Growth hormone and testosterone interact positively to enhance protein and energy metabolism in hypopituitary men

We investigated the impact of growth hormone (GH) alone, testosterone (T) alone, and combined GH and T on whole body protein metabolism. Twelve hypopituitary men participated in two studies. Study 1 compared the effects of GH alone with GH plus T, and study 2 compared the effects of T alone with GH plus T. IGF-I, resting energy expenditure (REE), and fat oxidation (F(ox)) and rates of whole body leucine appearance (R(a)), oxidation (L(ox)), and nonoxidative leucine disposal (NOLD) were measured. In study 1, GH treatment increased mean plasma IGF-I (P < 0.001). GH did not change leucine R(a) but reduced L(ox) (P < 0.02) and increased NOLD (P < 0.02). Addition of T resulted in an additional increase in IGF-I (P < 0.05), reduction in Lox (P < 0.002), and increase in NOLD (P < 0.002). In study 2, T alone did not alter IGF-I levels. T alone did not change leucine R(a) but reduced L(ox) (P < 0.01) and increased NOLD (P < 0.01). Addition of GH further reduced L(ox) (P < 0.05) and increased NOLD (P < 0.05). In both studies, combined treatments on REE and F(ox) were greater than either alone. In summary, GH-induced increase of circulating IGF-I is augmented by T, which does not increase IGF-I in the absence of GH. T and GH exerted independent and additive effects on protein metabolism, F(ox) and REE. The anabolic effects of T are independent of circulating IGF-I.