Plasma arginine kinetics in adult man: Response to an arginine-free diet

To explore the response of whole-body arginine metabolism to a change in arginine intake, plasma arginine kinetics were investigated in eight healthy adult men who received an l-amino acid diet supplying an Arg-rich or Arg-free intake for 6 days before undergoing a tracer study on day 7. The tracer protocol lasted for 8 hours. For the first 3 hours subjects remained in the postabsorptive (fasted) state, and during the following 5 hours they consumed small meals at 30-minute intervals. Primed continuous intravenous infusions of l-[guanidino-¹³C]arginine, l-[5,5,5-²H₃]leucine, and [¹⁵N₂]urea were administered to estimate plasma amino acid fluxes and the rate of urea production. For the fasted and fed states, plasma arginine fluxes (μmol · kg^-1 · h^-1, mean ± SD) were 69 ± 8 and 87 ± 12 (P < .01), respectively, for the Arg-rich diet and 63 ± 14 and 51 ± 7 (P < .01, from Arg-rich) for the Arg-free diet. Compared with the Arg-rich results, fed-state plasma arginine and ornithine concentrations were decreased (P < .01) and citrulline concentration was increased (P < .01) during the Arg-free diet period. Leucine fluxes and rates of urea production did not differ between the diet groups. The lower fed-state arginine flux in subjects receiving the Arg-free compared with the Arg-rich diet appears to be entirely due to the decreased rate of entry of arginine from the intestine in the former group. From these results, we conclude that the de novo rate of arginine synthesis, with a mean of about 16 μmol · kg^-1 · h^-1, is not affected by an Arg-free diet for 6 to 7 days.