What is HDL Cholesterol?

It has been recognized for many years that high HDL is associated with lower risk of heart disease, and the National Cholesterol Education Program has recommended that an HDL level of less than *35 mg/dl should be regarded as a risk factor. In people lucky enough to have a very high HDL (85 or more) the risk of heart disease remains less than half the average level even when the LDL is quite high (220). And conversely, when the HDL is very low, the risk may be high even when the LDL is low. In other words, the best predictor of risk is provided by the ratio of the LDL to HDL.

There is a two-way traffic of cholesterol in and out of the tissues; high LDL cholesterol levels in the blood promote the entry of cholesterol into the vessel walls and the formation of atheromatous plaque, while high HDL levels lead to its removal. HDL probably has two other beneficial effects: first, it may inhibit the transformation of LDL cholesterol to its more toxic oxidized form, and second, it may make blood platelets less sticky. All of these actions would tend to retard the formation of atherosclerotic plaque.

Like all the other blood lipids, your HDL is partly determined by your genes, but there are other influences, as shown below.

Factors Affecting HDL

Raise HDL	Lower HDL
Exercise	Physical inactivity
Alcohol	Smoking
Weight loss	Obesity
Statin drugs	Very low fat diet
Niacin	Diuretics (small effect)
Gemfibrozil	Beta blockers (small effect)

HDL levels are often low in people with a sedentary lifestyle, and high in marathoners. To improve your HDL with exercise, you need to do enough to make you start to lose weight. Another potent way to raise it is to drink alcohol, although few doctors would recommend it as a remedy, because of its other potentially harmful effects. Diet has relatively little effect on HDL, although weight loss, whether produced by diet or exercise, does tend to raise it. Quitting smoking may also help. Some blood pressure lowering medications (for example beta blockers) may lower HDL, but the effect is usually very small, and often transient. HDL levels can be raised by the statin drugs, gemfibrozil (Lopid), and niacin.

The evidence that raising HDL reduces your risk of coronary heart disease comes mainly from two studies. The first was the Coronary Drug Project, which established that nicotinic acid, which raises HDL in addition to lowering triglycerides, can cut the risk of having a heart attack. The second was the Helsinki Trial, which used another HDL-raising medication (gemfibrozil), and reported a reduction of heart attacks of 38 per cent. In this study the greatest benefit was seen in patients who started out with a high blood triglyceride and low HDL.

*Since the source of this material was published, the National Cholesterol Education Program Expert Panel has redefined the threshold for low HDL as < 40 mg/dL.