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Biomedical subjects

V R Young

Publications and source records attributed to V R Young.

At least 55 records · Page 3Linked to original sources
PubMed1998

Methionine and cysteine kinetics at different intakes of methionine and cysteine in elderly men and women.

Earlier nitrogen balance studies led to the conclusion that requirements for methionine in older individuals are much higher than those in younger adults. Hence, we examined the kinetics of whole-body methionine, cysteine, and leucine metabolism postabsorptively using a continuous intravenous infusion of L-[C2H3, 1-(13)C]methionine, L-[2H3]leucine, and [3,3-2H2]cysteine in 12 elderly men (n = 5) and women (n = 7) given as a 3-h infusion after a 12-h fast (study 1) and in 8 elderly men (n = 4) and women (n = 4) as an 8-h infusion according to a 3-h fasted, 5-h fed protocol (study 2) for 6 d. Before tracer infusion, each of 3 L-amino acid diets supplying the following nominal, but known, amounts (mg x kg(-1) x d(-1)) of methionine and cysteine, respectively, were used in study 2: diet 1: 13 and 0; diet 3: 6.5 and 5.2; and diet 5: 6.5 and 21. Studies 1 and 2 gave values for plasma methionine flux that agreed with the leucine flux data, which, in turn, also appeared to be comparable with findings in healthy younger adults. In study 2, methionine oxidation rates were the same across all diets in the fasted state and the same with diets 1 and 3 in the fed state but lower with diet 5, suggesting a modest sparing effect of dietary cystine on methionine oxidation. Estimated daily methionine balance was at equilibrium for diet 1 and negative (significantly different from zero, P<0.05) with diets 3 and 5. The results were evaluated against our previous findings in younger adults.

Age Factors↗
PubMed1998

Twenty-four-hour intravenous and oral tracer studies with L-[1-13C]-2-aminoadipic acid and L-[1-13C]lysine as tracers at generous nitrogen and lysine intakes in healthy adults.

BACKGROUND: This is a continuation of investigations of the relations between amino acid kinetics and amino acid dietary requirements in healthy adults. OBJECTIVE: The aim was to investigate the 24-h pattern and rate of the metabolism of an L-[1-13C]-2-aminoadipic acid ([13C]AAA) tracer and of whole-body L-[1-13C]lysine ([13C]lysine) oxidation and balance in healthy, young adults receiving a generous intake of lysine. DESIGN: Thirteen healthy adults were given an adequate, L-amino acid-based diet supplying 77 mg lysine x kg(-1) x d(-1) for 6 d before the tracer studies. Two subjects received [13C]AAA intravenously and 2 received it orally; 3 subjects received [13C]lysine intravenously and 6 received it orally. We measured 13CO2 output, plasma [13C]AAA and [13C]lysine enrichment, and urinary [13C]AAA. RESULTS: [13C]AAA oxidation was estimated to be higher after the orally administered than after the intravenously administer tracer; plasma [13C]AAA was similar to urinary [13C]AAA. Whole-body lysine oxidation showed a rhythm that was induced by meal feeding. The intravenous [13C]lysine tracer gave mean estimates of lysine balances (lysine intake minus oxidation) that apparently were too low (-15.7 mg x kg(-1) x d(-1)) or too high (16.6 mg x kg(-1) x d(-1), P < 0.05 from zero balance) on the basis of urinary [13C]AAA or plasma [13C]lysine estimates of oxidation, respectively. For the orally administered tracer and plasma [13C]lysine enrichment, the mean balance was slightly positive (8.7 mg x kg(-1) x d(-1), P < 0.05 from zero). CONCLUSIONS: Use of urinary [13C]AAA as an index of the enrichment of the precursor pool did not appear to significantly improve the estimate of the fasting and feeding components of daily lysine balance. For estimates of daily, whole-body lysine oxidation, we propose use of plasma [13C]lysine with a 24-h, orally administered tracer protocol.

2-Aminoadipic Acid↗
PubMed1998

The 24-h whole body leucine and urea kinetics at normal and high protein intakes with exercise in healthy adults.

In healthy adult men adapted to a diet/exercise regimen for 6 days, the effects of small, frequent meals supplying daily protein intakes of 1 (n = 8) or 2.5 g . kg-1 . day-1 (n = 6) on leucine oxidation, urea production, and whole body protein synthesis (PS) and degradation (PD) have been compared with the use of a 24-h continuous intravenous [13C]leucine and [15N,15N]urea infusion protocol. Two 90-min periods of exercise (approximately 50% maximal O2 consumption) were included during the fasting and the fed periods of the 24-h day. Subjects were determined to be at approximate energy, nitrogen, and leucine balances on both diets. Increased protein intake raised the urea production rate; the absolute rate of urea hydrolysis was the same on both diets. When the first-pass splanchnic uptake of leucine was taken to be 25% of intake, PS was stimulated by feeding (after an overnight fast) at both protein intake levels (P < 0.05 and P < 0.01), whereas PD declined significantly (P < 0.01) at both protein levels. Protein gain at a high protein intake appears to be the result of both a stimulation of PS and a marked decline in PD, whereas at a less generous intake, the gain appears to be a result of a fall in PD with a less evident change in PS. Exercise moderately decreased PS during and/or immediately after exercise at each protein level, and there was a postexercise-induced increase (P < 0.01) in PD, which was more dramatic when feeding was at the higher protein intake level.

Adult↗
PubMed1997

Mechanisms permitting nephrotic patients to achieve nitrogen equilibrium with a protein-restricted diet.

UNLABELLED: Clinical experience suggests nephrotic patients are at risk for malnutrition. To determine if nephrotic patients can adapt successfully to a protein-restricted diet, nephrotic (glomerular filtration rate, 52+/-15 ml/min; urinary protein [Uprot.], 7.2+/-2.2 grams/d) and control subjects completed a crossover comparison of diets providing 0.8 or 1.6 grams protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day. Nitrogen balance (BN) was determined and whole body protein turnover measured during fasting and feeding using intravenous -[1-13C]leucine and intragastric -[5,5, 5- 2H3]leucine. BN was positive in both nephrotic and control subjects consuming either diet and rates of whole-body protein synthesis, protein degradation, and leucine oxidation did not differ between groups. In both nephrotic and control subjects anabolism was due to a suppression of whole-body protein degradation and stimulation of protein synthesis during feeding. The principal compensatory response to dietary protein restriction was a decrease in amino acid oxidation and this response was the same in both groups. With the low protein diet leucine oxidation rates during feeding correlated inversely with Uprot. losses (r = -0.83; P < 0. 05). CONCLUSIONS: (a) a diet providing 0.8 gram protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day maintains BN in nephrotic patients; (b) nephrotic patients activate normal anabolic responses to dietary protein restriction (suppression of amino acid oxidation) and feeding (stimulation of protein synthesis and inhibition of protein degradation); (c) the inverse correlation between leucine oxidation and Uprot. losses suggests that proteinuria is a stimulus to conserve dietary essential amino acids.

Adult↗
PubMed1997

Continuous 24-h L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine oral-tracer studies at an intermediate phenylalanine intake to estimate requirements in adults.

The daily rates of whole-body phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were determined in young men (n = 10) receiving [13C]phenylalanine and [2H2]tyrosine via primed constant oral infusion (four also received simultaneously [2H4]tyrosine and [2H3]leucine via primed constant intravenous infusions) continuously for 24 h (first 12 h fast and then 12 h fed). The subjects were given a diet supplying a proposed requirement phenylalanine intake (six subjects: 39 mg phenylalanine.kg-1.d-1 without tyrosine; four subjects: 36 mg phenylalanine plus 6.8 mg tyrosine), based on an otherwise adequate L-amino acid mixture for 6 d before the tracer study. Our hypothesis was that the subjects would be in approximate body phenylalanine equilibrium at these intakes. Estimates of the daily rate of phe-ox were 26.9 +/- 7.5 mg.kg-1.d-1 (17.2 +/- 5.2 and 9.7 +/- 3.2 mg.kg-1.d-1 during the 12-h fast and fed periods, respectively), and for phe-OH they were 32.1 +/- 11.9 mg.kg-1.d-1 (21.7 +/- 10.5 and 10.4 +/- 2.5 mg.kg-1.d-1 during the 12-h fast and fed periods, respectively). The daily phenylalanine balance was approximately neutral (P > 0.05) when based on phe-ox or phe-OH (+4.73 +/- 7.34 and -0.41 +/- 12.6 mg.kg-1.d-1, respectively). In comparison with recent, comparable 24-h tracer studies at deficient (22 mg.kg-1.d-1) and generous (100 mg.kg-1.d-1) phenylalanine intakes, these results support the hypothesis that a phenylalanine intake of 39 mg.kg-1.d-1 (without significant tyrosine) approximates the mean requirement in healthy adults. This contrasts with the upper requirement value of 14 mg.kg-1.d-1 for the total of the aromatic amino acids proposed in 1985 by FAO/WHO/UNU.

Administration, Oral↗
PubMed1997

Effect of cystine intake on methionine kinetics and oxidation determined with oral tracers of methionine and cysteine in healthy adults.

There is evidence based on nitrogen balance that dietary cystine spares, from approximately 16% to 89%, the total methionine requirement. In a previous study we did not detect, by tracer techniques, a sparing effect of cystine when the diet provided methionine at a limiting intake (requirement level: 13 mg.kg-1.d-1). One reason could be that we used an intravenous infusion of the tracer, which may not, therefore, have labeled the carbon dioxide derived from the splanchnic oxidation of dietary methionine. The aim of this study was to compare methionine metabolism and oxidation in eight healthy adults given for 6 d each of three different diets: 13 mg (87.0 mumol) methionine.kg-1.d-1 and no cystine (diet A); 5 mg (33.5 mumol) methionine.kg-1.d-1 and no cystine (diet B); and 5 mg (33.5 mumol) methionine.kg-1.d-1 and 6.5 mg (52.4 mumol) cystine.kg-1.d-1 (diet C). On day 7, tracers ([1-13C, methyl-2H3]methionine and [2H2]cysteine) were administered orally at 30-min intervals for 8 h. Blood and breath samples were obtained for analysis during 3-h fasting and consecutive 5-h feeding periods. During fasting, methionine oxidation and methionine methyl (Qm) and carboxyl (Qc) fluxes and plasma concentrations were not affected by the amount of sulfur amino acids in the three diets. In the fed state methionine oxidation was significantly lower during diets B (3.0 +/- 0.5 mumol.kg-1.h-1) and C (2.8 +/- 0.6 mumol.kg-1.h-1) than during diet A (4.1 +/- 0.9 mumol.kg-1.h-1); there were no significant differences between diets B and C. Qm and Qc decreased with decreased methionine intake but no effect was observed by adding cystine. Cysteine flux (Qcys) was not affected by diet composition but it was lower during feeding than during fasting. In conclusion, replacing approximately 60% of the total requirement for methionine with cystine over a short diet period did not result in a detectable sparing of methionine oxidation.

Adult↗
PubMed1997

Moderate exercise at energy balance does not affect 24-h leucine oxidation or nitrogen retention in healthy men.

Short-term metabolic experiments have revealed that physical exercise increases the oxidation of leucine, which has been interpreted to indicate an increased requirement for dietary protein in physically active subjects. Because it may be inaccurate to extrapolate measurements of amino acid oxidation made over a few hours to the entire day, we have carried out a continuous 24-h intravenous [1-13C]leucine/[15N]urea tracer study in eight healthy adult men. Their diet supplied 1 g protein.kg-1.day-1, and exercise (mean maximal O2 consumption 46%) was for 90 min during the 12-h fast and 12-h fed periods of the day. Subjects were adapted to the diet and exercise regimen for 6 days. Then, on day 7, they were dressed in the University of Uppsala energy metabolic unit's direct calorimeter suit, were connected to an open-hood indirect calorimeter, and received the tracers. Exercise increased leucine oxidation by approximately 50 and 30% over preexercise rates for fast and fed periods, respectively. This increase amounted to approximately 4-7% of daily leucine oxidation. Subjects remained in body leucine equilibrium (balance -4.6 +/- 10.5 mg.kg-1.day-1; -3.6 +/- 8.3% of intake; P = not significant from zero balance). Therefore, moderate exercise did not cause a significant deterioration in leucine homeostasis at a protein intake of 1 g.kg-1.day-1. These findings underscore the importance of carrying out precise, continuous, 24-h measurements of whole body leucine kinetics; this model should be of value in studies concerning the quantitative interactions among physical exercise, energy/protein metabolism, and diet in humans.

Adult↗
PubMed1996

Evidence for a recommended dietary allowance for vitamin C from pharmacokinetics: a comment and analysis.

The current recommended dietary allowance (RDA) for vitamin C, as proposed by the Food and Nutrition Board/National Research Council in 1980 and reconfirmed in 1989, is 60 mg daily for nonsmoking adult males. Levine et al. [Levine, M., Conry-Cantilena, C., Wang, Y., Welch, R. W., Washko, P. W., et al. (1996) Proc. Natl. Acad. Sci. USA 93, 3704-3709], based on a study of vitamin C pharmacokinetics in seven healthy men, have now proposed that the RDA should be increased to 200 mg daily. I have examined, in brief, the experimental and conceptual bases for this new recommendation and its implications for public health and nutrition policy and programs. Using, for illustrative purposes only, data extracted from each of two recent dietary surveys of noninstitutionalized adult males living in households in the Netherlands and the United States, it is predicted that the prevalence of intakes inadequate to meet the individual's own requirement would be about 96% or 84%, respectively, if the criteria of adequacy used for derivation of the 200 mg RDA are accepted. Depending upon the particular average requirement value for ascorbic acid that might be derived from their data, the proposal by Levine et al. would mean a desirable increase in mean intakes in these two populations by as much about 2-to 3-fold. Hence, before an action of this kind is to be recommended, an answer must be sought to the question whether current experimental data including the criteria selected (saturation kinetics) are adequate to establish a new set of requirements for vitamin C, which then carry such profound policy implications. This will require critical assessment of all of the available evidence emerging from laboratory, clinical, and epidemiological studies to determine whether it provides a sufficient rationale for accepting criteria of vitamin C adequacy such as those proposed by Levine et al. and the requirement estimates so derived.

Adult↗
PubMed1996

Whole body nitric oxide synthesis in healthy men determined from [15N] arginine-to-[15N]citrulline labeling.

The rates of whole body nitric oxide (NO) synthesis, plasma arginine flux, and de novo arginine synthesis and their relationships to urea production, were examined in a total of seven healthy adults receiving an L-amino acid diet for 6 days. NO synthesis was estimated by the rate of conversion of the [15N] guanidino nitrogen of arginine to plasma [15N] ureido citrulline and compared with that based on urinary nitrite (NO2-)/nitrate (NO3-) excretion. Six subjects received on dietary day 7, a 24-hr (12-hr fed/12-hr fasted) primed, constant, intravenous infusion of L-[guanidino-15N2]arginine and [13C]urea. A similar investigation was repeated with three of these subjects, plus an additional subject, in which they received L-[ureido-13C]citrulline, to determine plasma citrulline fluxes. The estimated rates (mean +/- SD) of NO synthesis over a period of 24 hr averaged 0.96 +/- 0.1 mumol .kg-1.hr-1 and 0.95 +/- 0.1 mumol.kg-1.hr-1, for the [15N]citrulline and the nitrite/nitrate methods, respectively. About 15% of the plasma arginine turnover was associated with urea formation and 1.2% with NO formation. De novo arginine synthesis averaged 9.2 +/- 1.4 mumol. kg-1.hr-1, indicating that approximately 11% of the plasma arginine flux originates via conversion of plasma citrulline to arginine. Thus, the fraction of the plasma arginine flux associated with NO and also urea synthesis in healthy humans is small, although the plasma arginine compartment serves as a significant precursor pool (54%) for whole body NO formation. This tracer model should be useful for exploring these metabolic relationships in vivo, under specific pathophysiologic states where the L-arginine-NO pathway might be altered.

Adult↗
PubMed1996

Measurement of muscle protein synthesis by positron emission tomography with L-[methyl-11C]methionine.

Positron emission tomography (PET) with L-[methyl-11C]methionine was explored as an in vivo, noninvasive, quantitative method for measuring the protein synthesis rate (PSR) in paraspinal and hind limb muscles of anesthetized dogs. Approximately 25 mCi (1 Ci = 37 GBq) of L-[methyl-11C]methionine was injected intravenously, and serial images and arterial blood samples were acquired over 90 min. Data analysis was performed by fitting tissue- and metabolite-corrected arterial blood time-activity curves to a three-compartment model and assuming insignificant transamination and transmethylation in this tissue. PSR was calculated from fitted parameter values and plasma methionine concentrations. PSRs measured by PET were compared with arterio-venous (A-V) difference measurements across the hind limb during primed constant infusion (5-6 h) of L-[1-13C, methyl-2H3]methionine. Results of PET measurements demonstrated similar PSRs for paraspinal and hind limb muscles: 0.172 +/- 0.062 vs. 0.208 +/- 0.048 nmol-1.min-1.(g of muscle)-1 (P = not significant). PSR determined by the stable isotope technique was 0.27 +/- 0.050 nmol-1.min-1.(g of leg tissue)-1 (P < 0.07 from PET) and indicated that the contribution of transmethylation to total hind limb methionine utilization was approximately 10%. High levels of L-[methyl-11C]methionine utilization by bone marrow were observed. We conclude that muscle PSR can be measured in vivo by PET and that this approach offers promise for application in human metabolic studies.

Animals↗
PubMed1996

Twenty-four-hour intravenous and oral tracer studies with L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine at a tyrosine-free, generous phenylalanine intake in adults.

The daily rates of whole-body phenylalanine oxidation and hydroxylation were determined in young men receiving [1-13C]phenylalanine and [2H2]tyrosine via primed, constant intravenous (n=3) or oral (n=5) infusion for 24 consecutive hours (12-h fast followed by 12-h fed period), and given a generous phenylalanine (100 mg.kg-1.d-1), tyrosine-free, but otherwise adequate L-amino acid-based diet for 6 d before the tracer study. Our hypothesis was that subjects would be in whole-body phenylalanine equilibrium. Estimates of the daily rates of phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were significantly higher for the subjects receiving the oral compared with intravenous tracer (P<0.01 for both comparisons), with the estimates of phe-ox obtained with oral tracer during the 12-h fast period being close to those predicted from similar 24-h leucine kinetic studies. The precision of the agreement between the measured 24-h rates of phe-ox and phe-OH compared with the predicted daily rates by extrapolation from the last hour of the 12-h fast and fifth hour of the fed period was poor. From the 24-h data, daily phenylalanine balances were estimated to be positive for both the intravenous and oral tracer protocols, although it was less positive for the oral tracer group. These results imply that the [13C]phenylalanine probe underestimated whole-body irreversible loss of phenylalanine, and suggest that daily phenylalanine balance in earlier 24-h phenylalanine-tyrosine tracer studies at low phenylalanine intakes may have been overestimated. Studies involving [13C]tyrosine as tracer will be required to further assess whole-body aromatic amino acid balance.

Administration, Oral↗
PubMed1996

Kinetics of plasma arginine and leucine in pediatric burn patients.

The dynamic status of whole-body arginine and leucine was investigated in eight severely burned (mean 55% of body surface area) pediatric patients (mean age 5.3 y) at a mean of 16 d after their initial injury. Plasma amino acid kinetics were estimated by using primed constant intravenous infusions of L-[13C-guanidino]arginine and L-[I-13C]leucine given for 4 h. Each patient was studied twice within 2 d. The patients were studied either in a "basal" state, which involved removal of amino acids from the total parenteral nutrition (TPN) solution for 8 h before the tracer study, or while receiving complete TPN. Nitrogen intake was 0.58 +/- 0.08 g.kg-1.d-1 with nonprotein energy intake equivalent to 197 +/- 29 kJ.kg-1.d-1. Plasma leucine and arginine fluxes (mumol.kg-1.h-1) were 208 +/- 35 and 108 +/- 18 for basal and 290 +/- 38 and 195 +/- 22 for TPN periods, respectively. Leucine oxidation was 42 +/- 7 and 59 +/- 9 mumol.kg-1.h-1 for basal and TPN periods, respectively, indicating a higher rate of leucine loss in the absence of a leucine intake than that expected for healthy individuals. The arginine kinetic data implied little net de novo arginine synthesis and further suggested increased rates of arginine degradation from burn injury. The expected rate of urea excretion, based on the basal rate of leucine oxidation, agreed closely with the measured output of urinary urea. These findings suggest that arginine is a conditionally indispensable amino acid for maintaining body protein homeostasis and nutrition in severely burned pediatric patients. The metabolic response of these children appears to be quantitatively similar to that for severely burned adult patients.

Amino Acids↗
PubMed1996

Effects of age on energy expenditure and substrate oxidation during experimental overfeeding in healthy men.

Relatively little is known about the influence of age on energy regulation during energy imbalance. We compared the effects of overfeeding on changes in energy expenditure, substrate oxidation, and energy deposition between young men (age 23.7 +/- 1.1 [SEM] years) and older men (age 70.0 +/- 7.0) of normal body weight who were leading unrestricted lives. Changes in total energy expenditure, resting energy expenditure (REE), the thermic effect of feeding (TEF), respiratory quotient (RQ), and body energy content were determined in response to overeating by 4.09 +/- 0.07 Megajoule (MJ)/day for 21 days in 16 healthy subjects consuming a typical diet. After excluding data from one young subject with unusual results and adjusting for individual differences in excess energy intake, there was a tendency towards a smaller increase in REE in older men compared to the young men (p = .07) which was accounted for by their lower fat-free mass (p = .016). There was also a significantly smaller increase in resting energy expenditure averaged over fasting and fed states (i.e, REE + TEF) with overfeeding in older men than in young men (p < .01). Combined, these smaller increases in energy expenditure with overfeeding in the older subjects averaged an estimated 365 kilojoule (kJ)/day (8.9% of the excess energy intake) (p < .02). There were also significant effects of age on fasting RQ (p < .001) and the change in RQ with overfeeding (p < .001), but no significant increase in energy expenditure for physical activity and thermoregulation with overfeeding in either age-group. These results are consistent with the suggestion that older individuals experience both a reduction in the ability to increase energy expenditure, and an alteration in the pattern of substrate utilization, in response to overfeeding. These changes may promote cumulative increases in body energy during normal cycles of positive energy balance unless compensated for by adaptive variations in energy intake.

Adult↗
PubMed1996

Effects of age on energy expenditure and substrate oxidation during experimental underfeeding in healthy men.

Relatively little is known about the influence of age on energy regulation during energy imbalance. We compared the effects of underfeeding on changes in energy expenditure, substrate oxidation, and body energy loss between young men (age 22.0 +/- 0.9 [SEM] years) and older men (age 66.0 +/- 1.8) who were leading unrestricted lives. Changes in total energy expenditure (TEE), resting energy expenditure (REE), the thermic effect of feeding (TEF), respiratory quotient, and body energy loss were determined in response to undereating by 3.17 +/- 0.16 Megajoule (MJ)/day for 21 days in 19 healthy subjects consuming a typical diet. No significant effects of age were observed in changes in TEE, REE, TEF, or body energy loss in response to underfeeding. Thus, older men do not appear to have any impairment of energy conservation during negative energy balance compared to young men. This normal pattern of energy conservation during undereating together with the previously demonstrated reduction in energy dissipation during overeating can be predicted to promote body fat deposition in older men during the cycles of positive and negative energy balance that occur during normal life. This finding may help to explain the increased body fat mass in older individuals.

Adult↗
PubMed1996

Pattern of amino acid requirements in humans: an interspecies comparison using published amino acid requirement recommendations.

We undertook an interspecies comparison of essential (indispensable) amino acid requirements, in relation to those for total protein, to examine whether the current, internationally proposed human amino acid requirement patterns are significantly different from those of other animals. Data were compiled in their original form and then expressed as mg amino acid/g total dietary protein (N X 6.25) required (amino acid requirement pattern). Patterns of requirements within the various species were organized according to four, arbitrary age-developmental groupings (very young, early growth, growth and mature), and these were compared with the requirements for (a) human infants, (b) 2-5 year old children, (c) 10-12 year old children and (c) adults, respectively. Statistical comparisons determined if significant differences existed between humans and other species, at apparently similar age-development stages, for both the total and for individual indispensable amino acids. The sum of the specific indispensable amino acid requirements for humans was significantly different than that for other species at infancy, growth and adulthood; the greatest differences, however, were for the amino acid requirement pattern in adulthood. The change between the very young and adult was the greatest for humans. Some of this difference might be due to experimental factors, including the biological status of a species within the arbitrary groupings described and the criteria used to determine "requirements." Nevertheless, it is difficult to escape the conclusion that the current international human amino acid requirements, for adults in particular, appear to be anomalous when judged against data for other animal species.

Aging↗
PubMed1996

Plasma methionine and cysteine kinetics in response to an intravenous glutathione infusion in adult humans.

Glutathione (GSH), a tripeptide (gamma-glutamyl-cysteinyl-glycine), is thought to be both a storage and a transport form of cysteine (Cys). In a previous study (T. Hiramatsu, N.K. Fukagawa, J.S. Marchini, J. Cortiella, Y.-M. Yu, T.E. Chapman, and V.R. Young. Am. J. Clin. Nutr. 60: 525-533, 1994), the direct tracer-derived estimate of Cys flux was considerably higher than that predicted from estimates of protein turnover. To further examine the components of plasma Cys flux, seven normal-weight healthy adult men and women (26 +/- 2 yr) received stable isotope tracer infusions of L-[methyl-2H3;1-13C]methionine, L-[3,3-2H2]cysteine, and L-[methyl-2H3]leucine for 460 min. After a 3-h baseline period, GSH was administered at approximately 32 mumol.kg-1.h-1 until the end of the study. Expired breath and blood samples were obtained at timed intervals and analyzed for isotope enrichment using mass spectrometry. Leucine, alpha-ketoisocaproate, and methionine (carboxyl carbon, methyl moiety, remethylation, and transsulfuration) turnover were reduced during GSH administration (P < 0.01). In the final hour of GSH administration, Cys flux increased by 61% from 55.1 +/- 1.7 to 88.7 +/- 5.2 mumol.kg-1.h-1 (P < 0.01), which was essentially equivalent to the rate of exogenous GSH infusion. These data suggest that GSH breakdown accounts for approximately 50% of tracer-derived Cys flux basally and for all of the increase in measured Cys turnover during exogenous GSH infusion.

Adult↗