Adult patients with GH deficiency have reduced lean body mass (LBM), muscle mass, and muscle strength, suggesting an underlying abnormality of protein metabolism. As acute GH administration has previously been reported to decrease protein oxidation and increase protein synthesis in GH-deficient (GHD) adults, we investigated whether the converse might occur in untreated GH deficiency by undertaking studies of whole body protein turnover in 10 GHD and 13 normal subjects using a 3-h primed constant infusion of 1-[13C]leucine. Dual energy x-ray absorptiometry was used to quantify LBM and fat mass (FM). In normal subjects, LBM was the major, independent determinant of leucine appearance (Ra; r=0.80; P=0.0009), leucine oxidation (r=0.81; P=0.0008), and leucine incorporation into protein (r=0.75; P= 0.003). However, in an analysis of covariance, FM was also a significant independent determinant of leucine Ra (P=0.002) and leucine incorporation into protein (P=0.003). After correcting for LBM and FM, GHD patients had significantly reduced rates of leucine Ra (109.9+/-4.4 vs. 125.5+/-3.7 micromol/min, respectively; P=0.02) and leucine incorporation into protein (87.0+/-3.9 vs. 100.3+/-3.3 mmol/ min; P=0.02) compared to normal subjects. There was no significant difference in the corrected rates of leucine oxidation between the two groups (22.9+/-1.3 vs. 25.2+/-1.0, GHD vs. normal; P=0.20). In summary, GHD adults have reduced rates of protein synthesis and protein breakdown, but normal rates of irreversible oxidative loss; these findings are discordant with what was predicted from the acute changes in protein metabolism observed with GH administration. We conclude that normalization of protein oxidation may be a homeostatic mechanism that operates to constrain protein loss in GHD adults.