Muscle Fractional Synthetic Rate Research Articles

Combined ingestion of protein and free leucine with carbohydrate increases postexercise muscle protein synthesis in vivo in male subjects

The present study was designed to determine postexercise muscle protein synthesis and whole body protein balance following the combined ingestion of carbohydrate with or without protein and/or free leucine. Eight male subjects were randomly assigned to three trials in which they consumed drinks containing either carbohydrate (CHO), carbohydrate and protein (CHO+PRO), or carbohydrate, protein, and free leucine (CHO+PRO+Leu) following 45 min of resistance exercise. A primed, continuous infusion of L-[ring-13C6]phenylalanine was applied, with blood samples and muscle biopsies collected to assess fractional synthetic rate (FSR) in the vastus lateralis muscle as well as whole body protein turnover during 6 h of postexercise recovery. Plasma insulin response was higher in the CHO+PRO+Leu compared with the CHO and CHO+PRO trials (+240 +/- 19% and +77 +/- 11%, respectively, P < 0.05). Whole body protein breakdown rates were lower, and whole body protein synthesis rates were higher, in the CHO+PRO and CHO+PRO+Leu trials compared with the CHO trial (P < 0.05). Addition of leucine in the CHO+PRO+Leu trial resulted in a lower protein oxidation rate compared with the CHO+PRO trial. Protein balance was negative during recovery in the CHO trial but positive in the CHO+PRO and CHO+PRO+Leu trials. In the CHO+PRO+Leu trial, whole body net protein balance was significantly greater compared with values observed in the CHO+PRO and CHO trials (P < 0.05). Mixed muscle FSR, measured over a 6-h period of postexercise recovery, was significantly greater in the CHO+PRO+Leu trial compared with the CHO trial (0.095 +/- 0.006 vs. 0.061 +/- 0.008%/h, respectively, P < 0.05), with intermediate values observed in the CHO+PRO trial (0.0820 +/- 0.0104%/h). We conclude that coingestion of protein and leucine stimulates muscle protein synthesis and optimizes whole body protein balance compared with the intake of carbohydrate only.

Protein anabolic effects of insulin and IGF-I in the ovine fetus.

We determined the effect of insulin and/or recombinant human (rh)IGF-I infusion on ovine fetal phenylalanine kinetics, protein synthesis, and phenylalanine accretion. The chronically catheterized fetal lamb model was used at 130 days gestation. All studies were performed while fetal glucose and amino acid concentrations were held constant. Experimental infusates were 1). saline, 2). rhIGF-I plus a replacement dose of insulin (40 nmol), 3). insulin (890 mIU/h), and 4). IGF-I plus insulin (40 nmol IGF-I/h and 890 mIU insulin/h). Both hormones increased glucose and amino acid utilization, with insulin having a greater effect. The major effect on phenylalanine kinetics was a pronounced fall in phenylalanine hydroxylation, again with insulin having the greatest effect. Whole body protein breakdown was not significantly altered by either hormone; whole body protein synthesis was significantly increased during the combined infusion. Protein accretion was increased by both hormones, with the greatest increase during combined infusion. The fractional synthetic rate (FSR) of circulating albumin was increased by IGF-I but not by insulin. Both hormones significantly increased skeletal muscle FSR without a synergistic effect. The anabolic effects of insulin and IGF-I were more pronounced in these studies than in previous studies where amino acid concentrations were not maintained. The present data also suggest that insulin and IGF-I promote fetal growth through distinct, organ-specific mechanisms.

Acute response of net muscle protein balance reflects 24-h balance after exercise and amino acid ingestion.

The purpose of this study was to determine if the acute anabolic muscle response to resistance exercise and essential amino acids (EAA) reflects the response over 24 h. Seven subjects participated in the following two 24-h studies: 1) resting (REST) and 2) rest plus resistance exercise and consumption of EAA (ES). Net balance (NB) across the leg was determined for four amino acids. [(13)C(6)]phenylalanine was infused to determine mixed muscle fractional synthetic rate (FSR). Twenty-four-hour FSR was significantly greater for ES than for REST (P = 0.003). Exchange of phenylalanine across the leg was -194 +/- 74 (SE) mg for ES and -371 +/- 88 mg for REST (P = 0.07) over 24 h and 229 +/- 42 mg (ES) and 28 +/- 15 mg (REST; P < 0.01) over 3 h corresponding to exercise and EAA consumption for ES. The difference in phenylalanine exchange between REST and ES was not different for measurements over 24 and 3 h. Increases in NB during ES were primarily the result of increases in protein synthesis. Results for other amino acids were similar. The acute anabolic response of muscle to EAA intake and exercise is additive to the response at rest and thus reflects the 24-h response.

Parenteral amino acids increase albumin and skeletal muscle protein fractional synthetic rates in premature newborn minipigs.

Early administration of parenteral amino acids increases whole body nitrogen retention in premature infants. Tracer kinetic studies suggest an increase in whole body protein synthesis as a possible mechanism for this increase in nitrogen retention. However, the effect of early parenteral amino acids on synthesis of specific proteins remains uncertain. Using premature newborn minipigs as a model for human premature newborns, we investigated the effects of parenterally administered amino acids on albumin and skeletal muscle protein fractional synthetic rates. Fifteen Yucatan minipigs were delivered by cesarean section 6 days before the mean expected delivery date (day 106 of gestation; expected gestation, 111-113 days) and randomized to two groups immediately after birth: 7 piglets received a mixture of amino acids (0.4 g. kg. h ) and glucose (0.8 g. kg. h ) for 5 hours, and 8 piglets (control group) received glucose only. All piglets received a continuous primed infusion of 1-[ C]valine. Arterial plasma free C-valine enrichment was measured by gas chromatography/mass spectrometry, and protein synthetic rates were determined by measuring incorporation of C-valine into albumin and skeletal muscle protein using gas chromatography/combustion/isotope ratio mass spectrometry. Administration of amino acids increased albumin (87.0% +/- 42.1% [mean +/- SD] vs. 37.6% +/- 6.8% per 24 hours; < 0.05) and skeletal muscle fractional synthetic rates (11.60% +/- 6.9% vs. 6.5% +/- 1.5% per 24 hours; < 0.05). We conclude that parenteral amino acids increase albumin and skeletal muscle fractional synthetic rates in premature piglets on the first day of life.

Parenteral Amino Acids Increase Albumin and Skeletal Muscle Protein Fractional Synthetic Rates in Premature Newborn Minipigs

ABSTRACTObjectivesEarly administration of parenteral amino acids increases whole body nitrogen retention in premature infants. Tracer kinetic studies suggest an increase in whole body protein synthesis as a possible mechanism for this increase in nitrogen retention. However, the effect of early parenteral amino acids on synthesis of specific proteins remains uncertain. Using premature newborn minipigs as a model for human premature newborns, we investigated the effects of parenterally administered amino acids on albumin and skeletal muscle protein fractional synthetic rates.MethodsFifteen Yucatan minipigs were delivered by cesarean section 6 days before the mean expected delivery date (day 106 of gestation; expected gestation, 111–113 days) and randomized to two groups immediately after birth: 7 piglets received a mixture of amino acids (0.4 g · kg−1 · h−1) and glucose (0.8 g · kg−1 · h−1) for 5 hours, and 8 piglets (control group) received glucose only. All piglets received a continuous primed infusion of 1‐[13C]valine. Arterial plasma free 13C‐valine enrichment was measured by gas chromatography/mass spectrometry, and protein synthetic rates were determined by measuring incorporation of 13C‐valine into albumin and skeletal muscle protein using gas chromatography/combustion/isotope ratio mass spectrometry.ResultsAdministration of amino acids increased albumin (87.0% ± 42.1% [mean ± SD] vs. 37.6% ± 6.8% per 24 hours;P &lt; 0.05) and skeletal muscle fractional synthetic rates (11.60% ± 6.9% vs. 6.5% ± 1.5% per 24 hours;P &lt; 0.05).ConclusionWe conclude that parenteral amino acids increase albumin and skeletal muscle fractional synthetic rates in premature piglets on the first day of life.

THE EFFECT OF INCREASED DIETARY PROTEIN ON SKELETAL MUSCLE PROTEIN UTILIZATION FOLLOWING EXERCISE IN ENDURANCE RUNNERS

Higher protein diets have been advocated for enhancing performance, however, the physiological implications of increased protein intake on skeletal muscle protein utilization have not been elucidated in endurance athletes. This investigation examined the impact of increased dietary protein on skeletal muscle fractional synthetic and breakdown rates (FSR/FBR) following an endurance run (70% VO2peak/75 min). Six endurance trained, male runners (mean ± SEM, age, 20 ± 0.6 y; VO2peak, 71 ± 1.9 ml/kg/min; body mass, 70 ± 1.2 kg; height, 180 ± 5 cm; %body fat, 9 ± 0.5) participated in a crossover design study following randomization to either a moderate protein (MP; 1.8 g/kg) or high protein (HP; 3.6 g/kg) diet for 4 weeks. Diets were isocaloric and the macronutrient composition for carbohydrate, protein and fat were ∼55%, 15%, 30% and 40%, 30%, 30% for MP and HP, respectively. Protein oxidation at rest was estimated via indirect calorimetry. Mixed muscle protein utilization was determined by use of primed, constant infusions of [2H5] phenylalanine (FSR) and [15N] phenylalanine (FBR). Plasma concentrations of insulin, glucagon, cortisol, free testosterone, growth hormone, and free fatty acids were also determined following the endurance run. Protein oxidation more than doubled with HP (52% ± 6.1 vs. 25% ± 1.3, p < 0.01). FSR post-exercise was greater (0.06 ± 0.01% hr) for MP compared to HP (0.03 ± 0.01% hr) (p < 0.05). Similarly, FBR was higher for MP (0.06 ± 0.01%/hr vs. 0.03 ± 0.01%/hr) (p = 0.11). As a result, the net balance (FSR-FBR) was essentially the same for MP and HP (0.017 ± 0.1%/hr vs. 0.022 ± 0.02%/hr), while remaining positive for both diet interventions. No differences were noted between the diets for any of the hormones post-exercise. Findings might suggest that consumption of a HP diet provides no substantial benefit to skeletal muscle protein utilization post-exercise. However, it is still unclear what, if any value, might be attributed to reduced skeletal muscle FSR and FBR during recovery from prolonged exercise in endurance runners habitually consuming increased levels of dietary protein. We propose that prolonged consumption of increased dietary protein may augment amino acid availability and subsequently reduce the demand for proteolysis in support of protein synthesis post-exercise. Supported by National Cattlemen's Beef Association

Effect of Intracerebroventricular Injection of Tumor Necrosis Factor α on Gut Mucosal Protein Turnover in Mice Fed Enterally

Objective. The objective of this study was to determine whether mucosal protein turnover is enhanced by intracerebroventricular (ICV) injection of tumor necrosis factor α (TNF-α) in mice fed enterally.Methods. Male B6C3F1 mice (n = 37, 20.0–25.0 g) were catheterized in the ICV space using stereotaxic coordination, and subsequently a catheter was inserted into the stomach for enteral feeding on Day 0. The animals were fed a standard mouse diet and water ad libitum for 4 days. On Day 4, enteral feeding was begun with a standard liquid diet, and either artificial cerebrospinal fluid (control) or recombinant human (Rh) TNF-α (5 ng/h) was continuously given via the ICV catheter using an osmotic pump. On either Day 5 or 7, the mice were sacrificed 10 min after injection of [U-14C]phenylalanine (5 μCi/mouse, ip). Fractional synthetic rates (FSRs) of jejunal mucosa and muscle were measured by the pool flooding technique. Interleukin-6 (IL-6) levels in the plasma were measured by ELISA.Results. Body weight was significantly reduced by ICV injection of TNF-α, because muscle FSR was decreased with TNF-α. There were no differences in IL-6 levels in the plasma between the two groups. The FSR of jejunal mucosa was elevated by infusion of TNF-α as compared with control mice on both Days 5 and 7 [Day 5: (control) 99.5 ± 17.6%/day vs (TNF) 150.5 ± 20.5%/day, P < 0.05; Day 7: (control) 54.8 ± 8.8%/day vs (TNF) 102.7 ± 19.1%/day, P < 0.05).Conclusion. ICV injection of TNF-α enhanced the mucosal protein synthesis rate without elevating IL-6 levels as compared with control mice, suggesting that the central nervous system links to the gut to regulate mucosal protein turnover through cytokine in the brain, but plasma IL-6 is not involved in this linkage.

TIME COURSE OF MIXED MUSCLE PROTEIN SYNTHESIS FOLLOWING RESISTANCE EXERCISE IN HUMANS 1321

The time course of changes in muscle protein synthetic rate following resistance exercise is unknown. In addition, the effect of contraction type(concentric or eccentric) on muscle protein synthesis is also unknown. Hence, the purposes of the current study were: 1) To examine the time course of muscle protein fractional synthetic rate (FSR); and 2) To determine the effect of contraction, eccentric or concentric on muscle FSR. FSR was determined in 8 untrained volunteers (4 male and 4 female; 22±2 yr; means±SD). Subjects were studied on 4 consecutive days: rest, and 3h, 24h, and 48h post-exercise. The exercise was 8 sets of 8 concentric or eccentric knee extension repetitions at 80% of the subject's 1 RM. Muscle FSR was determined using a primed constant infusion of d5-phenylalanine and the tracer incorporation, muscle biopsy (vastus lateralis) method. There was no effect of contraction mode on muscle protein FSR. Exercise resulted in a stimulation of FSR at 3h post-exercise to 112% greater than resting (rest = 0.055±0.016%/h, 3h = 0.118±0.027%/h; P<0.05). FSR fell(P<0.05) at 24h (0.092±0.024%/h) and 48h (0.075±0.022%/h), versus 3h, but remained elevated versus rest by 65% and 34%, respectively. There was no difference in FSR between 24 and 48h. These results suggest that exercise results in a stimulation of synthesis that peaks immediately following exercise, but is still elevated 48h after the initial stimulus. The stimulation of FSR by exercise is unaffected, however, by the type of muscular contraction performed during exercise.

Infusion of labeled KIC is more accurate than labeled leucine to determine human muscle protein synthesis.

The leucine constant infusion method is the most commonly used method for measuring fractional synthetic rates (FSR) of muscle protein. However, this method has been criticized because of the uncertainty involved in measuring precursor pool enrichment. We present an alternative method for measuring muscle FSR by giving a constant infusion of alpha-[1-13C]ketoisocaproate (alpha-[1-13C]KIC). We infused alpha-[1-13C]KIC and [5,5,5-2H3]leucine for 4 h in five volunteers and took plasma samples half-hourly and muscle biopsies at 1 and 4 h of isotope infusion. When KIC was infused, intramuscular free leucine enrichment was the same as arterial leucine enrichment. However, when labeled leucine was infused, intramuscular free leucine enrichment was only 76 +/- 3% of arterial KIC enrichment, which agrees with previous reports that plasma KIC enrichment does not accurately reflect intramuscular leucine enrichment. We obtained an FSR of 2.25 +/- 0.12%/day by use of this method, which agrees with a previous report using tRNA bound leucine as the precursor. In conclusion, the KIC infusion method overcomes the theoretical limitations of the leucine infusion.

The effect of increasing levels of fish oil-containing structured triglycerides on protein metabolism in parenterally fed rats stressed by burn plus endotoxin.

This report investigates the effect of various levels of medium-chain/fish oil structured triglycerides on protein and energy metabolism in hypermetabolic rats. Male Sprague-Dawley rats (192 to 226 g) were continuously infused with isovolemic diets that provided 200 kcal/kg per day and 2 g of amino acid nitrogen per kilogram per day. The percentage of nonnitrogen calories as structured triglyceride was varied: no fat, 5%, 15%, or 30%. A 30% long-chain triglyceride diet was also provided as a control to compare the protein-sparing abilities of these two types of fat. Nitrogen excretion, plasma albumin, plasma triglycerides, and whole-body and liver and muscle protein kinetics were determined after 3 days of feeding. Whole-body protein breakdown, flux, and oxidation were similar in all groups. The 15% structured triglyceride diet maximized whole-body protein synthesis (p < .05). Liver fractional synthetic rate was significantly greater in animals receiving 5% of nonprotein calories as structured triglyceride (p < .05). Muscle fractional synthetic rate was unchanged. Plasma triglycerides were markedly elevated in the 30% structured triglyceride-fed rats. The 30% structured triglyceride diet maintained plasma albumin levels better than those diets containing no fat, 5% medium-chain triglyceride/fish oil structured triglyceride, or 30% long-chain triglycerides. Nitrogen excretion was lower in animals receiving 30% of nonnitrogen calories as a structured triglyceride than in those receiving 30% as long-chain triglycerides, but this difference did not reach statistical significance (p = .1). These data suggest that protein metabolism is optimized when structured triglyceride is provided at relatively low dietary fat intakes.

Effects of different lipid sources in total parenteral nutrition on whole body protein kinetics and tumor growth.

This study examined the short-term effects of three total parenteral nutrition solutions, each containing a different lipid source, on host and tumor protein metabolism in a rat cancer model. Each diet contained 220 kcal/kg per day, including 2 g of nitrogen/kg per day and 50% of nonprotein calories as either a structured lipid of medium-chain triglycerides and fish oil, a physical mix of medium-chain triglycerides and fish oil, or Liposyn II, a long-chain triglyceride. A 3-day intravenous feeding infusion began on day 7 after tumor implantation. Tumor growth rate, nitrogen balance, energy expenditure, and plasma albumin, glucose, and free fatty acids were measured, and whole body protein kinetics and fractional synthetic rates in liver, muscle, and tumor tissues were assessed using a constant infusion of 14C-leucine. The results revealed that tumor growth rate was slowed in structured lipid-fed animals (p = .06, one-way analysis of variance) with significant increases in rates of tumor protein synthesis and tumor protein breakdown (p < .001, one-way analysis of variance). Although muscle fractional synthetic rates were significantly decreased in tumor-bearing animals (p < .05, two-way analysis of variance), the rates in structured lipid-fed animals were restored. Nitrogen balance improved significantly in structured lipid-fed animals. The results demonstrate that the source of lipid in total parenteral nutrition solutions can influence tumor and host protein metabolism, and that a structured lipid composed of medium-chain triglycerides and fish oil seems to improve protein metabolism in host tissue without stimulating tumor growth.

Total Parenteral Nutrition with Short- and Long-Chain Triglycerides: Triacetin Improves Nitrogen Balance in Rats

Little is known about the long-term metabolic effects of parenteral administration of short-chain triglycerides. These studies were undertaken to investigate triacetin, the water-soluble triglyceride of acetate when it is incorporated into nutritionally balanced total parenteral nutrition formulas. Male Sprague-Dawley rats (n = 22) were fed an isovolemic, isocaloric and isonitrogenous diet for 7 d. The lipid energy represented 30% of the nonprotein energy with short-chain triglycerides representing 0, 50 or 90% of the lipid energy. Plasma acetate concentration was determined as well as indicators of protein metabolism: daily and cumulative nitrogen balance, whole body leucine kinetics and rectus muscle and liver fractional protein synthetic rates. No overt toxic effects were observed at any point during the study. As the proportion of short-chain triglycerides in the diet increased from 0 to 50 or 90% of the lipid energy, cumulative nitrogen balance increased 50 or 120%, respectively (P < 0.05). Whole-body and tissue leucine kinetics (determined during the last 2.5 h of the 7-d study) were unaffected by the lipid composition of the diet. Plasma acetate concentration was not significantly different among groups. These results indicate that incorporation of the short-chain triglyceride, triacetin, in nutritionally balanced total parenteral nutrition formulas improves nitrogen balance with no overt toxic effects. These data indicate that triacetin may have a future role as a parenteral nutrient, and that further studies of its use are warranted.

Effect of Glutamine Infusions on Glutamine Concentration and Protein Synthetic Rate in Rat Muscle

Studies were undertaken in sedated and unsedated rats to raise the depleted intramuscular glutamine concentrations produced by aseptic abscesses, and to assess the effect of this change on muscle protein fractional synthetic rate. Age- and weight-matched control animals were also included in the study. The rats were infused for up to 5 hours via the lateral tail vein with 1 mL/100 g of body weight per hour of either saline or 0.22 M glutamine. The intramuscular concentration of glutamine (mmol/L of intracellular water), which was reduced by 45% after turpentine in the sedated animals, was restored to within 79% of control values, but the muscle fractional protein synthetic rate, which was also reduced by 41% in these animals, was not improved by the glutamine infusions. Glutamine administration also failed to increase muscle protein synthesis in unsedated rats and in those supplemented with a liquid meal. It is concluded that acute elevations in muscle glutamine concentrations do not increase protein synthesis in this tissue and that therefore glutamine is unlikely to be a mediator in the control of muscle protein synthesis under these circumstances.

Effect of Total Parenteral Nutrition with Xylitol on Protein and Energy Metabolism in Thermally Injured Rats

The use of xylitol as an alternative carbohydrate calorie source in total parenteral nutrition may offer unique pharmacologic and nutritional properties in the therapy of the thermally injured. Male Sprague-Dawley rats (250 g) received a 15-second dorsal scald injury (25-30% BSA) and were parenterally fed isovolemic diets (60 ml/day) that provided 200 kcal/kg/d, 9.68 g of amino acids/kg/d, and 23.5% nonprotein calories (NPC) as fat for 3 days. The balance of NPC were provided as dextrose (Dex) or 50% xylitol:50% dextrose (Xyl/Dex). Rectus muscle and liver fractional protein synthetic rates (FSR, %/day), whole body leucine appearance (Flux), oxidation (OX), protein breakdown (PB), and synthesis (PS) were estimated using a 4-hour iv infusion of [1-14C]leucine on day 3. Mean values (+/- SE) for leucine kinetics (mumol leucine/hr/100 g), cumulative nitrogen balance (mg N) and plasma insulin concentration (Table I). (microU/mL). The partial replacement of dextrose calories with xylitol did not significantly alter whole body and tissue leucine kinetics, daily and cumulative nitrogen balance, insulin concentration, and energy expenditure (indirect calorimetry). These data indicate that xylitol may be useful as an alternative carbohydrate calorie source in parenteral nutrition to avoid possible deleterious side effects of glucose overfeeding in the critically ill but did not improve protein metabolism under the conditions of this study.

Protein metabolism after injury with turpentine: a rat model for clinical trauma

Subcutaneous injections of turpentine induced discrete aseptic abscesses in rats without detectable injury to other tissues. Hypoalbuminemia and high circulating concentrations of alpha 2-macroglobulin were present after 48 h. Liver size, protein content, and protein fractional synthetic rate (FSR) were all increased, whereas loss of muscle protein occurred together with similar (45-50%) reductions in both intramuscular glutamine concentrations ([Gln]i, mmol/l intracellular water) and FSR. The rats became anorexic, but pair feeding (50% ad libitum for 48 h) produced a 22% increase in muscle [Gln]i and only an 18% reduction in muscle FSR. Dietary restriction therefore did not demonstrate the positive relationship between muscle [Gln]i and FSR, which has been observed in animal trauma studies. It is concluded that subcutaneous injections of turpentine produce many of the features of the acute-phase response to injury, and therefore this technique is recommended as a convenient laboratory model for studying various aspects of the protein metabolic response to injury.

Effect of exercise and recovery on muscle protein synthesis in human subjects

Previous studies using indirect means to assess the response of protein metabolism to exercise have led to conflicting conclusions. Therefore, in this study we have measured the rate of muscle protein synthesis in normal volunteers at rest, at the end of 4 h of aerobic exercise (40% maximal O2 consumption), and after 4 h of recovery by determining directly the rate of incorporation of 1,2-[13C]leucine into muscle. The rate of muscle protein breakdown was assessed by 3-methylhistidine (3-MH) excretion, and total urinary nitrogen excretion was also measured. There was an insignificant increase in 3-MH excretion in exercise of 37% and a significant increase (P less than 0.05) of 85% during 4 h of recovery from exercise (0.079 +/- 0.008 vs. 0.147 +/- 0.0338 mumol.kg-1.min-1 for rest and recovery from exercise, respectively). Nonetheless, there was no effect of exercise on total nitrogen excretion. Muscle fractional synthetic rate was not different in the exercise vs. the control group at the end of exercise (0.0417 +/- 0.004 vs. 0.0477 +/- 0.010%/h for exercise vs. control), but there was a significant increase in fractional synthetic rate in the exercise group during the recovery period (0.0821 +/- 0.006 vs. 0.0654 +/- 0.012%/h for exercise vs. control, P less than 0.05). Thus we conclude that although aerobic exercise may stimulate muscle protein breakdown, this does not result in a significant depletion of muscle mass because muscle protein synthesis is stimulated in recovery.

Enteral nutrition with structured lipid: effect on protein metabolism in thermal injury

The effect of total enteral nutrition with structured and conventional lipids on protein and energy metabolism was assessed in gastrostomy-fed burned rats (30% body surface area) by measuring nitrogen balance, serum albumin, energy expenditure, and rectus muscle and liver fractional synthetic rates of protein (FSRs). Male Sprague-Dawley rats weighing 200 +/- 10 g received isovolemic diets that provided 50 kcal/d, 2 g/d amino acids, and 40% nonprotein calories as lipid for 3 d. The lipid source was either long-chain triglycerides (LCTs), medium-chain triglycerides (MCTs), structured lipid (SL), or a physical mix (PM) of the oils used in SL. Burned rats enterally fed either SL (p less than 0.01) or PM (p less than 0.05) yielded significantly higher daily and cumulative nitrogen balances and rectus muscle and liver FSRs than those fed either LCTs or MCTs. Rats fed SL or MCTs maintained higher serum albumin concentrations than rats fed either PM or LCTs. This study shows that the enteral administration of a mixed fuel system containing SL or its PM improves protein anabolism and attenuates net protein catabolism after thermal injury.

Whole-body leucine and muscle protein kinetics in rats fed varying protein intakes.

Whole-body leucine kinetics and rectus muscle synthetic rates were evaluated in postabsorptive rats fed semipurified diets that varied in the casein content. Rats were allowed to consume ad libitum a 2% casein diet or were pair-fed or ad libitum-fed 6, 20, or 40% casein diets for 14 days. After overnight starvation, rates of whole-body leucine kinetics and rectus muscle synthetic rates were determined with a 2-h constant intravenous infusion of L-[1-14C]leucine. The postabsorptive response to inadequate protein intakes included a significant reduction in the release of leucine from whole-body protein degradation as well as subsequent reutilization for protein synthesis. In contrast, dietary protein intake at levels greater than required for maximal growth were not associated with any increases in leucine incorporation into whole-body protein or muscle fractional synthetic rates. Rates of whole-body leucine oxidation based on plasma leucine specific radioactivities underestimated total oxidation by 22-27%, and this was relatively constant as the protein component of the diet was varied. In addition, the muscle acid-soluble leucine specific radioactivity was similar to the plasma alpha-ketoisocaproate enrichment, regardless of dietary protein intake.