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In the 1950s, researchers in California were studying risk factors for coronary heart disease as were the Framingham study researchers near Boston. The Californians, however, had more sophisticated methods for blood cholesterol analysis and separated blood cholesterol into the various lipoprotein fractions, measuring cholesterol within low-density lipoprotein (LDL), high-density lipoprotein (HDL) and other fractions.

As expected, a strongly positive correlation was shown between LDL levels and risk of coronary heart disease. An unexpected finding was that cholesterol in the HDL fraction had the opposite relationship: high levels of HDL were associated with low coronary heart disease risk, and low levels of HDL were associated with increased coronary heart disease risk.

The significance of this discovery was not realized at the time, and it was not until the mid-1970s that other research workers discovered a similar relationship between increased HDL levels and reduction in coronary heart disease risk. The Framingham researchers then showed similar results. Predicted coronary heart disease risk over a four-year period was inversely related to HDL levels, and the relationship was dependent on both systolic blood pressure and LDL levels.

HDL acts as a protective fraction because it is able to pick up cholesterol from the arterial wall and return it to the liver, where the cholesterol can be converted to other substances or excreted in the bile.

The rate of cholesterol accumulation in arteries is therefore dependent on the blood levels of LDL and HDL. Because of the opposite effects of HDL and LDL, blood levels of HDL and LDL have opposite effects on coronary disease risk. The blood cholesterol/HDL ratio (total cholesterol level divided by HDL level) is a convenient way to incorporate the opposite risks of HDL and LDL into a single figure.

The range of blood cholesterol/HDL ratios varies from 2.6 at birth to over 30 in very rare cases of genetic high blood cholesterol levels. Populations with average blood cholesterol ratios below 3 do not suffer from coronary heart disease. Those with average blood cholesterol ratios of 5 or above have high rates of coronary heart disease.

Blood HDL levels may be altered by diseases, medications, overweight and lifestyle habits, including alcohol consumption, cigarette smoking, dietary fats, dietary cholesterol, dietary fish oils and physical exercise. Hence, it is a good idea to check your blood cholesterol level regularly at your doctor's place to ensure your HDL levels does not fall below the ideal range.

Reference:
1. Craig, IH 1987, "What Every Man and Woman Should Know About Cholesterol Control", Methuen Haynes Publications.

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