In this study, we will be studying the effects of acute calorie deprivation (a 10-day fast) on bone marrow fat, other fat stores and bone parameters. We will be measuring changes in bone marrow fat using magnetic resonance spectroscopy and will use MRI to measure subcutaneous and visceral fat depots. Our hypothesis is that levels of bone marrow fat will increase after an acute fast.

Inclusion Criteria:

• Men and women, ages 21-45 years
• 101-130% of ideal body weight as defined by the 1983 Metropolitan Life Insurance Height and Weight Tables
• Normal thyroid function
• Regular menses (women)
• Normal liver transaminases

Exclusion Criteria:

• Any disease known to affect bone metabolism, including untreated thyroid dysfunction, Cushing's syndrome, or renal failure
• Any medication known to affect bone metabolism -- including systemic steroids or immunosuppressants -- within three months of the study, excluding estrogen and progestins. Patients receiving depot medroxyprogesterone (Depo-Provera) will be excluded from participating for six months after their last injections. Bisphosphonates must have been discontinued for at least one year before participation
• Serum potassium <3.0 meq/L
• Pregnant and/or breastfeeding (women)
• Diabetes mellitus
• Active substance abuse, including alcohol
• Contraindications to MRI: cardiac pacemaker, metal implants, claustrophobia