Strong evidence links obesity to gastrointestinal, breast cancers

There is strong evidence linking adiposity to esophageal adenocarcinoma, multiple myeloma, and cancer of the colon, rectum, biliary tract, pancreas, endometrium, kidney, and postmenopausal breast, according to the authors of an umbrella review published in the Feb. 28 edition of BMJ.

“Several meta-analyses support the link between obesity and cancer, but substantial heterogeneity exists between studies. The reported associations may be causal, but they may also be flawed, as inherent study biases such as residual confounding and selective reporting of positive results may exaggerate the effect of obesity on cancer.”

In this umbrella review, researchers analyzed 49 papers that included a total of 204 meta-analyses, which in turn summarized 2,179 individual study estimates from 507 unique cohort or case-control studies.

However, only 95 of the meta-analyses used cohort studies that used a continuous scale to measure adiposity, such as body mass index, waist circumference, hip circumference, or waist-to-hip ratio. Of these, 12 (13 percent) were supported by strong evidence, the authors reported.

When researchers applied a threshold for significance of P less than .000001, the summary random effects were significant in 35 meta-analyses; 31 of these found increased risk with adiposity of esophageal adenocarcinoma, multiple myeloma, and cancers of the colon, rectum, liver, biliary tract system (cancers of gallbladder, extrahepatic bile duct, and ampulla of Vater), pancreas, postmenopausal breast, endometrium, and kidney.

“The effect of obesity on the incidence and mortality of cancer is well recognized and was evident in our umbrella review, with approximately 77 percent of the included meta-analyses reporting a nominally statistically significant summary random effects estimate,” the authors reported.

Overall, the summary estimates were similar between men and women for esophageal adenocarcinoma, esophageal squamous cell carcinoma, multiple myeloma, leukemia, and gastric lung, kidney, and thyroid cancers.

However, men had a 30 percent higher risk of colon cancer per 5-kg/m increase of body mass index, compared with a 9 percent increase in risk in women for the same rise in BMI. Men also showed an increased risk of melanoma with increasing BMI, whereas women did not.

Women who had never used hormone therapy showed an 11 percent increase in the risk of postmenopausal breast cancer with each five kg of weight gained. Similarly, each 0.1 increase in waist-to-hip ratio in these women was associated with a 21 percent increase in the risk of endometrial cancer.

The analysis also revealed an inverse relationship in four meta-analysis for esophageal squamous cell carcinoma and lung cancer.

The authors said their findings agree with those of the World Cancer Research Fund, which currently states there is a convincing casual relationship with obesity for esophageal adenocarcinoma  and cancers of the pancreas, colorectum, postmenopausal breast, endometrium, kidney, and liver.