Researchers then began comparing people’s weights and other measurements from one clinic visit to the next. Based on BMI, about 7 percent of men and women were obese within about six years of their first visit to the clinic.
However, BMI is a rough approximation of body composition and not always an accurate measure of obesity. Therefore, the researchers also checked changes in people’s waist circumference and body fat percentage to see if they had become obese. With a waistline greater than 40 inches for men and 35 for women, or body fat levels greater than 25 percent for men and 30 percent for women, up to 19 percent of participants developed obesity over the years.
However, weightlifting changed these results, the researchers said, and significantly lowered the risk of someone becoming obese by any measure. Men and women who reported strengthening their muscles a few times per week for a total of one to two hours per week were about 20 percent less likely to become obese over the years, based on BMI, and about 30 percent less likely based on waist size or body fat percentage.
The benefits persisted when researchers checked age, gender, smoking, general health, and aerobic exercise. People who exercised aerobically and lifted weights were much less likely to become obese. But also those who exercised almost exclusively and reported little, if any, of aerobic exercise.
The results suggest that “little” strength training can be of great benefit, says Angelique Brellenthin, professor of kinesiology for the State of Iowa, who led the new study.
Of course, the study was observational and doesn’t prove that resistance training prevents weight gain, just that they are linked. Nor was it considered people’s diet, genetics, or health attitudes that could affect obesity risk.
Perhaps most importantly, it doesn’t tell us how muscle gain affects weight, although resistance training is likely to build and maintain muscle mass, says Dr. Brellenthin. A metabolically active tissue, muscle burns calories and easily increases our metabolism. Interestingly, the desirable effect of muscle mass may also explain why fewer strength athletes avoided obesity when researchers used BMI as a measure. The BMI does not distinguish muscle from fat, emphasizes Dr. Brellenthin. As you build muscle through weight training, your BMI can go up.