Lipids refers to a broad class of molecules that includes fatty acids, triglycerides, phospholipids, cholesterol, waxes and the fat-soluble vitamins, A, D, E and K. The Institute of Medicine recommends that the diet include between 20 and 35 percent of total energy calories as lipids. The process of lipid metabolism encompasses lipid digestion, transport, storage, breakdown and synthesis.
Lipid Digestion
Lipids are fat-soluble and in order to be processed by enzymes in the small intestine they must be first surrounded by bile salts. Bile salts are produced in the liver and secreted from the gall bladder into the small intestine. The bile salts act similar to soap and surround fats producing little bubbles of lipids. The pancreas secretes enzymes that breakdown the lipids into smaller molecules that are absorbed by cells in the intestine. Inside these cells the lipids are packaged up into large molecules that contain fats, cholesterol and proteins.
Lipid Transport
The large lipid and protein carrier molecules are distributed to other cells in the body by the circulatory system. If there is an excess of lipids in the blood the liver takes them up and repackages them into lipid protein complexes, called low density lipoproteins or LDLs. The LDLs transport the lipids to muscle where they are used to make energy or to fat tissue, called adipose where they are stored.
Lipid Storage
The adipose cells take up the lipids from the carrier molecules in the blood and convert all lipids to triglycerides. The triglycerides are packed together in lipid droplets in adipose cells. In contrast to the storage of glucose, storage of triglycerides requires no water and therefore is much less dense.
Lipid Breakdown
When energy is required by the body, the triglycerides stored in adipose cells are broken down into fatty acids and transported to the parts of the body that require energy. To make energy cells breakdown the fatty acids completely in a process called beta-oxidation. This process yields about nine kilocalories of cellular energy, which is more than twice the yield from carbohydrates or proteins.
Lipid Synthesis
If there is an excess of glucose in the blood the liver takes up the glucose and converts it to fat. This process is called lipogenesis. Glucose and rarely amino acids from proteins are metabolized in the liver to molecules that contain two carbons. The two-carbon molecules are assembled into long chain fatty acids. The fatty acids are attached to a glycerol forming triglycerides, which are then transported via LDLs to adipose tissue for storage.
Lipid Functions
All cells in the body are separated from their surroundings by membranes. The major components of membranes are a type of lipid called phospholipids and cholesterol. As mentioned, lipids in the form of triglycerides are the main way the body stores energy. Other functions of stored triglycerides include organ protection and insulation. Many types of lipid molecules act as chemical messengers and regulate cell growth, metabolism and immune system function among many other actions. Cholesterol, a type of lipid is the precursor to steroid hormones like testosterone and estrogen and the skin synthesizes vitamin D from cholesterol. The bile acids that aid in lipid digestion are actually made from cholesterol. The fat-soluble vitamins, A, D, E and K perform many essential functions in the body like maintaining vision, immune system function, reducing oxidative stress and blood clotting.
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