Does smoking influence the physical activity and lung cancer relation? A systematic review and meta-analysis

Research suggests an inverse association between physical activity and lung cancer. However, whether the relation is modified by degree of smoking adjustment has not been summarized. We conducted a meta-analysis of physical activity and lung cancer focusing on evaluating whether smoking status and the degree of smoking adjustment influenced the association. Comparing high versus low physical activity levels from 25 observational studies yielded a lung cancer summary relative risk (RR) of 0.79 [95 % confidence interval (CI) = 0.72-0.87], with RRs of 0.87 (95 % CI = 0.80-0.94) for cohort studies and 0.57 (95 % CI = 0.46-0.71) for case-control studies. In further analyses restricted to cohort studies, physical activity was inversely related to lung cancer among former smokers (RR = 0.68, 95 % CI = 0.51-0.90) and current smokers (RR = 0.80, 95 % CI = 0.70-0.90), whereas the association was null among never smokers (RR = 1.05, 95 % CI = 0.78-1.40, p interaction = 0.26). The degree of smoking adjustment did not modify the association (p interaction = 0.73). Physical activity was unrelated to lung cancer among never smokers but it was inversely associated with lung cancer among former and current smokers. Although the physical activity and lung cancer relation was not modified by smoking status or degree of smoking adjustment, residual confounding by smoking remains a possible explanation for the relations observed.