Americans need to reduce the cholesterol in their bloodstreams by as much as 25 percent to help avoid heart disease, an expert study group concluded yesterday.

Even children, the group said, eat too many fatty foods. The experts found that the cholesterol levels of American youngsters, as well as adults, are alarmingly high and should be cut to help them attain normal life spans.

These conclusions were reached at the close of a two-day workshop convened by the American Health Foundation in New York City. Among the 42 conferees were several world leaders in studies of the way heart and blood vessel diseases develop, both in individuals and whole populations.

In summary, the group strongly endorsed the theory that high levels of blood lipids-waxy, fat-like substances such as cholesterol and some others-clog arteries and either cause or help cause heart attacks and strokes in some persons.

Some doctors and scientists disagree with this idea. But most experts in heart and blood vessel diseases accept it.

And they think people should consume less fatty meats, high-fat milk products, egg yolks, sugar and salt. The experts recommend more fish, lean meats, low-fat milk products, fruits, vegetables and cereals-and more exercise, to limit both harmful blood lipids and excess weight.

"What has caused confusion among doctors," said Dr. Ernst Wynder, the Health Foundation's president, "is just what advice to give patients about cholesterol levels-just when to say their cholesterol is too high."

The average American adult male's cholesterol per deciliter, or one-tenth of a liter, of blood, with the majority someplace between 150 and 280.

The average woman's level is the same until menopause, when the the women's levels tend to rise.

The average level in childhood and youth is 160 to 180.

Many doctors have simply told their adult patients that their cholesterol is too high if it is above the average 220. Others consider a reading of 250 as a danger level. They have had few guidelines at all for advice about children.

The New York conferees agreed that:

The ideal level for any population, including America's, should be 150 to 160, on the average, which would mean that individuals might range between 100 and 200. Dr. Henry Blackburn, director of the Laboratory of Physiological Hygiene at the University of Minnesota and chairman of one of three working groups, called the 150-160 figure "the ideal with respect to freedom from heart attacks and artery disease."

On a practical level, however, the average person's level should be no more than 180 to 190, with individuals in the population ranging between 120 and 240.

For children up to age 18, a cholesterol level of 120 to 140 would be both ideal and practical.

The New York conference dealt with blood lipids in general, not just cholesterol, Blackburn said.

"But we can use the term cholesterol in a generic sense to represent them all," he said. "Generally speaking, a blood test for cholesterol is strongly predictive of individual risk up to age 50. After that, we've been learning, total cholesterol is no longer strongly predictive, and it helps to measure 'HDL', or high-density lipo-protein," a factor discovered only in recent years.

At older ages, Blackburn said, a high HDL level generally indicates a lower risk, while a high LDL-or low-density lipoprotein-level may mean a higher risk.

Many doctors now test their adult patients for cholesterol, HDL and triglycerides (another class of lipids). More and more doctors, Blackburn added, are screening children, too for cholesterol, because children's doctors "are more and more interested in preventing heart and blood vessel diseases" when these children become adults.

Among other well-known participants in the New York meeting were Dr. William Kannel of Boston University, director of the U.S. Public Health Service's study of heart disease risks in the city of Framingham, Mass; Dr. Jeremiah Stamler of Northwestern University, an early advocate of low-fat, low-cholesterol diets, and Dr. Lionel Tiger, Rutger University's social anthropologist.