Your body requires energy to function: each cell relies on a source of energy to drive the chemical reactions required for digestion and metabolism, cellular communication, cell division and growth, hormone synthesis and several other physiological processes. Your body can use both carbohydrates and lipids, or fats, to store energy.
Both carbohydrates and lipids serve as sources of energy, but these compounds contain different capacities for energy storage. Each gram of carbohydrates stores 4 calories of energy, whereas each gram of lipid stores 9 calories. As a result, lipids serve as a more compact way to store energy, since it contains more energy per gram than carbohydrates. As a result, your body tends to use fat to store energy over long periods of time and uses carbohydrates to store energy short-term.
In general, your body stores lipids in the form of triglycerides. Each triglyceride contains three fatty acid compounds, all bound to a glycerol chemical backbone. Most trigylcerides are stored within adipose tissue, made up of fat cells distributed throughout your body, though your blood also contains small amounts of tryiglycerides. Carbohydrates in your body are stored as glycogen, a large carbohydrate molecule made up of hundreds or thousands of smaller units of glucose, a simple sugar. Your body can also convert carbohydrates into fat for long-term energy storage.
Glycogen in your system serves as a source of glucose for your cells. A number of your tissues, including your brain and muscles, utilize glucose as a source of energy to support metabolic functioning. When your body requires glucose, your liver and muscles begin to break down their glycogen stores, releasing glucose. Some of this glucose may be used directly in your liver or muscle tissue, while other glucose is released into your bloodstream to be taken up and used by other tissues throughout your body.
Your body can also break down triglycerides as a source of energy. If your body requires energy and glucose isn't available, your adipose tissue begins to break down fatty acids into molecules that your cells can utilize to produce useable energy. Lipid breakdown proves important to weight loss. Your body utilizes energy from stored fat to maintain your tissues, so you lose fatty tissue and can potentially lose weight.