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1 From the US Department of Agriculture, Agricultural Research Service, Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston (MR, WCH, and FJ), and the Tropical Metabolism Research Unit, Tropical Medicine Research Institute, University of the West Indies, Kingston, Jamaica (MR, AB, and TF).
2 Supported by grant RO1 HD34224–01A1 from the NIH, grants from the International Atomic Energy Agency and The Wellcome Trust, and the USDA Agricultural Research Service (Cooperative Agreement no. 58-6250-6001). 3 Address reprint requests to F Jahoor, Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, 1100 Bates Street, Houston, TX 77030-2600. E-mail: fjahoor{at}bcm.tmc.edu.
ABSTRACT
Background: Although the reduction in whole-body protein turnover and net protein loss induced by protein-energy malnutrition (PEM) has been well documented, it is unclear whether the protein-sparing mechanisms elicited by chronically inadequate intakes of dietary protein and energy are affected by the protein catabolic response to infection.
Objective: The objective of this study was to determine whether the presence of infection alters the PEM-induced reduction in whole-body protein metabolism.
Design: We determined whole-body leucine kinetics in 4 boys and 3 girls aged 6–15 mo with edematous PEM and infection 3 d after admission (study 1), when they were both infected and malnourished; 11 d after admission (study 2), when infection had resolved but they were still anthropometrically malnourished; and at recovery (study 3), when weight-for-length was at least 90% of that expected.
Results: The children had significantly less leucine flux in both study 1 and study 2 than they had in study 3. There were no significant differences in the amount of leucine released from protein breakdown or used for protein synthesis between study 1 and study 2. There were no significant differences in leucine balance or in either the amount or percentage of enteral leucine extracted by the splanchnic tissues among the 3 studies.
Conclusions: When subjects are in the fed state, severe PEM induces a marked reduction in whole-body protein synthesis and breakdown rates, and the presence of infection does not alter this adaptation and hence the overall protein balance. A corollary is that children with severe PEM do not mount a protein catabolic response to infection.
Key Words: Leucine kinetics • protein metabolism • edematous protein and energy malnutrition • stable isotope • children
INTRODUCTION
In persons with protein and energy malnutrition (PEM), there is a marked reduction in whole-body protein synthesis and breakdown rates (1), a reduction in urea excretion, and an increase in urea recycling (2–4). These adaptive responses improve the nitrogen economy and hence prolong survival in the face of reduced dietary protein and energy intakes. The protein-metabolic response to the stress of infection, on the other hand, is generally characterized by accelerated rates of turnover of endogenous protein and a concomitant increase in the loss of nitrogen, primarily from skeletal muscle (5, 6). Although this stress-induced change in protein metabolism usually is well tolerated by healthy persons, a further loss of lean body mass by persons whose nutritional status is suboptimal (eg, patients with severe PEM) increases their risk of morbidity and mortality (7, 8). Therefore, it is important to know whether the PEM-induced protein-sparing adaptations in whole-body protein kinetics are modified by the catabolic response to infection.
It is well established that infection causes a marked stimulation in protein turnover and a negative protein balance in healthy children (5), but the effect in children with severe PEM is not clear. Tomkins et al (9) found that children with severe PEM and infection had lower rates of protein turnover and nitrogen loss than did mildly undernourished children with infection; however, both protein turnover and net protein loss were nearly twice as great in children with both PEM and infection as in uninfected children with PEM. In contrast, Manary et al (10) reported that children with edematous PEM and infection had lower rates of protein turnover and urea production than did children with edematous PEM whose infection had resolved. These contrasting results may reflect differences between the 2 studies in the tracer techniques used, the type and severity of PEM studied (eg, edematous or nonedematous PEM), the prandial state of the subjects, or the prior protein intakes of the subjects.
The present study was designed to evaluate whether infection alters the PEM-induced adaptive changes in whole-body protein metabolism; it was performed in fed children who had edematous PEM and infection. The studies were performed with subjects in the fed state because we were also interested in the effect of infection on splanchnic uptake and the efficiency of the use of dietary leucine for protein synthesis. To control for the confounding effects of edema and diet, total body water (TBW) was measured and all studies were performed while the subjects were receiving a standardized diet.
SUBJECTS AND METHODS
Subjects
Seven children (4 boys and 3 girls aged 6–15 mo) were admitted to the Tropical Metabolism Research Unit, University of the West Indies, for the treatment of severe PEM. As shown in Tables 1 and 2, each subject had a deficit in body weight-for-age of >20% and clinical evidence of infection (ie, presence of 1 of the following: leukocyte count >11 x 109 cells/L; temperature on admission >37°C or <35.5°C; abnormal chest X-ray; positive blood, urine, skin, or stool culture). The diagnosis of edematous PEM, ie, kwashiorkor or marasmic kwashiorkor, was based on the Wellcome classification (11).
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TABLE 1 . Clinical characteristics of the 7 edematous malnourished children on hospital admission1
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TABLE 2 . Anthropometric measurements, body water, and fat-free mass in the 7 edematous malnourished children at each study1
This study was approved by the Medical Ethics Committee of the University Hospital of the West Indies and the Baylor Affiliates Review Board for Human Subject Research of Baylor College of Medicine. Written, informed consent was obtained from at least one parent of each child enrolled.
Treatment
During hospitalization, the children were treated according to a standard protocol that divided their treatment into phases. The acute resuscitation and maintenance phase of treatment extended from admission until the appetite returned, the edema was lost, and the infection was cleared. The mean duration of this phase was 11 d. During this period, fluid and electrolyte imbalances were corrected and infections were treated with broad-spectrum antibiotics, usually parenteral penicillin and gentamicin, plus oral metronidazole. The children were fed a resuscitative diet that was made with 61 g commercial milk powder (NAN; Nestlé SA, Vevey, Switzerland), 36 g corn oil, and 903 g water. The energy content of the feed was 2623 kJ/kg and the macronutrient composition per kg of feed was 7.6 g protein, 47 g lipid, and 31.5 g carbohydrate. The energy distribution of the feed was 75% from fat, 20% from carbohydrate, and 5% from protein. The amount offered was intended to provide 417 kJ·kg-1·d-1 and 1.2 g protein·kg-1·d-1 (12). The feed was given as a bolus every 3 h throughout the day or as a smaller bolus every 2 h when the child was having problems tolerating the feed.
The next phase in the clinical care of the children was the rapid catch-up growth phase. The children were fed an energy-dense, milk-based formula that provided 625–750 kJ·kg-1·d-1 and 3 g protein·kg-1·d-1 until the growth rate reached a plateau and the weight-for-length was 90% of expected. The high-energy feed given during the rapid catch-up growth phase was made from coconut oil and NAN. The energy content was 6053 kJ/kg, and the macronutrient composition per kg of feed was 90 g lipid, 111 g carbohydrate, and 26.63 g protein. The energy distribution of the feed was 62% from fat, 30.5% from carbohydrate, and 7.5% from protein. The children were fed every 4 h ad libitum. During this phase, the energy intake was 626–750 kJ·kg-1·d-1.
In addition, both diets were supplemented with vitamins (Tropivite; Federated Pharmaceuticals, Kingston, Jamaica) and a mineral mix prepared in the Tropical Metabolism Research Unit’s metabolic kitchen. Each child received 2 mL Tropivite solution/d, which contained 6000 IU vitamin A (palmitate), 1600 IU vitamin D (calciferol), 2 mg thiamine, 3.2 mg riboflavin, 120 mg vitamin C (ascorbic acid), 4 mg vitamin B-6 (pyridoxine), and 28 mg nicotinamide. They also received 5 mg folic acid/d and 2 mL of a mineral mix·kg-1·d-1. The mineral mix consisted of 37.28 g KCl + 50.84 MgCl2·6H2O + 3.36 g (CH3COO)2 Zn·2H2O/L H2O (BDH Chemicals, Poole, United Kingdom). During the rapid catch-up growth phase, but not during the maintenance phase, the children also received 60 mg FeSO4.
Weight and length were monitored throughout hospitalization, the former daily with an electronic balance (Model F150S; Sartorius, Göttingen, Germany) and the latter weekly with a horizontally mounted stadiometer (Holtain Ltd, Crymych, United Kingdom).
Study design
We determined whole-body and splanchnic leucine kinetics by the simultaneous infusion of 2 different isotopes of leucine at 3 times during hospitalization: 3 d after admission, when the subjects were both infected and malnourished but clinically stable, as indicated by blood pressure, pulse, and respiration rates; 11 d after admission, when the subjects were still severely malnourished (anthropometrically) but no longer infected (ie, all clinical features of the infective episode had resolved), had lost edema, and had better affect and appetite; and 71 d after admission, when the rate of catch-up growth had started to plateau and weight-for-length was at least 90% of that expected. The TBW was also measured at each time by the dilution of deuterium oxide.
The resuscitative diet was fed during all 3 studies. The subjects had been on this diet for 3 d at study 1, for 11 d at study 2, and for 3 d at study 3. To ensure that the same amounts of energy and protein were given throughout the course of the isotope infusions, 42% of each child’s daily intake was given by continuous intragastric (IG) infusion over the 10-h period of the isotope-infusion protocol (Figure 1).
FIGURE 1. . Diagram of the isotope-infusion protocol. IV, intravenous; IG, intragastric.
Infusion protocol
A diagrammatic illustration of the infusion protocol is shown in Figure 1. At 0700, a nasogastric tube was inserted into the child’s stomach and a Flexiflo Magna-Port Y-Port connector (Ross Products Division, Abbott Laboratories, Columbus, OH) was attached to the proximal end. Approximately 42% of the child’s daily intake was then given over the next 10 h by continuous IG infusion into one limb of the Y-port by use of an enteral infusion pump (Flexiflo companion enteral nutrition pump, Ross Products Division, Abbott Laboratories). Two intravenous (IV) access sites were established in opposite arms by the insertion of 22-gauge or 24-gauge catheters after preparation of the access sites with a topical anesthetic (EMLA cream; Astra Pharmaceuticals Ltd, Langley, United Kingdom). One IV catheter was used for the infusion of the labeled substrates and the other for blood sampling.
Sterile solutions of [2H3]leucine, [1-13C]leucine, and NaH13CO3 (99.9%; Cambridge Isotope Laboratories, Woburn, MA) were prepared in 9 g NaCl/L. At 0900, after a 2-mL blood sample was drawn and baseline breath samples were collected, a priming dose of NaH13CO3 (3.6 µmol/kg at studies 1 and 2 and 5.7 µmol/kg at study 3) was administered intravenously; this was followed immediately by continuous infusion of the NaH13CO3 solution at a rate of 5.9 µmol·kg-1·h-1 for 2 h. Simultaneously, 100 mg deuterium oxide/kg (99.9%; Cambridge Isotope Laboratories) was given intragastrically as a bolus through the other port of the nasogastric tube. This was followed by a 2-mL flush of 9 g NaCl/L, immediately after which an 8-h continuous IG infusion of [2H3]leucine was begun at a dose rate of 26.7 µmol·kg-1·h-1. At 1100, the NaH13CO3 infusion was discontinued and a primed continuous IV infusion (prime = 6 µmol/kg, infusion rate = 6 µmol·kg-1·h-1) of [1-13C]leucine was started; it was maintained for 6 h.
During the infusions, additional 1-mL blood samples were drawn at 1200, 1300, and 1400 for the measurement of deuterium content and at 1600, 1615, 1630, 1645, and 1700 for the measurement of -ketoisocaproic acid (KIC) and leucine enrichments. Before each blood sample was taken, the arm was warmed for 10 min by the application of a latex glove containing water heated to 40°C. For each aliquot of blood removed, an equal volume of 9 g NaCl/L was infused. At 15-min intervals from 1000 to 1100 and from 1600 to 1700, breath samples were taken into a rebreathing bag connected to a pediatric facemask with a valve (valve #1500; Rudolph, Kansas City, MO). The facemask was applied snugly to the face of the child, covering the nose and the mouth, for 1 min. Duplicate samples were transferred to evacuated tubes attached to the bag.
Sample analyses
The blood samples were drawn into chilled tubes containing Na2EDTA and a cocktail of sodium azide, thimerosal, and soybean trypsin inhibitor. The samples were centrifuged immediately at 1000 x g for 10 min at 4°C, and the plasma was removed and stored immediately at –70°C for later analysis.
The 2H2 content of plasma water was determined by reducing the water extracted from 10 µL of plasma with zinc in quartz vessels and determining the 2H2 abundance of the resulting hydrogen gas by gas isotope ratio–mass spectrometry (Delta-E; Finnigan MAT, San Jose, CA). Plasma leucine was isolated by ion exchange (Dowex 200x) chromatography and converted to the n-propyl ester heptafluorobutyramide derivative. The tracer-to-tracee ratio was measured by negative chemical ionization gas chromatography–mass spectrometry with a Hewlett Packard 5890 quadruple mass spectrometer (Palo Alto, CA) and selective monitoring of ions at mass-to-charge ratios (m/z) 349–352. The plasma KIC tracer-to-tracee ratio was measured by negative chemical ionization gas chromatography–mass spectrometry of its pentafluorobenzyl derivative and monitoring of ions at m/z 129–132. The breath samples were analyzed in duplicate for 13C abundance in carbon dioxide by gas isotope ratio–mass spectrometry (Europa Scientific, Crewe, United Kingdom) and monitoring of ions at m/z 44 and 45.
Calculations
Carbon dioxide flux (RaCO2) was calculated from the steady state equation:
RESULTS
One child was HIV positive, but data obtained for this child were not different from data obtained for others; thus, for all analyses, these data were included. All subjects were anemic and hypoalbuminemic and had evidence of infection on admission (Table 1). The mean age of the children on admission was 11.8 ± 2.1 mo (Table 2). They were severely malnourished with a mean body weight of 5.8 ± 0.2 kg and mean weight-for-age and weight-for-length of 62 ± 3.5% and 82 ± 2% of that expected, respectively (Table 2). According to the Wellcome classification (11), 3 children had marasmic kwashiorkor and 4 had kwashiorkor.
At study 2, after 11 d of antibiotic therapy, all the subjects’ infections had cleared as determined by normalization of temperature and respiration and pulse rates and resolution of clinical features of the infective episode (eg, cessation of diarrhea). There were no significant differences in the mean body weight, weight-for-age, or weight-for-length between study 2 and study 1. At study 3, when the subjects had recovered, the mean weight-for-age and weight-for-length were 76.8 ± 4.5% and 99 ± 3.3% of that expected, respectively (Table 2).
Total body water, fat-free mass, and catch-up growth
At study 2, TBW tended to be 0.3 L (9%) less than at study 1, which reflected the loss of edema (Table 2). However, this decrease in body water was not significant. At study 3, when the subjects had fully recovered, TBW was significantly greater (P < 0.05) than at study 2. The water content of body tissue tended to be highest (68%) at study 1, when the children were edematous, and lowest (58%) at study 3, when they were fully recovered. This difference, however, was not significant.
FFM was estimated at studies 2 and 3 (not at study 1 because of the presence of edema) by use of the 2-compartment model and the age- or sex-specific hydration constants of Fomon et al (18). As expected, FFM at study 3 was significantly higher (P < 0.03) than that at study 2 (Table 2). The proportion of body weight composed of FFM decreased from 85% at study 2 to 81% at study 3. As shown in Figure 2, the rate of tissue deposition between study 2 and study 3, ie, during the catch-up growth phase, was 45 ± 8.8 g/d, of which 60% was fat-free tissue.
FIGURE 2. . Rate of tissue deposition during rapid catch-up growth in 7 children recovering from edematous protein-energy malnutrition. Values are means ± SEM.
Leucine kinetics
As shown in Figures 3 and 4, the ratios of both M+1 to M+0 and M+3 to M+0 isotopomers (normalized for infusion rate of the tracer) of plasma-free leucine and KIC reached a plateau during the final hour of the [1-13C]leucine and [2H3]leucine infusions, respectively. In all 3 studies, the steady state ratios of M+3 to M+0 isotopomers of leucine were significantly lower (P < 0.01) than the corresponding ratios of M+1 to M+0 isotopomers, which indicated appreciable uptake and metabolism of the enterally administered [2H3]leucine. In the 3 studies, the M+3 to M+0 ratios of leucine were, respectively, 71%, 68%, and 66% of the values of the M+1 to M+0 ratios at the plateau. With the IV [1-13C]leucine tracer, the KIC tracer-to-tracee ratio was always lower than that of leucine, ranging from 88% of the latter in study 1 to 92% in study 3 (Figure 5). In contrast, with the IG [2H3]leucine tracer, the KIC tracer-to-tracee ratio was always higher than that of leucine, ranging from 105% of the latter in study 2 to 119% in study 1 (Figure 5). In each study, the ratio of KIC to leucine (tracer-to-tracee ratio) obtained with the IG tracer was significantly higher (P < 0.05) than the ratio obtained with the IV tracer (Figure 5), which suggested that an appreciable amount of the enteral leucine tracer extracted by splanchnic tissues was being transaminated and released into the circulation as [2H3]KIC.
FIGURE 3. . Steady state tracer-to-tracee molar ratios (mol% above baseline) of plasma leucine (Leu) during an intravenous infusion of [13C]leucine (M+1 isotopomer, solid symbols) and an intragastric infusion of [2H3]leucine (M+3 isotopomer, open symbols) in 7 edematous malnourished children on postadmission days 3 (study 1: , ), 11 (study 2: •, ), and 71 (study 3: , ). The values for the M+3 isotopomer were normalized to the infusion rate of the intravenous tracer. Mean plateau tracer-to-tracee molar ratios are shown on the right side of the figure. Values are means ± SEM. *Significantly different (P < 0.01) from the corresponding M+3 value by repeated-measures ANOVA.
FIGURE 4. . Steady state tracer-to-tracee molar ratios (mol% above baseline) of plasma -ketoisocaproic acid (KIC) during an intravenous infusion of [13C]leucine (solid symbols) and an intragastric infusion of [2H3]leucine (open symbols) in 7 edematous malnourished children on postadmission days 3 (study 1: , ), 11 (study 2: •, ), and 71 (study 3: , ). The values for the M+3 isotopomer were normalized to the infusion rate of the intravenous tracer. Mean plateau tracer-to-tracee molar ratios are shown on the right side of the figure. Values are means ± SEMs. *Significantly different (P < 0.01) from the corresponding M+3 value by repeated-measures ANOVA.
FIGURE 5. . Ratio of steady state tracer-to-tracee ratios of plasma -ketoisocaproic acid (KIC) and leucine (Leu) during an intragastric infusion of [2H3]leucine and an intravenous infusion of [13C]leucine in 7 edematous malnourished children on postadmission days 3 (study 1), 11 (study 2), and 71 (study 3). Values are means ± SEMs. *Significantly different (P < 0.05) from the corresponding ratio of M+1 to M+1 by repeated-measures ANOVA.
As shown in Table 3, there were no significant differences in any measures of whole-body leucine kinetics between study 1, when the children were malnourished and infected, and study 2, when they were still malnourished but clear of infection. However, leucine flux to protein synthesis and leucine balance tended to be higher at study 1 (by 10% and 17%, respectively) than at study 2. At study 3, when the children had recovered, leucine flux from protein breakdown and leucine flux to protein synthesis were significantly greater (P < 0.01) than they were at studies 1 and 2. Similarly, total leucine flux was significantly greater at study 3 than at study 2. Total leucine flux also was 27% greater at study 3 than at study 1, but this difference was not significant. Although there were no significant differences in leucine oxidation and balance among the 3 studies, leucine oxidation tended to be 30% higher at studies 1 and 2 than at study 3. As a consequence, leucine balance tended to be better at study 3, both in absolute terms and as a percentage of either total leucine intake or dietary leucine intake. There were no significant differences in any measures of splanchnic leucine kinetics among the 3 studies (Table 4). The proportion of enteral leucine intake extracted by the splanchnic tissues was 30% at all 3 studies.
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TABLE 3 . Whole-body leucine kinetics in 7 edematous malnourished children on postadmission days 3 (study 1), 11 (study 2), and 71 (study 3)1
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TABLE 4 . Splanchnic leucine kinetics in 7 edematous malnourished children on postadmission days 3 (study 1), 11 (study 2), and 71 (study 3)1
DISCUSSION
The aim of this study was to determine whether superimposed infection in children with PEM alters the PEM-induced reduction in whole-body protein metabolism. We measured leucine kinetics in young children when they were both malnourished and infected, when their infections had cleared but they were anthropometrically still malnourished, and when they had recovered. The results show that severe PEM is associated with a marked reduction in rates of whole-body protein synthesis and breakdown and that the presence of concurrent infection does not have a significant impact on this adaptation and hence on overall protein balance in the fed state.
The finding of slower leucine fluxes but positive leucine balances at studies 1 and 2, when the children were severely malnourished, than at study 3, when the children had recovered, corroborates earlier findings that PEM elicits a marked suppression of protein turnover that facilitates a positive protein balance (1, 9). It has been proposed that this reduction in protein turnover is a necessary adaptation to improve the nitrogen and energy economies and hence to prolong survival in the face of reduced intakes of dietary protein and energy (19, 20). For example, Golden et al (1) showed that both whole-body protein synthesis and breakdown in children with severe PEM who were fed a maintenance diet of 397 kJ and 0.6 g protein·kg-1·d-1 were only 60% as fast in the malnourished state as they were at recovery. Furthermore, whereas this marginal protein intake was sufficient to facilitate a positive nitrogen balance and growth when the children were malnourished and their protein turnover was slower, it was insufficient to support nitrogen balance and growth once the children had recovered. Tomkins et al (9) also showed that protein synthesis and breakdown rates were lower in severely malnourished children with concurrent infection than in mildly undernourished children (75–90% weight-for-age) with measles. Again, the lower protein turnover rate in the group with severe PEM was associated with a positive nitrogen balance, which supported the idea that the lower protein turnover rate seen in subjects with severe PEM is an adaptation that improves the efficiency of use of dietary protein.
We found leucine kinetics to be similar at study 1 and study 2, when the children were both severely malnourished and infected and when they were still anthropometrically malnourished but free of signs and symptoms of infection, respectively, which suggests that the protein-sparing mechanisms elicited by severe PEM are not significantly altered by the presence of infections. In other words, children with severe PEM do not appear to mount a protein catabolic response to infection. At study 2, although the children were still anthropometrically malnourished, the loss of edema, the normalization of vital signs, and the improved appetite implied that their functional capacities were already improving. Yet the protein turnover rate remained lower at study 2 than at study 3, which indicated that these protein-sparing metabolic adaptations to PEM persisted even though the metabolic capacity of the children had begun to improve.
In contrast to the present findings and those of Golden et al (1) and Tomkins et al (9), Manary et al (10), using a combined [13C]leucine and [15N]urea oral-tracer method to measure whole-body protein kinetics, found that fasted children with edematous PEM and acute lower respiratory tract infections had lower rates of protein breakdown and protein synthesis than did a similar group of malnourished children whose infections had resolved. This suggested that the presence of infection induced a further decrease in protein turnover and hence nitrogen loss.
It is difficult to explain why Manary et al (10) observed lower rates of protein synthesis and breakdown in infected malnourished children than in uninfected malnourished children, whereas Golden et al (1), Tomkins et al (9), and we found either no difference between the two groups or somewhat higher rates in infected children than in uninfected malnourished children. Possible explanations include the use of different tracer techniques, the effect of feeding as opposed to fasting on protein kinetics, the difference in the ages of the study populations, and the different dietary protein and energy intakes of the 2 groups. For example, Manary et al (10) studied 2 groups of subjects: some were edematous and some were losing edema. Both groups were studied in the fasted state using orally administered [13C]leucine tracer. Further, leucine flux was calculated from plasma leucine enrichments, and leucine oxidation, an index of protein loss, was estimated indirectly from urea production. Perhaps more important is the fact that the protein and energy intakes of the 2 groups were markedly different.
Whereas our subjects received the same intakes of energy and protein (417 kJ·kg-1·d-1 and 1.2 g·kg-1·d-1, respectively) before and during studies 1 and 2, the 2 groups of subjects studied by Manary et al (10) received markedly different intakes. In their studies, the infected and malnourished group received 336 kJ·kg-1·d-1 and 1.2 g protein·kg-1·d-1, and the noninfected and malnourished group received 712 kJ·kg-1·d-1 and 5.8 g protein·kg-1·d-1. On the basis of considerable evidence that prior protein intake will affect protein turnover in both the fed and fasted states (20–23), we feel that this 4-fold difference in dietary protein intake is the most likely explanation for the higher rate of protein turnover reported by Manary et al (10) in infection-free children than in infected children with PEM. For example, Jackson et al (20) reported a 25% lower rate of protein synthesis in young children consuming 0.7 g protein·kg-1·d-1 than in those consuming 1.7 g·kg-1·d-1, and Thorpe et al (23) reported higher phenylalanine flux and oxidation in adult subjects who previously consumed 2 g protein·kg-1·d-1 than in those who consumed 1.4 g·kg-1·d-1. The same observation has been made in subjects in the fasted state by Pacy et al (21), who reported faster leucine and phenylalanine fluxes in adult subjects when they were on high protein intakes (1.59 g·kg-1·d-1) than when they were on marginal protein intakes (0.77 g-1·kg-1·d-1), and by Motil et al (22), who reported that increasing protein intake from a marginal to an adequate and then to a surfeit level elicited parallel increases in leucine flux in both the fed and fasted states. Together, these findings indicate that systems involved in the metabolism of protein are up-regulated during adaptation to a higher protein intake, and this supports our contention that the most likely explanation for the markedly higher leucine kinetics observed by Manary et al (10) in infection-free children than in infected malnourished children is the 4-fold dietary protein intake of the infection-free group.
Unlike Manary et al (10), Tomkins et al (9), using a different tracer approach, ie, the [15N]glycine end product method, reported a higher protein turnover rate and decreased net protein synthesis in infected children than in uninfected malnourished children, which suggested that the protein-sparing adaptations to chronic inadequate food intakes were negatively affected by the presence of the infections. In our present study, although there was a trend toward modestly higher protein turnover rates in the infected state, as indicated by an 5% increase in leucine flux and an 10% increase in leucine used for protein synthesis in study 1 over that in study 2, leucine balance, an index of net protein synthesis, was not lower. Possible explanations for the greater magnitude of the increase in protein turnover observed by Tompkins et al may lie in the severity of the infections in their group of study subjects as well as in differences in study design.
Both the magnitude and duration of the protein catabolic response to infection or injury are directly related to the severity and nature of the infection or injury (24, 25). The fact that 4 of the 6 infected children studied by Tomkins et al (9) had pneumonia, whereas only 1 of our subjects did, suggests that the children Tomkins et al studied may have had a greater infective stress and hence a greater protein catabolic response. Further, those investigators employed a cross-sectional study design, whereas we used a longitudinal design. Thus, the observed differences in protein kinetics reported by Tomkins et al (9) may merely reflect the impact of unmeasured differences in the characteristics of the 2 groups.
ACKNOWLEDGMENTS
We are grateful to the physicians and nursing staff of the Tropical Metabolism Research Unit for their care of the children and to Hyacinth Gallimore, Bentley Chambers, Sharon Howell, Margaret Frazer, and Melanie Del Rosario for their excellent work and support in the conduct of the studies and analysis of the samples.
REFERENCES