Lung Cancer, Smoking, and Obesity: It’s Complicated

Obesity is an established cause of cancer and causally linked specifically to cancers of selected sites, including, for example, endometrial cancer, kidney cancer, and esophageal cancer (adenocarcinoma) (1). Three main mechanisms have been postulated as underlying the increased cancer risk associated with obesity: 1) the actions of sex hormones affected by obesity; 2) insulin resistance and insulin-like growth factor 1; and 3) adipokine pathophysiology and systemic inflammation (2). The hormonal hypothesis has site specificity while the latter two mechanisms are general to carcinogenesis. For lung cancer, the worldwide leading cause of cancer death, the evidence from single studies and from systematic reviews and meta-analyses have been mixed, but consistent, in indicating an inverse relationship between body mass index (BMI) and lung cancer risk. However, the evidence has been limited to current and former smokers in most studies. Interpretation of this inverse association has been constrained by potential confounding, as smoking both causes lung cancer and affects body weight; current smokers tend to weigh less than never and former smokers while former smokers gain weight after successful cessation (3,4). Reverse causation may also affect the association between smoking and lung cancer risk, as the presence of lung cancer may affect body weight and potentially lead to a decision to quit.

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