New research has shown that boys who become obese as teenagers may double their risk of bowel cancer by the time they are in their 50s.

The study – published in the journal Gut, a British Medical Journal publication – was carried out by scientists from Harvard University and from Sweden, and focused on a large group of young Swedish men conscripted into military service aged 16-20.

 The 240,000 recruits had their height and weight measured upon entering the military, and were also given a test known as ESR (erythrocyte sedimentation rate), which assesses the level of inflammation in the body and can show a predisposition to certain diseases. Their body mass index at the time of conscription was then related to the incidence of bowel cancer 35 years later.

At the time of conscription, about 12% of the men were underweight and almost 81% were of normal weight. About 5% were moderately overweight, while 1.5% were very overweight and 1% were obese.

The study showed that over the average 35 years of follow-up, 885 of the men developed bowel cancer, 384 of which were rectal cancers. Those who were very overweight – with a BMI ranging between 27.5 and 30, as opposed to a normal BMI of 18.5 to 25 – were twice as likely to develop bowel cancer.

Those who had a BMI over 30 in early adulthood were shown to have a risk of developing bowel cancer that was 2.38 times that of those who were of normal weight when conscripted. It was also shown that those with a high inflammation rate in the ESR test had a 65% greater chance of bowel cancer than those with a low rate.

The precise reasons for obesity in adolescence having such an impact on later cancer risk is not yet fully understood, but the study’s authors theorise: “Late adolescence marks the transition from childhood to adulthood and is a period of accelerated growth, especially among men. Thus, this period may represent a critical window for exposure susceptibility among men.”

Justin Davies, Consultant Colorectal Surgeon at Addenbrooke’s Hospital, Cambridge and Clinical Adviser at Check4Cancer, comments: “We know that obesity among adults has a significant impact on bowel cancer risk, but this is the first time a link has been made specifically to obesity in adolescence. It shows that the factors affecting cancer risk are often determined years or even decades before the cancer itself develops, and emphasises the need for good all-round health education early on to help avoid a cancer legacy. It also gives us another criterion for determining those who may be at greater risk of bowel cancer – and therefore another tool for targeting testing and increasing the early detection rate. Early detection is critical to the outcome of all cancer treatments, and in the case of bowel cancer, sufferers have a 97% chance of survival if the disease is detected an its earliest stage. If it is diagnosed at an advanced stage, however, that survival rate drops to just 7%.”

Around 41,000 people are diagnosed with bowel cancer in the UK each year, with 16,000 dying from the disease.