Background To promote and maintain health, all adults are recommended to do moderate-intensity aerobic activity a minimum of 30 min on 5 days, or vigorous-intensity activity of 20 min on 3 days, each week. Whether these levels prevent long-term weight gain is uncertain.

Objective To assess the relationship between physical activity and long-term weight gain.

Study design An observational prospective cohort study.

Methods Weight and physical activity were measured in the Nord-Trøndelag Health Study in 1984–1986, 1995–1997 and 2006–2008. Participants (n=19 127) were classified based on physical activity into inactive, below recommended level, recommended level or above recommended level. We carried out adjusted mixed model regression analyses with weight as outcome.

Results Men maintaining physical activity above the recommendations for 33 years increased 5.6 kg, while inactive men increased 9.1 kg. For women, corresponding numbers were 3.8 kg in those above recommended physical activity levels, and 9.5 kg in inactive. In adjusted analyses, physical activity above the recommendations was associated with 2.1 kg (95% CI 1.8 to 2.4) less weight gain in men over any 11-year period, compared with inactive. Women exceeding the recommendations gained 1.8 kg (CI 1.5 to 2.2) less than inactive. Compared with inactive, the ORs of gaining meaningful weight of ≥2.3 kg were 0.79 (CI 0.69 to 0.91) and 0.70 (CI 0.60 to 0.81) if exceeding the recommendations in men and women, respectively.

Conclusions Physical activity above the current recommendations for health benefits was associated with significantly lower risk of weight gain.