"In this sample of middle-aged adults, drawn from the general population, physical activity was inversely associated with BMI [body mass index] and body fat percentage. For people with the same BMI, those who were more active had a lower body fat percentage."
Those were the conclusions made by Kathryn Bradbury and colleagues  (open-access) drawing on data derived from "cross-sectional analysis of participants recruited into UK Biobank in 2006–2010." Said results continue a research theme where physical activity figures in the aforementioned dataset.
So: "119 230 men and 140 578 women aged 40–69 years, with complete physical activity information, and without a self-reported long-term illness, disability or infirmity" were included for study - not an under-powered study by any means. Height and weight of participants were measured by trained staff "using standardised techniques." Physical activity estimates were gathered via self-report; specifically the use of 'touchscreens' as information gatherers regarding "walking, moderate physical activity and vigorous physical activity" and how often in a typical week participants "did each of the activities for 10 min or more" then onward for how many minutes during a day. Such data was number-crunched to provide something called "excess metabolic equivalent (MET)-hours/week of physical activity during work and leisure time." The authors also report on some efforts to off-set the potential unreliability of self-reported physical activity and several other variables (whether occupation involved primarily sitting or standing, tobacco smoking status, alcohol consumption, etc) were also thrown into the statistical mixer.
Results: well, who'd have thunk it? Those reporting higher levels of physical activity (via questionnaire responses) tended to have a lower BMI and a lower body fat percentage. They were also likely to eat more fruit and vegetables that those with low levels of physical activity. Diet, occupation type and education level did not however seem to affect the primary findings.
Accepting again that self-reported physical activity is not a great substitute for more objective measures and that BMI, whilst a good rough-and-ready indication of weight status, tends not to differentiate between fat and muscle, these are important results. They imply that far from not being able to outrun a bad diet (see here) physical activity still has an important place in maintaining a sensible weight and thus reducing the risk of a myriad of adverse health outcomes. Added to other findings indicating that we all really need to move quite a bit more (see here), the message seems to be that the human body was made for moving so move it. And the latest figures on obesity and physical activity highlight the challenges being faced in this area.
And so to close, a song to help with that 'move it' sentiment...
 Bradbury KE. et al. Association between physical activity and body fat percentage, with adjustment for BMI: a large cross-sectional analysis of UK Biobank. BMJ Open 2017; 7: e011843.
Bradbury, K., Guo, W., Cairns, B., Armstrong, M., & Key, T. (2017). Association between physical activity and body fat percentage, with adjustment for BMI: a large cross-sectional analysis of UK Biobank BMJ Open, 7 (3) DOI: 10.1136/bmjopen-2016-011843