Nitrogen Homoeostasis in man: The Diurnal Responses of Protein Synthesis and Degradation and Amino Acid Oxidation to Diets with Increasing Protein Intakes

1. The diurnal changes in whole body protein turnover associated with the increasing fasting body nitrogen (N) losses and feeding gains with increasing protein intake were investigated in normal adults. [¹³C]Leucine, [²H₅]phenylalanine and [²H₂]tyrosine kinetics, were measured during an 8h primed, continuous infusion during the fasting and feeding phase together with fed-state N turnover assessed with [¹⁵N]glycine after 12 days of adaptation to diets containing 0.36 (LP), 0.77 (MP), 1.59 (GP) and 2.07 (HP) g of protein day⁻¹ kg⁻¹. Measurements were also made of fasting and fed resting metabolic rate and plasma hormone levels.

2. Resting metabolic rate in the fasted and fed state was not influenced by dietary protein intake, but was increased by feeding (11-13%, P <0.01) with no influence of dietary protein concentration. Fasting plasma insulin levels were not influenced by protein intake and were increased by feeding independent of protein intake. Fasted but not fed values of insulinlike growth factor-1 increased with protein intake, although no feeding response was observed. Thyroid hormones (free and total tri-iodothyronine) did not change in any state.

3. For leucine with increasing protein intake the increasing fasting losses reflected increasing rates of protein degradation, although the changes were small and only significant between GP and MP intakes. The increasing leucine gain on feeding was associated with increasing rates of protein synthesis and falling rates of protein degradation, reflecting a progressive inhibition of degradation with feeding, and a change from inhibition of synthesis (LP diet) to stimulation (GP and HP diets). Mean daily rates of synthesis and degradation did not change with protein intake.

4. Phenylalanine and tyrosine kinetics were calculated from adjusted values based on leucine kinetics and published data of the hepatic/plasma enrichment ratio. With the increased protein intake, the increasing fasting losses of phenylalanine (GP > MP) were mediated by increasing rates of degradation (paired t-tests). The increasing phenylalanine gain (GP > MP > LP) was due to increasing fed-state rates of synthesis and falling rates of degradation, reflecting a progressive inhibition of degradation, a stimulation of hydroxylation and a variable response of synthesis ranging from inhibition at the lowest intake to stimulation at higher intakes. For tyrosine a similar progressive inhibition of degradation with intake was shown. Mean daily rates of synthesis and degradation (phenylalanine) and degradation (tyrosine) did not change with protein intake.

5. Estimation of protein turnover from ¹⁵N excretion in urea and ammonia during 9 h after 1 h intravenous infusion of [¹⁵N]glycine in the fed state on the LP, MP and GP diets indicated that neither synthesis nor degradation were influenced by dietary protein level. Synthesis estimated from ¹⁵N kinetics was significantly correlated with that determined from leucine kinetics (r = 0.78, n = 14, P <0.01) and from phenylalanine kinetics (r = 0.53, n = 14, P <0.05), and degradation estimated from ¹⁵N kinetics was significantly correlated with that determined from leucine kinetics (r = 0.60, n = 14, P <0.05). Thus the [¹⁵N]glycine, [¹³C]leucine and [²H₅]phenylalanine methods gave broadly comparable results.

6. We conclude that the feeding response of protein synthesis, degradation and amino acid oxidation reflects the combined impact of insulin and tissue amino acid levels with insulin inhibiting degradation and with amino acids both stimulating synthesis and oxidation and also further inhibiting degradation. Although the dietary protein level influences the extent of these feeding responses, it does not influence the mean daily rate of protein turnover. The rate of whole body protein turnover per se is unlikely to provide an indicator of protein nutritional status.