Leucine kinetics at graded leucine intakes in young men

Human Amino Acid Requirements: A Re-Evaluation

The most recent internationally stated estimates of the amino acid requirements in adult humans are those given in the 1985 report of the Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements. In this review we present, in brief a number of scientific concerns and problems that lead us to conclude that these current recommendations for amino acid requirements are not valid and that the appropriate values are likely to be considerably higher. Following a short review of the C-labelled amino acid tracer studies carried out at the Massachusetts of Technology (MIT) and designed to reassess the requirements for specific indispensable amino acids, we focus particular attention on the lysine requirement in adults. When various criteria and methods are used to estimate this requirement, it appears that a cohesive body of data indicates the mean requirement value for lysine in healthy adults to be about 30 mg/kg/day or 50 mg/g protein. Although this value contrasts with the FAO/WHO/UNU value of 12 mg/kg/day or 16 ma/g protein, this new, tentative requirement value is consistent with findings from studies carried out earlier at MIT on the nutritional quality of wheat proteins. We propose that it would be prudent to apply the MIT amino acid requirement pattern (see Food and Nutrition Bulletin 1990;12:298–300), rather than the 1985 FAO/WHO/UNU adult amino acid requirement pattern, in the design and implementation of sound nutrition policies and programmes that include considerations of the amount and quality of the protein component of national and regional diets.

Protein quality & amino acid requirements in relation to needs in India

The relevance of protein and its constituent amino acids (AAs) in the structure and function of the human body is well known. Accumulating evidence has conferred specific functional and regulatory roles for individual AAs, adding relevance to their requirements across different age groups. The methods for measuring AA requirements have progressed from the classical nitrogen balance to the current stable isotope-based AA balance methods. Requirements of most of the indispensable AA (IAA) have been estimated in healthy Indian population by the best available balance method and has shown to be higher than earlier 1985 WHO/FAO/UNU (World Health Organization/Food and Agriculture Organization/United Nations University) recommendations. In addition, potential changes in the requirement, through adaptation to chronic undernutrition or to infection, have also been evaluated. In 2007, the WHO/FAO/UNU released a recommendation that increased the daily IAA requirement, based on primary evidence from Indian balance studies. This meant that to ensure that the new IAA requirements were met, individual foods or mixed diets needed to be assessed for their protein quality, or their ability to deliver the required amount of IAA. The recent FAO report on protein quality evaluation recommends the use of a new chemical AA score, the digestible IAA score (DIAAS), to replace the earlier protein digestibility corrected AA score. The DIAAS requires the determination of individual AA digestibility at the ileal level. A minimally invasive dual stable isotope tracer-based approach has been developed in India and has been used to determine digestibility of various foods in Indian adults and children. The increase in IAA requirements and subsequent protein quality requirements have implications for national regulatory frameworks, growth and development, and in turn, for economic and agricultural policy.

Nutrition from the Inside Out

Nearly 50 years ago, I set out to investigate the clinical problem of hypoglycemia in children with illnesses that limited their food intake. My goal was to gather accurate and precise measurable data. At the time, I wasn't interested in nutrition as a discipline defined in its more general or popular sense. To address the specific problem that interested me required development of entirely new methods based on stable, nonradioactive tracers that satisfied the conditions of accuracy and precision. At the time, I had no inclination of the various theoretical and practical problems that would have to be solved to achieve this goal. Some are briefly described here. Nor did I have the slightest idea that developing the field would result in a fundamental change in how human clinical investigation was conducted, with the eventual replacement of radiotracers with stable isotopically labeled ones, even for adult clinical investigation. Additionally, I had no inclination that the original questions would open avenues to much broader questions of practical nutritional relevance. Moreover, only much later as the editor of The American Journal of Clinical Nutrition did I appreciate the policy implications of how nutritional data are presented in the scientific literature. At least in part, less accurate and precise measurements and less than full transparency in reporting nutritional data have resulted in widespread debate about the public policy recommendations and guidelines that are the intended result of collecting the data in the first place. This article provides a personal recollection (with all the known faults of self-reporting and retrospective memory) of the journey that starts with measurement certainty and ends with policy uncertainty.

Lysine requirements of moderately undernourished school-aged Indian children are reduced by treatment for intestinal parasites as measured by the indicator amino acid oxidation technique

BACKGROUND

Lysine requirements of well-nourished children from developing regions have been found to be similar to those of children from developed regions (33.5 mg · kg⁻¹ · d⁻¹). However, intestinal parasites have been shown to increase lysine requirements in undernourished adults, and it is not known if a similar phenomenon occurs in undernourished children from poor and unsanitary environments.

OBJECTIVE

Our objective was to measure the lysine requirement of moderately undernourished school-aged Indian children by the indicator amino acid oxidation technique before and after successful treatment for intestinal parasites.

METHODS

Twenty-one undernourished school-aged children (∼8 y of age) with z scores between -2 SD and -3 SD for height-for-age or weight-for-age, who tested positive for intestinal parasites, were studied before and after successful antiparasite treatment. Children were fed any 2 of 7 levels of lysine intakes (5, 15, 25, 35, 50, 65, and 80 mg · kg⁻¹ · d⁻¹) in random order. The lysine requirement was determined by applying a 2-phase linear regression crossover analysis on the fractional oxidation rate of the tracer L-[1-¹³C] phenylalanine in response to the graded lysine intakes.

RESULTS

The lysine requirement of undernourished children with intestinal parasite infestations was determined to be 42.8 mg · kg⁻¹ · d⁻¹ (95% CI: 32.6, 53.1 mg · kg⁻¹ · d⁻¹), and after successful antiparasitic treatment it was determined to be 35.5 mg · kg⁻¹ · d⁻¹ (95% CI: 25.5, 45.5 mg · kg⁻¹ · d⁻¹). The results were significantly different (P < 0.05), although the 95% CIs overlapped.

CONCLUSIONS

The lysine requirement in undernourished children is similar to that of well-nourished children, and intestinal parasitic infestation increased the lysine requirement by ∼20%.

Protein intakes in India

Indian diets derive almost 60 % of their protein from cereals with relatively low digestibility and quality. There have been several surveys of diets and protein intakes in India by the National Nutrition Monitoring Board (NNMB) over the last 25 years, in urban and rural, as well as in slum dwellers and tribal populations. Data of disadvantaged populations from slums, tribals and sedentary rural Indian populations show that the protein intake (mainly from cereals) is about 1 gm/kg/day. However, the protein intake looks less promising in terms of the protein digestibility corrected amino acid score (PDCAAS), using lysine as the first limiting amino acid, where all populations, particularly rural and tribal, appear to have an inadequate quality to their protein intake. The protein: energy (PE) ratio is a measure of dietary quality, and has been used in the 2007 WHO/FAO/UNU report to define reference requirement values with which the adequacy of diets can be evaluated in terms of a protein quality corrected PE ratio. It is likely that about one third of this sedentary rural population is at risk of not meeting their requirements. These levels of risk of deficiency are in a population with relatively low BMI populations, whose diets are also inadequate in fruits and vegetables. Therefore, while the burden of enhancing the quality of protein intake in rural India exists, the quality of the diet, in general, represents a challenge that must be met.

Amino acid supplements and recovery from high-intensity resistance training

The purpose of this study was to investigate whether short-term amino acid supplementation could maintain a short-term net anabolic hormonal profile and decrease muscle cell damage during a period of high-intensity resistance training (overreaching), thereby enhancing recovery and decreasing the risk of injury and illness. Eight previously resistance trained males were randomly assigned to either a high branched chain amino acids (BCAA) or placebo group. Subjects consumed the supplement for 3 weeks before commencing a fourth week of supplementation with concomitant high-intensity total-body resistance training (overreaching) (3 x 6-8 repetitions maximum, 8 exercises). Blood was drawn prior to and after supplementation, then again after 2 and 4 days of training. Serum was analyzed for testosterone, cortisol, and creatine kinase. Serum testosterone levels were significantly higher (p < 0.001), and cortisol and creatine kinase levels were significantly lower (p < 0.001, and p = 0.004, respectively) in the BCAA group during and following resistance training. These findings suggest that short-term amino acid supplementation, which is high in BCAA, may produce a net anabolic hormonal profile while attenuating training-induced increases in muscle tissue damage. Athletes' nutrient intake, which periodically increases amino acid intake to reflect the increased need for recovery during periods of overreaching, may increase subsequent competitive performance while decreasing the risk of injury or illness.

Lysine requirement of healthy, school-aged Indian children determined by the indicator amino acid oxidation technique

We recently reported the lysine requirement of school-aged children living in Canada consuming a mixed diet to be 35 mg.kg(-1).d(-1). Because the majority of children in the world live on cereal-based diets in developing countries, we measured the daily lysine requirement in healthy children living in India and consuming a mostly cereal-based diet. Our objective in this study was to determine the lysine requirement in healthy, school-aged children in the developing world by using the indicator amino acid oxidation method with l-[1-(13)C] phenylalanine. Six healthy, school-aged children consumed 7 levels of lysine (5, 15, 25, 35, 50, 65, and 80 mg.kg(-1).d(-1)) each in a random order along with an amino acid mixture providing energy and protein intakes of 1.7 x resting energy expenditure and 1.5 g.kg(-1).d(-1), respectively. The mean lysine requirement was determined by applying a 2-phase linear regression crossover analysis on tracer oxidation (F(13)CO(2)) data, which identified a breakpoint (requirement) at minimal F(13)CO(2) in response to the graded lysine intakes. The mean lysine requirements with the upper 95% CI for children were determined to be 33.5 and 46.6 mg.kg(-1).d(-1), respectively, by breakpoint analysis of the F(13)CO(2) data. The mean lysine requirements of Indian children were almost identical to that of Canadian children (35 mg.kg(-1).d(-1)). There is no evidence for any adaptation in lysine requirements in children from developing countries such as India.

An approach to defining the upper safe limits of amino acid intake

The existing data on the safe upper limits of amino acid intake in humans is essentially observational; how much do individuals ingest and what side effects do they have? There are numerous studies in humans comparing the effects of high doses of amino acids given as protein bound vs. as free amino acids. These studies have shown that protein-bound amino acids have much less effect on plasma levels of the test amino acid, because protein intake stimulates protein synthesis as another sink for the increased amino acid intake. In practice, the highest amino acid intakes occur with free amino acid supplements that may be ingested by athletes who believe that the amino acids will benefit them in training and/or performance. Previously, in a piglet study, we were able to define the point at which maximal phenylalanine oxidation occurred, above which plasma phenylalanine concentration and body balance rose exponentially. We regard this value of maximal disposal (oxidation) of an amino acid as one metabolic marker of the upper limit of intake. Recently, others have demonstrated a similar maximal oxidation rate for leucine in rats. Based on these experimental data and the paucity of published human data in controlled experiments, we think that a systematic approach needs to be undertaken to define the maximal oxidation rate for all dietary indispensable amino acids and other amino acids that may be ingested in excess by humans. We believe that this will provide a rational basis to begin to define the upper limits of tolerance for dietary amino acids. However, some amino acids, such as threonine and methionine, will be more difficult to study, because they have more than 1 route of disposal or very complex metabolic regulation, in which case defining their upper limits will be more multifaceted.

Minimum protein requirements in liver cirrhosis determined by nitrogen balance measurements at three levels of protein intake

Nitrogen balance at three levels of protein intake was measured in eight patients with cirrhosis of the liver; moreover, at each level of protein intake, the effects on nitrogen balance of branched-chain amino-acid enriched protein and natural protein were compared. From these nitrogen balance data, minimum protein requirements were calculated by linear regression analysis. The patients were in a negative nitrogen balance on a 40 g protein diet (-0.75 +/- 0.15 gN.), and in positive nitrogen balance on 60 g (+1.23 +/- 0.22 gN.) or 80 g of protein per day (+2.77 +/- 0.20 g N.). Their mean minimum protein requirement (48 +/- 5 g of protein/day or 0.75 g/kg/day) is higher than expected in healthy people; the safe level of protein intake (mean + 2 sd) is 58 g per day or 1.2 g/kg/day. Nitrogen balances and protein requirements were not different on branched-chain amino-acid enriched diets. The physical condition of the patients improved when they came into positive nitrogen balance; the higher rates of protein intake were well tolerated without onset of encephalopathy. We conclude that protein requirements are elevated in cirrhosis of the liver; diets supplying less than 60 g of protein per day should not be prescribed in long term treatment of cirrhotic patients.

Fat and protein metabolism in growing steers fed either grass silage or dried grass

Cattle fed grass silage diets have been reported to have high carcass fat:protein ratios. The effect of grass silage and dried grass diets, fed at different levels of intake to ensure a range of equivalent metabolisable energy intakes (MEI) from 1 .1 x metabolisable energy requirement for maintenance to ad libitum, on fat and protein metabolism in twenty-four Hereford x Friesian steers was investigated. After about 84 d of dietary treatment rates of whole-body fat and protein metabolism were measured, as were rates of lipogenesis in omental, perirenal and subcutaneous adipose tissue. Carcass composition was determined. Animals fed silage had greater (P<0 .001) carcass fat:protein ratios than animals fed dried grass at equivalent levels of MEI. Animals fed silage had lower (P<0 .001) rates of protein gain. Rates of leucine entry and oxidation were lower (P<0 .001) in animals fed silage, but there was no dietary difference in the rate of whole-body protein synthesis. There was no dietary difference in the rate of carcass fat gain, but rates of lipogenesis in perirenal adipose tissue were significantly (P=0 .007) higher in animals fed silage. There was no dietary difference in the rate of palmitate and glycerol entry or palmitate oxidation. There were no interactions between MEI and diet, indicating that increments of energy were utilised with the same efficiency from both diets. It was concluded that the high carcass fat:protein ratios of young growing steers was due to limited rates of protein accretion and not to elevated rates of carcass fat accretion.

Branched-chain amino acid requirements in healthy adult human subjects

There is now an expanding body of evidence to recommend, in the case of adult humans, the use of revised indispensable amino acid requirement values; these are approximately 2 to 3 times higher than the current international recommendations. The earlier methodologies for determining amino acid requirements, based on nitrogen balance, were criticized because of their design and the associated high energy intakes. The 1985 World Health Organization/Food & Agriculture Organization/United Nations University requirement for leucine has been demonstrated to be too low by short- and long-term (24-h) tracer-derived estimates of leucine oxidation and balance. The best values for leucine requirements come from 24-h direct amino acid oxidation (DAAO) and direct amino acid balance (DAAB) studies. Finally, we also collated all available data from studies on fed-state leucine oxidation with an adequate dietary adaptation period to assess the inflection on the leucine oxidation-leucine intake curve. The mean requirements for leucine, valine, and isoleucine are likely to be 40, 17-25, and 19 mg . kg(-1) . d(-1), respectively. This adds up to a total of approximately 84 mg . kg(-1) . d(-1), which is much lower than the lowest estimate of the total BCAA requirement of approximately 110 mg . kg(-1) . d(-1) made by the short-term indicator amino acid oxidation (IAAO) method, which determined the BCAA requirement from the pattern of oxidation of an indicator amino acid (phenylyalanine) at different levels of BCAA intake. An additional estimate of the leucine requirement was also made by a meta-analysis of all available 24-h DAAO/DAAB data from different studies. This resulted in a higher value for the leucine requirement than that obtained by the specific studies that utilized the 24-h DAAO/DAAB approach; however, even adding this value to the total BCAA requirement does not account for the difference in the total BCAA requirement estimates and the summed individual BCAA estimates.

What are the essential elements needed for the determination of amino acid requirements in humans?

The aims of this introductory article are to survey and critically evaluate the concepts and approaches that have been used to assess amino acid adequacy and to hypothesize about possible future directions of research. The issue in question is extensive, consequently this article will be limited to: 1) definitions of amino acid requirements; 2) available techniques to assess amino acid requirements; 3) actual recommendations for healthy adults; 4) factors influencing requirements; and 5) requirements in acute and chronic wasting diseases. Recommendations for amino acid intakes for healthy adults were proposed by the FAO/WHO expert committee in 2001. They have not yet been published. The major factors affecting amino acid requirements are the stage of development, reproductive state, environmental factors, digestibility of dietary proteins, genotype of the individual, and pathological conditions. Remarkably, there are no conclusive data relative to changes in requirements induced by infection, injury, trauma, and renal or liver failure. Future research using modern methods to evaluate requirements must thus receive a high priority. Wasting diseases are associated with deficiencies and imbalances of particular amino acids causing specific changes in requirements. Consequently, a new approach has been used to categorize amino acids as conditionally indispensable according to their functional and physiological properties. Kinetic measurements of plasma amino acids might help to estimate qualitative requirements. Measurement of tissue intracellular free amino acid deficiencies or excesses is another method to estimate qualitative requirements. Based on these measurements tentative values for conditionally indispensable amino acids during disease are given in the article.

Different approaches to define individual amino acid requirements

A full review of the strengths and limitations of the various methods used to define amino acid requirements is provided. The focus is on the recent development of carbon oxidation techniques such as indicator amino acid oxidation and 24-h amino acid balance to determine dietary indispensable (essential) amino acid needs in adults. All approaches depend on the change in a metabolic parameter in response to graded intake of the test amino acid. In humans, the within-subject variance is less than the between-subject variance, which has led to an appreciation of the need to study each subject across a range of intakes, above and below the mean requirement level. The data can then be analyzed using two-phase linear regression crossover and a precise population mean requirement can be determined. Several approaches have been used to define the variance of the mean requirement. Finally, a minimally invasive indicator amino acid oxidation model has been developed which allows the determination of dietary essential amino acid requirements in children and other vulnerable populations.

The total branched-chain amino acid requirement in young healthy adult men determined by indicator amino acid oxidation by use of L-[1-13C]phenylalanine

Previous recommendations for branched-chain amino acids (BCAA), based on nitrogen balance studies, were found to be low in a series of stable isotope-labeled amino acid studies. The BCAA requirement was increased in the new dietary reference intake (DRI) report on the basis of a series of stable isotope studies examining the requirement of leucine and valine individually, but not isoleucine. To reduce the possibility of interactions among these amino acids and imbalances in the mixture affecting the estimate of requirements, we decided to determine the requirement for the total BCAA of young healthy adult men, receiving a mixture of BCAA based on the proportion of these amino acids in egg protein, by use of indicator amino acid oxidation. Seven men were assigned to receive nine graded intakes of a BCAA mixture in random order: 34, 50, 66, 80, 100, 120, 140, 160 and 180 mg/(kg. d). The rate of release of (13)CO(2) from the oxidation of L-[1-(13)C]phenylalanine (F (13)CO(2)) was measured and a two-phase linear regression crossover model was applied to determine total BCAA requirement. The mean requirement and population-safe level (upper limit of 95% confidence interval) of the total BCAA were 144 and 210 mg/(kg. d), respectively. Based on the balance of BCAA in egg protein, our estimate for the mean leucine requirement is 55 mg/(kg. d), which is substantially higher than the 34 mg/(kg. d) recommended by the DRI.

Leucine requirement and splanchnic uptake of leucine in chronically undernourished adult Indian subjects

BACKGROUND

We showed previously by the 24-h direct amino acid balance (DAAB) method that the leucine requirement of well-nourished Western and South Asian subjects is approximately 40 mg x kg(-1) x d(-1).

OBJECTIVE

It is not known whether this estimated leucine requirement is applicable in chronic undernutrition; therefore, we assessed the leucine requirement in Indian men with chronic, but stable, undernutrition.

DESIGN

We studied 26 chronically undernourished men during 2 randomly assigned 7-d diet periods consisting of an L-amino acid diet (n = 20) and supplying either 14 and 30 (n = 10) or 22 and 40 (n = 10) mg leucine x kg(-1)x d(-1) or consisting of the subjects' habitual cereal-and-lentil-based diets (n = 6). The 24-h DAAB was estimated on day 6 by using a [13C]leucine tracer infusion. The splanchnic uptake of leucine was determined at an intake of 40 mg x kg(-1) x d(-1) by administering [2H3]leucine orally.

RESULTS

By using mixed-models linear regression of leucine balance against leucine intake, we estimated a zero leucine balance at a leucine intake of 39.6 mg x kg(-1) x d(-1). The splanchnic first-pass uptake of [2H3]leucine was 22.7% and 11.5% of the intake in the fasted and fed phases, respectively. The subjects were in neutral leucine balance with their habitual cereal-and-lentil-based diets.

CONCLUSION

On the basis of the 24-h DAAB approach, a mean leucine requirement of 40 mg x kg(-1) x d(-1) is proposed for healthy and for chronically undernourished Indian adults.

Intravenously infused 13C-leucine is retained in fasting healthy adult men

We have proposed a leucine requirement of 40 mg/(kg. d) in adults, based on 24-h direct amino acid balance (24-h DAAB) studies in which leucine intake is calculated as the sum of diet and tracer intake. However, it is possible that the tracer intake that is given during the fasting state in the 24-h studies is oxidized, thereby not contributing to the effective daily leucine intake and thus lowering the intake and, consequently, the requirement estimate. We assessed the fasting state leucine oxidation with different leucine infusion rates ( approximately 2.5-5% of the leucine flux rate) in well-nourished Indian men. Healthy subjects (n = 10) in a fasting state were studied during three randomly administered infusions of different, known amounts of leucine, supplying 4.1, 6.6 or 8.3 mg/(kg. 12 h) during the 12-h fast. Mean 12-h leucine oxidation rate and leucine flux for the different levels of leucine infused did not change significantly (P > 0.1) for the three leucine infusion rates. The plasma leucine concentrations increased significantly after 12 h of leucine infusion, rising from between 20 and 50 micromol/L by the end of the infusions over the range of tracer input. We conclude that tracer leucine infused in the fasting state does not measurably increase leucine oxidation at the doses studied. Thus, tracer intake during the 12-h fast contributes to the effective leucine intake in 24-h DAAB studies.

Effects of wine intake on postprandial plasma amino acid and protein kinetics in type 1 diabetes

BACKGROUND

Alcohol may impair protein turnover and insulin sensitivity in vivo.

OBJECTIVE

The acute effects of moderate wine intake on amino acid kinetics and on the fractional synthetic rate (FSR) of albumin and fibrinogen in patients with type 1 diabetes were studied.

DESIGN

Six patients with type 1 diabetes ingested an elementary mixed meal (46 kJ/kg) over 4 h, first without and 3 mo later with approximately 300 mL red wine. Postprandial glucose concentrations were maintained at <10 mmol/L.

RESULTS

Postprandially, the FSR of fibrinogen was approximately 30% greater (21.5 +/- 6.6% compared with 14.1 +/- 3.6% of pool/d; P < 0.01) and glucagon concentrations were approximately 40% greater (103 +/- 20 compared with 61 +/- 13 ng/L; P < 0.015) with wine than without wine. However, the FSR of albumin and the rates of appearance of total and endogenous phenylalanine and leucine were not significantly different between treatments. First-pass splanchnic uptake (in micromol*kg(-1)*min(-1)) of dietary phenylalanine (0.22 +/- 0.02 compared with 0.19 +/- 0.02) and leucine (0.25 +/- 0.04 compared with 0.14 +/- 0.02) were greater with wine (P < 0.05), whereas dietary phenylalanine oxidation was lower with wine, by approximately 25% (0.10 +/- 0.02 compared with 0.14 +/- 0.01 micromol.kg(-1).min(-1); P < 0.05). Selected amino acid concentrations were significantly lower but glutamate concentrations were significantly higher with wine.

CONCLUSIONS

In insulin-infused patients with type 1 diabetes, moderate wine intake with a meal resulted in 1) a higher fibrinogen FSR, glucagon concentration, and first-pass splanchnic uptake of leucine and phenylalanine; 2) lower dietary phenylalanine oxidation; 3) selective changes in plasma amino acid concentrations; 4) and no impairment in endogenous proteolysis and albumin synthesis.

Daily requirement for and splanchnic uptake of leucine in healthy adult Indians

BACKGROUND

The 1985 FAO/WHO/UNU requirement for leucine is too low according to tracer-derived estimates of leucine oxidation and balance in adults from developed regions.

OBJECTIVE

The leucine requirement in populations in developing countries was assessed with use of the 24-h tracer balance method and on the basis of nitrogen balances.

DESIGN

Twenty healthy Indian men were studied during their consumption for 6 d of 2 L-amino acid diets that supplied either 14 and 30 (n = 10) or 22 and 40 (n = 10) mg leucine x kg(-1) x d(-1) in random order. At 1800 on day 7, a 24-h constant intravenous [13C]leucine tracer-infusion protocol was conducted to determine leucine oxidation and daily leucine balance. During the intake of 40 mg leucine/d, [2H3]leucine was given orally to assess the splanchnic uptake of leucine.

RESULTS

Mean 24-h leucine oxidation rates were 29.8, 30.6, 33.6, and 39.3 mg x kg(-1) x d(-1) at leucine intakes of 14, 22, 30, and 40 mg x kg(-1) x d(-1), respectively; daily leucine balances were -16.5, -9.0, -3.3, and 0.5 mg x kg(-1) x d(-1), respectively. Mixed-models linear regression of balance against leucine intake resulted in a zero balance at a leucine intake of 37.3 mg x kg(-1) x d(-1). Nitrogen balances were -12.7, -17.9, -3.9, and 1.0 mg x kg(-1) x d(-1) at leucine intakes of 14, 22, 30, and 40 mg x kg(-1) x d(-1). Regression of nitrogen balance against intake resulted in a zero balance at a leucine intake of 37.6 mg x kg(-1) x d(-1). The first-pass splanchnic uptake of leucine was 45.7% and 33.9% in the fasted and fed periods, respectively.

CONCLUSION

A tentative mean leucine requirement of 40 mg x kg(-1) x d(-1) is proposed for healthy Indian adults, as it is for Western subjects.

Threonine requirement of young men determined by indicator amino acid oxidation with use of L-[1-(13)C]phenylalanine

BACKGROUND

Threonine is an indispensable amino acid with a complex degradative pathway. Use of the indicator amino acid oxidation technique should provide an estimate of the threonine requirement that is not affected by its metabolic pathway.

OBJECTIVE

Our objective was to determine the requirement for threonine in men by using the indicator amino acid oxidation method and to provide statistical estimates of the population mean and 95% CIs of the threonine requirement. We hypothesized that the current World Health Organization estimate of the threonine requirement, 7 mg*kg(-)(1)*d(-)(1) (based on nitrogen balance studies), is too low.

DESIGN

Six healthy men each received 6 different threonine intakes while consuming an energy-sufficient diet with 1.0 g L-amino acid mixture*kg(-)(1)*d(-)(1). The effect of graded alterations in dietary threonine intake on phenylalanine flux and oxidation was studied by using L-[1-(13)C]phenylalanine as the indicator amino acid.

RESULTS

The results of two-phase linear regression crossover analysis showed that the mean threonine requirement, based on indicator oxidation, was 19.0 mg*kg(-)(1)*d(-)(1) with an upper safe intake of 26.2 mg*kg(-)(1)*d(-)(1).

CONCLUSIONS

This is the first application of the indicator amino acid oxidation technique in humans to study the requirement for an indispensable amino acid with a complex degradative pathway. We found that the upper safe intake for 95% of the population is almost 4-fold higher than the current World Health Organization estimate.

Metabolic demands for amino acids and the human dietary requirement: Millward and rRvers (1988) revisited

In 1988, Millward and Rivers reappraised existing metabolic models for amino acid requirements. The metabolic demand for amino acids was reviewed in relation to both obligatory metabolic consumption and adaptive pathways of amino acid oxidation. The obligatory demand pattern was deemed unknowable from first principles except that the level of one amino acid would be similar to its concentration in an amount of tissue protein equivalent to the obligatory nitrogen loss. The adaptive demand pattern was predicted to vary in relation to the amount and the periodicity of food protein intake that influenced the amplitude of the diurnal cycle of gains and losses. A regulatory influence of protein intake on anabolism, the anabolic drive, was identified in animal studies; benefit appeared to derive from intakes in excess of the minimum for balance, which could facilitate definition of an optimal requirement. The inherent and design-related limitations of both nitrogen and stable isotope balance studies of requirement were recognized as a major problem in identifying secure values for indispensable amino acid requirements. A decade of research of increasing methodological sophistication has generated much new information, confirming the adaptive diurnal model of balance regulation and allowing development of the anabolic drive into a general protein-stat theory for coordinated control of growth and maintenance of the lean body mass. However, notwithstanding several new estimates of amino acid requirement values, definition of a widely accepted human amino acid requirement pattern remains unresolved. Although a case can be made for an adjusted 1985 FAO adult requirement pattern being a reasonable estimate of the obligatory indispensable amino acid requirements for human maintenance, the problems posed by adaptation, methodological inadequacies and lack of independent measures of adequacy mean that assessment of the adequacy of the human diet to satisfy amino acid needs remains inherently difficult.

Determination of amino acid requirements by indicator amino acid oxidation: applications in health and disease

Indispensable amino acid requirements for children, patients and pregnant women are largely unknown. We describe a new, non-invasive technique that can estimate requirements in vulnerable populations. Future applications will lead to optimal nutritional care for populations in which an appropriate balance of amino acids is essential for metabolic health and growth.

Phenylbutyrate-induced glutamine depletion in humans: effect on leucine metabolism

The present study was designed to determine whether sodium phenylbutyrate (phi B) acutely induces a decrease in plasma glutamine in healthy humans, and, if so, will decrease estimates of whole body protein synthesis. In a first group of three healthy subjects, graded doses (0, 0.18, and 0.36 g.kg-1.day-1) of phi B were administered for 24 h before study: postabsorptive plasma glutamine concentration declined in a dose-dependent manner, achieving an approximately 25% decline for a dose of 0.36 g phi B.kg-1.day-1. A second group of six healthy adults received 5-h infusions of L-[1-14C]leucine and L-[1-13C]glutamine in the postabsorptive state on two separate days: 1) under baseline conditions and 2) after 24 h of oral treatment with phi B (0.36 g.kg-1.day-1) in a randomized order. The 24-h phenylbutyrate treatment was associated with 1) an approximately 26% decline in plasma glutamine concentration from 514 +/- 24 to 380 +/- 15 microM (means +/- SE; P < 0.01 with paired t-test) with no change in glutamine appearance rate or de novo synthesis; 2) no change in leucine appearance rate (Ra), an index of protein breakdown (123 +/- 7 vs. 117 +/- 5 mumol.kg-1.h-1; not significant); 3) an approximately 22% rise in leucine oxidation (Ox) from 23 +/- 2 to 28 +/- 2 mumol.kg-1.h-1 (P < 0.01), resulting in an approximately 11% decline in nonoxidative leucine disposal (NOLD = Ra-Ox), an index of protein synthesis, from 100 +/- 6 to 89 +/- 5 mumol.kg-1.h-1 (P < 0.05). The data suggest that, in healthy adults, 1) large doses of oral phenylbutyrate can be used as a "glutamine trap" to create a model of glutamine depletion; 2) a moderate decline in plasma glutamine does not enhance rates of endogenous glutamine production; and 3) a short-term depletion of plasma glutamine decrease estimates of whole body protein synthesis.

Tyrosine kinetics and requirements during total parenteral nutrition in the neonatal piglet: the effect of glycyl-L-tyrosine supplementation

Tyrosine may be a conditionally indispensable amino acid in the neonate; however, the provision of aromatic amino acids to neonates receiving total parenteral nutrition (TPN) is complicated by the poor solubility of crystalline tyrosine. In the present study, we investigated tyrosine kinetics and requirements during TPN, when tyrosine was supplied as the soluble dipeptide, glycyl-L-tyrosine in a neonatal piglet model. Fifteen 3-d-old male Yorkshire piglets were fitted with external jugular and femoral catheters and randomized to one of five tyrosine intakes: 0.11, 0.31, 0.41, 0.51 and 0.71 g.kg-1.d-1. Total parenteral amino acid and energy intakes were 15.0 g.kg-1.d-1 and 1.1 MJ.kg-1.d-1, respectively. Piglets were maintained on TPN for 6 d, with nitrogen balance measured over the final 3 d of the study. On the final study day, tyrosine kinetics were measured during a 4-h primed-constant infusion of L-[l14C]tyrosine. Nitrogen retention was 67% at the lowest tyrosine intake and increased significantly (p < 0.05) at intakes of 0.31 g.kg-1.d-1 and above (84, 86, 87, and 88% for intakes of 0.31, 0.41, 0.51, and 0.71 g.kg-1.d-1, respectively). Plasma tyrosine concentrations and tyrosine oxidation (expressed as either a percentage of the dose oxidized or when corrected for flux) were low and similar at the two lowest intakes, but increased significantly at the higher intakes. Two-phase regression analysis of the data (plasma tyrosine, tyrosine oxidation) yielded estimates of a mean tyrosine requirement of 0.31 and 0.35, respectively, with estimated safe intakes (upper 95% confidence limit) of 0.44 and 0.42 g.kg-1.d-1. The present work also indicates that oxidation techniques may be suitable for the estimation of amino acid requirements during TPN in the neonate.

Compartmental approach for evaluation of plasma kinetics and (13)co(2)-exhalation after oral loading with L-[1-(13)c]leucine

Abstract A seven compartment model was applied for evaluation of oral L-[1-(13)C]leucine loading tests (38 μmol/kg body wt.) in healthy volunteers. The model comprises transport and absorption in stomach and gut into a central L-leucine-compartment which is connected to a protein compartment and to the compartment of the corresponding 2-oxo acid. CO(2) release from the latter occurs in a fast and a slow compartment into the central CO(2) compartment for exhalation. Using the fmins routine of MATLAB for parameter estimation, a good agreement was obtained between calculated and actually measured kinetics of (13)C-labelled metabolites and a mean in vivo L-leucine oxidation of 0.365 ± 0.071 μmol/kg per min (n = 5) was computed. Plausibility of the model was checked by predicting in vivo leucine oxidation rates from primed continuous infusion tests (priming: L-[1-(13)C]leucine, 5 μmol/kg; NaH(13)CO(2), 1.2 μmol/kg; infusion: L-[1-(13)C]leucine, 5 μmol/kg per h). In 5 tested volunteers, the experimental L-leucine oxidation rate amounted to 0.358 ± 0.105 μmol/kg per min versus predicted 0.324±0.099 μmol/kg per min. Possible causes for some observed intraindividual variations are discussed.

Effects of dietary protein restriction on regional amino acid metabolism in insulin-dependent diabetes mellitus

We studied subjects with insulin-dependent diabetes mellitus (IDDM) and controls by administering primed continuous infusions of L-[1-13C,15N)]leucine and L-[2,3-13C2]alanine to measure whole body and forearm metabolism of these amino acids during ample protein intake and again after 4 wk of moderately restricted protein intake. Decreased rates of whole body protein degradation, leucine transamination, leucine oxidation, and increased forearm alanine release produced by dietary protein restriction occurred equivalently in IDDM subjects under short-term tightly managed glycemia and in controls. Dietary protein restriction did not affect whole body alanine appearance or forearm leucine appearance, disposal, or balance in IDDM subjects or controls. IDDM subjects differed from controls only in that normal forearm leucine balance was maintained at higher rates of leucine appearance and disposal. We conclude that IDDM subjects adapt normally to dietary protein restriction. Undernutrition during moderate protein deprivation in these patients likely occurs during episodes of poor glycemic control.

Ethanol impairs post-prandial hepatic protein metabolism

The effects of acute ethanol ingestion on whole body and hepatic protein metabolism in humans are not known. To simulate social drinking, we compared the effects of the association of a mixed meal (632 kcal, 17% amino acids, 50% glucose, 33% lipids) with a bottle of either table wine (ethanol content 71 g) or water on the estimates ([1-14C]-leucine infusion) of whole body protein breakdown, oxidation, and synthesis, and on the intravascular fractional secretory rates (FSR) of hepatically (albumin, fibrinogen) and extrahepatically (IgG) synthesized plasma proteins in two randomized groups (ethanol n = 7, water n = 7) of healthy nonalcoholic volunteers. Each study was carried out for 8 h. Protein kinetics were measured in the overnight post-absorptive state, over the first 4 h, and during a meal infusion (via a nasogastric feeding tube at constant rate) combined with the oral ingestion of wine or water, over the last 4 h. When compared with water, wine ingestion during the meal reduced (P < 0.03) by 24% the rate of leucine oxidation, did not modify the estimates of whole body protein breakdown and synthesis, reduced (P < 0.01) by approximately 30% the FSR of albumin and fibrinogen, but did not affect IgG FSR. In conclusion, 70 g of ethanol, an amount usual among social drinkers, impairs hepatic protein metabolism. The habitual consumption of such amounts by reducing the synthesis and/or secretion of hepatic proteins might lead to the progressive development of liver injury and to hypoalbuminemia also in the absence of protein malnutrition.

Dietary lysine requirement of young adult males determined by oxidation of L-[1-13C]phenylalanine

Lysine requirement was determined in seven adult males by examining the effect of varying dietary lysine intake on phenylalanine flux and oxidation under dietary conditions of adequate energy and phenylalanine (14 mg.kg-1 x day-1) and excess tyrosine (40 mg.kg-1 x day-1). Phenylalanine flux was determined from primed, constant intravenous infusions of L-[1-13C]phenylalanine (1.2 mg.kg-1 x day-1) and L-[ring-2H5]phenylalanine (0.5 mg.kg-1 x day-1) and measurement of isotopic enrichments of phenylalanine in plasma. Phenylalanine flux was not affected by graded increases in dietary lysine intake or by the isotope infused. Mean phenylalanine conversion to tyrosine was low (3.4%) and not significantly affected by lysine intake. Phenylalanine oxidation, estimated from the rate of 13CO2 released in expired air during the infusion of L-[1-13C]phenylalanine, decreased linearly as lysine intake increased to a break point that was interpreted as the mean dietary lysine requirement (37 mg.kg-1 x day-1). At lysine intakes of > 37 mg.kg-1 x day-1 phenylalanine oxidation was low and constant. Plasma lysine concentrations supported this estimate of requirement. These data show that: 1) indicator amino acid oxidation can be used as a new method to determine amino acid requirements of humans and 2) the lysine requirement of adult males is three times greater than the World Health Organization recommendation of 12 mg.kg-1 x day-1.

Meal stimulation of albumin synthesis: a significant contributor to whole body protein synthesis in humans

The present studies were performed to test the hypothesis that the liver, by increasing the synthesis of specific plasma proteins during the absorption of an amino acid meal, may play an important role in the temporary "storage" of ingested essential amino acids and to explore the effects of glucocorticosteroids and recombinant human growth hormone (rhGH) on these processes. The fractional synthetic rates of albumin and fibrinogen were determined using simultaneous infusions of intravenous [1-14C]leucine and intraduodenal [4,5-3H]leucine after 22 h fasting and during absorption of glucose and amino acids in four groups of normal subjects treated for 1 wk with placebo, prednisone (0.8 mg.kg-1.day-1), rhGH (0.1 mg.kg-1.day-1), or combined treatment. When compared with the fasted state and independent of the route of tracer delivery and hormonal treatment, albumin, but not fibrinogen, synthesis increased (P < 0.0001) during absorption of a mixed glucose amino acid meal in all groups. This increase in albumin synthesis accounted for 28% of the increase in whole body protein synthesis associated with feeding and for 24, 22, and 14% in the prednisone, rhGH, and combined treatment groups, respectively. These data suggest that the stimulation of albumin synthesis observed during feeding prevents irreversible oxidative losses of a significant fraction of ingested essential amino acids and may serve as a vehicle to capture excess dietary amino acids and transport them to peripheral tissues to sustain local protein synthesis.

Altered partition of threonine metabolism in pigs by protein-free feeding or starvation

Kinetic aspects of threonine (Thr) metabolism were examined in growing pigs fed a well-balanced diet (C), an isocaloric protein-free diet (PF), or starved (S) for 48 h. With the use of continuous simultaneous infusion of L-[1-13C]Thr, [1-14C]sarcosine, and 2-[1-14C]ketobutyrate (KB) for 10 h, estimates were made of rates of Thr incorporated into protein (S), released from body proteins (B), and oxidized through the catabolic pathways of L-Thr 3-dehydrogenase (TDG) and threonine dehydratase (TDH). In the C group S was 185, B was 138, Thr disposal to glycine (DRThr-Gly) was 47, and Thr disposal to KB (DRThr-KB) was 7 mumol.h-1.kg-1. Consequently, Thr balance was +48 mumol.h-1.kg-1. In the PF-fed pigs, S, B, DRThr-Gly, and DRThr-KB were significantly reduced by 38, 15, 74, and 75%, respectively. In the S group, S, B, and DRThr-Gly were significantly reduced by 47, 17, and 55%, respectively, but DRThr-KB was similar to the C group. DRThr-Gly in all groups was highly correlated with TDG enzyme activity measured in liver homogenates. By contrast with in vivo results, TDH enzyme activity was increased by 88% (P less than 0.05) in the S group and decreased by 27% (not significant) in the PF group compared with the C group. The TDH pathway accounted for 13, 12, and 27% of total Thr oxidation in the C, PF, and S groups, respectively. These results suggest that Thr conservation in protein-depleted states (PF and S groups) occurred mainly by a decrease of Thr oxidation and that the partition through these pathways was only altered when energy was completely withdrawn.

Isolated hypoisoleucinemia impairs whole body but not hepatic protein synthesis in humans

It is not known whether an acute decrease in the plasma concentration of any essential amino acid, as occurs during insulin infusion, impairs protein synthesis. To test this hypothesis in humans, selective hypoisoleucinemia or hypothreoninemia was induced by insulin infusion while maintaining normal or elevated plasma concentrations of the other amino acids via their selective infusion. The effects on protein synthesis were assessed using leucine kinetics and fractional synthetic rates of the two hepatic proteins albumin and fibrinogen. Results were compared with those of a combined insulin and complete amino acid infusion. Hypoisoleucinemia increased leucine oxidation (P less than 0.03) and decreased nonoxidative leucine disposal (P less than 0.04) and net leucine balance (P less than 0.03), whereas hypothreoninemia had no effect on any of these parameters. Neither hypoisoleucinemia or hypothreoninemia altered albumin and fibrinogen fractional synthetic rates when compared with the control study. Because of the known relationships between intra- and extracellular amino acid concentrations, the hypoisoleucinemia was most likely associated with a decreased intracellular concentration of isoleucine; such would not be the case for hypothreoninemia. Thus acute limited availability of a single essential amino acid can adversely affect nonhepatic protein synthesis.

Assessment of threonine metabolism in vivo by gas chromatography/mass spectrometry and stable isotope infusion

The fractional contributions (FC) of threonine to glycine and 2-ketobutyrate (KB) fluxes in fed pigs have been assessed by the constant infusion of L-[1-13C]-threonine. The analysis of the enantiomeric purity of labeled threonine by gas chromatography/mass spectrometric (GC/MS) analysis is reported as the N-TFA isopropyl ester derivative. The commercially available [1-13C]threonine comprised 98.7% of the L-enantiomer, enriched at 99 atom percentage excess (APE), and 1.3% of L-allo-threonine contaminant, also enriched at 99 APE. The enantiomeric purity of threonine in plasma of pigs infused for 10 h with [1-13C]threonine showed that the L-allo contaminant did not accumulate. The t-butyl dimethylsilyl derivatives of threonine, glycine, and 2-aminobutyrate (ABA) were used to measure the enrichment of these compounds in plasma and liver samples by GC/MS/selected ion monitoring analysis. Analyses were performed on between 1 and 5 nmol of each amino acid extracted from biological fluids and a 1:10 split injection. GC/MS parameters were assessed with standards at similar quantities and found to be satisfactory; e.g., injection of 1-10 nmol of glycine did not significantly alter the slope and the precision of the standard curve. The coefficient of variation of enrichment determination was less than 10% for standards enriched at 0.4 APE or more and biological samples enriched at 0.6 APE or greater. Within-animal coefficients of variation for four plasma samples obtained at equal intervals between 8 and 10 h of [1-13C]threonine infusion were 4, 21, and 24% for threonine, ABA, and glycine, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Leucine kinetics from [2H3]- and [13C]leucine infused simultaneously by gut and vein

In amino acid tracer kinetic studies of the fed state, ingested amino acid may be taken up during its initial transit through splanchnic tissues and thus not enter the plasma compartment where tracer is infused. To investigate this possibility, adult human subjects received simultaneous intravenous (iv) and intragastric (ig) leucine tracer infusions, first during a postabsorptive (PA) 4-h primed continuous ig infusion of L-[1-13C]-leucine and L-[5,5,5-2H3]leucine iv, followed on a separate day by a fed infusion, in which an ig infusion of a liquid formula was started 2 h before the tracer infusion and continued throughout the tracer study. Subjects were accustomed to a constant experimental diet supplying 1.5 g protein.kg-1.day-1 and 41-45 kcal.kg-1.day-1 for 7 and 12 days before the PA and fed studies, respectively. For the PA study, plasma enrichment for the ig tracer was 3.34 +/- 0.27 (SE) mol + excess and for the iv tracer it was 4.18 +/- 0.10 (P less than 0.02). Enrichments of alpha-keto-isocaproic acid (KIC) were 3.24 +/- 0.16 (ig) and 3.02 +/- 0.14 (iv), respectively [not significant (NS)]. For the fed study, plasma leucine enrichment for the ig tracer was 2.15 +/- 0.14 and for the iv tracer was 2.84 +/- 0.09 (P less than 0.02). KIC enrichments were 2.02 +/- 0.08 (ig) and 2.24 +/- 0.08 (iv), respectively (NS). In the PA study, the ratio of the plasma leucine enrichments for the ig and iv tracers was 0.80 +/- 0.06 and in the fed experiment, 0.76 +/- 0.05, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C]phenylalanine

Phenylalanine metabolism was determined in 41 studies of adult males (n = 10) consuming an energy-sufficient diet and receiving graded levels of dietary phenylalanine and excess tyrosine (40 mg.kg-1.day-1). After a dietary adaptation period to either 4.2 or 14.0 mg.kg-1.day-1 of phenylalanine; flux, plasma concentration, oxidation, and conversion to tyrosine were measured at test phenylalanine intakes of 5, 7, 10, 14, 21, 28, or 60 mg.kg-1.day-1. Oxidation was low and constant (1.3 mumol.kg-1.h-1) at intakes at or below 10 mg.kg-1.day-1 and increased linearly above this level. Conversion to tyrosine was minimal (2.1%) at these intakes. Breakpoint analysis showed the phenylalanine requirement with excess tyrosine to be 9.1 mg.kg-1.day-1. Plasma phenylalanine concentrations confirmed this estimate of requirement. Prior adaptation did not significantly affect overall flux, plasma concentration, or oxidation nor did it affect the requirement estimate. With the assumption that tyrosine can supply two-thirds of the aromatic amino acid requirement, these data suggest that the aromatic amino acid requirement should be 30 mg.kg-1.day-1 and the World Health Organization recommendation of 14 mg.kg-1.day-1 is an underestimate.

Quantitative partition of threonine oxidation in pigs: effect of dietary threonine

Kinetic aspects of threonine (Thr) metabolism were examined in eight pigs fed hourly with a diet containing either 0.68% (LT group) or 0.81% (HT group) of Thr (wt/wt), corresponding to 10 and 30% Thr excess, respectively, compared with an "ideal" diet. Primary production (PR) and disposal (DR) rates were obtained for Thr, glycine (Gly), and 2-keto-butyrate (KB) after a 12-h continuous infusion of L-[U-14C]-Thr together with [1-13C]Gly and a 6-h continuous infusion of [1-14C]KB. Transfer of Thr into secondary pools was also monitored, and from these the rates of Thr oxidation through the catabolic pathways of L-Thr 3-dehydrogenase (DR(Thr-Gly)) and threonine dehydratase (DR(Thr-KB)) were estimated. For the LT group the results were (mumol.kg-1.h-1) PR(Thr) 314 +/- 3, PR(Gly) 551 +/- 24, PR(KB) 41 +/- 3, DR(Thr-Gly) 22 +/- 2, and DR(Thr-KB) 7 +/- 1. For the HT group they were PR(Thr) 301 +/- 23, PR(Gly) 598 +/- 55, PR(KB) 39 +/- 4, DR(Thr-Gly) 32 +/- 2, and DR(Thr-KB) 8 +/- 1. The increase in Thr intake (14 mumol.kg-1.h-1, P less than 0.01) induced a commensurate increase in the sum of DR(Thr-Gly) and DR(Thr-KB) (14 mumol.kg-1.h-1, P less than 0.001) when liver was used as the precursor pool. This was mainly due to the increased DR(Thr-Gly) (13 mumol.kg-1.h-1, P less than 0.01); the change in DR(Thr-KB) was not statistically significant. By comparison of intracellular-to-plasma ratios of specific activities (or enrichments) for different tissues with each type of infusion, liver was shown to be the major site of production of Gly and KB from Thr. These data suggest that in fed growing pigs a 30% excess of Thr in the diet does not alter the partition of Thr oxidation, since 80% of Thr oxidation occurs through the L-Thr 3-dehydrogenase pathway for both LT and HT groups.

In vivo enzyme activity in inborn errors of metabolism

Low-dose continuous infusions of [2H5]phenylalanine, [1-13C]propionate, and [1-13C]leucine were used to quantitate phenylalanine hydroxylation in phenylketonuria (PKU, four subjects), propionate oxidation in methylmalonic acidaemia (MMA, four subjects), and propionic acidaemia (PA, four subjects) and leucine oxidation in maple syrup urine disease (MSUD, four subjects). In vivo enzyme activity in PKU, MMA, and PA subjects was similar to or in excess of that in adult controls (range of phenylalanine hydroxylation in PKU, 3.7 to 6.5 mumol/kg/h, control 3.2 to 7.9, n = 7; propionate oxidation in MMA, 15.2 to 64.8 mumol/kg/h, and in PA, 11.1 to 36.0, control 5.1 to 19.0, n = 5). By contrast, in vivo leucine oxidation was undetectable in three of the four MSUD subjects (less than 0.5 mumol/kg/h) and negligible in the remaining subject (2 mumol/kg/h, control 10.4 to 15.7, n = 6). These results suggest that significant substrate removal can be achieved in some inborn metabolic errors either through stimulation of residual enzyme activity in defective enzyme systems or by activation of alternate metabolic pathways. Both possibilities almost certainly depend on gross elevation of substrate concentrations. By contrast, only minimal in vivo oxidation of leucine appears possible in MSUD.

Effects of branched-chain amino acids on nitrogen metabolism in patients with cirrhosis

This study was conducted to determine whether an amino acid solution enriched with branched-chain amino acids altered protein catabolic rates and plasma ammonia in patients with cirrhosis. Nine stable subjects were given two peripheral intravenous infusions: a standard amino acid solution (solution A) and a branched-chain-enriched solution containing 97% more leucine (solution B). Each solution was given for separate 9-day (group 1, n = 6) or 3-day (group 2, n = 3) periods. Amino acid solutions delivered 0.7 gm protein.kg-1.day-1. Diets provided an additional 0.3 gm protein plus maintenance calories. Protein turnover was assessed by a primed continuous infusion of [1-14C] leucine in six patients (three patients in group 1 and three patients in group 2). Nitrogen balance and urinary 3-methyl histidine excretion were determined in group 1 patients. Compared with solution A, solution B increased leucine flux and leucine oxidation but had no significant effect on protein synthesis or catabolism based on the plasma specific activity of either leucine or alpha-ketoisocaproic acid. The additional leucine infused with solution B was quantitatively oxidized. Nitrogen balance did not differ with the two solutions and there was also no difference in the urinary excretion of 3-methyl histidine, suggesting that muscle protein catabolism was unchanged. Plasma ammonia concentration decreased significantly during the infusion of solution B and was associated with a slight fall in plasma glucagon concentration. The results indicated that a branched-chain-enriched amino acid solution did not alter protein synthesis or catabolism although it did lower the plasma ammonia when compared with a standard amino acid formula in stable cirrhotic patients.

Amino acid metabolism during exercise and following endurance training

Exercise results in marked alterations in amino acid metabolism within the body. The branched-chain amino acids, especially leucine, are particularly important since they contribute as energy substrates and as nitrogen donors in the formation of alanine, glutamine and aspartate. Leucine oxidation increases during whole-body exercise. Nonetheless, leucine's contribution as a muscle energy substrate is amll, being 3 to 4% at rest, and even lower (1%) during exercise. Traditional energy substrates (carbohydrates, lipid) remain most important. These rates of leucine oxidation can be readily attributed to skeletal muscle. Following endurance training, whole-body leucine oxidation is increased at rest and during exercise. Since its oxidation by muscle is not augmented, this whole-body increase is not due to muscle. Thus, other tissues within the body (i.e. liver) must account for this. Comparisons of leucine oxidation in rats and humans indicate that species differences exist. Much larger increases in leucine oxidation are brought about by exercise in humans. Calculations based on steady-state rates of leucine oxidation at rest and during exercise indicate that the recommended dietary intake of leucine is inadequate, since it is lower than measured whole-body rates of leucine oxidation. This inadequacy is exacerbated in individuals who are physically active.

Measurement of plasma leucine flux in rainbow trout (Salmo gairneri R.) using osmotic pump. Preliminary investigations on influence of diet

A method for the direct measurement of plasma amino acid flux, in rainbow trout, using the continuous infusion of L-[U-(14)C]-leucine with ALZET mini-osmotic pumps implanted into the peritoneal cavity, was developed. The fish were fed successively on three different diets (a commercial control diet, a semi-purified diet and a purified diet) during the 4 weeks of experiment. The amounts of radioactivity in the free pool and the protein of both the plasma and skin mucus were measured in these fish. The total flux of leucine was calculated either from the specific activity of leucine in the plasma (61.8 mg leucine. 100 g(-1).d(-1)) or from the amounts of labelled and unlabelled leucine flowing into the skin mucus protein (61.5 mg leucine. 100g(-1).day(-1)). The total plasma flux was not affected by changes in the diet. The contributions of total leucine oxidation and whole body protein turnover to plasma leucine flux (80% and 20% respectively) were estimated in fish fed the semi-purified diet.

Recovery of 13C in breath from NaH13CO3 infused by gut and vein: effect of feeding

Estimates of substrate oxidation obtained from appearance of 13C or 14C from tracers in breath must be corrected for retention of labeled carbon in the body. We aimed to determine the effect of a defined experimental diet and metabolic status on recovery of infused Na [13C]bicarbonate in breath. Six healthy male subjects consumed an experimental diet for 7 days before receiving a continuous infusion of formula without tracer on day 8 and received either an intragastric (ig) or intravenous (iv) infusion of Na [13C]bicarbonate on day 9 or 11 during a 4-h postabsorptive (PA), 4-h continuously fed period. A trend toward increasing PA breath enrichment during the first 7 diet days approached statistical significance (P = 0.051), whereas breath enrichments measured 3 h postbreakfast were consistently higher than PA values throughout and did not change over the 7-day period. Breath enrichments during a 4-h continuous ig infusion of formula without tracer on day 8 rose 2.0 +/- 0.5 atom percent excess (APE).10(-3) above base line (P less than 0.001, ANOVA). In the tracer studies, breath enrichments were similar for the ig and iv routes of tracer infusion. For the ig infusion the fraction of infused Na [13C]bicarbonate recovered in breath as 13CO2 was 0.74 +/- 0.02 for the PA period and 0.79 +/- 0.02 for the fed period. For the iv infusion the fraction recovered was 0.70 +/- 0.04 for the PA period and 0.82 +/- 0.03 for the fed period. Fractional recoveries were not significantly different for ig and iv routes of administration but were different for PA and fed periods (P less than 0.0001, 2-way ANOVA). The fractional recoveries for the fed period obtained here were similar to the value 0.81 reported in a number of other studies. Recovery of tracer in breath increased linearly with O2 uptake and CO2 production, suggesting that factors affecting respiratory gas exchange may alter recovery. We conclude that the primary factor determining label recovery is the immediate and recent nutritional status of the host.

Protein and amino acid requirements of the elderly

This paper has reviewed changes in body composition and protein metabolism that are associated with aging. Body protein mass decreases. Rates of both muscle and whole-body protein turnover/kg body weight decrease, and the percentage of turnover contributed by muscle to whole-body turnover decreases with age. Total plasma proteins, albumin, prealbumin, transferrin and ceruloplasmin levels decline with age, and, in the case of albumin levels, the decline continues when young-old are compared with old-old. Protein requirement of the elderly appears to have been underestimated by the factorial method. Nitrogen balance studies indicate a minimum of 0.7 g protein/kg/day in one study. Several studies show 0.8 g, whereas other studies show negative nitrogen balance at this level. The recommended protein intake is 1 g protein/kg/day, and 12-14% of total calories should be provided by protein. The few nitrogen balance studies that attempted to determine essential amino acid requirements are limited and contradictory.

Effects of branched-chain amino acids on protein turnover

Amino acid availability rapidly regulates protein synthesis and degradation. Increasing amino acid concentrations above the levels found in post-absorptive plasma stimulates protein synthesis in a dose-dependent manner at the level of mRNA translation-initiation and inhibits protein degradation by inhibiting lysosomal autophagy. The anabolic effects of insulin on protein synthesis and protein degradation are exerted at the same sites (i.e., peptide chain initiation and lysosomal stabilization) allowing for a rapid synergistic response when both amino acids and insulin increase after a protein-containing meal. In perfused liver preparations, protein anabolic effects are exerted by a group of amino acids acting in concert. The BCAA are among the amino acids required for stimulation of hepatic protein synthesis, but there is no evidence that BCAA or leucine alone are effective. Leucine alone is an important inhibitor of hepatic protein degradation, but maximal inhibition requires in addition several other regulatory amino acids. In heart and skeletal muscle in vitro, increasing the concentration of the three BCAA or of leucine alone reproduces the effects of increasing the supply of all amino acids in stimulating protein synthesis and inhibiting protein degradation. Skeletal muscle is the largest repository of metabolically active protein and a major contributor to total body nitrogen balance. Supplying energy alone (i.e., carbohydrate and lipids) cannot prevent negative nitrogen balance (net protein catabolism) in animals or humans; only provision of amino acids allows the attainment of nitrogen balance. In rats and in humans nourished parenterally, provision of balanced amino acid solutions or of only the three BCAA cause similar improvements in nitrogen balance for several days. There is some evidence that infusions of leucine alone can stimulate muscle protein synthesis in vivo; the effect may be transitory and was not observed by all investigators; provisions of excess leucine alone does not seem to affect total body or muscle protein degradation in vivo. In postabsorptive rats, in vivo, infusion of the three BCAA together stimulates muscle protein synthesis as much as the infusion of a complete amino acid mixture or of a mixture of essential amino acids; the in vivo effect requires coinfusion of glucose or of small (physiological) doses of insulin, suggesting synergism between insulin and amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)