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Human Nutrition and Metabolism

Increasing Habitual Protein Intake Accentuates Differences in Postprandial Dietary Nitrogen Utilization between Protein Sources in Humans

Céline Morens, Cécile Bos¹, Maria E. Pueyo, Robert Benamouzig^*, Nicolas Gausserès, Catherine Luengo, Daniel Tomé and Claire Gaudichon

Institut National de la Recherche Agronomique, Unité de Physiologie de la Nutrition et du Comportement Alimentaire, Institut National Agronomique Paris-Grignon, 75005 Paris, France; ^* Service de Gastro-entérologie, Hôpital Avicenne, 93000 Bobigny, France; and Danone VITAPOLE, 91767 Palaiseau, France

¹To whom correspondence should be addressed. E-mail: bos{at}inapg.inra.fr.

	ABSTRACT

TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

 
It is appropriate to characterize the nutritional value of dietary proteins in humans through the specific study of dietary nitrogen metabolism during the postprandial period. However, the influence of the habitual protein intake on this variable has not been studied. We aimed to describe the influence of prior protein intake on the specific metabolic utilization of dietary nitrogen in humans. Healthy men and women were adapted for 7 d to two diets with a normal [NP, 1 g/(kg · d)] and high protein content [HP, 2 g/(kg · d)]. After each period, they were studied for an 8-h postmeal period after ingesting a single ¹⁵N-labeled mixed meal (0.41 g/kg protein) containing either milk (n = 12) or soy protein (n = 8). The HP diet reduced the peak of dietary N incorporation into free serum amino acids in the soy group but had no effect in the milk group. The incorporation of dietary N into plasma protein was higher after soy than after milk protein, but habitual protein level had no effect. The postprandial retention of milk protein was reduced by the HP diet compared with the NP diet by only 5% and that of soy protein was diminished by 13% (protein source: P < 0.0001, protein level: P < 0.0001, interaction: P < 0.001). In conclusion, the efficiency of the meal N postprandial retention was lower after HP adaptation, but this decrease was much more pronounced for soy than for milk protein, indicating that increasing the habitual protein intake accentuates differences in metabolic utilization among dietary proteins.

KEY WORDS: • dietary protein • high protein diets • protein quality • nitrogen retention • humans

The postprandial metabolism represents a critical step for protein and amino acid homeostasis because it ensures the anabolic dietary-stimulated repletion that counteracts protein and amino acid losses during the postabsorptive period (1,2). The efficiency of this anabolic stimulation is dependent on the characteristics of the meal, including the protein and essential amino acid (AA)¹ content and the kinetics of AA delivery from the gut (3–5); it also depends on the metabolic state of subjects with respect to protein and AA, which is influenced by their metabolic adaptation to the habitual protein and energy intake (6). However, uncertainties remain concerning the precise effect of increasing the protein level on the postprandial kinetics of dietary and endogenous nitrogen and AA.

The metabolic consequences of an increased habitual protein intake on whole-body nitrogen kinetics have been studied extensively. The principal change observed was a marked activation of AA catabolism, together with an increase in the daily cycling of protein gains and losses (6–16). However, the existing body of data was obtained using steady-state tracer methodologies based on the classical two-pools paradigm for amino acid kinetics studies. The specific behavior of dietary and recycled AA may differ in response to the habitual protein level, given that the activation of catabolic enzymatic systems in this situation might directly affect the time course and levels of AA availability (17).

In addition, few data are available concerning the consequences of dietary changes on the specific metabolism and nutritional efficiency of utilization of different dietary protein sources. We already demonstrated differences in postprandial metabolism between milk and soy proteins, and showed that a lower fraction of soy protein-derived AA was digested and/or absorbed than observed for milk protein-derived AA, whereas a larger fraction of the absorbed N was transferred to urea in the postmeal phase (18–21). As recently shown, this higher rate of soy protein deamination results from the massive and rapid appearance of dietary AA in the blood after ingestion, leading to an earlier and stronger catabolism (4), as was described earlier for variations in the absorption rates of proteins (3,22). We have thus hypothesized that in the context of activated catabolic activity due to habituation to a high protein diet, soy or milk protein metabolism could respond differently to variations in the protein intake.

The goal of the present work was to assess in humans the degree of influence of the habitual protein intake on the metabolic fate of either milk or soy protein. For this purpose, human volunteers were adapted successively for 1 wk to a normal protein [NP, 1 g/(kg · d)] and then a high protein [HP, 2 g/(kg · d)] intake, in a parallel design. Metabolic and hormonal variables were determined at the end of each adaptation period. Subjects received the same mixed test meal containing one of the two ¹⁵N-labeled dietary proteins, so that we could measure the net postprandial protein utilization of each protein at the two levels of habitual intake.

	SUBJECTS AND METHODS

TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

 
Subjects and study design.

Subjects (n = 20; 10 men and 10 women), in good health, as determined from a full medical history, a physical examination and routine biochemical tests, volunteered for the study. The subjects were 29 ± 5 y old and had a mean BMI of 21.6 ± 2.3 kg/m² (Table 1). Their body composition [total body water (TBW), fat-free mass (FFM) and fat mass] was determined using multiple frequency Bio-Impedance Analysis (BIA) (Analycor 5w; Spengler, Cachan, France). Subjects were studied only if they met the established criteria, i.e., BMI between 18 and 25 kg/m² and hemoglobin >12 g/L. The protocol had previously been approved by the Institutional Review Board at the St-Germain-en-Laye Hospital (St-Germain-en-Laye, France) and all volunteers gave their informed consent before participating in the study.

Adaptation diets.

The NP and HP diets were designed to be isoenergetic [138 kJ/(kg · d)] and supply the same amount of carbohydrate [4.5 g/(kg · d)]. An isoenergetic exchange existed between protein [NP: 1 g/(kg · d) and HP: 2 g/(kg · d)] and fat [NP: 1.2 g/(kg · d) and HP: 0.8 g/(kg · d)]. An increase in the protein content of the HP diet was achieved through a higher consumption of fish, white meat, milk, dairy products and leguminous plants, compensated in terms of energy by a decrease in the fat content of dairy products and meat. There was thus a shift in the distribution between animal and plant proteins from the NP period (59 and 41%, respectively) to the HP period (73 and 27%, respectively). The volunteers consumed the adaptation diets at home. They were provided with a detailed diet program for each day, weighing scales (accurate to 1 g) and daily record sheets. For each meal on each day, the type and quantity of food was specified, and the subjects were asked to adhere to three main meals with few intermittent snacks. Subject compliance was high for both the NP and HP diets, which provided a mean total energy of 8.70 ± 1.74 and 8.73 ± 1.80 MJ/d, respectively. For the NP diet, the total energy partition was as follows: 54.5% carbohydrate, 33.2% fat and 12.3% protein, whereas in the HP diet it was 54.7%, 20.9% and 24.4%.

Test meals.

The test meals were semisynthetic, liquid mixed meals providing 46 kJ/kg body, i.e., one third of the previous day’s intake. The composition of this standard meal corresponded to dietary allowances of 15% of total energy as protein, 55% as carbohydrate (one fourth in the form of sucrose and three fourths in the form of maltodextrins) and 30% as fat (sunflower oil). The test meal sizes were adjusted for body weight and provided 0.41 g/kg of protein (27.9 and 22.4 g in the milk and soy groups, respectively), 1.51 g/kg of carbohydrate (99 and 90 g in the milk and soy groups, respectively) and 0.38 g/kg of fat (24 and 22 g in the milk and soy groups, respectively). The mean nitrogen and energy intakes were 4.85 mmol/kg and 46 kJ/kg in both groups. Proteins were intrinsically and uniformly ¹⁵N-labeled and in the form of either total cow’s milk protein or a soy protein isolate, as previously used and described (4,19,23). The isotopic enrichment of concentrated milk proteins and soy proteins reached 0.606 and 0.67 atom % (AT), respectively.

Fasting and postmeal investigation of volunteers.

The volunteers were admitted to hospital on the mornings of d 8 and 16 after an overnight fast. A baseline blood sample was drawn from all subjects. As for the 20 subjects completing the postprandial metabolic test, a catheter was inserted into a superficial forearm vein for blood sampling. After a basal urine collection at 0900 h, these 20 subjects ingested the test meal, and blood samples were drawn every 30 min for 3 h and then hourly for 5 h. Plasma and serum were immediately separated by centrifugation at 4°C (2400 x g, 20 min), divided into aliquots and stored at -20°C. Subjects were asked to void every 2 h for the 8 h after meal ingestion; total urine was collected, measured for volume, sampled with thymol crystals and paraffin as preservatives and stored at 4°C.

Analytical procedures.

Urea concentrations in serum and urine samples and creatinine concentrations in urine were determined using an enzymatic method (Dimension automate; Dupont de Nemours, Les Ulis, France). Ammonia was measured in urine and plasma (Kone automate, Evry, France). Plasma glucose was assayed by a glucose oxidase method (kit Glucose GOD-DP; Kone, Evry, France). RIA were performed on plasma samples to determine concentrations of insulin (Bio-Rad, Marnes la Coquette, France), glucagon (Nichols, Paris, France), insulin-like growth factor I (IGF-I) (DSL, Webster, TX) and cortisol (Diasorin, Antony, France). ¹⁵N-enrichment was measured in urinary urea, serum urea, amino acids and protein fractions, as previously detailed (19). Briefly, urinary urea, together with deproteinized serum urea, were extracted on a Na/K cation-exchange resin. ¹⁵N-enrichment was determined by Isotopic Ratio Mass Spectrometry (Optima; Fisons Instruments, Manchester, UK), coupled to an elemental analyzer (NA 1500 series 2, Fisons Instruments). HPLC combined with a postcolumn ninhydrin derivatization (Biotek Instruments, St Quentin en Yvelines, France) was used to measure serum amino acid concentrations on 5-sulfosalicylic acid deproteinized serum samples. Separation was performed on a cation exchange resin column and the amino acids were detected at 570 nm (440 nm for proline).

	Calculations

TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

 
Postprandial responses of plasma amino acids, glucose and hormones.

Areas under the curve (AUC) were calculated using the trapezoidal method and were reported for the 8 h following meal ingestion, taking account of the positive areas above the baseline values.

Incorporation of dietary N into body N pools.

The time course of dietary N incorporation (expressed as a percentage of the ingested amount) into the different body N pools monitored (serum free amino acids, serum protein, body urea, urinary urea) was evaluated by the following equation:

where N_tot (t) is the N content of the pool (mmol N) at each time point t, E(t) is the ¹⁵N-enrichment (expressed as AT) in the N pool sampled at time t, E (0) is the ¹⁵N-enrichment at time 0 (AT), E_meal is the ¹⁵N-enrichment of the meal (AT) and N_ingested is the N content of the meal (mmol N). For urinary urea, N_tot was calculated as the product of the urinary urea N concentration and the volume of urine excreted. N_tot in the serum free AA or protein pool was determined as the serum concentration of free -amino nitrogen or protein N, respectively, multiplied by the serum volume, estimated to be 5% of body weight (24). The body urea N pool size was calculated as the product of the urea concentration and body weight, measured by BIA as described above. A factor of 0.92 was used to take into account the water content of serum.

Total, dietary and endogenous urea production.

The calculation of urea production [UP, in mmol N/(2 h · kg)] was based on the amount of urinary urea excretion, corrected for the change in the body urea pool size (5,25). Total (dietary + endogenous) UP was calculated for each 2-h period after meal ingestion (0–2, 2–4, 4–6 and 6–8 h):

where UU_{tot t-(t+2)} is the cumulative amount of urinary urea excreted between time t and t + 2 h and BU_{tot t} and BU_{tot t+2} represent the body urea pool sizes at t and t + 2 h. The body urea pool size was calculated as the product of the urea concentration and TBW, measured using BIA as described earlier. The TBW for each 2-h period was corrected for the volume of water excreted in the urine and that drunk by the subject. Urea production of dietary origin [UP_diet, in mmol N/(2 h · kg)] was calculated for each 2-h period as follows:

where UU_{diet t-(t+ 2)} is the cumulative amount of dietary N excreted in urinary urea between time t and t + 2 h and BU_{diet t} and BU_{diet t+2} represent dietary N in the body urea pool at t and t + 2 h. Endogenous urea production [UP_endo, in mmol N/(2 h · kg)] was estimated for each 2-h period from the difference between total urea and dietary urea productions:

Postprandial retention of dietary N.

At the end of each period, the amount of dietary N retained in the body for the 8-h period was calculated as follows:

The true ileal digestibility of milk and soy protein was taken from previous measurements in humans as 0.95 and 0.915, respectively (19,23,26). Dietary protein digestibility was assumed to be independent of the level of protein intake given the similarity of the NP and HP diets in fiber and antinutritional factors and on the basis of previous findings in rats in which a threefold increase in dietary protein led to a 1% decrease in true protein digestibility (17).

The net postprandial protein utilization (NPPU) of milk and soy proteins was measured for the 8 h after meal ingestion as follows:

Statistical analysis.

Results are presented as mean ± SD. Single planned comparisons for variables measured at the end of each adaptation diet were made using paired Student’s t tests (SAS/STAT, 6.11; SAS Institute, Cary, NC). In the 20 subjects studied during the postmeal session, comparisons between dietary adaptations (Diet = NP or HP) and protein sources (Protein = milk or soy) were evaluated using a two-way ANOVA with time and diet as repeated factors and protein as a between-group factor (General Linear Models, SAS). Interactions between diet, protein and time were also tested. For measurements in which there was a significant interaction, the post-hoc testing of differences between protein sources at each time point was performed using Tukey’s test (SAS/STAT). A probability of P < 0.05 was considered to be significant.

	RESULTS

TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

 
Plasma glucose, hormones and serum N compounds in fasting subjects.

The compliance of subjects with the two diets was good, as evaluated from their daily dietary records (Table 2). There were no variations in body weight during the study, indicating that both diets enabled volunteers to maintain their energy balance. The 7 d of adaptation to each protein level did not influence the fasting concentrations of glucose and insulin. Fasting IGF-I plasma levels tended to increase (P = 0.053).

The postmeal responses of glycemia, insulinemia and glucagonemia after ingestion of the milk protein– or soy protein–based standard test meal after both the NP and HP periods expressed as AUC were not significantly influenced by the protein level or the protein source (not shown). Total serum AA concentrations after the ingestion of the milk or soy protein meals were not significantly affected by the level of the prevailing protein intake (Fig. 1). Independent of the protein level, there was a protein source x time interaction (P < 0.05), indicating that the ingestion of soy or milk protein was associated with different time courses of serum AA concentrations. Indispensable AA (IAA) kinetics were characterized by a significant protein x time interaction. Differences between circulating IAA levels after milk protein or soy protein ingestion were significant only after the HP adaptation, i.e., they were higher after the ingestion of soy than after milk 1 h after the meal and lower at 4 h. By contrast, the levels of dispensable AA (DAA) were influenced mainly by the prevailing protein intake (P < 0.05, repeated-measures ANOVA) and not by the protein source. Regardless of the protein source, the level of circulating DAA was lower after the HP than after the NP period.

View larger version (27K): [in this window] [in a new window]   FIGURE 1 Serum total, indispensable and dispensable amino acid (AA) concentrations in humans in response to a mixed meal containing either milk protein (n = 12) or soy protein (n = 8) after adaptation to a normal protein (NP) or high protein (HP) diet. Values are mean ± SD. There was a significant protein x time interaction for total and indispensable AA time courses (P < 0.05, repeated-measures ANOVA). Within the soy and milk protein groups, NP and HP did not differ. *Different from the milk protein group at that time after HP adaptation, P < 0.05.

Dietary N incorporation into the serum free AA pool and serum protein pool was traced during the postmeal period (Fig. 2). In the serum free AA N pool, adaptation to the HP level produced no change in the milk protein group, whereas it reduced the peak of appearance of dietary N in this pool after soy protein ingestion. In the serum protein pool, the prevailing protein level had no effect on the percentage of dietary N incorporation, but the protein source had a significant effect, whatever the adaptation level (P < 0.01). Soy protein ingestion led to the incorporation of 7.6–8.0% of dietary N into plasma protein versus 7.0–7.2% after the ingestion of milk protein.

View larger version (20K): [in this window] [in a new window]   FIGURE 2 Time course of dietary N incorporation into the serum amino acid pool (A) and serum protein pool (B) in humans in response to a 15N-labeled protein meal after adaptation to a normal protein (NP) or high protein (HP) diet. The test meals were based on milk protein (n = 12) or soy protein (n = 8). Values are mean ± SD. There was a significant effect of the protein source and a significant protein x time interaction on the incorporation of dietary N into serum protein (P < 0.05, repeated-measures ANOVA). *,#Soy and millk protein groups differ at that time after NP and HP adaptation, respectively, P < 0.05 (post-hoc Tukey’s test).

View larger version (26K): [in this window] [in a new window]   FIGURE 3 Time course of urea production from endogenous or dietary origin in humans [in mmol N/(2 h · kg)] in response to a 15N-labeled protein meal after adaptation to a normal protein (NP) or high protein (HP) diet. The test meals were based on milk protein (n = 12) or soy protein (n = 8). Values are mean ± SD. There was a significant global effect of the diet (NP or HP) on the transfer of dietary N to urea (P < 0.0001, repeated-measures ANOVA). The protein source and the protein source x diet interaction were also significant (P < 0.05, repeated-measures ANOVA). *,#Soy and millk protein groups differ at that time after NP and HP adaptation, respectively, P < 0.05 (post-hoc Tukey’s test).

The amounts of dietary N ingested, absorbed and retained from the meals are summarized in Table 4. After ingestion of the same test meal containing either milk or soy protein, there was a considerable influence of the prevailing protein intake (Diet effect) on the level of UP, either endogenous or of dietary origin. The protein source (milk or soy) significantly influenced the amount of meal N absorbed and retained, whatever the prevailing protein intake level. The effect of increasing the protein intake was more pronounced for the metabolism of meal N from soy protein than from milk protein, as indicated by the significant Protein x Diet interaction for total UP, dietary UP and dietary N retention. As a result, the NPPU of milk protein was reduced by 5% when subjects switched from an NP diet to an HP diet (74.4 ± 2.5% to 70.6 ± 3.6%, P < 0.0001) and that of soy protein was diminished by 13% (70.7 ± 3.7% to 61.2 ± 3.0%, P < 0.0001) (Fig. 4).

View larger version (12K): [in this window] [in a new window]   FIGURE 4 Net postprandial protein utilization (NPPU) of milk (n = 12) and soy protein (n = 8) by humans as a function of the prevailing protein intake (normal protein, NP or high protein, HP) assessed after ingestion of a 15N-labeled protein meal. There was a significant influence of diet (P < 0.0001), protein source (P < 0.0001) and their interaction (P < 0.001) on NPPU, as assessed by a two-way ANOVA.*,**Soy and millk protein groups differ at each protein level, P < 0.02 and P < 0.0001, respectively (post-hoc Tukey’s test).

	DISCUSSION

TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

Our study showed that adaptation to a high protein intake stimulated dietary N deamination and lowered the whole-body postprandial fractional dietary N retention, i.e., the ratio of N retained/N ingested. In the two nutritional situations examined, regardless of the protein source in the meal, repeated ingestion of the test meal after the two protein levels produced differences mainly in the amounts of dietary amino acids entering catabolic pathways, as reflected by the increase in postprandial deamination. Interestingly, the augmentation of transfer into urea of both dietary and endogenous N was within the same range after transition from the NP to the HP diets (Table 4). The distribution of dietary N retention to different body tissues after NP or HP adaptation is difficult to assess experimentally in humans. However, using isotopic determination of ¹⁵N-enrichment of the serum protein pool after the ¹⁵N-meal, we were able to measure the specific incorporation of dietary N into this protein pool, which was present at 7% of the meal N content, but which was not affected by adaptation to a HP diet after ingestion of a fixed protein dose, regardless of the protein source. These findings are consistent with studies indicating that the fractional splanchnic uptake of leucine or lysine remains relatively stable for markedly different habitual dietary protein levels (10), even with very low intakes (30). In rats offered a high protein meal for the first time, the incorporation of dietary AA into plasma protein was higher than that after chronic high protein consumption, indicating that the proportion of dietary AA incorporated into exported hepatic protein is strongly influenced by the large amount of dietary AA entering the portal flow, maybe even more than the regulation produced by the usual protein level in the diet (17,31). In contrast, the incorporation of dietary N into plasma protein was influenced by the protein source because it was higher after soy protein ingestion than after milk protein ingestion. Because amino acids from soy are absorbed more rapidly than amino acids from milk (4,21), a higher transfer of dietary amino acids into plasma protein occurs. Taken together, these data support the idea that the albumin pool may represent a buffer when an severe excess of AA is supplied by the diet (32).

A further interesting finding was that the two dietary proteins studied during this investigation (milk or soy protein isolates) were not affected to the same extent by increasing the prevailing protein intake. The postprandial retention of the meal N was lower after HP adaptation, but this decrease was much more pronounced in subjects that consumed soy protein than in those who consumed milk protein. Adaptation to a high protein intake only slightly influenced the postprandial response to milk protein ingestion, i.e., both the time course and level of circulating total AA and dietary AA-derived N were similar during the two periods. The deamination of dietary AA was increased by 17% in this group when they switched from the NP to the HP diet. By contrast, the ingestion of soy protein was associated with a trend toward reduced circulating AA levels. This was due primarily to changes in dispensable AA kinetics and a reduction in the fractional incorporation of dietary N into the circulating AA, but also to a sharp increase in dietary AA deamination, i.e., 54% between the NP and HP periods. We showed that differences in the postprandial metabolic utilization of soy or milk protein result to a great extent from their different kinetics of appearance in the plasma pool, presumably because of different intestinal and absorption rates (4). As described for "fast" protein, soy-derived AA arrive early and in large quantities in the splanchnic zone after ingestion. This favors their catabolism and probably also their entry into synthetic pathways (3,4,21,22). Under the conditions of highly stimulated catabolic systems, as was the case for an HP diet, this phenomenon was probably exacerbated. It is obvious from the AA kinetics that the peak level was lowered (Fig. 2) and was accompanied by a large increase in dietary urea production during the first 2 h after soy protein meal ingestion. However, the incorporation of dietary AA from soy into hepatic exported protein was not enhanced by the HP adaptation, which could indicate that the stimulation of synthetic pathways by the increased habitual protein level was very moderate, as discussed earlier and shown in rats (33). Our present work thus confirms that the postprandial utilization of dietary protein is mainly under the control of kinetic factors and is not modulated to the same extent by nutritional or physiologic factors.

The data presented here also imply that the prevailing protein intake has a direct influence on the assessment of the nutritional quality of dietary protein based on retention measurements. We found a moderate change in the NPPU of milk protein that depended on the prevailing protein intake [from 74 to 71% when shifting from a NP to a HP diet] whereas the NPPU of soy protein fell from 71 to 61%). These data indicate that the differences in metabolic utilization between milk and soy protein sources are based mainly on digestibility at moderate habitual protein levels (4). At high levels of protein intake, these metabolic differences increase and the gap between the nutritional values of soy and milk proteins increases. On the basis of these observations, we conclude that the habitual protein intake of subjects must be taken into account in human studies measuring AA or N kinetics, a finding that agrees with previous authors (34).

In conclusion, we have shown that habituation to a high protein intake affects the postprandial utilization of dietary amino acids, leading to a slight reduction in the ratio of retained N/ingested N. Nevertheless, increasing the habitual level of protein intake influenced the assessment of dietary protein retention and widened the gap between the two dietary protein sources studied, i.e., the postprandial utilization of soy protein was reduced to a much greater extent by an increase in the prior protein intake than that of milk protein. We assume that the difference in metabolic behavior between dietary proteins was related to their kinetics, with more rapid soy AA absorption leading to a greater catabolism, and conclude that the prior protein intake should be monitored carefully in human studies measuring AA or N kinetics.

	ACKNOWLEDGMENTS

 
We thank Françoise Ferrière and staff members in the Biochemistry Laboratory at the Avicenne Hospital for their participation in biochemical assays. Sophie Daré is gratefully acknowledged for her technical assistance.

	FOOTNOTES

 
² Abbreviations used: AA, amino acid; AUC: area under the curve; AT: atom percent; BIA: bioimpedance analysis; DAA: dispensable AA; FFM: fat-free mass; HP, high protein; IAA: indispensable AA; IGF-I: insulin-like growth factor I; NP, normal protein; NPPU: net postprandial protein utilization; TBW: total body water; UP: urea production.

Manuscript received 5 May 2003. Initial review completed 27 May 2003. Revision accepted 16 June 2003.

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TOP ABSTRACT SUBJECTS AND METHODS Calculations RESULTS DISCUSSION LITERATURE CITED

 

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