Posted Feb 20, 2010
When we try to lose weight by cutting calories, our bodies seem programmed to resist by burning less energy. A new study at Oregon Health & Science University adds to the evidence that aerobic exercise is the best way to overcome the body’s resistance to losing weight.
Researchers at OHSU’s Oregon National Primate Research Center in Beaverton fed rhesus monkeys a diet with 30 percent fewer calories per day. The 18 female monkeys were raised to serve as stand-ins for typical post-menopausal women in the U.S; researchers had surgically removed their ovaries and fed them a high-fat diet for more than two years.
Four weeks on the low-calorie diet produced no significant weight loss. Rather than burn off fat, the monkeys slowed down their activities and used fewer calories. Researchers tracked the animals’ movements around the clock with instruments attached to their collars. At intervals during the study, the researchers also calculated each monkey’s precise metabolic rate by measuring the amount of oxygen consumed and carbon dioxide produced during one 24-hour period.
A second month of more severe dieting caused the animals to lose about 6 percent of their starting weight. Energy use fell by a total of about 68 calories per day. Decreased activity accounted for about 60 percent of the decrease in energy use, and simply having less body mass to move around accounted for about 30 percent of the decrease. Dieting also prompted a general decrease in resting metabolic rate, which accounted for about 9 percent of the decrease in energy use.
To test whether exercise could counteract the diet-induced drop in activity, the researchers trained three rhesus females to run on a treadmill and put them on a one-hour, five-days-per-week workout schedule. The workouts didn’t cause the monkeys to be less active when they weren’t on the treadmill. And after three months, the exercising monkeys lost about 6 percent of their starting weight.
“It’s hard to lose weight if all you are doing is dieting,” says lead author Judy Cameron, senior scientist at the Oregon National Primate Research Center and a professor of psychiatry at the University of Pittsburgh.
“Staying active while you are dieting will prevent the decrease in how many calories one burns each day due to becoming more sedentary, and it will also help counteract the decrease in basal metabolic rate,” Cameron says. Her study appears in the American Journal of Physiology — Regulatory, Integrative and Comparative Physiology.
Eric Ravussin, a researcher at the Pennington Biomedical Research Center in Baton Rouge, La., said rhesus monkeys are far more resistant to dieting-induced weight loss than humans. But he said Cameron’s general conclusion fits with studies using human volunteers.
“If you cut calories, you have a lowering of your energy expenditure and this ‘protects’ you against weight loss,” Ravussin says. “If you want to lose weight, you should progressively include an exercise program.”
Date: Jan 25, 2010 To see more of The Oregonian, or to subscribe the newspaper, go to http://www.oregonian.com.
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