In several studies, people in control groups who did not diet were not that much worse off — and in many cases were better off — than those who did diet, she said.

If dieting doesn't work, what does?

"Eating in moderation is a good idea for everybody, and so is regular exercise," Mann said. "That is not what we looked at in this study. Exercise may well be the key factor leading to sustained weight loss. Studies consistently find that people who reported the most exercise also had the most weight loss."