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 +/- 0.01, 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 corrected for changes in the body urea pool, and estimated faecal and miscellaneous losses. [13C]Leucine and [2H5]phenylalanine balances were measured during a primed, continuous infusion of the two amino acids during the fasting and feeding phase on the second day. 2. Increasing fasting N losses were observed (47 +/- 7, 60 +/- 6, 95 +/- 15 and 140 +/- 36 mg day-1 kg-1) on the four intakes, with corresponding increasing 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 [13C]leucine and [2H5]phenylalanine, whereas increasing fasting amino acid losses were confirmed with [13C]leucine. 4. The N equivalent of the leucine oxidation rate was mostly in the range of 10-50% lower than expected from the N excretion rates. This may reflect the timing of the amino acid balance measurements and non-uniform rates of gain and loss throughout the 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 demand for repletion of fasting losses which increases with increasing habitual protein intake.
Price
G M
GM
Nutrition Research Unit, London School of Hygiene and Tropical Medicine, St Pancras Hospital, U.K.
Halliday
D
D
Pacy
P J
PJ
Quevedo
M R
MR
Millward
D J
DJ
D000328:Adult; D000596:Amino Acids / Q000378:metabolism; D002940:Circadian Rhythm; D004044:Dietary Proteins / Q000008:administration & dosage / Q000378:metabolism*; D005215:Fasting / Q000378:metabolism; D005260:Female; D006706:Homeostasis; D006801:Humans; D007930:Leucine / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D009584:Nitrogen / Q000378:metabolism* / Q000652:urine; D010649:Phenylalanine / Q000378:metabolism
D000596:Amino Acids; D004044:Dietary Proteins; D010649:Phenylalanine; D007930:Leucine; D009584:Nitrogen