Recent revisions in the 2008 (USA) Physical Activity Guidelines were prompted by evidence that the health benefits (both in terms of morbidity and mortality) of moderate physical activity (at least 150 min per week) are substantial1 ,2 The authors of this paper do not directly address the problem of health and survival benefits associated with physical activity, but rather take an indirect route: they examine to what extent moderate amounts of exercise are enough to prevent weight gain over time, taking the adverse impact of weight gain on morbidity and mortality for granted.

The study follows 34 079 middle-aged women, whose self-reported physical activity, weight and height measures were obtained at baseline and five follow-up interviews spaced 2–3 years apart. The main outcome variables are individual weight changes occurring in any of the five 2- or 3-year intervals, and the odds of gaining at least 2.3 kg in weight during the same intervals. The main …