Exercise Impedes Weight Loss, Study Finds : Health: Losing pounds adds to energy efficiency, report says.
People who are exercising to lose weight may find that the benefit of exercise shrinks as they lose more weight, a study indicates.
Women who lost weight became more energy-efficient in walking, so they had to do more work to burn the same amount of energy-storing fat, the report said.
It’s known that metabolic rates fall when people lose weight, largely because smaller bodies need less energy to keep going, said researcher Zebulon V. Kendrick, director of the Research Laboratory at Temple University in Philadelphia.
But the new study found that the drop in the energy expenditure was greater than what would be expected based on weight loss alone.
“When the body is reaching a new status quo, it appears the body is more efficient,” Kendrick said.
The report in the American College of Sports Medicine journal, Medicine and Science in Sports and Exercise, looked at 11 obese female dieters in a 26-week study. The women had an average age of 40 and started the program at an average weight of more than 230 pounds. At the end of the program, their weight had fallen to 184 pounds.
The women were given a liquid formula diet, then weaned to conventional foods, and received counseling on weight control.
At nine weeks, they began walking two to three times a week, eight to 12 minutes per session, at a relatively modest 40% to 60% of predicted maximum heart rate.
By 18 weeks, they had worked up to three to five times a week, 20 to 40 minutes per session, at an active 60% to 80% of maximum heart rate.
The women were tested at the study’s start, at nine weeks, and at 22 weeks--two weeks after the active dieting phase had ended--to see how much energy they burned while walking. This was compared to the smaller amount of energy they used while standing quietly; the difference was considered the energy cost of walking.
Statistical analysis revealed that the energy cost of walking was lower after 22 weeks than could be accounted for by just changes in body weight, Kendrick said.
The differences were not big--body weight accounted for 90% of the change in overall energy cost after 22 weeks. But it indicates that weight loss does make the body more conservative in its use of energy while exercising, Kendrick said.
The study did not determine how this happened, but Kendrick speculates the weight loss made the women more efficient walkers. The limbs of obese people rub together as they walk, and he thinks this energy-wasting friction was reduced as the women got thinner.
“Even though each step is minuscule, the amount of calories from the friction of scraping, when you take it over one half-hour, is more significant,” Kendrick said.
Obese people sweep their arms and legs out farther as they walk to avoid chafing, said a separate researcher, Ann Ward of the University of Wisconsin in Madison.
“It’s harder to move your legs when you are fat,” said Patty Freedson, a professor in the Department of Exercise Science at the University of Massachusetts, Amherst. But there’s no evidence that she knows of to show that the reduction in friction creates the change in energy expenditure, she said.
Kendrick does not believe the increased energy level needed to burn the same amount of calories should discourage women from continuing to exercise to lose weight. An exerciser may simply need to add 15 minutes or so to the workout as the program continues in order to hold the rate of weight loss constant.
