1. The adaptation of the diurnal cycle of nitrogen (N) homoeostasis during a change in protein intake was investigated with diurnal measurements of N and leucine balance and turnover during a reduction from a high to a moderate protein intake in normal adults.

2. In experiment 1, during a 9 day period after a reduction from 1.82 to 0.77 g of protein day−1 kg−1, N excretion fell slowly at a similar rate in fed and fasted states so that the lowered intake was unable to replete any of the postabsorptive losses for 3 days. There was a marked negative N balance, which persisted throughout the study, although with a significant reduction in N losses in both fed and fasted states on day 4, balances during days 4–9 (−32.8 ± 28.3 mg of N day−1 kg−1) were less negative than during days 1–3 (−79.1 ± 60.4 mg of N day−1 kg−1).

3. In experiment 2, during a 14 day period after a reduction from 1.89 to 0.77 g of protein day−1kg−1, [1-13C]leucine oxidation and turnover were measured by primed intravenous infusion, during fasting and feeding in subjects before and on days 3, 7 and 14 after the dietary change. Leucine oxidation fell by 32% (P < 0.05) on day 3 in the fed state and by 12% (P < 0.05) during fasting, falling further in each case by day 7 with improved balance. N losses predicted from the leucine oxidation fell with the same time course as the measured N excretion in experiment 1, but the predicted losses were lower, so that the overall negative leucine balance on day 3 (−22 ± 17 mg of N day−1 kg−1) became positive by day 7 (+ 15 ± 21 mg of N day−1 kg−1).

4. Rates of protein synthesis and degradation calculated from leucine kinetics indicated that feeding the high protein diet resulted in a 60% inhibition of degradation and a 15% (P < 0.05) stimulation of synthesis. With the moderate protein diet the improving fasting balance between days 3 and 7 was the result of a non-significant fall in degradation. Feeding the moderate protein diet abolished the stimulation of synthesis and significantly lowered the inhibition of degradation (31 ± 9%). The improving fed balance between days 3 and 7 was the result of changes in the rates of synthesis and degradation which were below the detection limits of the methods. The daily sum of fasted and fed rates of synthesis (4.33 g day−1 kg−1) did not change with the lowered intake, whereas the rate of degradation increased significantly from 3.61 to 4.26 g day−1 kg−1).

5. Thus, in response to a lower protein intake, adaptation of amino acid oxidation and N excretion occurs, with a similar pattern of gradual reductions in fed and fasted periods during the first week. Thus protein requirements are influenced by the habitual protein intake, which influences the nutritional demand for protein for repletion of fasting losses, the efficiency of such repletion and the consequent dietary need.

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