Plasma immunoreactive beta-endorphin in exercise-associated amenorrhea.

During 2 hours of bed rest, plasma samples were taken at 15-minute intervals from nine women with exercise-associated amenorrhea and in 11 control women in the follicular phase of a normal menstrual cycle. Concentrations of immunoreactive beta-endorphin, cortisol, prolactin, luteinizing hormone, follicle-stimulating hormone, and estradiol were determined. During the first hour, cortisol levels decreased significantly in both groups and reached a plateau during the second hour, which period was considered to represent resting levels of the hormones. The amenorrhea group showed higher mean (+/- SE) resting levels of immunoreactive beta-endorphin (11.0 +/- 0.8 versus 8.3 +/- 0.6 pg/ml, p less than 0.05) and cortisol (274 +/- 35 versus 134 +/- 14 nmol/L, p less than 0.001) and lower mean resting levels of prolactin (2.4 +/- 0.3 versus 5.7 +/- 1.1 ng/ml, p less than 0.01), luteinizing hormone (4.0 +/- 0.7 versus 10.5 +/- 1.8 IU/L, p less than 0.01), and estradiol (0.09 +/- 0.01 versus 0.19 +/- 0.07 nmol/L, p less than 0.05) than the control group. These results suggest that exercise increases basal endorphin secretion in amenorrheic women and support the theory that increased opioid activity may be involved in the pathophysiology of exercise-associated amenorrhea.