The purpose of this study is to examine the effects of increased food intake on the menstrual cycle and bone health in physically active women who have irregular or absent menstrual cycles. This study will examine whether a 12 month period of increased food intake will cause menstrual cycles to resume and help bones get stronger.

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.

Comparison: Premenopausal women with irregular or absent menstrual periods will be assigned to either receive additional calories or serve as controls. A group of premenopausal women with normal menstrual periods will also be used for comparison.

Inclusion Criteria for Ovulatory Control Volunteers:

• 18-35 years
• BMI 16-25 kg/m2
• At least 2 hr/wk of aerobic exercise
• Gynecological age >/= 5 years
• Weight stable (+/- 2 kg) last 6 months
• History of regular menses for 6 months

Inclusion Criteria for Women with Irregular or Absent Menses:

• 18-35 years
• BMI 16-25 kg/m2
• At least 3 hr/wk of aerobic exercise
• Gynecological age >/= 5 years
• Weight stable (+/- 2 kg) last 6 months
• No menses within past 3 months or 6 or less menses in last 12 months
• Low to normal bone mass (L1-L4 Z score \