"A daily aspirin may slash bowel cancer risk for the obese," the Daily Mail reports. But the headline fails to make it clear that this latest research didn't involve people in the general public who were obese.
It actually involved people at high risk of bowel cancer as the result of a rare hereditary condition known as Lynch syndrome. The majority of those with the condition will develop bowel cancer at some point in their adult lives.
The main finding of this research was that being overweight or obese was associated with a further increase in these people's risk of bowel cancer. However, the study also found body mass index (BMI) did not appear to affect bowel cancer risk in people who had been taking aspirin. This suggested that aspirin might be offsetting the risk of obesity in people with Lynch syndrome.
However, this trial may not be representative of the majority of people, who do not have Lynch syndrome. In addition, there are potential risks associated with regular long-term aspirin use, such as gastrointestinal bleeding. This means the risks of the general public taking aspirin to combat bowel cancer may outweigh any benefits.
Aiming to achieve or maintain a healthy body weight, as well as eating a healthy diet, taking regular exercise and not smoking, are ways you can help to reduce your bowel cancer risk. You should not start taking aspirin regularly without discussing it with your GP first.
Where did the story come from?
The study was carried out by researchers from Newcastle University and other international research centres.
It was funded by the UK Medical Research Council, Cancer Research UK, the European Union, the Cancer Council Victoria in Australia, the Technology and Human Resources for Industry Programme South Africa, the Sigrid Juselius Foundation, and the Finnish Cancer Foundation.
Bayer and National Starch and Chemical provided drugs and placebos for free and made donations towards running the study.
The study was published in the peer-reviewed Journal of Clinical Oncology and was covered by the Daily Mail, the Daily Mirror and The Times.
None of the newspapers' headlines made it clear that the study was only in people with a rare genetic disease that increases their risk of bowel cancer. This may limit how directly relevant findings are for people who do not have this condition.
The papers also focus on the aspirin results, when the main focus of the study was on assessing the impact of being overweight or obese with Lynch syndrome. However, each paper's reporting did contain useful advice about the risk of self-prescribing without first consulting a doctor.
What kind of research was this?
Being overweight or obese has been linked to an increased risk of bowel cancer in the general population. This study assessed whether being overweight or obese influenced bowel cancer risk in people with Lynch syndrome. Estimates of its prevalence vary from 1 in 660 to 1 in 2,000.
Also known as hereditary non polyposis colorectal cancer (HNPCC), this condition greatly increases the risk of bowel cancer. The majority of those carrying the gene mutation develop bowel cancer at some point in their adult lives. For this reason, some people with the condition may have preventative treatment to remove all or part of the bowel to reduce their risk.
The study analysed data from a randomised controlled trial (RCT) in people with Lynch syndrome. The RCT (called CAPP2) assessed whether regularly taking aspirin or a form of starch that resists digestion (resistant starch) could reduce bowel cancer risk in these people.
The overall results of this trial, which have already been published, found regularly taking aspirin reduced bowel cancer risk. Behind the Headlines analysed these results in 2011.
The researchers aimed to look at this trial population to see whether being overweight or obese influenced the risk of bowel cancer compared with being a normal weight.
What did the research involve?
The RCT randomly allocated people with Lynch syndrome to receive 600mg aspirin, 30g resistant starch, both, or inactive placebos every day for up to four years (average about two years). Participants were followed for up to 10 years (average 4.6 years) to see if they developed bowel cancer.
People in the trial were 44.9 years old on average and had successfully had cancerous bowel tissue removed without removal of the entire bowel before entry to the trial. They had their BMI measured at the start of the trial – 34% were overweight (BMI 25 to 29.99) and 15% were obese (BMI greater than 30). BMI data was not available for all people in the trial.
In the current analysis, the researchers compared the risk of developing non-cancerous bowel tumours or bowel cancer during the trial in people with different BMIs.
These analyses were adjusted for age, gender, what interventions they were receiving (aspirin or resistant starch), where they lived, and what genetic mutation caused their condition. The researchers also looked at whether the effects of taking aspirin on risk of bowel cancer were influenced by BMI.
What were the basic results?
About 6% of people developed bowel cancer during follow-up. People who were obese were more than twice as likely to develop bowel cancer as those who were normal weight or underweight (hazard ratio [HR] 2.34, 95% confidence interval [CI] 1.17 to 4.67). There was a slight increase in risk in those who were overweight, but this did not reach statistical significance (HR 1.09, 95% CI 0.57 to 2.11).
When the researchers analysed the groups receiving different interventions separately, they found increased BMI was associated with increased bowel cancer risk in those taking the placebo, but not those taking aspirin:
- Overall, each unit increase in BMI was associated with a 7% increase in risk (HR 1.07, 95% CI 1.02 to 1.13)
- In the group taking placebo, each unit increase in BMI was associated with a 10% increase in risk (HR 1.10, 95% CI 1.03 to 1.17)
- In the group taking aspirin, each unit increase in BMI was not associated with a statistically significant increase in risk (HR 1.00, 95% CI 0.90 to 1.12)
How did the researchers interpret the results?
The researchers concluded that obesity increases the risk of bowel cancer in individuals with Lynch syndrome, but aspirin reduces this risk. They say that these people are likely to benefit from measures to prevent them becoming obese, as well as from taking aspirin regularly.
This study followed up on a previous trial that found taking aspirin regularly reduced the risk of bowel cancer in people with the genetic condition Lynch syndrome (or HNPCC), which puts them at an increased risk of developing the disease. The study found being obese appears to further increase bowel cancer risk among people with this condition.
It also found that BMI did not appear to have an effect on bowel cancer risk among those taking aspirin. While this might indicate that aspirin removes the effect of BMI, ideally a comparison of aspirin versus placebo in the different BMI groups is needed to further assess this. It is likely that the number of people in this trial who fell into the individual BMI categories was not large enough to show an effect.
However, this trial may not be representative of what would happen if obese members of the general public took aspirin regularly. The people in this trial were at high risk of bowel cancer because of their condition, and obesity appeared to increase this risk further.
Even if taking aspirin can reduce risk in the general public, people may not gain the same benefit as those with Lynch syndrome, and the potential risks associated with aspirin – such as an increased risk of gastrointestinal bleeding – may outweigh any benefits.
We know that being overweight or obese has been linked to an increased risk of bowel cancer and also carries other health risks. Aiming to achieve or maintain a healthy body weight by having a healthy diet that includes plenty of fibre, exercising regularly and not smoking are ways you can help to reduce your risk of bowel cancer. Do not start taking aspirin regularly without discussing the issue with your GP or the doctor in charge of your care.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Daily Mail, 18 August 2015
Daily Mirror, 17 August 2015
Links to the science
Journal of Clinical Oncology. Published online August 17 2015