Low levels of estrogen found in physically active, premenopausal women with irregular or absent menstrual periods is likely caused by insufficient energy (calorie) intake compared to energy expenditure. Premenopausal women with menstrual disturbances and amenorrhea suffer from reductions in bone mineral density, particularly in the lumbar spine. Bone loss observed in amenorrheic women may be serious enough to result in osteoporotic fractures, but is also associated with a high prevalence of stress fractures. Increased calorie intake should help improve energy status, menstrual status, and bone health.