The association of body fat composition with risk of breast, endometrial, ovarian and colorectal cancers among normal weight participants in the UK Biobank

Background: The association between body fat composition and risk of cancer in normal weight individuals (body mass index (BMI) 18.5–24.9 kg/m²) is unclear. Methods: We examined the association of measures of adiposity with risk of incident cancers of the breast (postmenopausal), endometrium, ovary and colon/rectum among 149,928 normal weight individuals (40–70 years) who were enrolled in the UK Biobank cohort between 2006 and 2010. Results: All of the body fat measures were positively associated with invasive postmenopausal breast cancer risk (hazard ratios (HR) for the uppermost quintile (Q5) versus the lowest quintile (Q1) ranged from 1.32 (95% CI: 1.09–1.60) for waist circumference (WC) to 1.56 (1.28–1.90) for BMI). Trunk fat mass index (HR_{Q5 vs Q1}: 1.72, 95% CI: 1.02–2.89) and WC (HR_{Q5 vs Q1}: 1.65, 95% CI: 1.01–2.70)) were positively associated with risk of endometrial cancer. Among males, trunk fat:trunk fat free mass ratio, trunk fat:leg fat mass ratio and (HR_{Q5 vs Q1}: 1.63, 95% CI: 1.02–2.60; 1.92, 1.20–3.07 and 1.68, 1.05–2.66, respectively) were positively associated with colon cancer risk. None of the body fat measures was associated with risk of ovarian cancer or colorectal cancer in women. Conclusion: The findings of this study suggest that the current normal weight category based on BMI includes individuals who are at increased risk of some obesity-related cancers.