The epidemiology and endocrinology of the menarche in relation to breast cancer.

The search for major endocrine abnormalities as causes for breast cancer has not been successful, whether it has been directed to patients with this disease, or to different groups or populations at risk, such as women who have had an early menarche. There were, however, clear-cut differences in a number of variables when the pubertal development of girls with an early menarche was compared with that in other girls. The girls with an early menarche, compared to those with a late menarche, had an early increase in serum concentrations of follicle stimulating hormone, higher circulating oestradiol concentrations before and for several years after menarche and lower serum sex hormone-binding globulin concentrations for several years after menarche, the last two leading to higher 'free oestradiol indices' in the serum. Early menarche was characterized by early onset of ovulatory cycles. We found that the times from menarche until 50% of the cycles were ovulatory were about 1, 3 and 4.5 years, when the ages at menarche were less than 12, 12-12.9 and more than or equal to 13 years, respectively. In addition, the luteal phase of the menstrual cycles of subjects with an early menarche was adequate more frequently than that of subjects with a later menarche. These findings are compatible with the view that girls with an early menarche have a more profound decrease in the sensitivity of the hypothalamic-pituitary axis to the negative feedback of circulating steroids. The associations between endocrine variables and body weight and percentage of fat seem to be primarily determined by the endocrine variables. Altogether the data on early menarche suggest that early-onset regular cyclic ovarian function is related to the increased risk of breast cancer that is associated with this category of women. Athletic activities may delay menarche, which may have relevance for reducing the risk of breast cancer.