Teenage boys who become very obese may double their risk of getting bowel cancer by the time they are in their 50s, research suggests.
Past studies have shown that obesity in adults increases the risk of bowel cancer, but little work has been done on whether weight is a risk in adolescents. Scientists from Harvard University and from Sweden investigated a large group of young Swedish men who were conscripted into military service aged 16-20 to see whether their body mass index (BMI, a relationship between weight and height) in their youth was related to their chances of getting bowel cancer 35 years later.
The 240,000 recruits underwent checks for height and weight when they joined up, as well as a test called ESR (erythrocyte sedimentation rate), which assesses the level of inflammation in the body and can show a predisposition to certain diseases.
At the time of conscription, nearly 12% of the men were underweight and almost 81% were of normal weight. About 5% were moderately overweight, 1.5% were very overweight and 1% were obese.
The scientists, publishing in the journal Gut - which is part of the British Medical Journal group, found 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 (normal BMI is 18.5 to 25), were twice as likely to develop bowel cancer.
Those who were obese in young adulthood, classified as having a BMI over 30, had 2.38 times the risk of developing bowel cancer as those who were of normal weight when they were recruited.
Systemic inflammation also raised the young men's chances of getting bowel cancer. Inflammation is part of the body's response to infection and wounds and important to the healing process. But chronic inflammation is linked to diseases including cancers, although why it happens is not well understood. Those with a high inflammation rate in the ESR test had a 65% greater chance of bowel cancer than those with a low rate.
However, not all the men who were obese had a high score in the ESR test, which suggests, say the scientists, that the effect of obesity on bowel cancer is not necessarily through inflammation as measured by ESR.
Bowel cancer is the third most common cancer in men worldwide, says the paper, but little is known about the factors early in life that may lead to an increased risk of developing it. "Late adolescence marks the transition from childhood to adulthood and is a period of accelerated growth, especially among men," it says. "Thus, this period may represent a critical window for exposure susceptibility among men."