Increased Amino Acid Oxidation Research Articles

Changes in urinary stable nitrogen isotope ratios during controlled short-term energy deficit: a proof-of-principle analysis

PurposeStable isotope ratios of nitrogen (δ15N) have previously been shown to increase in human hair during periods of catabolism. The goal of this study was to assess changes in δ15N in urinary urea (δ15Nurea) and Δ15N during a short-term controlled energy deficit.MethodsWe analyzed samples from 6 recreationally active men (25 ± 1 years, BMI: 23.5 ± 0.6 kg/m2) who participated in a repeated measures cross-over study involving 4 days of energy deficit (ED, ~ 15 kcal/kg FFM) without and with exercise (ED-EX, ED + EX) and control conditions in energy balance (CON-EX, CON + EX). δ15Nurea was analyzed from urine samples, and Δ15N was calculated as δ15Nurea–δ15Ndiet, with δ15Ndiet obtained from diet prescriptions.Resultsδ15Nurea was significantly elevated in ED-EX (4.4 ± 0.2‰) when compared to CON-EX (3.7 ± 0.1‰; p = 0.026) and CON + EX (3.34 ± 0.13‰, p = 0.001). As a consequence, Δ15N was positive in ED-EX (0.2 ± 0.2‰) and remained negative in ED + EX (− 0.6 ± 0.5‰), CON-EX (− 1.0 ± 0.2) and CON + EX (− 1.1 ± 0.2). Differences in Δ15N were significant between ED-EX and CON-EX (p = 0.005) and ED-EX and CON + EX (p = 0.006).ConclusionOur results suggest that δ15Nurea and subsequently Δ15N are responsive to a short-term energy deficit, likely due to increased amino acid oxidation to meet energy demands and preferable elimination of 14N.

Natural Isotope Abundances of Carbon and Nitrogen in Tissue Proteins and Amino Acids as Biomarkers of the Decreased Carbohydrate Oxidation and Increased Amino Acid Oxidation Induced by Caloric Restriction under a Maintained Protein Intake in Obese Rats.

A growing body of evidence supports a role for tissue-to-diet 15N and 13C discrimination factors (Δ15N and Δ13C), as biomarkers of metabolic adaptations to nutritional stress, but the underlying mechanisms remain poorly understood. In obese rats fed ad libitum or subjected to gradual caloric restriction (CR), under a maintained protein intake, we measured Δ15N and Δ13C levels in tissue proteins and their constitutive amino acids (AA) and the expression of enzymes involved in the AA metabolism. CR was found to lower protein mass in the intestine, liver, heart and, to a lesser extent, some skeletal muscles. This was accompanied by Δ15N increases in urine and the protein of the liver and plasma, but Δ15N decreases in the proteins of the heart and the skeletal muscles, alongside Δ13C decreases in all tissue proteins. In Lys, Δ15N levels rose in the plasma, intestine, and some muscles, but fell in the heart, while in Ala, and to a lesser extent Glx and Asx, Δ13C levels fell in all these tissues. In the liver, CR was associated with an increase in the expression of genes involved in AA oxidation. During CR, the parallel rises of Δ15N in urine, liver, and plasma proteins reflected an increased AA catabolism occurring at the level of the liver metabolic branch point, while Δ15N decreases in cardiac and skeletal muscle proteins indicated increased protein and AA catabolism in these tissues. Thus, an increased protein and AA catabolism results in opposite Δ15N effects in splanchnic and muscular tissues. In addition, the Δ13C decrease in all tissue proteins, reflects a reduction in carbohydrate (CHO) oxidation and routing towards non-indispensable AA, to achieve fuel economy.

A supplemental intravenous amino acid infusion sustains a positive protein balance for 24\xa0hours in critically ill patients

BackgroundProviding supplemental amino acids to ICU patients during a 3-h period results in improved whole-body net protein balance, without an increase in amino acid oxidation. The primary objective was to investigate if a 24-h intravenous amino acid infusion in critically ill patients has a sustained effect on whole-body protein balance as was seen after 3 h. Secondary objectives were monitoring of amino acid oxidation rate, urea and free amino acid plasma concentrations.MethodsAn infusion of [1-13C]-phenylalanine was added to ongoing enteral nutrition to quantify the enteral uptake of amino acids. Primed intravenous infusions of [ring-2H5]-phenylalanine and [3,3-2H2]-tyrosine were used to assess whole-body protein synthesis and breakdown, to calculate net protein balance and to assess amino acid oxidation at baseline and at 3 and 24 hours. An intravenous amino acid infusion was added to nutrition at a rate of 1 g/kg/day and continued for 24 h.ResultsEight patients were studied. The amino acid infusion resulted in improved net protein balance over time, from -1.6 ± 7.9 μmol phe/kg/h at 0 h to 6.0 ± 8.8 at 3 h and 7.5 ± 5.1 at 24 h (p = 0.0016). The sum of free amino acids in plasma increased from 3.1 ± 0.6 mmol/L at 0 h to 3.2 ± 0.3 at 3 h and 3.6 ± 0.5 at 24 h (p = 0.038). Amino acid oxidation and plasma urea were not altered significantly.ConclusionWe demonstrated that the improvement in whole-body net protein balance from a supplemental intravenous amino acid infusion seen after 3 h was sustained after 24 h in critically ill patients.Trial registrationThis trial was prospectively registered at Australian New Zealand Clinical Trials Registry. ACTRN, 12615001314516. Registered on 1 December 2015.

Safety and Efficacy of Early Parenteral Lipid and High-Dose Amino Acid Administration to Very Low Birth Weight Infants

To assess the efficacy and safety of early parenteral lipid and high-dose amino acid (AA) administration from birth onwards in very low birth weight (VLBW, birth weight <1500 g) infants. VLBW infants (n = 144; birth weight 862 ± 218 g; gestational age 27.4 ± 2.2 weeks) were randomized to receive 2.4 g of AA kg(-1) · d(-1) (control group), or 2.4 g AA kg(-1) · d(-1) plus 2-3 g lipids kg(-1) · d(-1) (AA + lipid group), or 3.6 g AA kg(-1) · d(-1) plus 2-3 g lipids kg(-1) · d(-1) (high AA + lipid group) from birth onwards. The primary outcome was nitrogen balance. The secondary outcomes were biochemical variables, urea rate of appearance, growth rates, and clinical outcome. The nitrogen balance on day 2 was significantly greater in both intervention groups compared with the control group. Greater amounts of AA administration did not further improve nitrogen balance compared with standard AA dose plus lipids and was associated with high plasma urea concentrations and high rates of urea appearance. No differences in other biochemical variables, growth, or clinical outcomes were observed. In VLBW infants, the administration of parenteral AA combined with lipids from birth onwards improved conditions for anabolism and growth, as shown by improved nitrogen balance. Greater levels of AA administration did not further improve the nitrogen balance but led to increased AA oxidation. Early lipid initiation and high-dose AA were well tolerated.

Prolonged maternal amino acid infusion in late-gestation pregnant sheep increases fetal amino acid oxidation

Protein supplementation during human pregnancy does not improve fetal growth and may increase small-for-gestational-age birth rates and mortality. To define possible mechanisms, sheep with twin pregnancies were infused with amino acids (AA group, n = 7) or saline (C group, n = 4) for 4 days during late gestation. In the AA group, fetal plasma leucine, isoleucine, valine, and lysine concentrations were increased (P < 0.05), and threonine was decreased (P < 0.05). In the AA group, fetal arterial pH (7.365 +/- 0.007 day 0 vs. 7.336 +/- 0.012 day 4, P < 0.005), hemoglobin-oxygen saturation (46.2 +/- 2.6 vs. 37.8 +/- 3.6%, P < 0.005), and total oxygen content (3.17 +/- 0.17 vs. 2.49 +/- 0.20 mmol/l, P < 0.0001) were decreased on day 4 compared with day 0. Fetal leucine disposal did not change (9.22 +/- 0.73 vs. 8.09 +/- 0.63 micromol x min(-1) x kg(-1), AA vs. C), but the rate of leucine oxidation increased 43% in the AA group (2.63 +/- 0.16 vs. 1.84 +/- 0.24 micromol x min(-1) x kg(-1), P < 0.05). Fetal oxygen utilization tended to be increased in the AA group (327 +/- 23 vs. 250 +/- 29 micromol x min(-1) x kg(-1), P = 0.06). Rates of leucine incorporation into fetal protein (5.19 +/- 0.97 vs. 5.47 +/- 0.89 micromol x min(-1) x kg(-1), AA vs. C), release from protein breakdown (4.20 +/- 0.95 vs. 4.62 +/- 0.74 micromol x min(-1) x kg(-1)), and protein accretion (1.00 +/- 0.30 vs. 0.85 +/- 0.25 micromol x min(-1) x kg(-1)) did not change. Consistent with these data, there was no change in the fetal skeletal muscle ubiquitin ligases MaFBx1 or MuRF1 or in the protein synthesis regulators 4E-BP1, eEF2, eIF2alpha, and p70(S6K). Decreased concentrations of certain essential amino acids, increased amino acid oxidation, fetal acidosis, and fetal hypoxia are possible mechanisms to explain fetal toxicity during maternal amino acid supplementation.

Anticatabolic Effects of Avoiding Preoperative Fasting by Intravenous Hypocaloric Nutrition

We tested the hypothesis that the avoidance of preoperative fasting by hypocaloric nutrition attenuates protein catabolism after surgery. Prolonged fasting before major abdominal procedures has been demonstrated to accentuate the catabolic response to surgery. Twenty-two patients undergoing colorectal cancer surgery were randomly assigned to receive glucose and amino acids intravenously starting either 20 hours before the operation or with surgical skin incision. Nutrition was administered until the second postoperative day, with glucose providing 50% and amino acids 20% of each patient's measured resting energy expenditure. Whole body leucine and glucose kinetics were assessed by L-[1-(13)C]leucine and [6,6-(2)H(2)]glucose before and after surgery. Fractional synthesis rates of muscle protein, albumin, and fibrinogen were determined using primed continuous infusions of L-[(2)H(5)]phenylalanine postoperatively, whereas the expression of mRNA of proteolytic genes in muscle (Mafbx/atrogin-1, ubiquitin, Murf 1) was determined by quantitative RT-PCR. Circulating concentrations of glucose, lactate, amino acids, insulin, glucagon, and cortisol were also measured. This study has been registered at ClinicalTrials.gov (Identifier: NCT00614133). Preoperative feeding inhibited endogenous protein breakdown (fasting group: 128 +/- 23 micromol . kg(-1) . h(-1); nutrition group: 96 +/- 22 micromol . kg(-1) . h(-1); P = 0.02) and blunted the increase in amino acid oxidation (fasting group: 27 +/- 5 micromol . kg(-1) . h(-1); nutrition group: 20 +/- 5 micromol . kg(-1) . h(-1); P = 0.03), resulting in positive whole-body protein balance after surgery (fasting group: -10 +/- 4 micromol . kg(-1) . h(-1); nutrition group: 1 +/- 3 micromol . kg(-1) . h(-1); P < 0.001). This anabolic response was associated with decreased muscle proteolytic gene expression and increased hepatic albumin synthesis. Total plasma protein, fibrinogen, and muscle protein synthesis were not affected. Hypocaloric nutrition decreases protein catabolism, with a contribution from the ubiquitin pathway in muscle, and stimulates albumin synthesis after colorectal surgery if initiated 1 day before the operation.

Postoperative Protein Sparing With Epidural Analgesia and Hypocaloric Dextrose

We examined the hypothesis that epidural analgesia prevents the increase in amino acid oxidation after elective colorectal surgery in patients receiving hypocaloric infusion of dextrose. Increased oxidative protein loss after surgery may adversely affect postoperative outcome. We have previously shown that effective segmental pain relief by epidural analgesia improves postoperative substrate utilization, resulting in less protein catabolism. We randomly allocated 10 patients to receive general anesthesia combined with epidural analgesia using bupivacaine/fentanyl and 10 to receive general anesthesia followed by patient-controlled analgesia with intravenous morphine. All patients received a peripheral 72-hour infusion of dextrose 10% from the day before until the second day after surgery. The dextrose infusion rate was adjusted to provide 50% of the patients' resting energy expenditure. The primary end point was whole-body leucine oxidation as determined by a stable isotope tracer technique (l-[1-C]leucine). In the intravenous analgesia group, leucine oxidation increased from 19 +/- 4 to 28 +/- 6 micromol kg h after surgery. Epidural analgesia prevented this increase of leucine oxidation (preoperative 21 +/- 6 micromol kg h, postoperative 21 +/- 5 micromol kg h). This difference was statistically significant (P = 0.01; analysis of variance for repeated measures). Perioperative epidural analgesia and hypocaloric dextrose infusion suppress the postoperative increase in amino acid oxidation, thereby saving more than 100 g of lean body mass per day.

Potential impact of nematode parasitism on nutrient partitioning forwool production, growth and reproduction in sheep

Most ovine studies indicate that protein supply is a key determinant of the ability to mount a strong immunity-associated response to gastrointestinal parasites. This paper quantifies the demands placed on protein supply by various physiological states, to determine why this is so. Protein requirements in fast growing lambs and ewes in late pregnancy and early lactation are substantially increased, amounting to a 1.43-, 0.69- and 2.50-fold increase in maintenance requirement for dietary protein. Infestation with internal parasites may result in a total net loss of about 12 g protein/day, which consists of a net loss of about 6 g from the gastrointestinal tract and a further loss of about 6 g through increased amino acid oxidation in the body. This loss from the infection accounts for about 0.57-, 0.71-, 0.14- and 0.77-fold the protein requirements respectively for body growth, pregnancy, lactation and wool production, and creates strong competition for protein. Such considerable demands for protein by activation of immunity-associated response diminishes the animal's nutrient status in late pregnancy and particularly in early lactation. The total demand for protein cannot be met by pastures during the reproductive season in Mediterranean environments, and it is this nutrient deficiency, relative to the demand of sheep, which results in weak immune responses over these periods. The immunity-associated response requires more protein rather than energy, particularly sulfur amino acids such as cysteine and probably methionine. Therefore, parasite infection has a more severe effect on wool- than meat-producing sheep. Body condition of sheep could be a predominant factor in determining immune status and current intake and quality of pastures needs to be considered to further boost immunity-associated response. Wool- and meat-producing genotypes, and parasite-resistant sheep in particular, may have different interactions with nutrition in response to parasite infection. These factors need to be integrated with anthelmintic treatment to establish an effective strategy to control parasite infection in Mediterranean environments.

Influence of the Protein Digestion Rate on Protein Turnover in Young and Elderly Subjects

It has long been recognized that numerous dietary parameters, such as the amount and type of protein and nonprotein energy sources, affect protein metabolism. More recently, we demonstrated that the protein digestion rate is an independent factor regulating postprandial protein gain. Indeed, in young men, using a non-steady-state approach and intrinsically labeled milk protein fractions [whey protein (WP) and casein (CAS)] we showed that a slow digested dietary protein (CAS) induced a greater protein gain than a fast one (WP). The mechanisms of this gain also differed according to the protein rate of digestion. WP stimulated amino acid oxidation and protein synthesis without modifying proteolysis, whereas CAS increased amino acid oxidation and protein synthesis to a lesser extent and strongly inhibited proteolysis. These results led to the concept of "slow" and "fast" protein and were confirmed by further experiments during which the meals tested presented different digestion rates but were otherwise identical in terms of amino acid profile. We also analyzed the effects of fat and carbohydrates added to CAS and WP. Our preliminary results suggest that added nonprotein energy sources to CAS and WP attenuated the differences in both the protein digestion rate and protein gain. Finally, and in contrast to young subjects, a "fast" protein may be more beneficial than a "slow" one in elderly subjects, to limit body protein loss. However, long-term studies are needed to confirm this age-related effect.

The determinants of protein catabolism in neonates on extracorporeal membrane oxygenation

Background/Purpose: Protein catabolism appears to be markedly elevated among neonates on extracorporeal membrane oxygenation (ECMO). The aim of this study was to determine the effect of dietary caloric intake on protein catabolism in neonates on ECMO to help construct therapies that may promote anabolism. Methods: Twelve total parenteral nutrition (TPN)-fed (88.1 ± 5.0 [SE] kcal/kg/d; range, 60 to 113 kcal/kg/d; 2.3 ± 0.2 g/kg/d protein) neonates were studied on ECMO at day of life 7.2 ± 0.8 d. Protein kinetics were determined using infusions of NaH 13CO 3 and 1-[ 13C]leucine. Results: As expected, C-reactive protein levels were significantly elevated compared with normal controls (44.0 ± 7.6 mg/L v 1.9 ± 1.1 mg/L; P < .001). Negative protein balance (−2.3 ± 0.6 g/kg/d; range, 1 to −6.4 g/kg/d) highly correlated ( r = −0.88, P < .001) with total protein turnover. Increased dietary caloric intake correlated with increased amino acid oxidation ( r = 0.85, P < .001), increased total protein turn-over ( r = 0.73, P < .01), continued negative protein balance ( r = 0.72, P < .01), increased whole-body protein breakdown ( r = 0.66, P < .05), and increased CO 2 production rate ( r = 0.73, P < .01). Conclusions: A surplus of dietary caloric intake does not improve protein catabolism and merely increases CO 2 production in these highly stressed neonates. Thus, judicious caloric supplementation is warranted.

Dexamethasone Negatively Impacts Nitrogen Metabolism and Growth in Newborn Piglets † 1550

Pharmacological doses of dexamethasone (dex) are commonly used to treat very low birth weight infants during the first weeks of life. Although its beneficial pulmonary effects are well recognized, detrimental effects are seen in other tissues. Our goal was to develop a piglet model in which to study the protein catabolic effects of dex. Experiment 1: Nine female piglets(1500-2165g) were removed from the sow within 12h of birth and fitted with umbilical catheters for blood sampling and dex administration. Pigs were housed in individual cages which allowed for control of intake and quantitative urine collection. A 5 day baseline preceded the 7 day treatment period. Five pigs received dex (1mg/(kg*d)) and 4 pigs served as saline controls. Daily fasting (7h post feeding) and fed (1h post) blood samples were obtained. Dex administration resulted in 40% reductions in weight gain (72 vs. 43 g/(kg*d), p=0.0004) and snout to rump length (1.15 vs. 0.61 cm/d, p=0.015). Fasting BUN (mg/dl) was elevated within 24h after dex (2.2 vs. 4.1, p=0.002), and continued to rise (d6: 2.2 vs. 11.7, p=0.0001). Cumulative urinary N excretion (g/kg) in the dex treated pigs was 3 times greater than in controls(1.7 vs. 5.5, p=0.0001). Fasting blood glucose was not altered by dex; however, the average fed state blood glucose was ≈20% higher. In the fed state, plasma insulin was elevated and a doubling of the insulin:glucose ratio was observed (12 vs. 26 pmol:mmol, p=0.02). Experiment 2: After catheters were secured, piglets returned to the sow for the entire study. The design was modified to include 4 day baseline, treatment, and recovery periods. Female piglets (n=21) were studied. One of 4 levels of dex (0, 0.25, 0.5, 1 mg/(kg*d)) was given. A 25-75% rise in BUN from baseline was evident by 12h in all dex groups (p=0.0001). BUN rose over time in the dex pigs with no differences between the 0.25 and 0.5 groups. With removal of dex, BUN returned to baseline in 3 to 4d. The ratio of insulin:glucose doubled by 12h of dex treatment, was maintained, and returned to baseline 24h into recovery. Linear growth was affected by dose during treatment (1.2, 0.66, 0.44, 0.29 cm/d); all pigs grew at the control rate during the 4d recovery. Dex treatment altered metabolism as seen by elevations in BUN and N excretion, and disruption of glucose and insulin homeostasis. Marked reductions in weight gain and linear growth were observed. It is anticipated that dex treatment will result in increased amino acid oxidation and decreased muscle protein synthesis in the piglet model.

Post-prandial protein metabolism

Current post-prandial studies of amino acid metabolism and utilization are consistent with a feeding mechanism mediated primarily by insulin and amino acids, with the balance between protein conservation and net deposition dependent on the amino acid supply [1-13C]leucine post-prandial kinetic tracer studies of leucine oxidation, non-oxidative disappearance and endogenous appearance allow study of the regulation of whole-body amino acid oxidation, protein synthesis and proteolysis. On the basis of these studies it appears that for leucine oxidation, the main determinant of the efficiency of protein utilization, the overriding regulatory influence is substrate availability rather than insulin. Such substrate sensitivity is manifest throughout the physiological range of insulin down to the lowest insulin levels observed suggesting that a basal insulin need is not an important part of regulation of this important catabolic pathway. The key protein turnover response is an inhibition of proteolysis sufficient to limit any increases in amino acid levels thus limiting any increase in amino acid oxidation. It appears that the influences of amino acids and insulin on proteolysis are separate and additive and may both be receptor mediated so that extracellular amino acid levels can regulate intracellular levels. It is likely that protein synthesis is regulated by intracellular amino acid levels but post-prandial stimulation through increases in amino acid levels appears to be unhelpful because of parallel increases in amino acid oxidation. Evidence for any influence of insulin on protein synthesis has yet to be unequivocally identified.

Glucocorticoids and acidosis stimulate protein and amino acid catabolism in vivo

We have shown that chronic metabolic acidosis in awake rats accelerates whole body protein turnover using stochastic modeling and a continuous infusion of L-[1-13C] leucine. To delineate the role that glucocorticoids play in mediating these catabolic responses, we measured protein turnover in awake, chronically catheterized, adrenalectomized rats in the presence or absence of glucocorticoids and/or a NH4Cl feeding regimen which induced chronic metabolic acidosis. In adrenalectomized rats receiving no glucocorticoids there was no statistical difference in amino acid oxidation, protein degradation or synthesis whether or not the rats had acidosis. In contrast, chronically acidotic, adrenalectomized rats receiving glucocorticoids demonstrated accelerated whole body protein turnover with a 84% increase in amino acid oxidation and a 26% increase in protein degradation, compared to rats not receiving glucocorticoids or those given the same dose of glucocorticoids but without acidosis. We conclude that metabolic acidosis accelerates amino acid oxidation and protein degradation in vivo, and that glucocorticoids are necessary but not sufficient to mediate the catabolic effects of metabolic acidosis.

NAD +]/[NADH] redox-state metabolites of freezed-clamped livers of rats fed casein or gelatin diets

Postprandial thermic effects of dietary protein meals have been hypothesized to be related to a diet-mediated increase in amino acid oxidation that may affect the in vivo redox states. The objective of the present study was to determine the effects of isoenergetic isonitrogenous restricted (80% of the ad libitum gross energy intake) diets containing either casein (30%) or gelatin (15% casein, and 12.7% gelatin) for 1, 3, or 28 days to Wistar rats on liver metabolites characterizing cytoplasmic and mitochondrial redox-states. Both casein and gelatin diets produced a progressive decline in the concentrations of lactate and an increase in the concentrations of acetoacetate with the duration of feeding. β-hydroxybutyrate concentrations were reduced on day 3 in relation to day 1 and were highest on day 28. Pyruvate levels changed moderately or stayed the same. The calculated redox ratios were increased in cytoplasm on day 3 and 28 in relation to day 1 and were insignificantly higher with the gelatin diet compared with the casein diet. The corresponding redox ratios in mitochondria were increased on day 3 and remained high on day 28 in relation to day 1 and were significantly higher on day 28 in the gelatin diet-fed group in relation to the casein diet-fed group. Adenine nucleotide content and energy charges were only slightly modified by the experimental conditions used. It is suggested that these results are in accordance with a stimulation of amino acid oxidation and reoxidation of reducing equivalents feeding the protein diets.

Protein requirements of soccer

Although the physical demands of soccer have been studied frequently over the years, there has been little attention to the dietary protein needs of soccer players. Recent data from both moderate-intensity, prolonged (endurance) and heavy-resistance (strength) exercise studies indicate that the current recommendations (0.8 g per kg body mass per day) for protein intake are probably suboptimal for individuals who are chronically active. Endurance athletes need more dietary protein than sedentary individuals to maintain an auxiliary fuel source which appears to become increasingly important as exercise is prolonged. Strength athletes can also benefit from a greater protein intake than is currently recommended because it appears that, in combination with heavy-resistance training, it can provide an enhanced stimulus for muscle development. Soccer is a high-intensity, intermittent activity which requires aspects of both strength and endurance over a period of 90 min. As a result, soccer players would be likely to benefit from protein intakes above current recommendations not only because of their potential to enhance strength, but also to provide a supply of amino acids for any increased amino acid oxidation that may occur during training and in competition. Based on the related exercise studies completed to date, it appears that a protein intake of 1.4-1.7 g kg-1 day-1 should be adequate for soccer players. Assuming free access to a wide variety of foods, this protein intake can be easily obtained by most soccer participants. Individuals at greatest risk of falling short of this intake include those who are growing (especially children in developing countries where suboptimal protein intake may be common) or those who consume proteins of lower quality. Although diets high in protein are frequently condemned because of possible kidney problems, it appears these concerns have been over-emphasized. There is no evidence that protein intakes in the range recommended will cause healthy individuals any concerns.

Effects of Glucose Infusion on Leucine Transamination and Oxidation in the Ovine Fetus

During fasting of the ewe, the rate of amino acid oxidation by the ovine fetus increases substantially. We hypothesized that the increase in amino acid oxidation derived mainly from reduced protein synthesis. We further hypothesized that fetal glucose supplementation would result in diminished amino acid oxidation. To test these hypotheses, nine ovine fetuses were infused with [15N,1-13C]leucine to determine the rates of leucine appearance and disposal. Simultaneously, the fetal uptake of leucine was determined. Animals were studied in the fed and fasted state. After baseline measurements, glucose was infused into the fetal inferior vena cava at a rate estimated to match the fetal glucose uptake. Results of these studies indicate that leucine nitrogen flux, leucine carbon flux and fetal leucine uptake were constant. Leucine oxidation was increased by 50% in the fasted state (6.3 versus 13.4 mumol/min); glucose infusion resulted in a 25% decline in oxidation (to 10.4 mumol/min) in the fasted state, but had no effect in the fed state. Mean leucine umbilical uptake during fasting was 9.3 mumol/min, 4.1 mumol/min less than leucine oxidation. These data suggest that leucine (and potentially other amino acids) may be in negative balance during maternal fasting, and can be spared by supplementation of the fetus with exogenous glucose.

Regulation of lipolysis in severely burned children.

In this study, the rates of lipid mobilization and of lipolysis have been quantified in severely burned children. In all 12 patients studied, the basal rates were determined. In seven patients, the lipolytic responsiveness to an infusion of epinephrine (0.015 micrograms/kg/min) was tested, and in the other five patients, the response to beta-adrenergic blockade (propranolol, 1 mg/kg) was tested. The rate of appearance (Ra) of free fatty acids (FFA) was quantified by means of the infusion of 1-13C-palmitate to determine the rate of lipid mobilization, and Ra glycerol was determined using d5-glycerol to assess the rate of lipolysis more directly. In five patients, body composition was determined after recovery by means of H2(18)O dilution. The basal rate of lipolysis was higher than normal in the burned children. In four of the seven patients infused with epinephrine, there was a pronounced increase in Ra glycerol. In all patients given beta-blockade, Ra glycerol decreased greatly. Changes in Ra FFA corresponded with the changes in Ra glycerol in each case. Total body fat was very low (approximately 2% body weight), reflecting the surgical removal of fat in the process of burn wound excision. From these data it is concluded that lipolytic responsiveness to catecholamines in severely burned children is variable, but not absent, despite chronically elevated levels of catecholamines. The total extent of lipolysis may be limited by the available fat mass in children treated with fascial excision. In such patients, the limitation in the ability to mobilize an adequate amount of FFA to fully meet energy requirements provides an important rationale for the clinical practice of providing nutritional support in hourly boluses, as opposed to infrequent meals, since any period of even a few hours in which nutrients are not being absorbed will result in an energy substrate deficiency and consequent increase in amino acid oxidation.

Protein Nutrition for the Athlete

Endurance exercise results in a protein catabolic state characterized by decreased protein synthesis, increased amino acid oxidation, and increased conversion of amino acids to glucose. The adaptive response to performance of strength exercise, on the other hand, results in an anabolic state in hypertrophying muscles, and the accretion of protein is the result of increased protein synthesis. Because of changes in protein metabolism there is an increased dietary requirement for protein in both endurance and strength exercise. However, the normal dietary intake of protein is adequate for athletes as long as the energy intake is sufficient to maintain body weight. There is little scientific evidence that consumption of large protein supplements will have any beneficial effect on muscle hypertrophy, muscular strength, or physical performance.

The importance of protein for athletes.

Although it is generally believed that carbohydrate and fat are the only sources of energy during physical activity, recent experimental results suggest that there are also significant alterations in protein metabolism during exercise. Depending on several factors, including intensity, duration and type of exercise, as well as prior diet, training, environment and perhaps even gender or age, these changes may be quite large. Generally, exercise promotes: a decrease in protein synthesis (production) unless the exercise duration is prolonged (greater than 4h) when increases occur; either an increase or no change in protein catabolism (breakdown); and an increase in amino acid oxidation. In addition, significant subcellular damage to skeletal muscle has been shown following exercise. Taken together, these observations suggest that the protein requirements of active individuals are greater than those of inactive individuals. Although the underlying reasons are different, this statement applies to both endurance and strength/power athletes. At present, it is not possible to precisely determine protein requirements. However, because deficiencies in total protein or in specific amino acids may occur, we suggest that athletes consume 1.8 to 2.0 g of protein/kg of bodyweight/day. This is approximately twice the recommended requirement for sedentary individuals. For some athletes this may require supplementation; however, these quantities of protein can be easily obtained in a diet where 12 to 15% of the total energy is from protein. Although the effect of exercise on protein metabolism has been studied for many years, numerous questions remain. Hopefully, with the recent renewed interest in this area of study, most of these answers will soon be available.

A rationale for administering leukocyte endogenous mediator to protein malnourished, hospitalized patients

Leukocyte endogenous mediator is a low molecular-weight protein synthesized by circulating monocytes and fixed macrophages of the reticuloendothelial system. Exogenous administration of leukocyte endogenous mediator to a well-nourished animal stimulates both specific and nonspecific immune function and replicates the protein metabolic response to infection, characterized by fever and increased amino acid oxidation, skeletal protein degradation and synthesis of “acute-phase” proteins. Leukocyte endogenous mediator administration also affords protection against semilethal doses of bacteremia in the well-nourished animal. In the protein-depleted host, synthesis or release of leukocyte endogenous mediator in response to infection appears to be reduced and the attenuated metabolic response may be attributed, in part, to a deficit in its production. However, nutritional repletion of the malnourished patient results in restoration of the capacity to produce leukocyte endogenous mediator usually within three to seven days, if adequate dietary protein is provided. Since protein malnutrition is associated with increased incidence and severity of bacterial infections, we postulate that the reduced synthesis and/or release of leukocyte endogenous mediator in protein malnutrition is detrimental. In those critically-ill, malnourished patients who cannot endogenously synthesize leukocyte endogenous mediator, and for clinical reasons cannot be repleted rapidly or are already infected and/or undergoing operative stress, exogenous administration of leukocyte endogenous mediator should be considered along with nutritional support. Administration of this protein to a seriously-ill malnourished individual should produce a metabolic profile of fever, increased urinary nitrogen excretion and falls in serum albumin concentrations that are generally considered pathologic. However, administration of leukocyte endogenous mediator over short periods of time should also provide the anabolic impetus for the augmented synthesis of proteins beneficial to recovery. In most cases, these countervailing forces of anabolism and catabolism should be of benefit to the host if the response to infection and injury is viewed as a physiologic redistribution of endogenous nutrients to meet the more critical and immediate needs of the stressed patient.