Vigorous physical training 1 – 5 and even moderate exercise 6 – 9 can interrupt the menstrual cycle, perhaps by suppressing the pulsatile release of gonadotropin-releasing hormone. 10 , 11 This effect of physical activity may lower a woman's cumulative exposure to estrogen and progesterone, thereby inhibiting carcinogenesis in the breast. 12 – 22 Energy balance might also influence the risk of breast cancer. Caloric restriction in rodents reduces the proliferative activity of the mammary glands 23 and inhibits carcinogenesis. 24 , 25 However, the effect of energy balance, as indicated by energy intake, body-mass index (the weight in kilograms divided by the square of the height in meters), and energy . . .