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Obesity strongly linked to heightened risk of certain cancers

Every 5kg in weight gain may make the difference

Caroline White

Wednesday, 01 March 2017

Obesity is strongly linked to the risk of developing certain major cancers, reveals a re-analysis of systematic reviews and meta-analyses,* published in The BMJ today.

The links are strongest for 11 cancers related to digestive organs and hormones, says the review. Obesity could also be linked to other cancers, but the quality of the evidence is not sufficiently strong to draw those conclusions yet, say the researchers.

More high quality research to unpick these issues might help to fine tune those most at risk, they suggest.

Obesity prevalence has more than doubled over the past 40 years, and the evidence to date suggests that it is linked to a heightened risk of developing particular cancers, but methodological flaws in some published studies have weakened the strength of the associations found.

To better gauge the quality of the evidence and the strength of these associations, the researchers comprehensively reviewed published studies looking at obesity and cancer risk.

From among 204 reviews that analysed obesity measurements, such as body mass index (BMI), weight gain, and waist circumference, and the risk of 36 cancers, 95 included continuous measures of obesity.

Only 13% of the associations for nine cancers were based on strong evidence, meaning the results were statistically significant and excluded bias.

Strong associations were found in studies that looked at heightened risk of oesophageal, bone marrow, colon (in men), rectal (in men), biliary tract system, pancreatic, endometrial (in premenopausal women), and kidney cancers.

The risk of developing cancer ranged from 9% for colorectal cancer among men, to 56% for biliary tract system, for every 5kg increase in BMI.

Similarly, risk of breast cancer among women who had gone through the menopause but who had never used hormone replacement therapy increased by 11% for each 5kg of weight gain. And the risk of endometrial cancer increased by 21% for each 0.1 increase in waist to hip ratio.

Strong evidence pointed to five additional associations when categorical measures of obesity were used. These included weight gain and heightened risk of colorectal cancer; and BMI and heightened risk of cancers of the gallbladder, gastric cardia, and ovaries, as well as a heightened risk of death from multiple myeloma.

Other studies had highly suggestive (18%), suggestive (25%), or weak (20%) evidence in support of a link between obesity and cancer risk, while in 25% there was no evidence of an association.

In a linked editorial,** Yikyung Park and Graham Colditz, from Washington University School of Medicine, explain that "though some specifics remain to be worked out, the unavoidable conclusion from these data is that preventing excess adult weight gain can reduce the risk of cancer.”

“Given the critical role of health care providers in obesity screening and prevention, clinicians, particularly primary care clinicians, can be a powerful force to lower the burden of obesity-related cancers as along with the many other chronic diseases linked to obesity such as diabetes, heart disease, and stroke,” they conclude.

 * Kyrgiou M, Kalliala I, Markozannes G, et al. Adiposity and cancer at major anatomical sites: umbrella review of the literature. BMJ 2017;356:j477. DOI: 10.1136/bmj.j477

** Park Y, Colditz GA. Fresh evidence links adiposity with multiple cancers. BMJ 2017;356:j908. DOI: 10.1136/bmj.j908

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