Amidst the scientific data supporting specific recommendations, the 1,000-page study recommends doubling the amount of physical activity previously advised - from a minimum of 30 minutes per day to 60 minutes per day - balancing energy intake and expenditure, and minimizing intake of saturated fats, cholesterol and trans fatty acids.

One of the most significant changes is a move away from one-size-fits-all recommendations for the intake of calories and macronutrients, that is, carbohydrates, fats and proteins, said panel member George Brooks, professor of integrative biology at UC Berkeley and an exercise physiologist. Instead, the panel members urge people to balance caloric intake with physical activity. The more active you are, the more calories you should consume, and the more flexible your daily diet can be in terms of fat and carbohydrate.

Overweight people need to reduce their overall intake, increase physical activity level and be more strict about dietary fat, since fat has more than twice the calories per gram as carbohydrate and protein.

"We're going to change the whole paradigm," Brooks said. "Instead of recommendations based on age, height and weight alone, we're basically saying, 'You tell me what exercise you do, and I'll tell you what you can eat.'"

The panel recommended an hour of moderately intense physical activity, such as brisk walking, each day - twice the recommended minimum of the 1996 Surgeon General's Report on Physical Activity and Health, which advised at least a half-hour daily of vigorous exercise.

The panel of 21 scientists and physicians was convened at the request of various U.S. agencies and Health Canada. The organizations wanted the National Academy of Sciences (NAS) to evaluate the literature on macronutrients in the diet and suggest any changes or updates to the nations' dietary recommendations. The panel was one of seven plus two subcommittees overseen by the Food and Nutrition Board of NAS, formed to look at all aspects of diet to develop dietary reference intakes.

Panel members spent two and a half years reviewing nutritional studies conducted in the past decade, and arrived at a consensus about what the public at large, as well as children and those with special needs, such as diabetics, should consume on a daily basis. Its recommendations will become the basis for setting policy in both countries, and will guide the Food and Drug Administration (FDA) and other organizations in their efforts to educate the public about healthy eating habits. The U.S. Department of Agriculture, for example, could use the data to make specific food recommendations or revise the well-known Food Pyramid.

Panel member Ronald M. Krauss, adjunct professor of nutritional sciences at UC Berkeley and a staff senior scientist at Lawrence Berkeley National Laboratory, cautioned that the study is not intended to replace current dietary guidelines.

"This is like an encyclopedia of what was already known about the macronutrients in our diet," said Krauss, who studies how genes determine the body's response to dietary fats and cholesterol. "Though there are recommendations in the report, I would not consider them recommendations for an overall diet. They are recommendations for considering the effects of specific nutrients. Our task was to put numbers out there that could help future dietary guideline committees."

One panel recommendation, that the public wean itself away from trans fatty acids, was released July 10 in order to help the FDA decide whether to require that food labels include trans fatty acid content. The panel could find no evidence that trans fatty acids - hydrogenated oils found primarily in processed and fried foods - are essential to the diet. Since, even in small amounts, they increase cholesterol levels and thus the risk of heart disease, the study suggested that consumption be minimized. This change has already spurred McDonald's USA to halve the amount of trans fat in the oil it uses to cook French fries and Chicken McNuggets.

Regarding the panel's recommendations about physical activity, Brooks acknowledged that doctors have always recommended exercise. Until recently, however, studies correlating dietary intake with physical activity have not shown a clear benefit from exercise, primarily because they have relied upon questionnaires and self reporting.

In the past 10 years, thanks to techniques such as stable, non-radioactive isotope tracer studies, scientists have been able to measure energy expenditure directly. As a result, they discovered that people have tended to overestimate their activity level and underestimate their consumption of food - in particular, fats and alcohol.

"For the first time, from stable, non-radioactive isotope tracer studies, we know how active people are when they have a healthy body weight, what proportions of carbohydrate, fat, and protein they use as energy sources, so we can link physical activity with dietary recommendations," Brooks said. "If people find themselves gaining weight, they need to do something. An hour a day keeps the disease away."

People must work this activity into their daily lives, he said, whether by taking the stairs instead of an elevator, or walking to the store instead of driving. He emphasized that the one hour is cumulative, so it can include 15 minutes of brisk walking in the morning, 10 minutes of stair climbing throughout the day, combined with recreational biking or other walking in the afternoon or evening.

"Previously, the Surgeon General's report talked about how much activity you need to minimize progression of chronic diseases such as coronary heart disease, Type II diabetes and some forms of cancer, whereas the National Research Council issued RDAs, which focused on what you should consume," Brooks noted. "Now, it's fair to say, this is the first time physical activity has been an intrinsic part of dietary recommendations. This is a product of our change in lifestyle."

There are two key concepts, he said. Because both physical activity and body fatness affect chances of developing chronic diseases, people need sufficient physical activity to reduce the morbidity and mortality of chronic diseases. Beyond that, we need enough physical activity to balance energy intake and expenditure to control body weight. If you are active about 60 minutes most days, maintain a stable weight and have a body mass index (ratio of weight to square of height, measured in kilograms per square meter) between 20 and 25, "you're doing great," he said.

Another significant change from previous recommendations is the panel's establishment of a range of acceptable carbohydrate, fat and protein intakes. Previous recommendations were mostly maximums or minimums.

"If you are in energy balance, a person doesn't need to worry on a daily basis, am I getting the right percentage of carbohydrate or fat. There's a broad range there," Brooks said.

For example, instead of recommending fewer than 35 percent of daily calories in the form of fat, the panel lists an acceptable range of 20 to 35 percent for total dietary fat, assuming energy balance in the overall diet. The acceptable range of carbohydrates is 45-65, while protein should comprise between 10 and 35 percent of daily calories.

Krauss worries that the report will be misconstrued as promulgating hard and fast levels of macronutrients in the diet. But that was not the goal of the panel, he said. It was, instead, to look at fats, carbohydrates and protein from a perspective more appropriate to micronutrients like vitamins and minerals. This approach was not always appropriate, he said.

"The panel worked very hard to apply criteria developed for micronutrients, present in generally small amounts in diets, to this larger issue of the major food components in the diet," he said. "But most macronutrients are not essential in the way that many micronutrients are. Most of us in the U.S. don't need all the carbohydrate we are eating, we don't need all the fat we are eating, even all the protein we are eating. A major role for these nutrients merely is to provide energy.

"Within these constraints, however, it doesn't make a whole lot of difference how you divide those extra calories between protein, carbohydrates and fat as long as your total energy intake is balanced by expenditure."