Energy balance and cancer: the role of sex hormones

Energy balance can affect the risk for hormone-related cancers by altering sex hormone levels. Energy intake and expenditure are difficult to measure in epidemiological studies, but a chronic excess of intake relative to expe nditure leads to a high BMI, which can be accurately measured. In premenopa usal women obesity has little effect on the serum concentration of oestradi ol, but causes an increase in the frequency of anovular menstrual cycles an d thus a reduction in progesterone levels; these changes lead to a large in crease in the risk for endometrial cancer, but little change, or a small de crease, in the risk for breast cancer. In post-menopausal women oestradiol levels are not regulated by negative feedback, and obesity causes an increa se in the serum concentration of bioavailable oestradiol; this factor cause s increases in the risk for both endometrial cancer and breast cancer. The development of ovarian cancer appears to be related more strongly to the fr equency of ovulation than to direct effects of circulating levels of sex ho rmones, and BMI is not clearly associated with the risk for ovarian cancer. In men, increasing BMI has little effect on bioavailable androgen levels, and any effect of obesity on prostate cancer risk is small.