Thermogenenic Responses to Fasting and Overfeeding in Women

Inclusion criteria for premenopausal women * Age 18-45 years. * Regular menses (no missed cycles in the previous year; cycle length 25-35 d). Inclusion criteria for postmenopausal women * No menses for at least 12 months * Follicular stimulating hormone (FSH) ≥50 mIU/ml. * Women who are within 2 years of the final menstrual period are preferred The primary exclusion criteria are: * Body mass index \> 30 kg/m2 * Diabetes (fasting glucose \>126 mg/dL). * Thyroid dysfunction, defined as an ultrasensitive thyroid stimulating hormone (TSH) \<0.5 or \>5.0 mU/L * Uncontrolled hypertension defined as resting systolic BP \>150 mmHg or diastolic BP \>90 mmHg. Additional exclusion criteria for postmenopausal women * History or current use of estrogen-based hormonal therapy * Women who have undergone surgical menopause. Additional exclusion criteria for premenopausal women * Current hormonal contraceptive use (past 6 mo.). * Pregnant, lactating, or intention to become pregnant during the study period. Participants who fail the thyroid and BP criteria will be considered for enrollment if they receive treatment for these conditions and are considered stable by their treating physician.