An Imperial-led analysis of more than 2,000 studies of obesity and cancer reveals significant links between high body mass index and certain cancers.

The study concludes there is strong evidence to support the association between obesity and 11 of the 36 cancers studied, consisting mainly of those related to digestive organs and hormone-related malignancies. The review is published today in the BMJ.

There could be associations between obesity and other cancers, but substantial uncertainty remains because the quality of evidence from studies is not strong, say the international team of researchers, led by Dr Maria Kyrgiou from the Department of Surgery & Cancer and Dr Kostas Tsilidis from the School of Public Health at Imperial College London.

The researchers say confirming the link is important for identifying those at risk.

Dr Kyrgiou said: “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 personalised primary and secondary prevention strategies.”

Cancer is a leading cause of death worldwide, and the prevalence of obesity has more than doubled over the past 40 years.

Previously published evidence supports the association between obesity and some cancers, but some studies may be flawed or biased due to weak study design and conduct.

Therefore, in a bid to determine the quality of evidence and the strength of these associations, the researchers conducted a comprehensive review of studies on obesity and risk of developing cancer. 

After a literature search, they identified 204 meta-analyses from 49 publications that analysed obesity measurements – such as body mass index, weight gain, and waist circumference – and 36 cancers and their subtypes.

Of the 95 meta-analyses that included continuous obesity measures, only 13 per cent of associations were supported by strong evidence, meaning the studies had statistically significant results and no suggestion of bias.

Other studies were evaluated to have:

• highly suggestive (18 per cent)
• suggestive (25 per cent)
• weak (20 per cent) evidence
• and 25 per cent had no evidence of an association

Increasing risks

Strong associations were found in studies that examined body mass index (BMI) with oesophageal, bone marrow, and colon (in men), rectal (in men), biliary tract system, pancreatic, endometrial (in premenopausal women), and kidney cancers.

Risk of developing cancer for every five-point increase in BMI ranged from 9 per cent for colorectal cancer among men, to 56 per cent for biliary tract system cancer.

Risk of postmenopausal breast cancer among women who never used HRT increased by 11 per cent for each five kilograms of weight gain. Risk of endometrial cancer increased by 21 per cent for each 0.1 increase in waist to hip ratio.

Five additional associations were supported by strong evidence when categorical measures of obesity were used. These included weight gain with risk of colorectal cancer risk and BMI with risk of gallbladder, gastric cardia, and ovarian cancer, and mortality from multiple myeloma.

Dr Tsilidis said: “It is now clear that preventing excess weight may reduce the risk of developing certain forms of cancer. With this information, it is crucial that the healthcare system intensifies efforts to reduce the burden of cancer associated with obesity.”

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'Adiposity and cancer at major anatomical sites: umbrella review of the literature' by Maria Kyrgiou et al is published in the BMJ.