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1 From the Department of Physiology and Division of Nutrition (AVK, TR, JV, and RK) and the Department of Biochemistry (JG), St John’s Medical College, Bangalore India, and the Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge (MMR and VRY).
2 Supported by NIH grants RR88, DK42101, and P30-DK40561. 3 Address reprint requests to AV Kurpad, Department of Physiology and Nutrition Research Center, St John’s Medical College, Bangalore 34, India. E-mail: a.kurpad{at}divnut.net.
ABSTRACT
Background: In earlier studies with well-nourished subjects that used a 24-h indicator amino acid oxidation or balance approach, we concluded that the 1985 FAO/WHO/UNU requirement for lysine (12 mg · kg-1 · d-1) was inadequate for healthy South Asian subjects and proposed a tentative requirement of 30 mg · kg-1 · d-1.
Objective: We assessed whether chronic undernutrition, with low habitual dietary protein and lysine intakes, leads to changed lysine requirements.
Design: Twenty-seven otherwise clinically healthy, chronically undernourished Indian men were studied during 2 randomly assigned 7-d diet periods supplying 12 and 30, 18 and 36, or 24 and 42 mg lysine · kg-1 · d-1, based on an L-amino acid diet. The subjects’ leucine intake was 40 mg · kg-1 · d-1. At 1800 on day 6, a 24-h intravenous [13C]leucine tracer-infusion protocol was conducted to assess leucine oxidation and daily leucine balance at each test lysine intake.
Results: A breakpoint was not identified in the lysine intake–leucine oxidation or balance response over the range of intakes studied. Mixed-models linear regression analysis indicated a mean requirement of 44 mg lysine · kg-1 · d-1 (95% CI: 36, 63) for the lysine intake–leucine balance relation.
Conclusions: The mean lysine requirement in chronically undernourished men is estimated to be higher than the value of 30 mg · kg-1 · d-1 proposed for well-nourished individuals. This may be related to body-composition differences. It also suggests that these subjects have not elicited a metabolic adaptation in response to their habitually low lysine intakes by substantially improving their efficiency of dietary lysine utilization.
Key Words: Chronically undernourished adults • lysine requirements • indicator amino acid oxidation • indicator amino acid balance • Indian men
INTRODUCTION
We developed a 24-h indicator amino acid oxidation and balance (IAAO and IAAB) method for the investigation of lysine and threonine requirements in healthy adults from the South Asia region, with the use of a stable-isotope-tracer protocol (1–4). This technique uses [13C]leucine as the indicator amino acid tracer, administered over an entire 24-h period, after subjects are adapted to their experimental diets for 1–3 wk. These more recent 24-h kinetic studies supported our earlier conclusion (5) that the 1985 FAO/WHO/UNU (6) requirement values for the indispensable amino acids in adult human nutrition were too low. For lysine, the 1985 FAO/WHO/UNU upper requirement value was set at 12 mg · kg-1 · d-1 for healthy adults (6), and we proposed a mean requirement value of 30 mg · kg-1 · d-1, on the basis of our earlier 24-h IAAO and 24-h IAAB estimates (1–3), as well as direct amino acid balance estimates made on adult men of a similar age at the Massachusetts Institute of Technology (MIT; 7, 8). This proposed requirement value is also consistent with results from short-term IAAO tracer studies carried out by researchers in Toronto (9, 10) and our (11) reassessment of the earlier nitrogen balance data of Jones et al (12). Finally, the recommendation of a lysine mean requirement of 30 mg · kg-1 · d-1 was tentatively accepted by a working group meeting on protein and amino acid requirements, held in Rome in July 2001, under the auspices of the FAO/WHO/UNU (Internet: http://www.fao.org/es/esn/require/upcoming.htm; accessed September 2001).
One argument against the global acceptability of the earlier MIT estimates of the lysine requirement (6, 7, 13) was that they were made in healthy young American males, with habitual intakes of lysine that were well in excess of the requirement. Our studies on well-nourished Indian men with habitual intakes of lysine that were less than those of the American subjects but still well above the minimum lysine requirement (1–3) gave a similar lysine requirement as did the MIT studies. However, it could still be argued that the estimate of the lysine requirement for global use should perhaps be determined in subjects whose habitual intakes of lysine are lower than these: in particular, in economically disadvantaged members of the population who are chronically undernourished and subsist on predominantly cereal-based diets. It is possible that in such subjects an adaptation—with or without cost—to the low lysine intakes may have occurred, leading to a lysine requirement that is lower than "normal."
Therefore, we designed the present 24-h IAAB and 24-h IAAO study, with leucine as the indicator amino acid, to assess the lysine requirement of poor, chronically undernourished Indian men recruited from urban slums. A robust estimate of the lysine requirement on the 24-h IAAB–lysine intake response curve (2, 9) was the main output required. This was determined earlier from a breakpoint analysis of the balance–lysine intake relation to be at a 30-mg lysine · kg-1 · d-1 intake in well-nourished Indian subjects (3). We also anticipated a lower requirement estimate in undernourished subjects than was previously found in healthy Indian adults, and so we chose to study 3 intakes below 30 mg · kg-1 · d-1 and 2 above this amount.
SUBJECTS AND METHODS
Subjects
Twenty-seven young men participated in this experiment. All the subjects, who were otherwise clinically healthy, were recruited from the slums of Bangalore City. The subjects had body mass indexes (BMIs; in kg/m2) of < 18.5 and heights that were < 1.65 m and were from the lowest socioeconomic class in India, as measured by a modified socioeconomic scale (14). They were semiskilled manual laborers and were on daily wages. Their habitual food intakes were measured by recall over 3 separate occasions.
The physical characteristics of the subjects are given in Table 1. All were free of acute disease as determined by medical history, physical examination, analysis for blood cell count, routine blood biochemical profile, blood serology for hepatitis and HIV, and urinalysis. Their habitual intake of lysine was estimated to be < 50 mg · kg-1 · d-1, as judged by dietary recall. It was not possible to find volunteer subjects who did not smoke cigarettes or consume alcohol. Both habits were variable within individuals and depended on their income at the time, because all the subjects were paid on a daily-wage basis. Therefore, we could not exclude subjects on the basis of smoking or alcohol intake but chose subjects who were prepared to do without alcohol or tobacco during each experimental dietary adaptation period. Their intake of caffeinated beverages/d was also variable, depending on their economic circumstances at the time, but was usually < 6 small cups (50–100 mL)/d. The purpose of the study and the potential risks involved were explained to each subject, in his language. Signed (or thumbprint) consent was obtained from each subject. The Human Ethical Approval Committee of St John’s Medical College approved the research protocol.
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TABLE 1 . Characteristics of chronically undernourished Indian men studied to determine their lysine requirements1
Anthropometric measurements
Anthropometric and skinfold-thickness measurements were carried out on the subjects at the beginning of the experiment. The subjects,wearing minimal clothing, were weighed with a digital scale (Soehnle-Waagen GmbH & Co, Murrhardt, Germany), which had a precision of 0.1 kg. All weights were measured twice and the mean expressed to the nearest 0.1 kg. The heights of the subjects were recorded with a vertically mobile scale (Holtain, Crymych, United Kingdom), expressed to the nearest centimeter, while the subjects were without footwear. The following skinfold-thickness measurements were carried out in duplicate with the subjects in the standing position, and the mean of each was taken for further calculation: biceps, triceps, subscapula, and suprailium. All skinfold-thickness measurements were standardized (15) and carried out to the nearest 0.2 mm, with the use of skinfold calipers (Holtain). The logarithm of the sum of the 4 skinfold thicknesses was used, in age- and sex-specific equations (16), to obtain an estimate of body density, from which percentage body fat and fat-free mass (FFM) were determined (17). We previously validated this method of measuring body fat against hydrodensitometric estimates of body density and fat (18). To characterize the subjects further, their muscle mass was also predicted from equations with the use of values of 24-h urinary creatinine excretion (19). This prediction was performed for each subject only when he was receiving adequate amounts of lysine and other nutrients. Therefore, each subject was studied at one time, when he was consuming lysine intakes that were at or above the putative requirement amount, that is, 30, 36, or 42 mg · kg-1 · d-1. The creatinine excretion was determined as the average of 3-d values (days 4, 5, and 6) of the experimental diet period.
Diet and experimental design
The tracer experiment was carried out after the 6-d diet period. All subjects were studied during 2 separate diet periods, during which they received a weight-maintaining diet based on an L-amino acid mixture with different daily lysine intakes (Table 2; L-amino acids obtained from Ajinomoto, USA, Inc, Washington, DC). Daily energy intakes were designed to maintain body weight, on the basis of the time and motion studies referred to below, and the energy requirement was calculated to be 1.6 x the basal metabolic rate from day 1 to day 6 and 1.35 x the basal metabolic rate on day 7 (tracer study day). The subjects were resident in the metabolic ward of the Division of Nutrition and during this time were maintained on an activity program that maintained their daily physical activity level. We previously estimated the physical activity level of such subjects in a free-living condition by the doubly labeled water method and found their physical activity levels to be 1.5–1.6 x the basal metabolic rate (20). However, to be certain that the subjects were in energy balance during the experimental diet period, each subject was followed all day by a laboratory assistant, who recorded his activities for each successive 10-min period, such that an estimate based on time and motion of his true physical activity could be made, against which his energy intakes were titrated. The observed physical activity level was 1.5. The major energy supply was given in the form of a sugar-oil formula and as protein-free wheat-starch cookies (Table 3). Nonprotein energy was provided as fat (43%) and carbohydrate (56%). The main source of carbohydrate was beet sugar and wheat starch, to attain a low 13C content in the diet so that a relatively steady background in breath 13CO2 enrichment over the 24-h period could be obtained. Breath 13CO2 enrichments obtained during the leucine-tracer studies were corrected to account for the small changes in background 13CO2 output that would have occurred without the [13C]leucine tracer.
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TABLE 2 . Composition of the L-amino acid mixtures used to supply 6 lysine intakes daily
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TABLE 3 . Composition of the experimental diet used to supply adequate energy with 1 of 6 test lysine intakes
The subjects were randomly assigned to 2 experimental diets (supplying 12 and 30, 18 and 36, or 24 and 42 mg lysine · kg-1 · d-1), given in random order. At the end of each diet period, a 24-h intravenous [13C]leucine tracer-infusion study was carried out. The leucine content of the diet was kept at a maintenance requirement level, at 40 mg · kg-1 · d-1 and was held constant for all lysine intakes (Table 2). The subjects were terminated from the experimental protocol at the end of the first 24-h tracer study, and the second diet period was begun within 1–4 wk. During this interval, the subjects consumed their free-choice diets and returned to their usual living conditions.
During the experimental period all other nutrients were provided in adequate amounts (Table 3). A choline supplement of 500 mg was given daily, and dietary fiber was provided in the form of 20 g isabgol (Sat-Isabgol; Charak Piramal Healthcare Ltd, Mumbai, India) when requested by the subject. The total daily food intake was consumed as 3 isoenergetic, isonitrogenous meals (at 0800, 1300, and 2000). Every morning, body weight and vital signs were monitored. All the subjects’ meals were consumed at the kitchen of the Nutrition Research Center, under the supervision of the dietary staff.
Twenty-four–hour tracer-infusion protocol
The primed tracer-infusion approach was used in this study, following a standard design in all subjects (2, 3). Briefly, [1-13C]leucine (99.3 atom%; MassTrace, Woburn, MA) was given as a primed, constant intravenous infusion at a known rate of 2.8 µmol · kg-1 · h-1 (the prime was 4.2 µmol/kg) into an antecubital vein. The bicarbonate pool was primed with 0.8 µmol sodium [13C]bicarbonate/kg (99.9 atom%; MassTrace).
Recovery of 13CO2 and the contribution of dietary 13C to breath 13CO2
Because the present study was conducted with diets that contained low amounts of 13C-enriched carbohydrate, the contribution to breath 13CO2 from the experimental diet was expected to be low, although a correction was made for this small contribution of endogenous 13C-substrate oxidation over the 24-h study period, in these specific subjects, 6 of whom were also studied in the 24-h [13C]leucine tracer protocol, as described above. In this case, however, these subjects did not receive any labeled amino acid tracer. Breath samples were taken every 30 min, for measurement of breath 13CO2, and the enrichment showed a diurnal variation, with the 13CO2 abundance falling during the fasting period and increasing during the fed period. However, at no time did this enrichment exceed a value of ± 0.001 atom percent excess (APE), and a correction was made for this small contribution of endogenous 13C-substrate oxidation over the 24-h study period.
The 13CO2 recovery was also measured in these undernourished subjects, because we did not know whether the 13CO2 recovery value would be the same as for the well-nourished subjects studied previously (1–4). The 13CO2 recovery study was carried out in 6 of the chronically undernourished young men in this study to determine the recovery, in expired air, of a 24-h tracer infusion with 13C-labeled sodium bicarbonate. The bicarbonate tracer protocol followed was similar to that used in the [13C]leucine tracer studies, in terms of feeding and fasting periods. Thus, an infusion of [13C]NaHCO3 (99.9%; MassTrace) was given for 24 h, and the recovery of 13CO2 in the breath was calculated for every 30-min interval. Indirect calorimetry was carried out to determine the CO2 output from the body and along with the 13CO2 enrichment of the breath gave the primary data required for determining total 13CO2 output. The infusates of the labeled bicarbonate were analyzed by using a spectrophotometric method (Pointe Scientific Inc, Lincoln Park, MI) to accurately measure the amount of bicarbonate infused. On the basis of this method in 6 subjects, the bicarbonate recovery during the 12-h fast was 79% and during the 12-h fed-state recovery was 80%; there was no significant difference between the fasting and fed states, and these values compare with those of 77% and 85%, respectively, found for healthy US men (21). Mean values for each time point were used to correct each 30-min estimate of 13CO2 production caused by oxidation of [1-13C]leucine.
Indirect calorimetry and collection and analysis of breath and blood samples
Total carbon dioxide production (
Leucine oxidation was computed for consecutive half-hourly intervals as described previously (3, 4, 23).
RESULTS
Anthropometry and habitual dietary intakes
Mean values for anthropometric indexes of the subjects are summarized in Table 1. The subjects’ BMI range was 14.2–18.3, and their body fat was low: 10%. Their mean 24-h creatinine excretion was 1.0 ± 0.1 g/d, with a mean CV of 11.0% (range: 0.3–22%). This predicted a mean muscle mass of 22.0 ± 2.5 kg, amounting to 55% of FFM, as estimated by measurements of skinfold thickness, and 51% of body weight. The subjects lost small but statistically significant (P < 0.001) amounts of weight during the 6-d diet adaptation periods: 0.26 ± 0.34 kg and 0.40 ± 0.27 kg during the first and second diet periods, respectively. We discuss the significance of these weight changes below. All subjects remained in apparent good health throughout. Their daily energy intake, as measured by dietary recall, was 7.4 ± 1.3 MJ/d, and protein intake was 0.82 ± 0.17 g · kg-1 · d-1. Lysine and leucine intakes were estimated to be 39 ± 13 mg · kg-1 · d-1 and 60 ± 13 mg · kg-1 · d-1, respectively. The dietary protein-energy ratio was 8%.
Leucine kinetics
Data for the primary variables measured, including CO2 output, breath 13CO2 enrichment, 13CO2 production, and plasma [13C]KIC enrichment, at each test lysine intake, were quantitatively similar to those published previously (1) and are presented in Table 4.
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TABLE 4 . Summary of primary variables [CO2 production rate, [13C]-ketoisocaproic acid (KIC), and 13CO2 enrichments] at 6 lysine intakes in chronically undernourished Indian men at day 7 of the diet period1
Oxidation
There was no significant interaction between lysine intake and metabolic phase (P = 0.45) (Table 5). The rates of leucine oxidation were significantly lower during the fasting than during the fed period (P < 0.001), without regard to lysine intake. The 12-h rate of oxidation differed among lysine intakes (P = 0.012) without regard to metabolic phase. The 24-h rates of oxidation at the 12 and 42-mg · kg-1 · d-1 intakes were significantly different from one another (P < 0.01).
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TABLE 5 . Summary of leucine oxidation, balance and flux at 6 lysine intakes in chronically undernourished Indian men at day 7 of the diet period1
Balance
With respect to leucine balance, the results were essentially the same whether expressed as an absolute balance or as a percentage of leucine intake (Table 5). Balances at the 12-, 18-, and 24-mg · kg-1 · d-1 intakes were significantly lower than zero (P < 0.05), and at the 30-mg · kg-1 · d-1 intake the balance tended to be lower than zero (P = 0.09).
Daily leucine balance was affected by lysine intake (P < 0.01) and was lower or tended to be lower at the 12-mg · kg-1 · d-1 intake than at the 30-, 36-, and 42-mg · kg-1 · d-1 intakes (P = 0.05, P < 0.15, and P < 0.05, respectively).
Flux
The interaction between lysine intake and metabolic phase was not statistically significant (P = 0.85). The main effect of lysine intake tended to be significant (P = 0.07), and the effect of metabolic phase was not statistically significant (P = 0.17).
Breakpoint analysis
The results of the statistical analysis of the relation between 24-h leucine oxidation or daily leucine balance and lysine intake are summarized in Table 6. With respect to leucine oxidation, the simulated likelihood ratio comparisons indicate that there is no improvement in model fit by adding additional parameters to create the two-phase regression lines as compared with the straight-line model. The allowance of a two-phase regression line in which the slope of the second line segment is restricted to be zero resulted in an estimated breakpoint of 33 mg · kg-1 · d-1 (95% CI: 24, 61); informally comparing –2ll illustrates that this model does not appropriately summarize the relation. The allowance of a two-phase regression without restriction on the slope of the second line segment resulted in an estimated breakpoint of 18 mg · kg-1 · d-1 (95% CI could not be reliably estimated); again the informal comparison of –2ll illustrates that this model does not appropriately summarize the relation. Thus, in the range of lysine intakes studied, the leucine oxidation–lysine intake relation [slope = –0.33 (95% CI: –0.49, –0.17)] is best summarized by a straight line without indication of a breakpoint in the relation.
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TABLE 6 . Breakpoint analysis of the leucine oxidation and balance–lysine intake relation
With respect to leucine balance, the simulated likelihood ratio comparisons similarly indicate that there is no improvement in model fit by adding parameters to create the two-phase regression lines as compared with the straight-line model. Thus, in the range of lysine intakes studied, the leucine balance–lysine intake relation is best summarized by a straight line without indication of a breakpoint in the relation. The linear relation [slope = 0.34 (95% CI: 0.18, 0.50)] predicts zero leucine balance at a lysine intake of 44.4 mg · kg-1 · d-1 (95% CI: 36, 63). This intake is therefore the estimated mean intake of lysine required to achieve whole-body leucine equilibrium, and we are 95% confident that the true mean is in the interval 36–63 mg · kg-1 · d-1. Hence, this value is taken to be the mean requirement for these undernourished subjects, which as indicated by the 95% CI is above the 30-mg · kg-1 · d-1 requirement for well-nourished Indian subjects (3).
DISCUSSION
The immediate question that we were interested in answering was whether the lysine requirements of weight-stable, chronically undernourished but otherwise clinically healthy subjects, when given an indicator amino acid (leucine) at an intake judged to be adequate for healthy subjects, was different from that determined to be sufficient for maintenance of adequate nutritional status (leucine balance) in well-nourished subjects. It was our hypothesis that if an adaptation to low amino acid intakes had occurred in the undernourished subjects they would make more efficient use of an equivalent amino acid intake than did well-nourished subjects. Hence when given leucine at a requirement intake for healthy subjects, the undernourished subjects would be expected to come into leucine balance at an intake of lysine that was less than that determined to be sufficient for well-nourished subjects; the latter is estimated to be 30 mg lysine · kg-1 · d-1 (2, 3). The subjects in the present study were chronically undernourished, as judged by a low BMI, but were otherwise free of recognizable clinical pathology. They were from a poor socioeconomic background and were significantly shorter than the subjects we studied previously (1–4); they had been weight stable over the past 6 mo. Chronic energy deficiency is similarly defined; a BMI of 18.5, thought to indicate an absence of acute disease in both men and women, is taken as the cutoff for defining chronic energy deficiency (25). Most of the adult Indian population has a BMI lower than this (26).
As noted above, there was a small weight loss in the subjects in the present study during the weeklong experimental diet period. Without precise estimates of body composition, we cannot determine the true nature of the weight loss. However, we examined the respiratory gas exchange data obtained during the 24-h tracer studies on day 7 to assess rates of fat and carbohydrate oxidation. Leucine oxidation was used as a surrogate for protein oxidation, on the basis of a leucine content in mixed body protein of 8% (27). The heat equivalents of body protein and fat (for the fasting state), as well as of the amino acid mixture and other components of the diet (for the fed state) were calculated according to the methods of Livesey and Elia (28). On the basis of this approach, the 24-h oxidation rates for carbohydrate and fat and their balances were also calculated. There was not a significant relation between weight loss and leucine balance across the different lysine intakes. However, the maximum change in FFM was 0.3 kg over the 7-d experimental feeding period for the lowest lysine intake. This was within the normal fluctuations in individual daily body weight and the changes caused by glycogen loss, as discussed below. This is possibly why body weight did not show a consistent relation with leucine balance under these conditions. In terms of substrate oxidation, although there were no differences between different lysine intakes, the mean carbohydrate balances across all lysine intakes were –1.08 and –1.28 g · kg-1 · d-1 during day 7 of the first and second diet periods, respectively. Similarly, there was a positive fat balance of 0.5 g · kg-1 · d-1 across all lysine intakes for both diet periods. Although there would be no net effect on body energy balance, these energy substrate alterations suggest that there was a carbohydrate (glycogen) loss from the body, of 50 g/d, which, along with a concomitant water loss associated with glycogen (29), might amount to a weight loss of 0.5 kg at the end of the week’s experimental feeding. This is generally consistent, given the assumptions, with the total weight loss seen during this experiment.
The estimated mean lysine requirement of 44 mg · kg-1 · d-1 in these chronically undernourished subjects is higher than the value of 30 mg · kg-1 · d-1 that we obtained previously in well-nourished Indians (1–3). The subjects were at equilibrium with respect to whole-body leucine balance at this lysine intake, so the higher lysine requirement is apparently not an expression of a protein-repletion phenomenon, because they were given a fully adequate diet, at least for fully replete, healthy subjects. A repletion phenomenon would involve one or more of the following responses: an increased body weight, a positive energy balance, or a positive nitrogen or leucine balance. The latter, in terms of the leucine balance–lysine intake curve, would be shifted downward, such that zero leucine balance would be achieved at lower lysine intakes. On the other hand, it would be interesting to determine whether a higher intake of the indicator, leucine, which was set at 40 mg · kg-1 · d-1, or close to the mean requirement for this amino acid in healthy subjects (13, 23), might have permitted a greater retention of leucine when lysine was given at intakes at and above 44 mg · kg-1 · d-1, together with perhaps a somewhat higher energy intake.
The habitual mean intake of lysine of the subjects in the present study was 40 mg · kg-1 · d-1; for healthy, well-nourished Indian adults in our previous studies, it was estimated to be 50–60 mg · kg-1 · d-1. These intakes are below the average in our MIT subjects, which was 100 mg · kg-1 · d-1. Hence we see no evidence that there was an adaptive decrease in the lysine requirement in the Indian subjects. This is important, because if this is the case and given that the mean requirement in the undernourished subjects is approximately equivalent to the mean intake by the population, then 50% of the members of the population would be at risk of not having met their lysine requirement. This could be caused, in part, by the relatively poor protein quality of the diet, with rice and millet supplying a major fraction of the dietary protein intake.
Comparisons of the mean requirement estimate for undernourished Indian subjects (44 mg · kg-1 · d-1 ) with that for well-nourished Indian subjects (30 mg · kg-1 · d-1; 1–3), should take into account the differences in body composition of these 2 populations, with requirements expressed in relation to FFM. Thus, for well-nourished subjects with 19% body fat, the mean lysine requirement is estimated to be 37 mg · kg FFM-1 · d-1; for undernourished subjects with 10% body fat, the mean lysine requirement is estimated to be 48 mg · kg FFM-1 · d-1. Hence on this basis, the requirement in undernourished subjects is still higher than for well-nourished subjects. However, the differential is somewhat less (30%) than when requirements are expressed per unit of body weight (47%). This higher requirement might be functionally related to the fact that the ratio of muscle to visceral weight of the FFM is lower in undernourished than in well-nourished individuals; in the undernourished subjects it is 0.55 and in the well-nourished subjects it is 0.62. Of importance are the observations by van Goudoever et al (30) that intestinal lysine oxidation in the neonatal pig can account for one-third of whole-body lysine oxidation, suggesting that intestinal lysine losses via catabolism might contribute to the difference in lysine requirements between well-nourished and undernourished subjects.
Higher rates of whole-body protein turnover are correlated with higher requirements for proteins and amino acids (31), and protein turnover rates are higher in the viscera than in muscle tissues (32). Thus, a lower whole-body rate of protein turnover, per unit body weight or FFM, might be anticipated for well-nourished compared with undernourished subjects and so could account for a lower need for dietary lysine, when expressed in relation to body weight or FFM. In the present set of studies, the isotopic enrichment of plasma KIC can be used to estimate the whole-body leucine flux as an index of whole-body amino acid and protein turnover (33). In well-nourished subjects, the average 12-h fasting-state whole-body leucine flux for subjects across all lysine intakes was 113 µmol · kg-1 · h-1. This is comparable, on a body-weight basis, to estimates that we and others obtained for healthy adults (34). In the undernourished subjects, the mean fasting leucine flux across all lysine intakes was 117 µmol · kg-1 · h-1. It appears, therefore, that there is no detectable difference between the well-nourished and undernourished subjects, as Soares et al (35) concluded previously. Thus, whole-body turnover differences per se do not appear to be a basis for the requirement differences.
In summary, this investigation of 24-h [13C]leucine tracer kinetics in clinically healthy but chronically undernourished Indian subjects studied with 6 test intakes of lysine, including the 1985 FAO/WHO/UNU recommended intake of 12 mg · kg-1 · d-1 (6), showed that, when the subjects received an intake of indicator leucine sufficient to meet the leucine requirement of healthy subjects, the lysine requirement value for the undernourished subjects was higher than for their well-nourished counterparts. It appears unlikely, therefore, that a lower-than-normal lysine intake in undernourished men caused an adaptation with reduced lysine requirements below those for well-nourished adults. However, whether the 30-mg · kg-1 · d-1 requirement for lysine, set as a provisional recommendation by the 2001 FAO/WHO/UNU Working Group on Protein and Amino Acid Requirements (meeting in Rome, July 2001) for well-nourished adults, should be used for assessing and planning diets for populations of chronically undernourished people is a challenging policy and public health question.
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