Wednesday March 1 2017
Cancers linked with obesity include bowel and breast cancer
"Increased risk of 11 types of cancer linked to being overweight," The Guardian reports.
A new review in the BMJ found strong evidence of a link between body weight and 11 types of cancer, most of them either digestive (such as bowel cancer) or hormonal (such as breast cancer).
The review was based on more than 200 summaries of studies looking at the link between excess body fat and a specific cancer.
It found strong evidence people who are overweight have an increased risk of developing 11 different cancers: pancreatic, kidney, ovarian, biliary tract, oesophagus, colon and rectum, bone marrow (multiple myeloma) and stomach cancers, as well as breast and endometrial cancer in women.
A high proportion of adults in the UK have excess fat (high adiposity) and are at risk, not just those who are classified as overweight or obese.
As a result of the ongoing obesity epidemic in the UK, the concern is obesity-related cancers will overtake lung cancer as the leading cause of cancer-related deaths. This would mean a lot of people develop cancer that could have been prevented.
Not everyone is aware of the link between excess fat and being at an increased risk of developing certain cancers, so it's important we all take steps towards leading a healthy lifestyle.
You should aim to maintain a healthy weight by eating a balanced diet and exercising regularly, and avoid smoking and drinking too much alcohol.
Where did the story come from?
The study was carried out by researchers from Imperial College London, Imperial Healthcare NHS Trust, Lancashire Teaching Hospitals and the University of Lancaster, all in the UK, and the University of Ioannina, Greece and the International Agency for Research on Cancer in France.
It was funded by the Genesis Research Trust, Sigrid Jusélius Fellowship, World Cancer Research Fund International Regular Grant Programme, Ovarian Cancer Action, the Imperial Experimental Cancer Medicine Centre, the Cancer Research UK Imperial Centre, and Imperial Healthcare NHS Trust.
None of the funders had any influence on the study. The authors declared no competing interests.
The study was published in the peer-reviewed British Medical Journal (BMJ) and is open access, meaning it's free to read online (PDF, 371kb).
The study attracted a great deal of coverage in the UK media, and the reporting was accurate.
What kind of research was this?
This umbrella review of systematic reviews and meta-analyses looked at the association between indicators of excess body fat and the risk of developing or dying from cancer.
An umbrella review involves looking at the evidence on a whole area, and examining multiple risk factors on a range of outcomes, by comparing and contrasting the findings of relevant systematic reviews and meta-analyses.
While a review is good at looking at all evidence in a certain area, it's only as good as the studies it includes. Any limitations of these studies will also be limitations of the review.
In this case, the main findings were based on meta-analyses of mostly cohort studies, which aren't able to prove cause and effect. However, this is probably the best type of study available to look at excess fat and cancer risk.
What did the research involve?
The study involved looking at 95 meta-analyses that examined studies using a continuous scale to measure fat.
The researchers then looked at the association between levels of fat and the risk of developing or dying from cancer at one of 28 different sites.
Continuous measures of fat included grouping people according to every 10cm increase in waist circumference, 5kg increase in body weight, or 5kg/m2 increase in body mass index (BMI).
To put this last measurement into context, this increase could take someone from a healthy weight (a BMI of 24) to being overweight (a BMI of 29), or alternatively, from being overweight (a BMI of 29) to being obese (a BMI of 34).
BMI was the most common measure of fat, used in 60% of meta-analyses.
The studies were categorised by their strength and validity of evidence into a:
- strong association
- highly suggestive association
- suggestive association
- weak association
All of these strengths were statistically significant, but some provided stronger evidence than others.
What were the basic results?
There was strong evidence in 12 studies for an association between excess body fat and risk of developing cancer.
Nine cancers showed strong evidence of a link to increased fat levels.
Each increase of 5kg/m2 in BMI was linked to a higher risk of developing:
- colon cancer in men
- rectum cancer in men
- a certain cancer of the gullet (oesophagus)
- pancreatic cancer
- kidney cancer
- cancer of the womb lining (endometrial cancer) in premenopausal women
- multiple myeloma – a type of bone marrow cancer
- three types of biliary tract system cancers – cancers of the gallbladder, extrahepatic bile duct and ampulla of Vater
Weight gain and waist-to-hip ratio were linked to developing breast cancer in postmenopausal women who never had hormone replacement therapy (HRT), as well as endometrial cancer.
- for every 5kg/m2 gain in BMI, there was a 9% increased risk of developing colorectal cancer in men (95% confidence interval [CI] 1.06 to 1.13)
- for every 5kg/m2 gain in BMI, there was a 56% increased risk of developing biliary tract system cancer (95% CI 1.34 to 1.81)
- for each 5kg of weight gain, postmenopausal women who never used HRT had an 11% increased risk of breast cancer (95% CI 1.09 to 1.13)
- for each 0.1 increase in waist-to-hip ratio, there was a 21% increased risk of endometrial cancer (95% CI 1.13 to 1.29)
There was also a strong link between obesity and stomach and ovarian cancer, compared with people who had a healthy BMI.
How did the researchers interpret the results?
The researchers concluded that, "Although the association of adiposity with cancer risk has been extensively studied, associations for only 11 cancers (oesophageal adenocarcinoma, multiple myeloma, and cancers of the gastric cardia, colon, rectum, biliary tract system, pancreas, breast, endometrium, ovary and kidney) were supported by strong evidence.
"Other associations could be genuine, but substantial uncertainty remains. Obesity is becoming one of the biggest problems in public health; evidence on the strength of the associated risks may allow finer selection of those at higher risk of cancer, who could be targeted for personalised prevention strategies."
The results of this study provide further evidence for the link between increasing levels of fat and the risk of developing certain cancers.
There was strong evidence for nine cancers, with another two – ovarian cancer and stomach cancer – included when comparing obesity with healthy weight.
This study is important in showing the significance of fat levels and obesity in cancer risk.
But there are some important things to consider:
- The study doesn't tell us how excess body fat might play a role in the development of certain cancers, just that there's a link.
- Some studies might have been missed, as the review relied on other researchers to include all the most recent and relevant studies in their meta-analyses.
- Other studies were statistically significant but of mixed quality, so it's possible excess fat is linked to other cancers, but the evidence was not quite as strong as for the cancers the study identified.
While the findings of this study may be worrying, it's important to focus on the positives.
Losing any excess weight should help reduce your risk of developing these types of cancer, as well as other chronic diseases like heart disease and type 2 diabetes.
Get more advice and weight loss tips.