Correlation between amino acid induced changes in energy expenditure and protein metabolism in humans

The relationship between energy expenditure and protein metabolism during amino acid (AA) administration wasevaluated in normal humans. A balanced AA solution was infused for 180 min at five different rates: 20 (study I), 40 (study II) , 80 (study III) , 160 (study IV), and 240 mg · m2−1 · min−1 (study V), on separate days, in seven normal, overnight-fasted subjects (age 25 ± 2 y; height 172 ± 5 cm; weight 68 ± 4 kg). Indirect calorimetry and [I-14C] leucine infusion techniques were employed. Basal total plasma AA concentration averaged 1827 ± 121 μmol/L and increased to 2192 ± 142, 2576 ± 158, 3677 ± 195, 5638 ± 237, and 7185 ± 261 μmol/L in studies I–V, respectively. Basal energy expenditure averaged 0.60 ± 0.02 kcal· M2−1· min−1 and increased slightly in studies I and II (to 0.62 ± 0.03, 0.63 ± 0.02, respectively), and significantly in studies III–V (to 0.65 ± 0.03, 0.70 ± 0.04, and 0.77 ± 0.05 kcal· m2−1 · min−1, respectively; all P < 0.01 versus basal; P < 0.05−0.01 for each study versus preceding study). Basal nonoxidative leucine disposal (NOLD), an index of protein synthesis, averaged 73 ± 3 μmol·m2−1·min−1 and increased, albeit not significantly, in studies I and II (to 75 ± 5, 76 ± 4, respectively). In contrast, a significant increase in NOLD was observed in studies III–V (to 87 ± 7, 103 ± 7, and 127 ± 9 μmol · ml2−1· min−1, respectively; all P < 0.01 versus basal; P < 0.05−0.01 for each study versus preceding study). Basal respiratory quotient averaged 0.81 ± 0.02 and did not change significantly in studies I–V (0.80 ± 0.02, 0.79 ± 0.02, 0.80 ± 0.03, 0.82 ± 0.02 and 0.82 ± 0.03, respectively). The thermic effect of AA administration, calculated as percent of the AA energy infused, was constant and averaged 24 ± 4, 19 ± 3, 17 ± 4, 17 ± 3, and 18 ± 3% in studies I–V, respectively. When AA-induced increase in protein synthesis was plotted with the increment in energy expenditure, a positive correlation was obtained (r = 0.792, P < 0.001). In summary, during AA administration (1) the absolute rise in energy expenditure is dose-dependent and does not show evidence of achieving a plateau; (2) it is positively correlated with AA-induced protein synthesis; and (3) the thermic effect is not dependent upon the AA dose administered. The data provide a quantitative assessment of AA-induced thermogenesis in normal humans and the energy needs associated with an acute stimulation of protein synthesis.