These men and women had completed the typical series of health and fitness measurements during their clinic visits and also filled out an exercise questionnaire that inquired about, among other things, weight training. They were asked if they did “muscle strengthening exercises” and, if so, how often and for how many minutes a week.
The researchers then began to collate the data, comparing people’s weights and other measurements between each clinic visit. Using BMI as a measure, about seven percent of the men and women had become obese within six years of their first clinic visit.
But BMI is a poor approximation of body constitution and is not always an accurate measure of obesity. So the researchers also looked at changes in waist thickness and body fat percentage to determine whether they had become obese. Based on the criteria of a waist diameter greater than 100 centimeters for men and 90 for women, or a percentage of body fat greater than 25 percent for men and 30 percent for women, up to 19 percent of the participants developed obesity over the years.
However, the researchers found that lifting weights changed those results, significantly reducing the risk that someone might be obese, no matter what measurement parameter they used. Men and women who reported strengthening their muscles several times a week, for a total of one to two hours per week, were 20 percent less likely to become obese over the years, based on BMI, and 30 percent less, depending on the thickness of the waist or the percentage of body fat.
The benefits did not change when the researchers controlled for variables of age, sex, smoking, general health, and aerobic exercise. People who did some aerobic exercise and lift weights were much less likely to become obese. But the same was true of those who only lifted weights and said they did little or no aerobic exercise.
The results suggest that “you can get a lot of benefits from lifting weights, even if you don’t do it a lot,” says Angelique Brellenthin, a professor of kinesiology at Iowa State University who led the new study.
Of course, the research was observational and does not show that resistance training prevents weight gain, only that both factors are related. It also did not consider people’s diet, genetics or health habits, which could affect the risk of obesity.