| Summary: | Background: Obesity remains a serious but preventable challenge of our time, and it has been linked to many comorbidities. This study uses body mass index (BMI) as a measure of obesity to investigate the relationship between low and high BMI and total cancer incidence, as well as some common specific cancers. These relationships were explored in relatively young subjects who may lose more life years to cancer. Method: A population-based cohort study was carried out using the third Tromsø survey of 1986-87 (Tromsø 3) with the Norwegian Cancer Registry (up to December 2010). The cohorts, which were year of birth based, were aged 20-61 years (men) and 20-56 years (women) in 1986. A total of 19,943 subjects (10,219 men and 9,724 women) were followed up for a mean period of 22.41 years. During the follow-up period, a total of 2,248 incident cancers were identified with 1,252 (55.7%) in men, and 996 (44.3%) in women. The relationship of the subjects’ BMI to the cancer incidence was explored using Cox proportional hazards regression to compute the hazard ratios (HR). In most of the analyses, subjects with BMI 20.0-24.9 kg/m2 were the reference category. Results: In men, a U-shaped relationship between BMI and total cancer incidence was observed, with men of BMI 20.0-24.9 kg/m2 having the lowest risk of cancer occurrence (BMI < 20.0 kg/m2: HR=1.41 [95% CI: 1.03-1.93]; BMI ≥ 30.0 kg/m2: HR=1.30 [95% CI: 1.03-1.63]). Unlike in men, there was essentially no relationship between BMI and the total cancer incidence observed in women. BMI appeared indifferent to prostate cancer risk, while BMI < 20.0 kg/m2 and ≥ 30.0 kg/m2 were associated with increased risk of lung cancer. In men, BMI may be a strong risk factor in colon cancer, with BMI < 25.0 kg/m2 having the lowest risk (BMI 25.0-29.9 kg/m2: HR=1.81 [95% CI: 1.19-2.74]; BMI ≥ 30.0 kg/m2: HR=1.83 [95% CI: 0.88-4.07]). In women, a null relationship was observed. However, when the women cohort were stratified into 2 by their mean age at baseline, 36 years, a relatively strong positive linear relationship was found between BMI and colon cancer risk in those younger than 36 years at the study baseline (BMI 25.0-29.9 kg/m2: HR=2.09 [95% CI=0.57-7.58]; BMI ≥ 30.0 kg/m2: HR=5.26 [95% CI: 1.15-24.06]). In men, a positive linear relationship was found between BMI and the risk of colorectal cancer. No marked fluctuation in the risk of colorectal cancer was observed in women. Conclusion: Low and high BMI have impacts on the total cancer risk in the relatively young and the middle aged population, as well as the risk of some of the other specific cancers studied. Therefore, any public health policies directed at reducing cancer incidents should address both ends of the BMI spectrum in the community.
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