The risk for a numbers of cancers could be raised by being overweight, researchers warn following a comprehensive review of over 200 studies. The cancer types consist mainly of those related to digestive organs and hormone-related malignancies, including those of the colon, breast, pancreas and ovary.
Associations between obesity and other cancers may also exist, but significant uncertainty remains due to the weak quality of the evidence, according to the international team of researchers, led by Maria Kyrgiou and Kostas Tsilidis from Imperial College London.
The team is calling for more research because 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.
Obesity And Cancers
Previous studies support the association between obesity and some cancers, but some may be flawed or biased due to weak study design and conduct. Cancer is a leading cause of death worldwide, and the prevalence of obesity has more than doubled over the past 40 years.
The researchers therefore conducted a comprehensive review of studies on obesity and risk of developing cancer.
After a literature search, they identified 204 studies from 49 publications that analysed the obesity measurements, such as body mass index (BMI), weight gain, and waist circumference, and 36 cancers and their subtypes.
Of the 95 studies that included continuous obesity measures, only 13% of associations were supported by strong evidence, meaning the studies had statistically significant results and no suggestion of bias.
Strong associations were found in studies that examined BMI with risk of oesophageal, bone marrow, and colon (in men), rectal (in men), biliary tract system, pancreatic, endometrial (in premenopausal women), and kidney cancers.
“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. Furthermore, emerging evidence suggests that excess body fat in early life also has an adverse effect on risk of cancer in adulthood,"
wrote Yikyung Park and Graham A Colditz in an accompanying editorial.
Observational Data Umbrella Review
The analysis involved an umbrella review of studies that used observational data, which is useful for bringing together evidence. However, no firm conclusions can be drawn about cause and effect when analysing observational studies.
Risk of developing cancer for every 5 kg increase in BMI ranged from 9% for colorectal cancer among men, to 56% for biliary tract system cancer.
Risk of postmenopausal breast cancer among women who never used hormone replacement therapy increased by 11% for each 5 kg of weight gain. Risk of endometrial cancer increased by 21% 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 and BMI with risk of gallbladder, gastric cardia, and ovarian cancers, and mortality from multiple myeloma.
Other studies were evaluated to have highly suggestive (18%), suggestive (25%), and weak (20%) evidence, and 25% had no evidence of an association.
Kyrgiou Maria, Kalliala Ilkka, Markozannes Georgios, Gunter Marc J, Paraskevaidis Evangelos, Gabra Hani et al. Adiposity and cancer at major anatomical sites: umbrella review of the literature BMJ 2017; 356 :j477
Image: Annie Cavanagh, Wellcome Images