"Teenage boys who become very obese may double their risk of getting bowel cancer by the time they are in their 50s," The Guardian reports. A Swedish study found a strong association between teenage obesity and bowel cancer risk in later adulthood.
The study involved over 230,000 Swedish males, who were conscripted into the military aged 16 to 20 years old. Those who were in the upper ranges of overweight and those who were obese at that time were about twice as likely to develop bowel cancer over the next 35 years as those who were a normal weight.
This study has a number of strengths, including its size, the fact that body mass index (BMI) was objectively measured by a nurse and that the national cancer registry in Sweden captures virtually all cancer diagnoses. However, it was not able to take into account the boys' diets or smoking habits – both of which affect bowel cancer risk.
Obesity in adulthood is already known to be a risk factor for bowel cancer, therefore the possibility that a person being obese from an early age also increases risk seems plausible. Maintaining a healthy weight at all ages will have a range of health benefits, such as reducing your risk of developing conditions including heart disease and type 2 diabetes, as well as a number of cancers.
Where did the story come from?
The study was carried out by researchers from Harvard School of Public Health and other research centres in the US, Sweden and the UK.
The study and researchers were funded by the National Cancer Institute, Harvard School of Public Health, Örebro University and the UK Economic and Social Research Council (ESRC).
The study was published in the peer-reviewed medical journal Gut.
The UK media covers this study reasonably well but did not discuss any limitations.
What kind of research was this?
This was a cohort study looking at whether there was a link between body mass index (BMI) and inflammation in adolescence, and risk of colorectal (bowel) cancer later in life.
Being obese and having long-lasting (chronic) signs of inflammation in the body as an adult have been linked to increased bowel cancer risk. However, few of the studies have assessed the effect of obesity in adolescence specifically, and none have been said to look at the impact of inflammation in adolescence.
This type of study is the best way to look at the link between a possible risk factor and an outcome, as people cannot be randomly assigned to have, for example, higher or lower body mass index (BMI) or inflammation.
However, as people are not randomly allocated, it does mean that a group of people with an exposure are likely to differ in other ways from those without that exposure.
It is difficult to disentangle the effects of each of these differences, but researchers can try to single out the effect of the factors they are interested in if they have enough information about the differences between the groups.
What did the research involve?
The researchers used data on BMI and inflammation collected from a very large group of Swedish adolescents and young men taking part in compulsory military service.
They used a national cancer registry to identify any of these men who later developed bowel cancer. They then analysed whether those who had higher BMIs or inflammation as youths were at greater risk.
The researchers analysed data from 239,658 men aged between 16 and 20 years old. These men had medical examinations when they were enlisted in compulsory military service between 1969 and 1976.
The marker (or sign) of inflammation the researchers had information on was the erythrocyte (red blood cell) sedimentation rate, or ESR. This measurement increases when there is inflammation.
Sweden has a national registry recording cancer cases diagnosed in the country, and researchers used this to identify men in the study who developed cancer from their enlistment up to January 2010. This gave an average of 35 years of follow-up for the men.
The researchers analysed whether BMI or signs of inflammation in late adolescence were linked to later risk of bowel cancer. They took into account confounding factors measured at the time of conscription that might affect results, including:
- household crowding
- health status
- blood pressure
- muscle strength
- physical working capacity
- cognitive function
What were the basic results?
The researchers identified 885 cases of bowel cancer.
Compared with those with a healthy weight BMI (from 18.5 to less than 25), those who were:
- underweight (BMI less than 18.5) or at the lower end of the overweight category (BMI 25 to less than 27.5) did not differ in their risk of bowel cancer
- at the upper end of the overweight category (BMI 27.5 to less than 30) had about twice the risk of developing bowel cancer during follow-up (hazard ratio [HR] 2.08, 95% confidence interval [CI] 1.40 to 3.07)
- obese (BMI 30 or more) were also more than twice as likely to develop bowel cancer during follow-up (HR 2.38, 95% CI 1.51 to 3.76)
Adolescents with "high" levels of inflammation were more likely to develop bowel cancer than those with "low" levels (HR 1.63, 95% CI 1.08 to 2.45).
However, those developing bowel cancer or inflammatory bowel disease (Crohn's disease or ulcerative colitis) in the first 10 years of follow-up were excluded, as this link was no longer statistically significant.
This suggested that the link with inflammation might at least in part be due to some men with high levels of inflammation already being in the early stages of inflammatory bowel disease, which is itself linked to higher bowel cancer risk.
How did the researchers interpret the results?
The researchers concluded that, "late-adolescent BMI and inflammation, as measured by ESR, may be independently associated with future CRC [colorectal cancer] risk".
This large cohort study found obesity in adolescence is linked to later colorectal cancer risk in men.
The very large size of this study is its main strength, along with the fact that BMI was objectively measured by a nurse, and that the national cancer registry in Sweden is estimated to record virtually all cancer cases.
As with all studies, there are limitations. For example, the study:
- only had information on BMI at one time point, and could not tell whether the men maintained their BMIs or not
- did not have information on diet or smoking, and these are known to impact bowel cancer risk
- only analysed one marker for inflammation – results may differ for other markers
- findings may not apply to women
Obesity in adulthood is already known to be a risk factor for bowel cancer, therefore the possibility that if a person is obese from an early age also increases risk seems plausible.
Research suggests that you can help lower your risk of bowel cancer by:
- cutting down on your consumption of red meat (no more than 70g a day) and processed meat
- eating lots of fibre-rich foods such as fruits and vegetables
- quitting smoking if you smoke
- sticking within recommended alcohol consumption levels
- taking regular exercise
Also, adults can take part in the NHS Bowel Screening Programme offered at specific ages (age 55 for one form of screening, and ages 60 to 74 for another).