Influence of increasing dietary intake on whole body protein kinetics in normal man

The whole body protein kinetic response to increasing dietary intake was studied in 20 normal adult male subjects receiving a defined formula diet orally. Each person received the same amount for 5 days at the rate ranging from 150 to 330 mgN/kg. day and 16 to 34 kcal/kg. day, keeping the calorie to nitrogen ratio as constant. Whole body protein flux was measured using a primed constant infusion of 15N glycine and determining isotopic enrichments in the urinary urea and ammonia. Whole body protein synthesis and breakdown rates were calculated from the flux measurement and nitrogen excretion and intake. The mean protein turnover (Q), synthesis (S) and breakdown (C) rates for all subjects were 3.72+/-0.42, 2.47+/-0.47 and 2.12+/-0.39 g protein/kg. day. These values increased with increasing dietary nitrogen intake up to 270+/-4 mgN/kg. day which is twice the daily recommended protein requirement for a normal adult man and then tended to decrease. Nitrogen intake in the range of 150 to 270 mg N/kg. day showed significant positive correlations with nitrogen balance, Q, S and S/C and the protein accretion was due to a relatively large increase in S compared to that in C. When the intake rate exceeded 270 mg N/kg. day, the nitrogen balance was still positive but now due to a larger decrease in C. These results show that the kinetic parameters of whole body protein metabolism in adult man appear to exhibit a maximum at a dietary nitrogen intake twice the daily requirement level. The mechanism of the maintenance of protein balance changes at this threshold.