Nitrogen Homoeostasis in man: Influence of Protein Intake on the Amplitude of Diurnal Cycling of Body Nitrogen

Abstract

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.

Adaptation of protein metabolism in relation to limits to high dietary protein intake

“…The rates of leucine oxidation determined by Price et al (1994) in the fed and fasting states show a similar pattern to the N excretion shown in Figure 4. However, when leucine oxidation rates were converted to equivalent rates of N excretion, by the same method as that employed by El Khoury et al (1994), described in the previous Section, the values are in all cases lower than expected; between 61% Figure 3 Simulated curves for leucine oxidation and net urea production rates, illustrating damping of the apparent rate of urea production by the body urea pool.…”

“…More recently, there has been a variety of studies of whole-body protein turnover at low to moderate levels of protein intake in humans (Robinson et al, 1990;Garlick et al, 1991;Campbell et al, 1995;Pannemans et al, 1995;Gibson et al, 1996) and also at high intake (Yang et al, 1986;Fern et al, 1991). However, a recent and systematic series of studies with levels of protein intake extending from low to high (0.36 ± 2.31 gakgad) has further developed the concepts of immediate effects of food intake and adaptive responses Price et al, 1994;Quevedo et al, 1994). As in earlier studies described above (Motil et al, 1981;Clugston & Garlick, 1982;Melville et al, 1989), the experimental protocol consisted of a day divided into two notional periods, 12 h of fasting (overnight) and 12 h of feeding, involving hourly small meals.…”

“…In addition, urine collections were made over the two times 12 periods of feeding and fasting, and nitrogen excretion, corrected for changes in the body urea pool, was determined. In the study by Price et al (1994), four levels of protein were given to volunteers for two weeks of adaptation before measurements were made. Figure 4 illustrates the relationship of nitrogen intake to nitrogen excretion during the fasting and fed periods.…”

“…As discussed before, when measurements are made over a complete 24 h period, good agreement between these two measurements has been demonstrated. Therefore, it seems likely that the discrepancy observed by Price et al (1994) relates to the speci®c experimental protocol. The main differences between protocols are that Price et al (1994) estimated, but did not measure non-urinary N losses, and they also measured leucine kinetics during 8 h rather than the whole day.…”

“…In experimental studies, investigators have used the term`high protein' for diets ranging from 1.8 gakgad ± 3.3 gakgad (Fisher et al, 1967;Allen et al, 1979;Fern et al, 1991;Price et al, 1994;Quevedo et al, 1994;Yang et al, 1986). However, in one notable study male volunteers consumed 8 gakgad ( $ 600 g proteinad) for 9 d (Calloway et al, 1971).…”