While it's no secret that obesity is at epidemic proportions in America or that obesity exacts a horrific toll in terms of diabetes, hypertension, and heart disease, it's less well known that being overweight (not merely obese) brings with it a cancer risk.
Far from being tenuous or uncertain, the link between overweight and cancer is very well established. In 2003, Calle et al. reported in The New England Journal of Medicine that increased body weight is associated with increased death rate for all cancers combined as well as for individual cancers specifically. This is based on research involving not just a few thousand people but 900,053 persons, spanning 16 years.
To quote from the study:
The heaviest members of this cohort (those with a body-mass index [the weight in kilograms divided by the square of the height in meters] of at least 40) had death rates from all cancers combined that were 52 percent higher (for men) and 62 percent higher (for women) than the rates in men and women of normal weight. For men, the relative risk of death was 1.52 (95 percent confidence interval, 1.13 to 2.05); for women, the relative risk was 1.62 (95 percent confidence interval, 1.40 to 1.87). In both men and women, body-mass index was also significantly associated with higher rates of death due to cancer of the esophagus, colon and rectum, liver, gallbladder, pancreas, and kidney; the same was true for death due to non-Hodgkin’s lymphoma and multiple myeloma. Significant trends of increasing risk with higher body-mass-index values were observed for death from cancers of the stomach and prostate in men and for death from cancers of the breast, uterus, cervix, and ovary in women. On the basis of associations observed in this study, we estimate that current patterns of overweight and obesity in the United States could account for 14 percent of all deaths from cancer in men and 20 percent of those in women.
The study, initiated in 1982 by the American Cancer Society, included men and women from all 50 states plus D.C. and Puerto Rico. The youngest participants were 30 years old; average age at inception was 57 years. By December 1998, 24% of participants had died, just over a quarter of them from cancer. In analyzing the results, researchers attempted to take into account such potential confounders as smoking status, alcohol use, aspirin use, status with respect to estrogen replacement therapy, and a wide variety of other factors that might otherwise skew the results.
The results were (and are) clear: The more you weigh, the greater your risk of dying of cancer (up to 52% higher risk for men, 62% for women). Positive associations of body mass index (BMI) and cancer mortality were found for almost all cancers. In men as well as women, the only cancers that did not show an overall statistically significant positive correlation with weight were lung cancer, brain cancer, melanoma, and bladder cancer (although in women only, lack of correlation was also noted for leukemia and esophageal cancer). For women, the strongest correlation was for uterine cancer, which is 6.5 times higher for women with a BMI of 40 or more. For men, the strongest weight correlation was with liver cancer (4.5 times higher in the most obese men) while the weakest positive correlation was with prostate cancer.
Smoking tends to obscure cancer/weight effects, because smokers tend to be more successful in keeping weight off than non-smokers, offsetting some of the risk. In non-smokers, lung cancer (as with most cancers) shows a clear positive correlation with body mass.
As for why extra weight leads to excess cancer mortality, there's no clear consensus. Decreased bioavailability of vitamin D in obese persons seems a likely factor. (Vitamin D is known to have a role in preventing colorectal and breast cancer and may well have broad-spectrum anti-cancer properties.) Also there's the simple fact that obesity complicates the management of cancer just as it does other conditions. On the other hand, the mechanisms by which obesity induces or promotes tumorigenesis do seem to vary by cancer site. For now, there is no simple answer to why obesity increases a person's cancer risk. All we know for certain is that the risk is real.
The 2003 study by Calle et al. is by no means the only study of its kind (although it's certainly the largest). For more data on obesity and cancer, see Percik and Stumvall (2009), Exp Clin Endocrinol Diabetes, or Basen-Engquist and Chang (2011) in Current Oncology Reports, or just spend a little time on Google Scholar while you're downing that burger.