Illustration: Edwin Fotheringham

Number of dieters: Low fat, 24; exercise, 19.

Pounds lost at six weeks: Low fat, 6; exercise, 2.

Final loss (three months): Low fat, 9 pounds; exercise, 3.

The details: In a 2008 study published in the Journal of Exercise Physiology, a group of dieters followed a low-fat plan, while exercisers stuck to their usual eating patterns and worked out at least three days a week for 50 to 60 minutes at a gym. Though the dieters shed more pounds, some of the loss was in the form of calorie-burning muscle (the exercisers kept their muscle mass). As the researchers point out, muscle is key to helping dieters maintain weight loss. The results demonstrate the need to combine exercise and dieting.