NITROGEN HOMEOSTASIS IN MAN - INFLUENCE OF PROTEIN-INTAKE ON THE AMPLITUDE OF DIURNAL CYCLING OF BODY NITROGEN

1. The diurnal nature of nitrogen (N) homoeostasis was investigated in adults fed increasing protein intakes. N balance was estimated during a 48 h period of consecutive 12 h periods of feeding hourly meals and fasting, after 12 days of adaptation to diets containing 0.36 +/- 0.0 1, 0.77 +/- 0.03, 1.59 +/- 0.08 and 2.31 +/- 0.65 g of protein day(-1) kg(-1). N losses were determined from measured urinary N excretion co rrected for changes in the body urea pool, and estimated faecal and mi scellaneous losses. [C-13]Leucine and [H-2(5)]phenylalanine balances w ere measured during a primed, continuous infusion of the two amino aci ds during the fasting and feeding phase on the second day. 2. Increasi ng fasting N losses were observed (47 +/- 7 60 +/- 6, 95 +/- 15 and 14 0 +/- 36 mg day(-1) kg(-1)) on the four intakes, with corresponding in creasing fed gains of 8.2 +/- 3.9, 40.2 +/- 7.1, 112 +/- 24 and 180 +/ - 56 mg day(-1) kg(-1). 3. Increasing fed-state amino acid gains with increasing protein intake were observed with both [C-13]lucine and [H- 2(5)]phenylalanine, whereas increasing fasting amino acid losses were confirmed with [C-13]leucine. 4. The N equivalent of the leucine oxida tion rate was mostly in the range of 10-50% lower than expected from t he N excretion rates, This may reflect the timing of the amino acid ba lance measurements and non-uniform rates of gain and loss throughout t he diurnal cycle. 5. We conclude on the basis of both N and amino acid balances that the amplitude of the diurnal cycling of body protein N in human adults increases with increasing dietary protein intake. Thus one component of the protein requirement for N balance reflects a dem and for repletion of fasting losses which increases with increasing ha bitual protein intake.