HealthDay News — Low body mass index (BMI) among men entering adult life is associated with an increased incidence of Crohn disease (CD) and ulcerative colitis (UC) up to 40 years later, according to a study published online April 23 in Scientific Reports.

Michael A. Mendall, MD, from Croydon University Hospital in the United Kingdom, and colleagues explored the impact of BMI in men entering adult life on their long-term risk for developing inflammatory bowel disease using data from 377,957 men born from 1939 to 1959. BMI was measured at draft boards (mean age, 19 years) with men followed from 1977 (or time of examination) through 2015.

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The researchers found that during 13 million person-years of follow-up, 1523 men developed CD and 3323 developed UC. Compared with men of normal weight, the risk for CD was as follows: hazard ratio, 1.35 (95% confidence interval, 1.12 to 1.62) for BMI <18.5 kg/m²; hazard ratio, 0.83 (95% confidence interval, 0.68 to 1.02) for BMI 25 to 29.9 kg/m²; and hazard ratio, 1.20 (95% confidence interval, 0.75 to 1.90) for BMI >30 kg/m². Until age 60 years, the increased risk for CD in underweight was maintained and was not explained by known effects of smoking. Minor inverse associations were seen for UC. A U-shaped association between BMI and CD was seen with restricted cubic splines, but not for UC.

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“Smoking to fully explain our findings would have had to be associated with an implausibly large effect on BMI,” the authors write.

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