Lipid metabolism breaks down fatty acids for energy or stores them for later use. Triglycerides are a component of fats and can be found in blood vessels and adipose tissue cells. They are energy-dense nutrients and are broken down by lipase into fatty acids and monoglycerides before being transported by chylomicrons. Triglycerides can be stored in liver cells or fat cells and used for energy when needed.
Lipid metabolism is the process by which fatty acids are digested, broken down for energy, or stored in the human body for later energy use. These fatty acids are a component of triglycerides, which make up most of the fats humans eat in foods such as vegetable oils and animal products. Triglycerides can be found in blood vessels and stored for future energy needs in adipose tissue cells, better known as body fat, and in liver cells. Although the body’s main source of energy is carbohydrates, when this source is depleted, the fatty acids in triglycerides will then be broken down as a backup energy source. Examples of times the body draws energy from fat metabolism are during exercise, when the supply of glycogen, or the stored form of glucose, is depleted, or when the diet does not contain enough carbohydrates to meet energy needs of the body.
Triglycerides, also known as lipids or fats, are well suited to their role as a form of stored energy as each gram provides 9 calories (37 kilojoules), while carbohydrates provide only 4 calories (17 kilojoules) per gram. Because calories are units of energy, fats are considered energy-dense nutrients. Triglycerides are made up of three chains of fatty acids linked to a hydrogen-containing compound called glycerol, fatty acids that can be released during lipid metabolism when the body requires these calories for energy.
The first step in lipid metabolism is the consumption and digestion of triglycerides, which are found both in plant foods such as olives, nuts and avocados, and in animal foods such as meat, eggs and dairy products. These fats travel through the digestive tract to the intestines where they cannot be absorbed as triglycerides. Instead, they’re split via an enzyme called lipase into fatty acids and, most often, a monoglyceride, which is a single chain of fatty acids attached to a glycerol. These broken down triglycerides can then be absorbed through the intestines and reassembled back into their original form before being transported by chylomicrons, a type of cholesterol-like substance known as lipoprotein, into the lymphatic system.
From the lymphatic system triglycerides enter the bloodstream, where the process of lipid metabolism can be completed in three ways, as they are transported to the liver, muscle cells or fat cells, where they are stored or used for energy. If they end up in liver cells, they are converted into a type of “bad” cholesterol known as very low-density lipoprotein (VLDL) and released into the bloodstream, where they work to transport other lipids. Triglycerides sent to muscle cells can be oxidized in those cells’ mitochondria for energy, while those sent to fat cells will be stored until needed for energy at a later date. This results in an increase in the size of fat cells, visible on a person as an increase in body fat.
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