We were previously told that 150 minutes/week of "moderate-intensity" exercise (5 days a week @ 30 min./day) would suffice in terms of preventing weight gain. It seems that number has now been pushed to 250 minutes/week (5 days a week @ 50 day). Keep in mind the key to exercise comes down to one word = INTENSITY. Just another reason to look into alternative training methods - see my post below on Tabata training.
To read the full story published by Reuters Health on the new American College of Sports Medicine guidelines regarding weight gain CLICK here.
If your interested in the research source please see Medicine & Science in Sports & Exercise, the February 2009 issue.