BMJ: Strong evidence supports the association between obesity
and some major types of cancer, consisting mainly of those related to
digestive organs and hormone-related malignancies, reveals a large
review published by The BMJ today. There
could be associations between obesity and other cancers, but
substantial uncertainty remains because the quality of evidence is not
strong, say the international team of researchers, led by Maria Kyrgiou
and Kostas Tsilidis from Imperial College London. They
call 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.”
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 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 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.
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.
This 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. 
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.
[Ends]
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