Background: \- In elderly individuals, an age-associated decline in the immune system s ability to function is believed to contribute to increased incidence of infection, autoimmune disorders, and cancer. This decline in immune system function may be related to the decline in the body s production of growth hormone, which helps regulate human development and may contribute to the health of the immune system. Researchers are interested in studying whether growth hormone, given as an infusion over time, can improve the function of the immune system and other body systems associated with good health. Objectives: \- To study the effects of growth hormone administration on the immune systems of healthy men. Eligibility: \- Healthy men between 25 and 50 years of age. Design: * This protocol will involve three separate studies: Study I, Study IB, and Study II. Participants in Study I and Study IB may participate in Study II as directed by the researchers. * Participants will be screened with a full medical history and physical examination, and will provide blood, urine, and stool samples; have a glucose tolerance test; and have other tests as required by the researchers. * Participants will have an infusion pump with a small catheter inserted beneath the skin to administer the study chemicals (either growth hormone or placebo). * Study I and Study IB participants will receive pulses of growth hormone through the infusion pump at regular intervals to monitor the body s response to the hormone. Study IB participants will receive a higher dose of growth hormone than Study I participants. * Throughout the study period, all participants will have frequent blood and urine tests, as well as tests of glucose tolerance and metabolism, imaging studies, and other tests as required by the researchers. * Study II participants will be divided into two groups. The first group will receive pulses of either growth hormone or placebo infusion at intervals throughout the day for 4 weeks, followed by an 8-week period without infusions. The second group will receive conventional once-a-day infusion of growth hormone or placebo for 4 weeks, followed by an 8-week period without infusions. * Throughout the study period, all participants will have frequent blood and urine tests, as well as tests of glucose tolerance and metabolism, imaging studies, and other tests as required by the researchers.
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GH administration may elicit clinical significant and relevant changes in the human immune system
Timeframe: 4 weeks