Barely measurable amounts of energy, released as body heat, could be the difference between holding the waistline or adding 10 pounds a year, say Cornell University researchers who turned couch-potato rats into exercising athletes.

The culprit: excess dietary protein and fat, particularly from animal-based foods. These appear to tip a delicate energy balance toward adding body fat instead of burning energy through a metabolic process called thermogenesis, says Cornell nutritional biochemist T. Colin Campbell. His theory of energy balance and obesity -- an old one now refined by new findings -- was developed through studies of diet and disease among rural Chinese and laboratory rats, and it could explain why so many attempts at dieting are doomed. His proposed solution: Eat a diet that is low in animal-based fats and proteins and rich in plant-based nutrients and fiber.

"Laboratory rats, fed diets comprised of substantially reduced intakes of protein, consume more energy but gain slightly less weight and exhibit increased thermogenesis due both to enhanced metabolic body heat and to diet-driven physical activity," said Campbell at the recent Conference on the Role of Diet and Caloric Intake in Aging, Obesity and Cancer, in Reston, Va. Also, he reported, these same rats show sharply reduced blood cholesterol concentrations and tumor development.

Details of the theory will be published in the American Journal of Clinical Nutrition by Campbell, the Jacob Gould Schurman Professor of Nutritional Science at Cornell, and by Dr. Junshi Chen of the Chinese Academy of Preventive Medicine's Institute of Nutrition and Food Hygiene.

The Cornell rat studies, in which lab animals ate diets ranging from 5 percent to 20 percent protein in the form of casein from cow milk, found another remarkable result: Rats on low-protein diets voluntarily exercised more. Given a choice of lounging around their cages or climbing on exercise wheels, the low-protein-fed rats spent more time burning calories compared with rats on moderate- and high-protein diets.

That increased exercise, as evidenced by Chinese who ride bicycles instead of driving to work, undoubtedly accounts for most of the expended energy. A small amount of expended energy, the "missing matter," is at the heart of an apparent dietary paradox: Humans and animals on lower-protein, lower-fat diets actually consume more calories in the form of carbohydrates but are less likely to convert the energy to body fat.

Studies at other institutions and at Campbell's Cornell laboratory, where the China Project http://www.human.cornell.edu/dns/chinaproject/ survey of thousands of Chinese families was based, are beginning to pinpoint this missing matter. Humans and animals on low-protein, low-fat diets burn the energy off through very slight increases in thermogenesis, and it is released as body heat instead of becoming body fat, according to Campbell.

"The difference is so slight and so difficult to measure that we have been missing it -- perhaps as little as 50 calories a day in a 2,500-calorie diet -- but those unburned calories can add up to 10 additional pounds a year," Campbell says.

Further research is needed to explain the role of low-protein diets in increased thermogenesis, the biochemist says, suggesting two possible mechanisms for the effect: "There is some evidence that it could be due to a small amount of a special tissue called brown adipose tissue, although this tissue is not thought to be as significant in humans as in some animals. In addition, most nutritionists regard animal protein as more 'efficient' than plant protein, and one of the efficiencies might result in converting the ingested energy directly to fat."

Campbell credits Eleanor Krieger, a Cornell undergraduate at the time of the research, with discovering the effect of low-protein diets in rats' voluntary exercise, as well as Fumiyika Horio and Rhonda Bell, a visiting professor and a former graduate student, respectively, with the thermogenesis in brown adipose tissue findings. The diet-disease studies are funded in part by the National Cancer Institute and the American Institute for Cancer Research.

Discovery of the missing matter should be should be good news for the 25 to 34 percent of overweight American adults who spend $30 to $40 billion a year on weight-loss programs and products, Campbell says. "Living a long and healthy life, in a body that uses food for energetic activities instead of creating more fat, calls for long-term commitment to lifestyle changes and not to fad diets. Changing and committing to a healthful diet isn't always fast and easy, but it is worth it -- both in weight control and in avoiding the diseases of extravagance, such as heart disease and

cancer."

Nor will a reduction in protein intake be harmful, the Cornell nutritionist says. Most Americans already consume considerably more protein than the U.S. government's Recommended Dietary Allowance (RDA) calls for, Campbell observes. "Even people on strict vegetarian or vegan diets get plenty of protein. Now that we're learning the subtle effects of excess protein, it's easier to see why people on healthful diets feel more energetic."