Most doctors will tell you that exercise alone is not effective for weight loss, but that’s because 98 percent of the information they have is from research that isn’t done in a controlled setting, said Joseph E. Donnelly, EDD, at the American Society for Nutrition (ASN) meeting in April 2012. 

Working with the theory that “the potential to get more out of exercise is generally more than we think,” Donnelly and his colleagues at the University of Kansas Medical Center conducted two randomized, clinical trials evaluating what type of exercise can result in a 5 percent weight loss, without any sort of diet restrictions. Previous studies have shown that losing 5 percent of your body weight can significantly improve your risk of cardiovascular disease, diabetes and other health conditions that are worsened by being overweight. 

The researchers conducted two studies called the Midwest Exercise Trials. In the first study, completed in 2003, overweight men and women ages 24 to 34 did supervised aerobic exercise 45 minutes a day, five days a week, for 16 months. All participants exercised at 75 percent of their maximum heart rate—defined as moderate aerobic exercise—and ate whatever they wanted throughout the study period.

The researchers targeted an energy expenditure of 400 calories per exercise session, Donnelly said. The men averaged 667 calories during each 45-minute session, while the women averaged 439 calories. At the end of the study, the men had lost an average of 11.5 pounds, but the women actually gained 1.3 pounds.

“We weren’t all that popular with the ladies,” Donnelly joked during his presentation at the ASN meeting, “but the advice to the men may be rather than diet and exercise, you can just do exercise.”

In an effort to discover whether women could lose weight by exercise alone, the researchers conducted a second Midwest Exercise Trial. This trial, which lasted 10 months and was completed in 2012, was tweaked so that the 141 men and women who participated stopped exercising after they had expended 400 or 600 calories in a session. As in the first trial, all participants did aerobic exercise at 75 percent of their heart rate five days a week, without restricting their diet.

Men who weighed an average of 220 pounds only had to exercise 37 minutes per session to reach the 600-calorie expenditure, while women who averaged 174 pounds had to work out for 85 minutes. But the good news is that after 10 months, the women lost an average of about 9 pounds with 400- or 600-calorie exercise, while the men lost about 13 pounds at 600 calories. In addition, most of that weight loss was fat. 

“I think we’re selling exercise a bit short—I do think exercise alone can provide about 5 percent weight loss for many individuals,” Donnelly concluded. “Gender differences do seem to change dramatically when expenditure of exercise is equivalent.”