Besides getting to the gym, you need to watch what you eat. We see the ads for low fat diets everywhere.  At the same time we wolf down burgers and pizza.  Feeling guilty you haven't started that low fat diet yet?  Well, the US Institute of Medicine says the low fat diet may be just as harmful as chowing down on those burgers and fries.

The Institute is issuing new guidelines for healthy eating that offer Americans more flexibility in balancing fat and carbohydrates. And in its latest report the Institute calls for at least an hour of daily physical activity, twice the amount suggested in a 1996 report from the surgeon general.

The previously recommended half-hour of exercise is insufficient to maintain recommended weight in adults, the study concluded. Instead, it urged a total of 60 minutes of moderate activity, such as swimming, brisk walking or jogging, to both maintain weight and obtain other health benefits.

The same amount was recommended for children and comes at a time when worry is increasing about the large number of obese youths. Indeed, former Surgeon General David Satcher has organized a national summit of health and education experts to discuss ways to trim the fat from young people.

In its dietary recommendations, the Institute edged away from previous guidelines that called for getting 50 percent or more of calories from carbohydrates and 30 percent or less from fat. "We established ranges for fat, carbohydrates and protein because they must be considered together," said Joanne Lupton, professor of nutrition at Texas A&M University, chair of the committee that prepared the recommendations.

The Institute, an arm of the National Academy of Sciences, said that because fats, carbohydrates and protein can all serve as sources of energy they can, to some extent, substitute for one another in providing calories. The newly recommended guidelines call for getting 45 percent to 65 percent of calories from carbohydrates, 20 percent to 35 percent from fat and 10 percent to 35 percent from protein. The protein recommendation is the same as in the past.

Lupton noted that studies have shown that when people eat very low levels of fat and very high levels of carbohydrates their so-called good cholesterol declines. Good cholesterol, or high-density lipoprotein, can reduce the likelihood of heart attack. On the other hand, she added, high-fat diets can lead to obesity and its health dangers.

The report is one of a series updating the dietary guidelines issued by the Institute in 1989. It is based on a review of thousands of studies of the effects of consumption of fats, protein and carbohydrates and the potential relationship with various diseases.

The study noted that fat is a major source of energy in the diet, but urged avoiding saturated fats as much as possible because they can increase the risk of heart disease. The main sources of saturated fats are baked goods, meat and full-fat dairy products.

Trans-fatty acids, often found in cookies, crackers and meats also pose a health risk. The Institute recommended in a report released earlier this year that trans-fatty acids be listed on food product labels so people can reduce their intake.

The report includes recommendations for daily intake of fiber, noting that diets low in fiber have been associated with an increased risk of heart disease. Adding fiber to the diet may also decrease likelihood of colon cancer, the study noted.

For adults under age 50 the report recommends a daily intake of 38 grams of fiber for men and 25 grams for women. Over age 50 the recommendations are 30 grams for men and 21 grams for women.

The National Academy of Sciences is an independent organization chartered by Congress to provide guidance to the government in scientific issues.