Background: Increased body mass index (BMI) is associated with an increase in overall mortality; however, the specific relationship of BMI to cancer is unclear. The most reliable evidence links increased BMI to cancers of the colon, endometrium, kidney, esophagus, and breast (in postmenopausal women). It is uncertain whether BMI influences the incidence of certain cancers, the risk of mortality, or both. Reeves and colleagues studied the relationship of BMI to cancer incidence and mortality in postmenopausal women.

The Study: The authors analyzed data from the Million Women Study, a large British cohort study of women 50 to 64 years of age. The 1.3 million participants were invited for a national breast screening campaign between 1996 and 2001. At recruitment, women who were diagnosed with a cancer (other than nonmelanoma skin cancer) were excluded from the study. The women completed questionnaires to record their height, weight, demographic factors, and other personal characteristics. BMI was calculated for each participant. Cancer registries and death certificates were subsequently monitored on the National Health Service registers to identify any cancer-related incident in participants. The association between BMI and incidence of the 17 types of cancers studied was assessed after an average follow-up of 5.4 years. The association between BMI and mortality caused by the cancers was examined after an average follow-up of seven years.

Results: The analysis included 1,222,630 women with an average age of 55.9 years at recruitment. The women experienced 45,037 new cancers and 17,203 cancer deaths during the study. BMI was significantly related to the relative risk of cancer incidence across all cancers, as well as cancers of the esophagus, pancreas, lung, endometrium, breast, kidney, and leukemia. The relationship of BMI to cancer mortality was generally similar to that of cancer incidence.

Although a trend emerged associating BMI to ovarian cancer, non-Hodgkin lymphoma, and multiple myeloma, the association was not statistically significant. For smoking-related cancers, the association between greater cancer risk and increased BMI was stronger when the analysis was restricted to never-smokers. An inverse relationship was found between BMI and the incidence of squamous cell carcinoma of the esophagus, lung cancer, and premenopausal breast cancer.

Overall, the researchers estimate that about 5 percent of cancers in postmenopausal women are attributable to excess weight. For endometrial cancer and adenocarcinoma of the esophagus, about one half of cases could be attributed to being overweight. This proportion was between 10 and 20 percent for leukemia, multiple myeloma, kidney cancer, and pancreatic cancer.

Conclusion: Increasing BMI is associated with increased incidence and mortality for at least 10 common cancers in postmenopausal women.