Although fat often gets a bad reputation as being unhealthy, it is actually a vital part of the human diet. Fats, or lipids, are the most energy-dense nutrients. Digestion breaks dietary lipids down into fatty acids, which the body uses in a variety of ways. Although high consumption of lipids may be harmful, the body needs a moderate amount to be healthy.
Types
The chemical structures of lipids vary, but all lipids share one common characteristic: They do not readily dissolve in water. The three major types of lipids are triglycerides, phospholipids, and sterols. Triglycerides, made from three fatty acids attached to a glycerol backbone, are the most common type of lipid in food and in the body. Phospholipids are structurally similar to triglycerides except that one fatty acid is replaced by a phosphorus-containing chain. Phospholipids play an important role in the digestion of dietary triglycerides. Sterols, such as cholesterol, have a ringed structure and are important in the formation of bile.
Digestion and Absorption
Enzymes throughout the digestive track are responsible for digesting dietary lipids. Although the stomach can digest smaller lipids, most digestion does not start until lipids reach the small intestine. In the small intestine, the enzyme lipase, secreted by the pancreas, acts in conjunction with bile, secreted by the gall bladder, to break lipids down into individual fatty acids and glycerol. Cells in the small intestine absorb the fatty acids and glycerol. At this point, the larger fatty acids are remade into triglycerides and transported by lipoproteins to the liver via the lymphatic circulatory system.
Lipoproteins
Because lipids don't mix with water -- the main component of blood -- they must move through the blood on transporters called lipoproteins. As the name suggests, lipoproteins have both a lipid and protein component. The four major types of lipoproteins are chylomicrons, very-low-density lipoproteins, or VLDL, low-density lipoproteins, or LDL, and high-density lipoproteins, or HDL. Chylomicrons transport remade triglycerides from the intestine to the liver and eventually into the bloodstream. In the blood, the remade triglycerides are broken down, allowing surrounding cells to absorb and use the free fatty acids. VLDL and LDL both carry lipids made by the liver to the cells in the body. HDL removes the lipids, allowing them to be excreted from the body.
Functions
After digestion and absorption, lipids perform a number of functions in the body. The body stores lipids as triglycerides, which serve as the main source of energy. Lipids in the small intestine help to transport fat-soluble vitamins, and both phospholipids and cholesterol are important parts of cell membranes. Cholesterol is also important in the production of steroid hormones.
References
- "Contemporary Nutrition"; Gordan Wardlaw, et al.; 2006
- Rensselaer Polytechnic Institute; Lipoproteins: Lipid Digestion and Transport
- Colorado State University: Absorption of Lipids; R. Bowen
