The study by Hamer et al.  in this issue of Obesity presents results from an analysis of the association between achievement of moderate and vigorous PA according to the public health guidelines, with BMI and waist circumference using prospective data among middle-aged men and women from The Whitehall II cohort study. Information on PA and adiposity were assessed on three occasions during 9.5-12.9 years of follow-up. The authors report that PA was unrelated to BMI and waist circumference when using only the baseline information on PA. In contrast, when taking full advantage of the three repeated assessments of PA, participants consistently meeting the 2.5 hours/week PA public health guideline during follow-up had lower BMI (−0.43 kg/m2) and waist circumference (−2.50 cm) over 10-years compared with participants rarely meeting the guideline in a multivariable model including important determinants of adiposity such as total energy intake and smoking. Interestingly, these findings are generally consistent with two other population-based prospective studies using multiple assessments of PA during follow-up [2, 3], although the data from Women's Health Study suggest that the total amount of PA should be approximately twice that of the current guideline for the prevention of gains in adiposity in women. Hamer et al.  also reported some evidence of a bi-directional association that adds to the evidence of a vicious cycle relationship in which low PA may influence adiposity gains and that gains in adiposity also may lead to a reduction in PA .
In the current public health guidelines for PA it is also recommended to additionally engage in muscle-strengthening activity at least two times/week. The authors did not address whether additional achievement of this component of the guidelines confers added and independent benefit for primary prevention of adiposity gains and to date this remains fairly unknown. In addition, the 2008 Physical Activity Guidelines for Americans  refers to PA beyond what is referred to as the “baseline activity” (the usual light or sedentary activities of daily living). Daily time spent on sedentary activities such as TV viewing and computer use is substantial for many people and is likely to displace other forms of “baseline activity” with greater energy expenditure. Estimating the independent effects of increasing “baseline activity” on prevention of adiposity gains in population samples is also an important topic for future studies.
In conclusion, the study by Hamer et al.  suggests that engagement in moderate and vigorous PA according to the minimum of 2.5 hours/week recommended by the current guidelines can prevent gains in adiposity in a population of middle aged men and women. The study adds to the evidence that addressing obesity prevention in populations should not only focus on limiting energy intake but also put emphasis on encouraging increases in PA levels.