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.

中文翻译：

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肺癌，吸烟和肥胖：这很复杂

肥胖是确定的癌症原因，并且与选定部位的癌症有因果关系，包括例如子宫内膜癌，肾癌和食道癌（腺癌）（1）。据推测，与肥胖相关的癌症风险增加的潜在原因有三个主要机制：1）受肥胖影响的性激素的作用； 2）由肥胖引起的性激素的作用。2）胰岛素抵抗和胰岛素样生长因子1；3）脂肪因子的病理生理和全身性炎症（2）。激素假说具有位点特异性，而后两种机制是致癌的一般机制。对于肺癌（全球癌症死亡的主要原因）而言，单项研究以及系统评价和荟萃分析的证据混杂在一起，但却是一致的，它们表明体重指数（BMI）与肺癌风险之间存在反比关系。但是，在大多数研究中，证据仅限于现在和以前的吸烟者。由于吸烟会引起肺癌并影响体重，因此对这种反向关联的解释受到了潜在混淆的限制。目前的吸烟者比以前的吸烟者体重更轻，而以前的吸烟者成功戒烟后体重会增加（3,4）。反向因果关系也可能影响吸烟与肺癌风险之间的关联，因为肺癌的存在可能会影响体重并可能导致戒烟决定。目前的吸烟者比以前的吸烟者体重更轻，而以前的吸烟者成功戒烟后体重会增加（3,4）。反向因果关系也可能影响吸烟与肺癌风险之间的关联，因为肺癌的存在可能会影响体重并可能导致戒烟决定。目前的吸烟者比以前的吸烟者体重更轻，而以前的吸烟者成功戒烟后体重会增加（3,4）。反向因果关系也可能影响吸烟与肺癌风险之间的关联，因为肺癌的存在可能会影响体重并可能导致戒烟决定。