The National Institutes of Health are expected for the first time today to propose cholesterol guidelines to help doctors decide when to treat people to reduce the risk of heart disease.

The recommendations are likely to target an estimated one in four American men and three in 10 women -- aged 20 to 74 -- as candidates for diet changes or, in some cases, drug treatment.

Heart disease is the leading cause of death in the United States.

Since Monday, a 14-member NIH conference committee -- charged with charting the sometimes controversial territory of blood cholesterol levels and their relationship to cardiovascular disease -- has been listening to more than two dozen reports from international researchers. The committee of researchers, doctors and consumer advocates is scheduled to issue the recommendations today, and they are likely to serve as guideposts for physicians and the public.

For the first time, researchers agreed that high cholesterol levels cause heart disease. The committee focused on what levels of cholesterol should be considered a health threat. They are expected to recommend that people with cholesterol levels 240 milligrams and above reduce fats to no more than 30 percent of their diet -- from the current average of 40 percent -- and if necessary take cholesterol-lowering drugs.

A public health effort is expected to include a drive to teach Americans what their cholesterol levels are, much as they've been taught to pay attention to blood pressure.

The committee has focused on three major questions:

* Do high blood levels of cholesterol cause heart disease?

* Will reducing cholesterol levels prevent heart disease?

* Should an attempt be made to reduce the blood cholesterol levels of the American public?

"The answers to those questions are 'Yes, yes and yes,' " says Dr. Jeremiah Stamler of Northwestern University. Stamler, who addressed the conference Monday, said Americans should make "a progressive reduction in the total amount of fat, saturated fat and cholesterol they eat . . . at the same time increasing the amount of vegetable products and fiber consumed."

Evidence presented this week shows a clear relationship between blood cholesterol levels and heart disease. The higher the cholesterol levels, the greater the incidence of heart disease. People in Greece, Yugoslavia, Japan and Italy have average blood cholesterol levels of 180 to 200 mg, compared with 210 to 220 mg in the United States, and those countries have much lower incidence of cardiovascular disease, according to the National Center for Health Statistics (NCHS). While cholesterol levels have dropped in this country over the past 10 years -- due to an increased health consciousness -- about 25 percent of men and 30 percent of women have levels above 240 mg, the NCHS reports.

Studies by Dr. Scott Grundy of the University of Texas suggest that people with mildly elevated levels of cholesterol -- in the range of 220 to 250 mg -- 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 says, lead to "four times the risk of developing heart disease." Levels above 300 mg translate to an eight-fold increase in risk, he says.

Lowering cholesterol levels can reduce the risk of coronary heart disease. And some preliminary evidence indicates that people who have suffered one heart attack may be able to cut their risk of having a second attack by decreasing cholesterol levels.

Findings from a major NIH study this year showed that "each 1-percent reduction in plasma cholesterol resulted in a 2-percent fall in fatal and nonfatal myocardial infarctions [heart attacks]."

Although it is a prime culprit in heart disease, cholesterol is also a "necessary substance for the body," Grundy says. Found in all cells, cholesterol is an important constituent of cell membranes. It is also required for the production of numerous hormones, including sex hormones, and for producing bile acid.

Since cholesterol is not soluble in the blood, it must be transported in a kind of chemical package known as a lipoprotein complex. There are three major categories of lipoproteins: HDL, LDL and VLDL. These stand for high-density lipoproteins, low-density lipoproteins and very-low-density lipoproteins.

Studies now show that a diet too high in cholesterol and saturated fats causes blood levels of LDL and VLDL to rise. Cells begin to accumulate LDL, leaving high levels of this type of cholesterol in the blood. With nowhere to go, LDL molecules start depositing their cholesterol packages in the cells lining blood vessels. The result is atherosclerosis -- hardening of the arteries -- and heart disease.

One way to prevent this process is by eating less cholesterol and saturated fats. Studies of countries where heart disease rates are low also report diets that include no more than 30 percent of daily calories as fat -- be it from the cholesterol and saturated fats found in meat, eggs, poultry and dairy products or from the polyunsaturated fats derived largely from vegetables. The American Heart Association already recommends such a diet -- with the caveat that polyunsaturated fats not exceed 10 percent of the total calories. High amounts of polyunsaturated fats have been linked to cancer.

More specifically, researchers like Grundy are recommending that the average American, who now eats about 400 to 450 mg of cholesterol a day, reduce that gradually to about 150 mg. Researchers told the committee that these diet alterations should be aimed at Americans 2 years and older.

"The only way to accomplish the dietary goals and the change in [cholesterol] levels I am recommending," said American Heart Association President Antonio Gotto, "is to begin early in life with a program of dietary education."