The researchers then began crosschecking, comparing people’s weights and other measurements from one clinic visit to the next. Based on B.M.I., about 7 percent of the men and women had become obese within about six years of their first visit to the clinic.
But B.M.I. is a loose approximation of body composition and not always an accurate measure of obesity. So the researchers also checked changes to people’s waist circumferences and their body-fat percentage to determine if they had become obese. By the yardsticks of a waist circumference greater than 40 inches for men and 35 for women, or a body-fat percentage above 25 percent for men and 30 percent for women, as many as 19 percent of participants developed obesity over the years.
Weight lifting, however, changed those outcomes, the researchers found, substantially lowering the risk that someone would become obese, by any measure. Men and women who reported strengthening their muscles a few times a week, for a weekly total of one to two hours, were about 20 percent less likely to become obese over the years, based on B.M.I., and about 30 percent less likely, based on waist circumference or body-fat percentage.
The benefits remained when the researchers controlled for age, sex, smoking, general health and aerobic exercise. People who worked out aerobically and lifted weights were much less likely to become obese. But so were those who lifted almost exclusively and reported little, if any, aerobic exercise.
The results suggest that “you can get a lot of benefit from even a little” weight training, says Angelique Brellenthin, a professor of kinesiology at Iowa State, who led the new study.
Of course, the study was observational and does not prove that resistance training prevents weight gain, only that they are linked. It also did not consider people’s diets, genetics or health attitudes, any of which could affect obesity risk.
Perhaps most important, it does not tell us how muscle strengthening influences weight, although it is likely that resistance training builds and maintains muscle mass, Dr. Brellenthin says. A metabolically active tissue, muscle burns calories and slightly increases our metabolic rate. Interestingly, the desirable effect of adding muscle mass may also explain why fewer lifters avoided obesity when the researchers used B.M.I. as a measure. B.M.I. does not differentiate muscle from fat, Dr. Brellenthin points out. If you add muscle with weight training, your B.M.I. can rise.