effect of exercise

An Hour of Daily Exercise Helps Ward Off Weight Gain in Middle-Aged Women (with BMI less than 25 - in heavier women there was no relation - controlling calories is the key there)
From JAMA

Roughly 34,000 women (mean age at enrollment, 54) in the Women's Health Study answered questionnaires about their weight and exercise habits at baseline and periodically over 13 years' follow-up.

After adjusting for reported diet at baseline, researchers found that over any 3-year period, women who exercised <150 minutes weekly or 150 to <420 minutes weekly were, respectively, 11% and 7% more likely to gain 2.3 kg (5 lb.), compared with women who exercised ≥420 minutes a week.

The authors point out that the association was observed only among women with a body-mass index less than 25, noting that "among heavier women, there was no relation, emphasizing the importance of controlling caloric intake for weight maintenance in this group."