Obesity and Breast Cancer Risk in Men: A National Case-Control Study in England and Wales

Background Breast cancer is rare in men, and information on its causes is very limited, from studies that have generally been small. Adult obesity has been shown as a risk factor but more detailed anthropometric relations have not been investigated. Methods We conducted an interview population-based case-control study of breast cancer in men in England and Wales including 1,998 cases incident during 2005–2017 at ages under 80 years and 1,597 male controls, with questions asked about a range of anthropometric variables at several ages. All tests of statistical significance were two-sided. Results Risk of breast cancer statistically significantly increased with increasing body mass index (BMI) at ages 20 (odds ratio [OR]=1.07, 95% confidence interval [CI] = 1.02 to 1.12 per 2-unit change in BMI), 40 (OR = 1.11, 95% CI = 1.07 to 1.16), and 60 (OR = 1.14, 95% CI = 1.09 to 1.19) years but there was also an indication of raised risk for the lowest BMIs. Large waist circumference 5 years before interview was more strongly associated than was BMI with risk, and each showed independent associations. Associations were similar for invasive and in situ tumors separately and stronger for human epidermal growth factor receptor 2 (HER2)-positive than HER2-negativetumors. Ninety-nine percent of tumors were estrogen receptor-positive. Conclusions Obesity at all adult ages, particularly recent abdominal obesity, is associated with raised risk of breast cancer in men, probably because of conversion of testosterone to estrogens by aromatase in adipose tissue. The association is particularly strong for HER2-expressingtumors.