NITROGEN HOMEOSTASIS IN MAN - DIURNAL CHANGES IN NITROGEN-EXCRETION, LEUCINE OXIDATION AND WHOLE-BODY LEUCINE KINETICS DURING A REDUCTION FROM A HIGH TO A MODERATE PROTEIN-INTAKE

1. The adaptation of the diurnal cycle of nitrogen (N) homoeostasis du ring a change in protein intake was investigated with diurnal measurem ents of N and leucine balance and turnover during a reduction from a h igh 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 protei n 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 b alance, which persisted throughout the study, although with a signific ant reduction in N losses in both fed and fasted states on day 4, bala nces 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(-1)kg(-1), [1-C-13]leucine oxidation and t urnover were measured by primed intravenous infusion, during fasting a nd feeding in subjects before and on days 3, 7 and 14 after the dietar y change. Leucine oxidation fell by 32% (P<0.05) on day 3 in the fed s tate 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 ox idation 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. Rate s of protein synthesis and degradation calculated from leucine kinetic s indicated that feeding the high protein diet resulted in a 60% inhib ition 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. Feedi ng the moderate protein diet abolished the stimulation of synthesis an d 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 detecti on limits of the methods. The daily sum of fasted and fed rates of syn thesis (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 si milar pattern of gradual reductions in fed and fasted periods during t he first week. Thus protein requirements are influenced by the habitua l protein intake, which influences the nutritional demand for protein for repletion of fasting losses, the efficiency of such repletion and the consequent dietary need.