From the Journals

‘Strong evidence’ links obesity to 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 the BMJ.

“Several meta-analyses support the link between obesity and cancer, but substantial heterogeneity exists between studies,” wrote Maria Kyrgiou, MD, of Imperial College London and her coauthors. “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%) were supported by strong evidence, the authors reported (BMJ 2017;356:j477).

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% 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% higher risk of colon cancer per 5-kg/m2 increase of body mass index, compared with a 9% 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% increase in the risk of postmenopausal breast cancer with each 5 kg of weight gained. Similarly, each 0.1 increase in waist-to-hip ratio in these women was associated with a 21% increase in the risk of endometrial cancer.

The analysis also revealed an inverse relationship in four meta-analyses 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 causal relationship with obesity for esophageal adenocarcinoma and cancers of the pancreas, colorectum, postmenopausal breast, endometrium, kidney, and liver.

While this umbrella analysis did not find strong evidence for an association with liver cancer, the authors said the evidence was “highly suggestive” but suffered from small study effects, excess significance bias, and substantial heterogeneity between studies.

“To draw firmer conclusions, we need prospective studies and large consortiums with better assessment of the changing nature of body fatness and with comprehensive standardized reporting of analyses,” they wrote. “As obesity becomes one of the greatest public health problems worldwide, evidence of the strength of the associations between obesity and cancer may allow finer selection of people at high risk, who could be selected for personalized primary and secondary prevention strategies.”

The study was supported by the Genesis Research Trust, Sigrid Jusélius Fellowship, the World Cancer Research Fund International Regular Grant Programme, Ovarian Cancer Action, the Imperial Experimental Cancer Medicine Centre, the Cancer Research UK Imperial Centre, Imperial Healthcare NHS Trust NIHR BRC. No relevant conflicts of interest were declared.

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