Teenagers who are very overweight may run double the risk of developing colorectal cancer when they reach middle age, according to research published Monday. At the time of conscription, around 12 percent of the men were underweight, more than 80 percent were of normal weight and five percent were moderately overweight. Of the remainder, 1.5 percent were very overweight — with a body mass index of between 27 and nearly 30 — and one percent were obese, with a BMI of more than 30.