Understanding Cholesterol

Did you know that the food you eat has a major influence on your cholesterol and other blood lipid levels? These blood lipid levels, along with many other less important factors, influence the health of your arteries. Most heart attacks and strokes in America result from clogged arteries, also known as atherosclerosis, the most common disease of the major arteries.

What are cholesterol and triglycerides?The two major blood lipids your doctor measures to help predict the health of your arteries are cholesterol and triglycerides. Cholesterol is a waxy-like substance that is found in all animal fats and is also part of all animal cell membranes. Triglyceride is simply the chemical name for fats and oils.The liquid part of human blood is basically salty water. Cholesterol and triglycerides do not dissolve in water. In order to transport triglycerides and cholesterol throughout the body, these lipids must be combined with special proteins. This combination of lipids and proteins are referred to as lipoproteins.What are HDL, LDL and VLDL?Doctors usually measure blood lipoprotein levels after an overnight fast. In the fasting state there are low-density lipoproteins (LDL), high-density lipoproteins (HDL) and very-low-density lipoproteins (VLDL). Density refers to the buoyancy of these particles. Fat floats and proteins sink in water. The VLDL carry most of the triglycerides, which make it less dense. The HDL particles have the most protein relative to lipid and so are the least buoyant or most dense.The VLDL is produced in the liver and functions to take triglycerides and other fat-soluble substances from the liver to fat cells and other tissues. After most of the triglyceride in the VLDL is removed it loses some of its proteins and becomes the LDL. Most of the LDL is then removed from the blood by special receptors located on liver cells.Some people however inherit a tendency to make fewer of these LDL-receptors and so LDL is more slowly cleared from the blood and begins to accumulate in the arteries to form plaque. While genetics does contribute to atherosclerosis, diets high in saturated fatty acids, trans fatty acids and/or cholesterol are the major cause of elevated LDL levels. When one consumes the average American diet, which is so high in these substances, the number of LDL receptors on liver cells drops. As a result, the LDL cholesterol particles keep circulating in the blood and more of them get picked up by the artery wall. In the artery these LDL particles can be modified by oxygen and a variety of other chemicals. Once modified, LDL becomes toxic and triggers atherosclerosis.The importance of HDLHDL particles are produced by the liver and small intestine. They start out as proteins, which are designed to pick up and remove excess cholesterol from tissues and bring it back to the liver for disposal. Low HDL-cholesterol levels make it more difficult for the body to keep excess cholesterol from building up in the arteries.Bottom LineDrs. Brown and Goldstein (who won the Nobel Prize for their work on LDL-receptors) stated “If the LDL receptor hypothesis is correct, the human receptor system is designed to function in the presence of an exceedingly low LDL level. The kind of diet necessary to maintain such a level would be markedly different from the customary diet in Western industrial countries and much more stringent than moderate low-cholesterol diets of the kind recommended by the American Heart Association (AHA). It would call for the total elimination of (fat containing) dairy products as well as eggs (yolks, not whites), and for a severely limited intake of meat and other sources of saturated fats.”1The table above shows typical blood lipid levels for middle-aged and older Americans. Note that AHA’s “desirable” levels and are not optimal levels. For example, someone with a total cholesterol of 195, an HDL of 35, triglycerides of 175 and an LDL of 125 would have a “desirable” reading according to NCEP and AHA. But they would have an above average risk of dying of a heart attack. However, the optimal blood lipid values shown below would prevent the build up of atherosclerotic plaque in most people. Indeed, research has shown that most people, even with very advanced atherosclerosis, can reverse this disease if they follow a healthy diet and lifestyle and maintain optimal blood lipid values.2

References1. Brown MS and Goldstein JL Scientific American November 1984; p58-662. Ornish D, et al. JAMA 1998;280:2001-7.