File a study published in this week's JAMA under "been there, done that" in which the researchers followed for 20yrs 3,554 men & women 18-30yo at baseline and concluded that those who were the most physically active gained the least in both weight & waist circumference over the following two decades.
The algorithm used to determine physical activity took into account intensity, frequency & duration leading to an activity score in exercise units, 300 of which was deemed equivalent to the suggested 150 minutes/week recommendation of moderate intensity. However, it turns out that men in the lowest tertile of physical activity averaged just this amount, while men in the highest tertile, those who gained the least in body weight & waist circumference, were more than twice as active. For the ladies, 300 exercise units only placed them in the middle tertile while twice that much garnered them a place in the highest tertile.
There are two take home points from this study. First, everyone gained weight & expanded their waist circumference as they aged from young adulthood into middle age, regardless of physical activity. Second, it takes quite a bit of activity to minimize said gain & expansion, more than double what is currently recommended.