Forget an apple a day — it’s an hour of exercise
Food intake, energy output considered together for the first time in a major study

By Robert Sanders, Public Affairs

11 September 2002 | A new study on healthy eating from the National Academies’ Institute of Medicine received widespread attention upon its release last week, including prominent coverage by the New York Times, Washington Post, and other national media. Given Americans’ propensity toward obesity (and parallel near-obsession with diets and physical appearance), that’s not surprising. Nor, given the weight and heft of the 1,000-page report itself, is it surprising that some of its recommendations are in one key regard downright revolutionary.

“We’re going to change the whole paradigm,” says George Brooks, one of two Berkeley faculty members on the panel that drew up the report. “Instead of issuing one-size-fits-all 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.’”

Brooks, professor of integrative biology at UC Berkeley and an exercise physiologist, says the report provides the most detailed criteria yet for deciding what levels of fat, carbohydrate, and protein Ameri-cans should aim for in their diet. It also recommends doubling the amount of physical activity believed advisable less than a decade ago (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.

As Brooks notes, one of the most significant changes is a move away from uniform recommendations for the intake of calories and macronutrients (i.e., carbohydrates, fats, and proteins). Instead, the report urges people to balance caloric intake with physical activity. The more active you are, the more calories you can and indeed should consume, and the more flexible your daily diet can be in terms of fat and carbohydrate consumption. (The body generates energy from both sources.) In contrast, overweight people need to reduce their overall intake, increase their physical activity level, and be stricter about dietary fat, which has more than twice the calories per gram as carbohydrate and protein.

Panel member Ronald M. Krauss, adjunct professor of nutritional sciences at Berkeley, cautions that the study is not intended to replace current dietary guidelines, issued periodically by various groups from the Food and Drug Administration to the American Heart Association.

“This is like an encyclopedia of what was already known about the macronutrients in our diet,” says 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 these 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 acknowledges 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, scientists conducting such studies have discovered that people tend to overestimate their activity level and underestimate their consumption of food — in particular, fats and alcohol.

“For the first time, we know both the diet and the activity level of healthy people — what proportions of carbohydrates, fat, and protein they use as energy — so we can link physical activity with dietary recommendations,” Brooks says. “If people find themselves gaining weight, they need to do something. An hour a day keeps the disease away.” He emphasizes that the one hour is cumulative, so it can include, say, 15 minutes of brisk walking in the morning and 10 minutes of stair climbing throughout the day, combined with recreational biking or other walking in the afternoon or evening. 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, says Brooks, “you’re doing great.”

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 says.

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 to 65 percent, 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 says. It was, instead, to look at fats, carbohydrates, and protein from the same perspective one might bring to assessing micronutrients like vitamins and minerals. This approach was not always appropriate, he says.

“The panel worked very hard to apply criteria developed for micronutrients, present in generally small amounts in [people’s] diets, to this larger issue of the major food components in the diet,” he says. “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 carbohydrates, fat, or even protein we are eating, A major role for these nutrients is merely to provide energy.

“Within these constraints,” he says, “it doesn’t make a whole lot of difference how you divide those extra calories among protein, carbohydrates and fat as long as your total energy intake is balanced by expenditure.”

“The key is energy balance and nutrient balance,” Brooks agrees. “Once you have that, you have a lot of latitude.”

For further information, plus access to a webcast of a Sept. 5 press briefing by the Institute of Medicine, visit www.nas.edu.