ArticlesBody-mass index and incidence of cancer: a systematic review and meta-analysis of prospective observational studies
Introduction
Excess bodyweight, whether in people who are overweight (defined as a body-mass index [BMI] of 25 to 29·9 kg/m2) or obese (BMI of 30 kg/m2 or greater), is increasingly recognised as an important risk factor for some common cancers.1, 2 Several meta-analyses3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 have assessed whether BMI is associated with cancer risk; most have investigated cancer at a particular site in the body. Some have examined the risk of cancer for incremental increases in BMI;3, 5, 8, 10, 11, 13, 15 others, the risk for overweight and obese categories in comparison with normal weight.7, 9, 12, 16, 17 Some meta-analyses incorporated results from case-control and cohort studies;3, 4, 5, 7, 9, 10, 12, 13, 15, 16, 17 others combined both incident cases and cancer deaths;3, 5, 8, 9, 10, 11, 12, 13, 15, 17 and others included studies that used diagnoses of obesity at discharge from hospital.9, 10, 13, 14, 17 Comparison of associations across studies, populations, and cancer sites is therefore difficult.
In 2007, the World Cancer Research Fund (WCRF)2 used a more standardised approach to review the evidence. This report concluded that the evidence that body fatness is associated with increased risk of oesophageal adenocarcinoma, and with cancers of the pancreas, colorectum, postmenopausal breast, endometrium, and kidney is convincing, and that a probably association exists between body fatness and risk of gallbladder cancer.2 However, several unanswered questions remain, including whether associations hold for less common malignancies, and whether associations differ between sexes and populations of different ethnic backgrounds. Several large cohort studies that were not included in previous reviews, including the Million Women study,20 studies from different continents,21, 22, 23 and studies of less common malignancies, have been published. We aimed to compare associations across cancer sites, and between sexes and populations to quantify the risk of different cancers associated with an incremental increase in BMI. We used uniform methods and definitions to do a systematic review and meta-analysis of prospective observational studies.
Section snippets
Search strategy and selection criteria
We systematically searched Medline and Embase (from their commencements to November 2007), with no language restrictions, for studies in humans of the association between bodyweight and cancer incidence for 20 cancer types in 15 sites in the body: colorectal (colon and rectum); gastro-oesopheageal (gastric, oesophageal adenocarcinoma, and squamous cell carcinoma); hepatobiliary (gallbladder and liver); leukaemia; lung; malignant melanoma; multiple myeloma; non-Hodgkin lymphoma; pancreas; renal
Results
Of 4285 citations, we identified 221 datasets from 141 articles which met the inclusion criteria (see webappendix 3). Figure 1 shows our search and selection process, and webappendix 4 lists excluded articles with reasons for their exclusion. Agreement between observers on which studies to include was good (Cohen's unweighted κ=0·86). All papers used in our analysis were published in English, except for one that was written in Chinese.34 Figure 2 shows that more than half the papers (73/141)
Discussion
Our large standardised meta-analysis shows that increased BMI is associated with an increased risk of several cancers in adults. Our findings extend the results of previous reports,1, 2 to show associations between increased BMI and cancer risk for less common malignancies, and evidence that associations might differ between men and women for some sites, in particular for colon cancer. The magnitudes of associations between increased BMI and cancer were similar across populations for most
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