Body mass index and mortality from lung cancer in smokers and nonsmokers: a nationally representative prospective study of 220,000 men in China

The mean baseline BMI was 21.7 kg/m 2 , and 2,145 lung cancer deaths were recorded during 15 years of follow-up. The prevalence of smoking was strongly inversely associated with BMI, but no apparent relationship was seen between amount smoked (or other measures of smoking intensity) and BMI among smokers. Overall there was a strong inverse association between BMI and lung cancer mortality (p < 0.0001 for trend) after excluding the first 3 years of follow-up. This association appeared to be confined mainly to current smokers, with no apparent relationship in nonsmokers (p < 0.001 for difference between slopes). Among current smokers, the inverse association appeared to be log-linear, with each 5 kg/m 2 lower BMI associated with a 35% (95% confidence interval: 24–46%; p < 0.0001) higher lung cancer mortality, and it persisted after excluding those who had reported poor health status or history of any disease or respiratory symptoms at baseline. In this relatively lean Chinese male population, low BMI was strongly associated with increased risk of lung cancer only among current smokers. ' 2009 UICC Lung cancer is among the commonest types of cancer throughout China, causing some 400,000 deaths annually. During the last few decades the disease rate has risen steadily, especially among males, as a consequence of the recent large increase in cigarette consumption. 1,2 Despite the rise in smoking-related diseases, the emerging tobacco hazard in China remains at an early stage due to the delayed effect of increase in cigarette consumption, with only about 12% of male adult deaths being attributed to smoking during the 1990s, a proportion similar to that in the USA during the early 1950s. 3 Currently about two-thirds of men aged 15 or over in China smoke, compared to fewer than 5% of women. 3,4 Although there is little variation in the smoking prevalence between different regions of China, the absolute lung cancer rate varies considerably, with the nonsmoker lung cancer rate in some parts of China being about 10 times greater than that among US nonsmokers. 3 This suggests that factors other than smoking contribute significantly to disease risk in the population. 3,5,6 Several observational studies have examined the relationship between lung cancer and body mass index (BMI), an indirect but useful measure of body fatness. 7‐23 Most of these studies have shown BMI to be inversely associated with the risk of lung cancer, with particularly high excess risk among very lean people (BMI < 18.5 kg/m 2 ). Questions remain, however, about the nature of the association, particularly the relevance to it of smoking. Some studies have attributed the inverse association of BMI and lung cancer risk entirely to incomplete control of confounding by smoking and/or of reverse causality due to pre-existing dis