All Americans 2 years and older should make significant dietary changes to cut their risk of heart disease, concluded a panel of experts convened by the National Institutes of Health last month to set new guidelines for cholesterol. But charting a new nutritional course can be a real challenge -- unless you know some key facts.

Among the most important: Reducing dietary cholesterol alone is not enough to lower blood cholesterol to the new levels recommended by NIH -- 180 mg or less for those 30 and younger, and 200 mg or less for those over 30.

To meet those levels, people will need to "lower total dietary fat, saturated fat and cholesterol," the NIH committee says.

The recommendations are aimed at reducing the average American's blood cholesterol by about 10 percent. They suggest consuming no more than 300 mg per day of dietary cholesterol -- about the amount in one egg yolk -- and limiting fat intake to no more than 30 percent of total calories. Of the fat, one-third should be from animal sources, and two-thirds from vegetable and plant sources.

That means on a 2,100-calorie-a-day diet, up to 630 calories could come from fats. Since each gram of fat contains nine calories, that's about 70 grams, or 2.5 ounces, of fat.

Based on the NIH guidelines, no more than 10 percent of these calories -- about 210 calories -- should come from saturated fats, found chiefly in animal and dairy products. The rest, some 420 calories, should be divided roughly between polyunsaturated fats, such as corn oil, and monounsaturated fat, such as olive oil.

Few foods -- with the exception of egg yolks and shrimp -- are high in cholesterol without also being high in saturated fat. So "the most efficient way to lower blood cholesterol is to decrease the amount of saturated fat in the diet," says Lynne W. Scott, chief dietitian of Baylor College of Medicine's Diet Modification Clinic.

Saturated fats give beef its marble and make lamb rich-tasting. They're found in poultry skin and in the cream that produces butter, cheese and ice cream. Among the few vegetable sources of saturated fats are coconut and palm oils, which often turn up in commercial and processed foods because they are inexpensive, have a long shelf life and can be used at high temperatures.

Polyunsaturated fat is considered beneficial because it clears saturated fat from the blood, lowering blood cholesterol levels.

Saturated fat is so tough to eliminate from the bloodstream that "for each gram of saturated fat, it takes two grams of polyunsaturated fat to counteract the effect on blood cholesterol," says Ron Goor of the National Heart, Lung and Blood Institute. "Thus, decreasing saturated fat has much more of an impact than increasing polyunsaturated fat."

There's another reason not to increase polyunsaturated fat too much: Studies suggest that you may trade protection against heart disease for a higher risk of cancer. University of Texas researcher Dr. Scott Grundy reports that high levels of polyunsaturated fat may work too well -- they may actually reduce levels of a protective fatty substance known as HDL, for high density lipoprotein -- which guards against heart disease by clearing cholesterol from the body.

Monounsaturated fats -- also derived from plant and vegetable sources -- may have other effects. Research suggests that consumption of these fats, which include olive and peanut oils, may lower blood cholesterol and keep levels of the protective HDL high. CC hoosing the right foods to reduce the risk of heart disease can be filled with C "pitfalls" says Goor, coordinator of the federally sponsored National Cholesterol Education Program. Some foods now bear large labels proclaiming that they are "cholesterol free," while in small print noting that they contain high amounts of saturated fat.

Saturated fats can crop up in unexpected places. The cholesterol-conscious person ordering popcorn without butter at the movies may not realize that most commercial popcorn is popped in highly saturated coconut oil or palm oil. Similarly, non-dairy creamer -- though cholesterol-free -- contains more saturated fat than real cream, says the U.S. Department of Agriculture.

But numerous studies show that people who reduce fat and cholesterol consumption can reap benefits. For each 1 percent reduction in blood cholesterol levels, there is a 2 percent drop in the risk of developing heart disease, reports an NIH study. Moreover, people who have already suffered one heart attack seem to have a lower chance of recurrence by modifying their diet. One researcher, who had a high-risk blood cholesterol level of 320 mg, reduced his blood cholesterol to 200 mg with diet changes alone.

While the NIH committee targeted dietary changes to reduce blood cholesterol levels, it noted that losing weight, exercising and limiting alcohol consumption are also important. Steps Toward Cutting Back

Beginning this year, the National Heart, Lung and Blood Institute will launch a National Cholesterol Awareness Program to teach people how to reduce their blood cholesterol levels. Patterned after a successful nationwide program for high blood pressure, it is expected to help cut heart disease the way detection and treatment of high blood pressure cut the rate of stroke.

Until that program is launched, NIH and the American Heart Assocation offer these tips:

* Know your blood cholesterol levels and those of your children, ages 2 and older. Find out where they fall within the new NIH guidelines.

* Maintain normal body weight. Added weight raises blood cholesterol levels.

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

Hydrogenated vegetable oils also are no fat bargain. Hydrogen is percolated back into these fats, often changing them from a polyunsaturated to saturated fat.

* Eat no more than 7 ounces per day of cooked lean meat, fish or poultry (without the skin).

* Eat limited amounts of food that contain more than 30 percent total fat and 10 percent saturated fat. Most fruit, vegetables and whole-grain foods would pass this test. Skinless turkey breast would also pass. Only 5 percent of the total calories come from fat, and less than 2 percent of those calories are from saturated fat. Lean beef chuck would fail. Fifty-six percent of its calories are fat, with 23 percent saturated fat.

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

* Eat no more than two egg yolks per week.

* Prepare food with minimal amounts of oil. Saute' foods in small amounts of water or unsaturated vegetable shortening, rather than butter.

* Eat several meatless lunches or dinners a week.

A new book by heart specialist Dr. Michael E. DeBakey, "The Living Heart Diet" (Simon & Schuster, $19.95), provides low-fat, low-cholesterol recipes and extensive lists of the fat content of foods.