"Strong evidence" supports the association between obesity and 11 cancers, which mostly comprise digestive organ tumors and hormone-related malignancies in women, according to a new analysis published online February 28 in BMJ.
"Other associations could be genuine as well, but there is uncertainty about them," said lead author, Maria Kyrgiou, PhD, MSc, from the Department of Surgery and Cancer, Imperial College London, United Kingdom, in an email to Medscape Medical News.
The new study is known as an "umbrella review" or a "meta-review" because it looks at previous meta-analyses and systematic reviews.
The umbrella review's conclusion — that excess body fat increases most digestive system cancers as well as endometrial and postmenopausal breast cancer — agrees with last year's report from the International Agency for Research on Cancer (IARC), point out a pair of researchers in an accompanying editorial.
However, the IARC has found associations with additional cancers (such as those of the liver, thyroid, and ovary) that the current study did not, write the editorialists, Yikyung Park, ScD, and Graham Colditz, MD, DrPH, from the Division of Public Health Sciences, Washington University School of Medicine, St Louis, Missouri.
Nonetheless, the data are "clear," say the pair. "The unavoidable conclusion from these data is that preventing excess adult weight gain can reduce the risk of cancer."
Clinicians — especially primary care providers — "can be a powerful force to lower the burden of obesity related cancers," given their role in obesity screening and prevention, the editorialists assert.
Excess body fat is potentially the second most important modifiable cancer risk factor after smoking, they say.
The new umbrella study looked at 95 meta-analyses that reported an association between excess body fat (as measured on a continuous scale) and the risk of developing or dying of cancer. Obesity was defined as a body mass index (BMI) >30 kg/m2.
Dr Kyrgiou explained that a "continuous measure is when the effect of the exposure on the outcome is measured as per unit change, i.e. risk of endometrial cancer per 5 kg/m2 increase in BMI."
There were seven indices of excess body fat/adiposity, including BMI, waist circumference, weight, and waist-to-hip ratio.
The international team of investigators judged that only 13% (12 of 95) of the studies identified in the umbrella review were based on strong statistical evidence (and avoided biases that may have exaggerated the effect of obesity on cancer). In other words, most studies had methodological flaws.
In the end, after analyzing these 12 studies, the team determined that there was an association between body fat and 11 cancer sites: esophageal adenocarcinoma; multiple myeloma; and cancers of the gastric cardia, colon (in men), rectum (in men), biliary tract system, pancreas, breast (postmenopausal), endometrium (premenopausal), ovary, and kidney.
The degree of risk varied. For example, the increase in the risk of developing cancer for every 5-kg/m2 increase in BMI ranged from 9% (relative risk, 1.09; 95% confidence interval [CI], 1.06 - 1.13) for rectal cancer among men to 56% (relative risk, 1.56; 95% CI, 1.34 - 1.81) for biliary tract system cancer.
The authors determined that the other 83 studies had highly suggestive (18%), suggestive (25%), and weak (20%) evidence; also, 25% had no evidence of an association.
Prospective studies are needed to draw "firmer conclusions" about which cancers are caused by excess body fat, say the study authors.
Who exactly is at high risk is unknown, they say. If that could be discerned, individuals could be selected for "personalised primary and secondary prevention strategies," the authors write.
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