The susceptibility to gain weight is highly variable even when caloric intake and physical activity are well controlled. Because basal metabolic rate (BMR) represents approximately 70% of total daily energy expenditure (TDEE), even a small difference in BMR can affect daily energy balance, thereby increasing the susceptibility for gaining weight. Our preliminary data indicate that high-normal growth hormone (GH) secretion is associated with resistance to weight-gain in rats when overfed and greater weight-loss in humans when underfed. Given that GH influences many of the key metabolic processes that contribute to BMR, we hypothesize that persons with high-normal GH will be resistant to weight gain because of a high BMR, resulting from accelerated rates of these processes. We will measure basal 24h GH secretion and BMR in 106 non-obese men and women. We will also measure protein synthesis, proteolysis, triglyceride/fatty acid cycling (all measured using stable isotope tracer methods) to determine the relationships among these processes, BMR, and GH [Specific Aim 1]. Subjects identified as having """"""""low-normal"""""""" (<1.5 ug/L) and """"""""high-normal"""""""" (>3 ug/L) 24h GH will then be admitted to the hospital for a 2 wk overfeeding protocol (approximately 2000 kcal/d >TDEE - with restricted physical activity), immediately followed by a 4 wk caloric restriction protocol (approximately 750 kcal/d
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