Exercise is more helpful for preventing weight gain than for inducing weight loss
From The New York Times:
It is a question that plagues all who struggle with weight: Why do some of us manage to keep off lost pounds, while others regain them?
Now, a study of 14 participants from the “Biggest Loser” television show provides an answer: physical activity — and much more of it than public health guidelines suggest.
On average, those who managed to maintain a significant weight loss had 80 minutes a day of moderate activity, like walking, or 35 minutes a day of vigorous exercise, like running.
My patients taught me this years ago.
Diabetic Mediterranean Diet 
So eat less and move more to stay thin.