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American Journal of Clinical Nutrition, Vol. 82, No. 2, 373-379, August 2005
© 2005 American Society for Clinical Nutrition

ORIGINAL RESEARCH COMMUNICATION

The daily valine requirement of healthy adult Indians determined by the 24-h indicator amino acid balance approach1,2,3,4

Anura V Kurpad, Meredith M Regan, Tony DS Raj, Justin V Gnanou, Vidya N Rao and Vernon R Young

1 From the Division of Nutrition (AVK, TDSR, and VNR) and the Core Biochemistry Laboratory (JVG), Institute of Population Health and Clinical Research, St John's National Academy of Health Sciences, Bangalore, India, and the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA (MMR and VRY)

2 VR Young is deceased.

3 Supported by NIH grant DK42101.

4 Reprints not available. Address correspondence to AV Kurpad, Division of Nutrition, Institute of Population Health and Clinical Research, St John's National Academy of Health Sciences, Bangalore 560034, India. E-mail: a.kurpad{at}iphcr.res.in.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background: The 1985 FAO/WHO/UNU requirement for valine was set at 10 mg · kg–1 · d–1 on the basis of nitrogen balance studies carried out in Western subjects. It is likely that the requirement is higher, however, because the requirement of another branched-chain amino acid, leucine, was found to be about 3 times as high (40 mg · kg–1 · d–1) as the 1985 FAO/WHO/UNU value (14 mg · kg–1 · d–1).

Objective: We assessed the valine requirement in healthy, well-nourished Indians by using 7 test valine intakes (5, 10, 15, 20, 25, 30, and 35 mg · kg–1 · d–1) and the 24-h indicator amino acid oxidation (24-h IAAO) and balance (24-h IAAB) method, with phenylalanine as the indicator amino acid, while maintaining leucine intake at 40 mg · kg–1 · d–1.

Design: Eighteen healthy, well-nourished Indian men were studied during each of 3 randomly assigned 7-d diet periods supplying valine intakes that were equally placed on either side of a putative mean valine requirement of 20 mg · kg–1 · d–1. Twenty-four–hour IAAO and 24-h IAAB were measured on day 7 by use of a 24-h [13C]phenylalanine tracer infusion. The breakpoint in the relation between these values and the valine intake was determined.

Results: Two-phase linear regression of daily phenylalanine oxidation or balance against valine intake estimated a breakpoint in the response curve at a valine intake of 17 mg · kg–1 · d–1 (95% Fieller's CI: 11, > 35 and 11, 28 mg · kg–1 · d–1, respectively).

Conclusion: From the 24-h IAAO/IAAB approach, a mean valine requirement of 17 mg · kg–1 · d–1 is proposed for healthy, well-nourished Indian adults.

Key Words: Well-nourished Indian adults • valine requirement • branched-chain amino acids • BCAA requirement • 24-h indicator amino acid oxidation • 24-h indicator amino acid balance


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The 1985 FAO/WHO/UNU (1) daily requirement for valine was set at 10 mg · kg–1 · d–1 on the basis of nitrogen balance studies carried out in healthy adults (2, 3). The nitrogen balance technique has several disadvantages (4), however; reanalyses of the nitrogen balance data by Hegsted (5) and Millward (6) gave mean valine requirement estimates of 17 and 14 mg · kg–1 · d–1, respectively. On the basis of an obligatory amino acid loss model, Young et al (4) suggested a value of 24 mg · kg–1 · d–1 for the daily valine requirement. Because the branched-chain amino acids (BCAAs) share common enzymes in their oxidative pathways, the valine requirement can also be theoretically estimated by an assumed proportionality with the leucine requirement on basis of the amino acid composition of body protein (7). Such a procedure yields a value of 26 mg · kg–1 · d–1 when a value of 40 mg · kg–1 · d–1 is used for the leucine requirement (8, 9). The response of plasma valine concentrations to different valine intakes has suggested a requirement value of 20 mg · kg–1 · d–1 (10).

Tracer studies with 13C-labeled valine to determine the valine oxidation rate (also called the direct amino acid oxidation, or DAAO, method) over the short term at different valine intakes in an egg-protein-based amino acid mix have suggested a valine requirement of {approx}16 mg · kg–1 · d–1 or greater (10). However, potential problems with the short-term DAAO approach exist, because the kinetics of the test amino acid are measured only over a few hours with extrapolations made for 24 h, and the test amino acid intake supplied by the intravenously administered tracer is not massless. The other type of tracer study, called the indicator amino acid oxidation (IAAO) method, offers significant advantages over the DAAO method in that the oxidation and balance of an independent amino acid, for which the kinetics are well characterized, are used to plot a response curve to graded intakes of the test (in this case, valine) amino acid. The IAAO method has been used in short-term measurements to measure the total BCAA requirement (11), and the total requirement of these amino acids was found to range from 122 to 144 mg · kg–1 · d–1 depending on the outcome (phenylalanine oxidation or a surrogate based on the fraction of tracer oxidized) used. The daily valine requirement can then be estimated to be 28 to 32 mg · kg–1 · d–1 on the basis of the proportion of valine (22.5%) in the egg protein amino acid mix used in this study (11). A later paper from the same group (12) suggested that the requirements for the BCAAs may have been overestimated by {approx}10%; if so, the tentative valine requirement derived from these studies would be in the range of 25 to 29 mg · kg–1 · d–1.

However, these IAAO studies were conducted in adults who were first adapted for 2 d to the experimental diet before the tracer study, which comprised measurements for a few hours in the fed state. We have refined the IAAO technique to include a 7-d adaptation period to the experimental diet as well as a 24-h measurement of IAAO and indicator amino acid balance (IAAB) to assess the requirements of lysine, methionine, and threonine in adult Indians (13-17). This approach might reasonably be considered to be the best method currently available to measure amino acid requirements in adults. Therefore, the present study was designed to assess the valine requirement in healthy, young adult Indian males by using the 7-d dietary adaptation period and the 24-h IAAO and IAAB approach, with [13C]phenylalanine as the indicator amino acid. Furthermore, because it is possible that dietary leucine intake may affect valine kinetics and hence the valine requirement, the dietary leucine intake was kept at its requirement level of 40 mg · kg–1 · d–1 (8, 9).


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Subjects and anthropometry
Eighteen healthy men participated in this experiment. The subjects were weighed to the nearest 0.1 kg, and their height was measured to the nearest 0.1 cm. The logarithm of the sum of 4 (biceps, triceps, subscapular, and suprailiac) skinfold thicknesses was used in age- and sex-specific equations (18) to obtain an estimate of body density, from which percentage body fat and fat-free mass (FFM) were determined (19; Table 1Go). The purpose of the study and the potential risks involved were explained to each subject, and written informed consent was obtained from each subject. The Human Ethical Review Board of St John's Medical College approved the research protocol.


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  		TABLE 1 . Characteristics of well-nourished Indian men studied for their valine requirements1

 
Diet and experimental design
Each subject was randomly assigned to 3 separate 6-d experimental diet periods during which he received a weight-maintaining diet based on an L-amino acid mixture as previously described (13-17), such that between 7 and 9 observations were made at each valine intake. The test valine intakes during the respective diet periods were chosen from 7 designated valine intakes of 5, 10, 15, 20, 25, 30, and 35 mg · kg–1 · d–1 (Table 2Go). The 3 valine intakes assigned to each subject were distributed around a putative intake requirement of 20 mg · kg–1 · d–1. The subjects received their daily intake as 3 isoenergetic, isonitrogenous meals (at 0800, 1300, and 2000), except on days 6 and 7 (see below).


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  		TABLE 2 . Composition of the amino acid mixtures used to supply 7 different valine intakes per day

 
Twenty-four–hour tracer-infusion protocol and sample collection
The indicator amino acid used was phenylalanine. A primed, 24-h, intravenous [13C]phenylalanine approach was used, with the protocol of indirect calorimetry and blood and breath sampling as previously described (13-17). Briefly, [1-13C]phenylalanine (99 atom%; Cambridge Isotope Laboratories, Inc, Andover, MA) was given as a primed, constant intravenous infusion at a known rate of {approx}4.5 µmol · kg–1 · h–1 (the priming dose was {approx}4.5 µmol/kg) into an antecubital vein. The bicarbonate pool was primed with 0.8 µmol/kg of [13C]sodium bicarbonate (99.9 atom%; MassTrace, Woburn, MA). The tracer administration began at 1700 on day 6, with the subjects having consumed their last meal of that day at 1500, and lasted until 1800 on day 7. Therefore, the tracer infusion was given for 25 h, although only the data from the last 24 h were used in calculating daily phenylalanine oxidation and balance. On the day of the infusion study, the subjects received, at hourly intervals, 10 isoenergetic, isonitrogenous small meals beginning at 0600 on day 7 and lasting until and including 1500 (which together were equivalent to the 24-h dietary intake for that day). A similar feeding pattern was imposed on the subjects on day 6, so that the infusion day feeding pattern was not suddenly different from the pattern on the previous day.

The analyses of breath for 13CO2 enrichment by isotope ratio mass spectrometry (EUROPA Scientific Ltd, Crewe, United Kingdom) were as previously described (13), and blood samples for the 13C enrichment of plasma phenylalanine were analyzed as described by Sanchez et al (20) with minor modifications by using a Varian 2000 ITD Mass Spectrometer coupled to a Varian 3800 Gas Chromatograph (Varian Inc, Palo Alto, CA).

Calculation of phenylalanine oxidation and balance
Phenylalanine oxidation (µmol · kg–1 · 30 min–1) was computed for consecutive half-hourly intervals as the ratio of the 13CO2 production rate (µmol · kg–1 · 30 min–1), corrected for recovery, to the isotopic abundance of the intracellular phenylalanine pool that was undergoing oxidation at that time, which was taken to be represented by plasma [13C]phenylalanine enrichment (mole percent excess). A surrogate measure of the rate of phenylalanine oxidation in the fed state was also computed as the proportion of administered tracer oxidized (F13CO2), and was calculated as the ratio of the 13CO2 production rate (µmol · kg–1 · 30 min–1) corrected for recovery to the [13C]phenylalanine infusion rate (µmol · kg–1 · 30 min–1). The F13CO2 was calculated for the first 6 h after feeding commenced. Phenylalanine balance (mg · kg–1 · d–1) was computed as the difference between phenylalanine input (dietary phenylalanine + intravenous tracer) and phenylalanine output (sum of phenylalanine oxidation over 24 h).

Statistical methods and data evaluation
Data are presented as means ± SDs. Weight change and phenylalanine flux were analyzed by using mixed-models analysis of variance. The model for weight change over the 6-d experimental diet periods included a factor for diet period. The model for 12-h phenylalanine flux included diet period, metabolic phase (fasted compared with fed), valine intake, and the intake-by–metabolic phase interaction. For the relations between valine intake and 24-h IAAO (phenylalanine), 12-h fed IAAO (phenylalanine), 24-h IAAB (phenylalanine), and F13CO2, two-phase linear random effects regression models were fit. The intercept and slope of the first line segment and the intercept of the second line segment were estimated; although biologically the slope of the second line segment should be restricted to zero, it was tested versus zero slope before making this restriction. The models were constrained such that the 2 line segments intersect at the unknown breakpoint. The breakpoint parameter was estimated as –1 times the ratio of the difference between intercepts divided by the difference between slopes (21). The 95% CI for the breakpoint was calculated by using Fieller's theorem. Model estimates and SEs are presented.

Model contrasts were used to make pairwise comparisons of interest as appropriate on the basis of the statistical significance of the model parameters and to make comparisons versus zero balance. A two-sided P value of 0.05 indicated significance for all tests of interaction and main effects; P values of pairwise comparisons were adjusted by using Holm's method (22). The data were analyzed by using SAS version 9.1 (SAS Institute Inc, Cary, NC).


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Anthropometry
The subjects' anthropometric measurements (Table 1Go) were comparable with those of subjects in our previous series of studies (13-17). During the 6-d experimental diet periods, the subjects experienced a small but significant (P < 0.01) weight loss of 0.42 ± 0.34 kg on average across diet periods. There was no significant difference in weight loss between diet periods.

Phenylalanine oxidation and breakpoint analysis
Phenylalanine oxidation and balance at the 7 valine intakes are shown in Table 3Go. Two-phase linear regression models were fit to the valine intake–24-h IAAO (phenylalanine) and valine intake–12-h fed phenylalanine oxidation relations; these results are summarized in Table 4Go. For 24-h IAAO there was no evidence of a nonzero slope in the second line segment (ß2 = –0.07 ± 0.16; P = 0.69) and thus this was restricted to be zero. The breakpoint estimated from this model was 17 mg · kg–1 · d–1 (95% CI: 11, >35 mg · kg–1 · d–1), which indicates that 24-h IAAO (phenylalanine) decreased linearly until a valine intake of 17 mg · kg–1 · d–1, above which 24-h phenylalanine oxidation was estimated as 29 ± 1.2 mg · kg–1 · d–1 at all higher valine intakes. Within this model, 24-h phenylalanine oxidation at the 5- and 10-mg valine intakes was also significantly higher than oxidation above the breakpoint (each P < 0.05) but at the 15-mg valine intake was not significantly different from oxidation above the breakpoint.


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  		TABLE 3 . Summary of phenylalanine oxidation, balance, and flux at 7 test valine intakes in well-nourished Indian men1

 

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  		TABLE 4 . Two-phase regression analysis of the relation between valine intake and phenylalanine oxidation, phenylalanine balance, and the rate of phenylalanine oxidation in the fed state (F13CO2)

 
Results for 12-h fed phenylalanine oxidation were similar to 24-h oxidation. There was no evidence of a nonzero slope in the second line segment (ß2 = –0.08 ± 0.09; P = 0.39) and thus this was restricted to be zero. The breakpoint was estimated as 18 mg · kg–1 · d–1 (95% CI: 13, >35 mg · kg–1 · d–1), which indicates that 12-h fed phenylalanine oxidation decreased linearly until a valine intake of 18 mg · kg–1 · d–1, above which it was estimated as 14 ± 0.6 mg · kg–1 · d–1 at all higher valine intakes. Within this model, 12-h fed phenylalanine oxidation at the 5- and 10-mg valine intakes was also significantly higher than oxidation above the breakpoint (each P < 0.01) but at the 15-mg valine intake was not significantly different from oxidation above the breakpoint.

Phenylalanine balance and breakpoint analysis
The results of fitting a two-phase linear regression model to the valine intake–phenylalanine balance relation are summarized in Table 4Go. There was no evidence of a nonzero slope in the second line segment (ß2 = 0.008 ± 0.17; P = 0.96) and thus this was restricted to be zero. The breakpoint estimated from this model was 17 mg · kg–1 · d–1 (95% CI: 11, 28 mg · kg–1 · d–1), which indicates that daily IAAB (phenylalanine) increased linearly until a valine intake of 17 mg · kg–1 · d–1, above which phenylalanine balance was estimated as 9.3 ± 1.2 mg · kg–1 · d–1 at all higher valine intakes. Within this model, phenylalanine balance was not significantly different from zero balance at an intake of 5 mg valine kg–1 · d–1 (P = 0.06) but was significantly different from zero balance at the 10- and 15-mg valine intakes and intakes above the breakpoint (each P < 0.01). Phenylalanine balance at the 5- and 10-mg valine intakes was also significantly lower than balance above the breakpoint (each P < 0.05) but at the 15-mg valine intake was not significantly different from balance above the breakpoint.

F13CO2 ratio and breakpoint analysis
The data for the F13CO2 ratio are presented in Figure 1Go, and the results of fitting a two-phase linear regression model to the valine intake–F13CO2 ratio relation are summarized in Table 4Go. There was no evidence of a nonzero slope in the second line segment (ß2 = –0.0003 ± 0.001; P = 0.77). The breakpoint estimated from the model with a fixed slope of zero in the second line segment was 20 mg · kg–1 · d–1 (95% CI: 12, >35 mg · kg–1 · d–1), which indicates that the daily F13CO2 ratio decreased linearly until a valine intake of 20 mg · kg–1 · d–1, above which the F13CO2 ratio was estimated as 0.11 ± 0.007 at all higher valine intakes. Within this model, the F13CO2 ratio at the 5- and 10-mg valine intakes was also significantly lower than the F13CO2 ratio above the breakpoint (each P < 0.05) but at the 15-mg valine intake was not significantly different from the F13CO2 ratio above the breakpoint.



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  		FIGURE 1. Rate of phenylalanine tracer oxidation in the fed state (F13CO2) versus valine intake. Results for F13CO2 are expressed as the ratio of 13CO2 output to the dose of [13C]phenylalanine administered. Individual values are represented by unfilled circles, and mean values are represented by filled squares.

 
Phenylalanine flux
There was no significant effect of valine intake on phenylalanine flux, but flux was significantly higher in the fasted than in the fed phase (Table 3Go; P = 0.02).


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The findings of the present study add to the earlier tracer-derived balance data that we generated by using the diet-adapted, 24-h IAAO and IAAB paradigm to quantify adult amino acid requirements (8, 9, 13-17, 23) in South Asian (Indian) and American subjects. This pattern of amino acid requirements (for leucine, lysine, methionine, and threonine) is similar to the amino acid pattern recommended for adults by a recently convened WHO/FAO/UNU Expert Consultation on Protein and Amino Acid Requirements (24).

A different indicator amino acid (phenylalanine) was used in the present study at an intake of {approx}38 mg · kg–1 · d–1, which was based on direct amino acid oxidation and balance (DAAO and DAAB) studies of the phenylalanine requirement with a tyrosine-free diet (20, 25). The phenylalanine balances in the present study were largely positive, which could be because balance was measured as the difference between phenylalanine intake and oxidation rather than the hydroxylation rate. At a phenylalanine intake that was similar to that in the present study (with zero tyrosine except for that administered as a tracer), the ratio between phenylalanine hydroxylation and oxidation rates was 0.9 in the 12-h fasted state and close to 1.0 in the 12-h fed state (25). Nevertheless, in that study, the difference in daily phenylalanine balance calculated from the phenylalanine oxidation rate was higher by {approx}10% of the phenylalanine intake ({approx}4 mg · kg–1 · d–1) than when the phenylalanine hydroxylation rate was used as the calculation parameter. The positive phenylalanine balances at valine intakes that were above the requirement could have also been due to the underestimation of phenylalanine oxidation with intravenous administration of the tracer (20). Unlike with leucine, for which there are no major differences in whole-body leucine oxidation when the tracer is given orally or intravenously (26), phenylalanine oxidation occurs predominantly in the liver (27, 28), and the first-pass uptake of this amino acid is greater than it is for leucine (20). However, in this initial investigation into the valine requirement, we preferred to use the intravenous route because it involved a relatively undisturbed period of sleep for the subjects, in contrast with an intermittent oral dosing regimen. We also used the enrichment of plasma phenylalanine to represent the precursor pool enrichment, because in long-term intravenous infusions of [ring 13C6]phenylalanine, it has been found that the ratio of enrichment of phenylalanine in a hepatic secretory protein (apolipoprotein B-100) to the enrichment in plasma phenylalanine approached unity in the fasted state (29).

A tyrosine-free diet was used in the present study, in contrast with earlier IAAO studies of the BCAA requirement, in which a generous intake of tyrosine was provided in the diet to reduce the conversion of phenylalanine to tyrosine (11). The labeled tyrosine formed from the hydroxylation of labeled phenylalanine tracer was partitioned into oxidation and protein synthesis and this partitioning could be variable depending on the needs of protein synthesis. In this respect, the provision of a tyrosine-free diet does not reduce the conversion of phenylalanine into tyrosine, and adds an additional variability to determination of the breakpoint estimate from estimates of phenylalanine oxidation. This might be truer particularly in respect of short-term fed state measurements or the use of surrogate indicators of oxidation such as the F13CO2. However, the desired primary aim of the present study was to assess 24-h phenylalanine oxidation and balance as an outcome of varying dietary valine intakes, and because the provision of {approx}38 mg · kg–1 · d–1 with a zero tyrosine intake has been shown to result in fairly neutral balances (25), it was our hypothesis that the net phenylalanine oxidation and balance under a defined and constant set of conditions would serve effectively as an indicator of valine equilibrium.

The valine requirement in the present study was similar to that suggested from earlier DAAO studies at different valine intakes (10). We chose to measure the valine requirement when a maintenance intake of leucine (8, 9) was supplied in the diet, because it is possible that the dietary leucine intake could influence the requirement for valine. This maintenance requirement of leucine is not a safe requirement that would meet the needs of 97.5% of the population. Because the BCAAs share common membrane transport systems (30, 31) and metabolic enzymes (32, 33), it is possible that the valine requirement was underestimated. In short-term animal experiments, a high leucine intake has been shown to result in increased valine oxidation (34), while also depressing free valine concentrations in plasma and tissue amino acid pools. Similar findings have been observed in humans, with leucine intake having an effect on plasma concentrations of valine and isoleucine, but with valine and isoleucine intake having little effect on the plasma leucine concentration (35, 36) or oxidation (37).

In contrast, however, it has been shown that varying leucine intake in the range supplied by normal diets (40 to 80 mg · kg–1 · d–1) has no effect on valine oxidation (38). This is corroborated by comparing the findings of the present study with those of an earlier study (10) in which the valine requirement was estimated by direct measurements of valine oxidation in the presence of generous amounts (egg-protein-based amino acid mixture) of leucine. In these 2 experiments that were based on different principles (24-h IAAO/IAAB and short-term DAAO), the valine requirement estimate was similar, even though the amount of leucine supplied in the diet varied twofold. Therefore, it appears that there may be no regulatory effect of dietary leucine on valine catabolism within the physiologic range of leucine intakes and that the estimated value of the valine requirement is likely to be the same with a range of normal diets.

An important feature of the present study is that we were able to compare short-term fed state measurements of the proportion of tracer phenylalanine oxidized (F13CO2 ratio, short-term IAAO) with conventional estimates of phenylalanine oxidation and balance measured over a day (24-h IAAO/IAAB). We preferred to use the F13CO2 ratio rather than the F13CO2 value (µmol · kg–1 · 30 min–1) alone, because the ratio normalized the data for small differences in tracer infusion rates. The short-term IAAO technique, which was developed initially to study amino acid requirements during growth (reviewed in 39), has been extensively used to measure the amino acid requirements of adults (11, 40-42). Its advantages lie in the short nature of the protocol and its relative noninvasiveness. However, there are critical differences between the short-term IAAO method and the longer-term 24-h IAAO and IAAB method. One of these is the length of the dietary adaptation period before the tracer study; it is 2 d in the short-term IAAO method but 7 d in the 24-h IAAO/IAAB method. A second important difference is that only fed state measurements are made in the short-term IAAO method, whereas the 24-h IAAO/IAAB method uses oxidation measurements over the whole day divided into 12-h fasted and 12-h fed states, such that daily amino acid balance can be accurately calculated.

The results of the present study show that the breakpoint estimate of the valine requirement, based on the F13CO2 data, was numerically {approx}20% higher than the estimates obtained from 24-h IAAO/IAAB within the same experiment (20 compared with 17 mg · kg–1 · d–1, respectively); however, this difference was not significant because the 95% CIs of the 2 estimates completely overlapped. Given such large CIs, it is clear that currently designed tracer investigations cannot be powered sufficiently to detect even 20% differences because of the costs involved. There are several theoretical reasons as to why a 24-h tracer protocol may be preferable to a short-term tracer protocol. For instance, earlier 24-h studies of leucine (13) and phenylalanine oxidation (20) showed that the oxidation rate changes in a complex manner throughout the fed period; thus, it is possible that the "window" of the fed period measurement in the short-term IAAO method can confer some variability in the breakpoint estimate. Furthermore, because adaptive changes in amino acid oxidation also occur in the postabsorptive state in response to changes in test amino acid intake, this can modulate the 24-h fast-fed oxidation value at different test amino acid intakes to change the 24-h response pattern. However, given the variability in the estimates of amino acid kinetics, it would appear that in the present state of the art, the short-term and 24-h tracer approach gave similar findings, and that breakpoint estimates from 12-h fed state oxidation rates were similar to those obtained from the 24-h estimates.

The estimate of the valine requirement from either the F13CO2 ratio or the 24-h IAAO/IAAB method in the present study was lower than that predicted from the total BCAA requirement by the short-term relatively unadapted IAAO technique (11) or by the obligatory amino acid loss (OAAL) method (4). It is not possible to state with any certainty the reason for this discrepancy. One possibility is that the short-term IAAO study (11) determined the total BCAA requirement and from that predicted the valine requirement based on a similar proportionality in the amino acid composition of body protein. A similar higher valine requirement value is predicted from the OAAL method based on the same principle (4), and it may be that the prediction of the valine requirement based on an assumed proportionality of the BCAAs in body protein is unwarranted. Another possibility is the length of dietary adaptation before the tracer study, because it is possible that the effect of a lack of adequate dietary adaptation might give a higher value for the requirement of the test amino acid, particularly if habitual intake were high.

In summary, the present investigation of 24-h [13C]phenylalanine tracer kinetics in healthy Indian subjects studied with 7 test valine intakes and at a maintenance intake of leucine indicates that the 1985 WHO/FAO/UNU requirement value of 10 mg · kg–1 · d–1 is not adequate for the healthy Indian population.


   ACKNOWLEDGMENTS
 
This manuscript is dedicated to the memory of VR Young.

AVK was involved in study design, data collection, sample and data analysis, and writing of the manuscript. VNR and TDSR were involved in looking after the subjects, data collection, and analysis. JVG was involved in data collection and sample analyses. MMR was involved in study design, data analysis, and writing of the manuscript. The authors had no conflicts of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. FAO/WHO/UNU Expert Consultation. Energy and protein requirements. World Health Organ Tech Rep Ser 1985;724:204.
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Received for publication November 24, 2004. Accepted for publication April 7, 2005.




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A. V Kurpad, M. M Regan, T. D. Raj, V. N Rao, J. Gnanou, and V. R Young
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A. V. Kurpad, M. M. Regan, T. Raj, and J. V. Gnanou
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