New York City, NY -- Now hear this: avocados, walnuts, salad dressings with oil, sautéed vegetables, fatty fish and some kinds of margarine may be back on the menu for health-conscious Americans, even for those trying to lose weight, if the findings of recent studies are to be believed.

For three decades now, Americans have been bombarded with advice to eat less fat for the sake of their hearts and their waistlines. One well-known expert, Dr. Dean Ornish, advocates stripping away all added fats and naturally fatty foods to achieve a diet containing no more than 10 percent of calories from fat, down from the 44 percent typically consumed by Americans in the 1960's and the 34 percent now consumed.

But now a growing number of nutrition, health and obesity specialists maintain that in trying to squeeze some of the heart-damaging grease from our high-fat diets, they have sent Americans the wrong message. It's not fat per se that's the problem, the experts now say, but the kinds of fats Americans eat and the other kinds of foods they fill up on when they cut back on appetite-satisfying fat. For while heart disease has indeed declined as many Americans shun artery-clogging saturated fats and cholesterol, waistlines have expanded significantly and obesity has risen by 50 percent since the big push to limit fat took off in the 1970's.

The very tactic viewed as the key to weight control -- stripping the diet of fat -- seems to have backfired. Food companies responded to fat phobia with a plethora of fat-free, low-fat and reduced-fat products, especially the dessert and snack foods that Americans covet. The result was an overdose of carbohydrates, ranging from fat-free pretzels, crackers, cookies, cakes and frozen desserts to dinner plates piled high with pasta.

Many obesity specialists, as well as popular diet advocates like Dr. Robert Atkins and Barry Sears, say these carbohydrates are the cause of the growing American girth. Even the potato, which once proclaimed "I am not fattening" in award-winning ads, now heads the hit list being circulated by carbohydrate bashers, some of whom, like Dr. Atkins, go to the opposite extreme by recommending that people can lose weight by eating all the fat they want as long as they eat few or no carbohydrates.

"The swing back to Atkins is a response to the fact that a low-fat diet hasn't worked for a lot of people because they stuff in carbohydrates," said Dr. Margo Denke, an associate professor of medicine and endocrinology at the University of Texas Southwestern Medical Center in Dallas.

Dr. Alice H. Lichtenstein, a professor of nutrition at Tufts University in Boston, agrees that "the low-fat pendulum swung too far."

"People assumed that if a food had no fat, they could eat as much of it as they wanted," she said. "But many low-fat and fat-free products have nearly as many calories as their full-fat versions. Reducing fat alone is no guarantee of weight loss. You must cut calories or increase physical activity."

And that is just what Americans have been doing: gaining weight on fat-free and low-fat foods consumed without regard to their caloric content. Instead of replacing some of the less desirable high-fat foods with nutrient-rich but low-calorie fruits and vegetables, they are filling up on foods loaded with added sugars and refined starches that have little to offer nutritionally besides calories, the experts lament.

"In making food choices, we must learn to eat foods that are nutritionally robust -- fruits, vegetables, legumes," said Dr. Robert H. Eckel, chairman of the American Heart Association's nutrition committee and professor of medicine and physiology at the University of Colorado. "There is strong evidence that these kinds of foods help to reduce disease, not just heart disease but also cancer, diabetes, hypertension and obesity."

Extreme reductions in dietary fat can deprive the body of vital nutrients that play a crucial role in health. Dietary fat transports the fat-soluble vitamins A, D, E and K. Unsaturated fat from vegetable and seafood sources supplies the body with essential fatty acids needed to produce nerve cells and hormones.

There is also growing evidence that unsaturated fatty acids help protect people from serious diseases. For example, studies in Spain, Italy, Greece, Sweden and the United States linked diets rich in olive oil to a reduced risk of breast cancer. The brain, too, may benefit from monounsaturates like olive oil. A study published this month in the journal Neurology by Italian researchers found that a high intake of extra-virgin olive oil was associated with preservation of cognitive functions in healthy elderly people.

The ongoing Nurses Health Study revealed that among 80,000 women initially aged 34 to 59, total fat consumption did not affect coronary risk, but the kinds of fats the women ate did. Each 5 percent increase in calories from saturated fats (primarily from meats and dairy products) raised their risk of coronary disease by 17 percent.

Polyunsaturates and particularly omega-3 fatty acids like fish oils, on the other hand, appear to protect against heart disease, especially sudden cardiac death. Fish oils have been shown to reduce the risk of blood clots and abnormal heart rhythms and improve blood cholesterol and triglyceride levels. Studies in the Netherlands and in the United States have indicated that eating just two fish meals, or seven ounces of fish, a week can reduce a man's risk of heart attack by 50 percent.

Another type of omega-3 fatty acid called alpha-linolenic acid, found prominently in canola oil, flaxseeds, soybean oil, walnuts and many dark green leafy vegetables, also appears to offer strong protection against sudden cardiac death, according to a new study by Dr. Frank Hu and associates at Harvard School of Public Health. Among 76,000 participants in the Nurses Health Study, those with the highest intake of alpha-linolenic acid had up to a 50 percent lower risk of fatal heart attacks when compared with women who consumed the least amount of this fat.

"People who choose fat-free salad dressings are missing out on this important fatty acid," Dr. Hu said. "Fat has been perceived as the enemy, but that's not true. Some fats are good, some fats are bad. We should be substituting good fats for bad ones rather than worrying about reducing the total amount of fat."