Two American doctors were awarded the 1985 Nobel prize for medicine this week for research on cholesterol, a fatty substance in the blood that has been linked to heart disease and stroke.

Dr. Michael S. Brown and Dr. Joseph L. Goldstein, molecular geneticists at the University of Texas Health Science Center in Dallas, received the award for work that the Nobel Institute said "revolutionized our knowledge about the regulation of cholesterol metabolism and the treatment of diseases caused by abnormally elevated cholesterol levels in the blood." Because of their research, Brown and Goldstein, wrote recently in Scientific American, "It may one day be possible for many people to have their steak and live to enjoy it, too."

But the negative effects of cholesterol are only part of the story. Cholesterol also is beneficial to the body. It is found in food and also is produced by almost all cells -- particularly by the liver and the intestine.

Without cholesterol, there would be no cell membranes -- the outer coats that protect each cell. Nor would the body be able to manufacture many important hormones, including the sex hormones. Without cholesterol, digestion would be difficult, since it also is required for the production of bile.

Since cholesterol is not soluble in the blood -- much the way oil doesn't mix with water -- it must be transported in a kind of chemical package known as a lipoprotein complex. Three major categories of lipoproteins carry cholesterol through the blood: HDL, LDL and VLDL. These stand for high-density lipoproteins, low-density lipoproteins and very-low-density lipoproteins. In the 20 years since researchers like Brown and Goldstein have been exploring the role of cholesterol in the development of heart disease, some 13 lipoproteins also have been identified as carriers of cholesterol.

In recent years, many consumers have learned to think of these three major groups of lipoproteins as "good" or "bad." Studies have shown that higher blood levels of "good" HDL offer protection against cardiovascular diseases, while increased amounts of "bad" LDL and VLDL are associated with a greater risk of developing a cardiovascular illness.

The process seems to work like this: A diet too high in cholesterol and saturated fats causes blood levels of LDL and VLDL to rise. Cells begin to accumulate LDL and VLDL, leaving high levels of these types of fat in the blood. With nowhere to go, these molecules start depositing their cholesterol packages in the cells lining blood vessels. The result is arteriosclerosis -- hardening of the arteries -- and heart disease.

HDL is considered protective because it seems to clear cholesterol from the blood. Blood levels of HDL can be increased by exercising.

The research by Nobel laureates Brown and Goldstein suggests why some people seem more vulnerable to cholesterol than others.They discovered receptors on the surface of cells in the human body that regulate the intake of LDL molecules. The receptors pick up packets of cholesterol, absorb them and break them up.

People with many receptors on their cells can eat more cholesterol-rich food, such as eggs and red meat, without raising their risk of heart disease. But those with fewer receptors tend to have higher blood levels of cholesterol, which accumulates along the artery walls and results in arteriosclerosis. This can impede blood flow, causing a stroke or heart attack.

These discoveries, the Nobel citation said, "have drastically widened our understanding of the cholesterol metabolism and increased our possibilities to prevent and treat arteriosclerosis and heart attacks."

The Nobel Institute expressed hope that their work would lead to new medications that could increase the number of LDL receptors and, in combination with dietary changes, reduce the risk of heart disease.

Last year, at a consensus conference on cholesterol and cardiovascular disease, the National Institutes of Health concluded that high levels of cholesterol in the blood cause heart disease and that scientific studies show "beyond a reasonable doubt" that lowering cholesterol levels, preferably through diet, will reduce the risk of heart attacks.

Studies by Dr. Scott Grundy of the University of Texas suggest that people with mildly elevated blood levels of cholesterol -- in the range of 220 to 250 milligrams -- face about twice the risk of developing heart disease as people with safer levels, which researchers cite as 200 mg and below. Cholesterol levels of 250 to 300 mg, Grundy told the NIH consensus conference, lead to "four times the risk of developing heart disease." Levels above 300 mg translate to an eight-fold increase in risk, he said.

And findings from a major NIH study published in 1984 showed that "each 1 percent reduction in plasma cholesterol resulted in a 2 percent fall in fatal and nonfatal myocardial infarctions heart attacks ."

Reducing blood cholesterol can be accomplished by a variety of methods. Among them:

*Reducing daily dietary cholesterol to no more than 300 mg -- about the amount in one egg yolk.

Limiting total fat intake to no more than 30 percent of total calories.

*Cutting down on the amount of saturated fats eaten. (Saturated fat most frequently comes from animal sources, although some vegetable sources -- such as palm and coconut oil -- are high in saturated fat.) Of total fat consumed, NIH has recommended that one third should be from animal sources and two thirds from vegetable and plant sources.

*Maintaining normal body weight.

*Reading labels carefully. Even foods low in cholesterol may contain high amounts of saturated fat. Although some products contain all vegetable shortening, they still may be high sources of saturated fat if they contain coconut or palm oil.

*Eating no more than seven ounces per day of cooked lean meat, fish or poultry (without the skin).

*Choosing dairy products made with skim milk -- including yogurt and cheese.

*Eating no more than two egg yolks per week.

*Preparing food with minimal amounts of oil.

*Eating several meatless lunches or dinners a week.

*These changes, NIH has concluded, would help achieve a national goal of a 10 percent reduction in blood cholesterol for Americans. The ideal: blood cholesterol levels of 180 mg or less for those 30 and younger and 200 mg or less for those over 30. The rewards: saving an estimated 100,000 lives a year and reducing deaths from heart disease by one fifth.