Body mass index and lung cancer risk in never smokers: a meta-analysis

Background: Obesity is found to increase the risk of most cancer types, but reduce lung cancer risk in many studies. However, the association between obesity and lung cancer is still controversial, mainly owing to the confounding effect of smoking. Methods: Eligible studies were identified from electric databases to July 1, 2017. Relevant data were extracted and pooled using random-effects models; dose-response and subgroup analyses were also performed. Results: Twenty-nine studies with more than 10,000 lung cancer cases in 15 million never smokers were included. Compared with normal weight, the summary relative risk (RR) was 0.77(95% confidence interval [CI]: 0.68-0.88, P < 0.01) for excess body weight (body mass index [BMI] >= 25 kg/m(2)). An inverse linear dose-response relationship was observed between BMI and lung cancer risk in never smokers, with an RR of 0.89(95% CI: 0.84-0.95, P < 0.01) per 5 kg/m(2) increment in BMI. The results remained stable in most subgroup analyses. However, when stratified by sex, a significant inverse association existed in women but not in men. Similar results were found in analyses for other categories of BMI. Conclusion: Our results indicate that higher BMI is associated with lower lung cancer risk in never smokers.