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Facts About Lipids

By Matthew Fox, MD

Molecules can be divided into two types: There are those that are polar and have magnetically positive and negative poles such as water. Non-polar molecules are the second type and include lipids. These fat-like molecules do not mix well with polar molecules -- like oil on water. Lipids play important roles in the functioning of cells and the health of the body; excessive lipids in the blood are linked to adverse health conditions.

Lipid Functions in Cells

At the cellular level, lipids function to store energy, protect cells and make signaling molecules. Fat molecules contain 9 dietary calories per gram, which is more dense than carbohydrates or protein. The fat can be stored in cells and used for energy when the body needs a source of calories not adequately supplied by the diet. The breakdown of lipids can contribute to the formation of ATP, the main energy-carrying molecule of the body. Lipids protect cells by composing most of the cell membrane. The cell membrane surrounding the cell is analogous to the skin. Since it is non-polar, the polar, water-based fluids inside and outside the cell do not penetrate the membrane. Finally, lipids are used to make hormones and other messengers that tell cells what functions to perform.

Types of Lipids

Common types of lipids include fatty acids, triglycerides and cholesterol. Fatty acids are long non-polar carbon chains with oxygen on the very end. Triglycerides have three long fatty acids linked to the small molecule glycerol. Cholesterol is a ringed structure that is mostly non-polar, but also has a polar end. In the blood, these lipid molecules are bound to proteins to make molecules called lipoproteins. Two of the more well known lipoproteins include low-density lipoprotein, or LDL, known as the "bad cholesterol" and high-density lipoprotein, or HDL, known as the good cholesterol.

Lipids in the Diet

Lipids on food labels are listed as fats and cholesterol. There are fats are bad for you and those that are good or better for you. You should avoid saturated and trans fats. Monounsaturated and trans fats are better for you. Cholesterol should be limited in the diet. Even though it is important for maintaining the flexibility of the cell membrane and making steroid hormones, the liver is able to synthesize enough of it; excessive cholesterol in the diet, along with saturated and trans fats increase the risks of disease.

Complications from High Lipids

High levels of triglycerides, LDL and total cholesterol in the blood are linked to adverse health conditions. These substances can lodge in the tissues. They contribute to the narrowing and hardening of the arteries, called arteriosclerosis. This increases the chances of developing restriction of the blood supply to the extremities, known as peripheral vascular disease; restriction of blood flow to the intestines, known as intestinal angina; kidney damage; heart attacks and strokes.

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