physical and chemical processes that occur inside the cells of the body and that maintain life. Metabolism consists of anabolism (the constructive phase) and catabolism (the destructive phase, in which complex materials are broken down). The transformation of the macronutrients carbohydrates, fats, and proteins in food to energy, and other physiological processes are parts of the fat metabolism process. ATP (adinosene triphosphate) is the major form of energy used for cellular metabolism.
The fuel that your body runs on is called glucose and fat metabolism is important for this. You get most of your glucose from carbohydrates. Ideally, you will only take in enough to use. If you take in more carbohydrates than you can break down you will store it as fat. If you begin to use more carbohydrates than you need to fuel your body, you have alternate pathways to break down fat that can be converted into glucose. So the fat is converted back into carbohydrate and used as fuel for your body.
If you’re not sure what fat metabolism actually is, here’s an easy definition: metabolism is the amount of energy your body uses to maintain itself. This maintenance includes processing, using, transporting, acquiring, and disposing of food and nutrients. So your body burns units of energy — better known as calories — to process the food you put into it.
Fat is the means through which your body stores energy in the form of large complex molecules. When you lose weight your body tapes into its energy stores (fat) and breaks down these molecules through fat metabolism. These molecules end up in all sorts of places, sometimes as amino acids, sometimes as other proteins, but most of the time they are broken apart and filtered through your metabolic process to produce energy.
Fats contain mostly carbon and hydrogen, some oxygen, and sometimes other atoms. The three main forms of fat found in food are glycerides (principally triacylglycerol [triglyceride], the form in which fat is stored for fuel), the phospholipids, and the sterols (principally cholesterol). Fats provide 9 kilocalories per gram (kcal/g), compared with 4 kcal/g for carbohydrate and protein. Triacylglycerol, whether in the form of chylomicrons (microscopic lipid particles) or other lipoproteins, is not taken up directly by any tissue, but must be hydrolyzed outside the cell to fatty acids and glycerol, which can then enter the cell as a part of fat metabolism.
Dietary fat (which is digested and then re-synthesized into triglycerides) is non-polar and must be carried in the circulation as lipoproteins. The protein molecules provide a polar coat for the non-polar lipid and thus enable transportation in the polar (water based) bloodstream. The lipoproteins, which transport triglycerides derived from the diet to adipose tissue, are called chylomicrons. In fat metabolism, storage in the adipose tissue is catalyzed by lipoprotein lipase, the activity of which is stimulated by insulin (the same hormone which stimulates storage of glucose as glycogen).