Twenty-four volunteers will be enrolled in this 2 year study. All participants will receive 12 months of treatment with human growth hormone. Participants will be randomly assigned to one of two study arms. Twelve participants (Arm 1) will receive growth hormone during the first year of the study (3 mg given daily by subcutaneous injection, with dose reduction to 1.5 mg after 6 months). Twelve participants (Arm 2) will be enrolled in an observational control arm (no placebo injections) that will cross over to growth hormone treatment after 1 year. Participants, whether in Arm 1 or Arm 2, will have as many as 24 scheduled study visits during the 2 years after enrollment. In general, study visits occur every every 1 to 3 months. Study visits will include physical exams, blood tests, CT scans, PET scans, and DEXA scans.