Rates Of Whole Body Protein Breakdown Research Articles

Quantification and interpretation of postprandial whole-body protein metabolism using stable isotope methodology: a narrative review.

Stable isotopes are routinely applied to determine the impact of factors such as aging, disease, exercise, and feeding on whole-body protein metabolism. The most common approaches to quantify whole-body protein synthesis, breakdown, and oxidation rates and net protein balance are based on the quantification of plasma amino acid kinetics. In the postabsorptive state, plasma amino acid kinetics can easily be assessed using a constant infusion of one or more stable isotope labeled amino acid tracers. In the postprandial state, there is an exogenous, dietary protein-derived amino acid flux that needs to be accounted for. To accurately quantify both endogenous as well as exogenous (protein-derived) amino acid release in the circulation, the continuous tracer infusion method should be accompanied by the ingestion of intrinsically labeled protein. However, the production of labeled protein is too expensive and labor intensive for use in more routine research studies. Alternative approaches have either assumed that 100% of exogenous amino acids are released in the circulation or applied an estimated percentage based on protein digestibility. However, such estimations can introduce large artifacts in the assessment of whole-body protein metabolism. The preferred estimation approach is based on the extrapolation of intrinsically labeled protein-derived plasma bioavailability data obtained in a similar experimental design setting. Here, we provide reference data on exogenous plasma amino acid release that can be applied to allow a more accurate routine assessment of postprandial protein metabolism. More work in this area is needed to provide a more extensive reference data set.

The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans

The belief that the anabolic response to feeding during postexercise recovery is transient and has an upper limit and that excess amino acids are being oxidized lacks scientific proof. Using a comprehensive quadruple isotope tracer feeding-infusion approach, we show that the ingestion of 100g protein results in a greater and more prolonged (>12 h) anabolic response when compared to the ingestion of 25g protein. We demonstrate a dose-response increase in dietary-protein-derived plasma amino acid availability and subsequent incorporation into muscle protein. Ingestion of a large bolus of protein further increases whole-body protein net balance, mixed-muscle, myofibrillar, muscle connective, and plasma protein synthesis rates. Protein ingestion has a negligible impact on whole-body protein breakdown rates or amino acid oxidation rates. These findings demonstrate that the magnitude and duration of the anabolic response to protein ingestion is not restricted and has previously been underestimated invivo in humans.

Even or skewed dietary protein distribution is reflected in the whole-body protein net-balance in healthy older adults: A randomized controlled trial

For older adults, the dietary protein intake has shown to be skewed towards the evening meal. Resultingly, the vital source of essential amino acids could be insufficient after some meals, while after the evening meal the dietary protein could be suboptimally utilized for protein synthesis. The present study explored if an even distribution of the protein intake could improve the dietary amino acid absorption and whole-body protein net-balance. Twenty-four healthy elderly males and females were included in a randomized controlled trial. Ten days of habituation to either an EVEN (n=12) or SKEWED (n=12) protein intake, was followed by a trial day. The total protein intake was controlled at 1.5g/kg LBM, divided into 30% at each main meal in EVEN, and into 15% at breakfast and lunch and 60% at dinner in SKEWED. Snacks with 5% of the protein intake were served between meals. Energy intake in the meals and snacks were equal in both groups. Intrinsically labelled 2H5-phenylalanine minced meat was served as the dietary protein to assess the amino acid absorption. On the trial day, infusion of 2H8-phenylalanine and 2H2-tyrosine, and blood samples taken over 11h were used to measure whole-body protein turnover. Vastus lateralis muscle biopsies were taken to measure 9h muscle protein FSR. Amino acid absorption rates and concentrations were greater in EVEN compared to SKEWED protein intake. Whole-body protein breakdown rates were lower with similar protein synthesis rates, and consequently the net-balance was greater in EVEN after breakfast and lunch compared to SKEWED and were the same in both groups after dinner. Muscle protein FSR were not different between EVEN and SKEWED. The whole-body protein net-balance was more positive in EVEN compared to SKEWED for an extended time of the measured period, driven by a lower whole-body protein breakdown in EVEN. NCT03870425, https://clinicaltrials.gov/ct2/show/NCT03870425.

Whole-body protein kinetic models to quantify the anabolic response to dietary protein consumption

<h2>Summary</h2> Determination of whole body rates of protein synthesis, breakdown and net balance in human subjects still has an important role in nutrition research. Quantifying the anabolic response to dietary protein intake is a particularly important application. There are different models with which to accomplish this goal, each with advantages and limitations. The nitrogen (N)-flux method in which tracer is given orally has distinct advantages in terms of lack of invasiveness. In addition, the calculated results include all aspects of whole-body protein synthesis and breakdown. However, the prolonged timeframe of the method eliminates the possibility of the "pre-post" experimental design whereby each subject serves as their own control in the evaluation of the response to a meal. Models based on the primed-constant infusion of an essential amino acid (EAA) tracer enable the determination of baseline whole-body protein kinetics within 2 h, and can quantify a dynamic change from the basal state. The greatest challenge when using an EAA model is distinguishing exogenous and endogenous sources of the tracee in the blood. One approach is to use an intrinsically-labeled protein. This method has the advantage that the exogenous tracee is clearly distinguished from endogenous tracee. On the other hand, the intrinsically-labeled protein method suffers from unmeasured dilution that is likely to cause the systematic underestimation of the rate of appearance of exogenous tracee and thus overestimate the rate of whole-body protein breakdown. Alternatively, the "bioavailability" approach estimates the contribution of exogenous tracee to the peripheral circulation from the amount of tracee ingested, the true ileal digestibility of the tracee, and the irreversible loss of tracee prior to entry into the peripheral circulation. Errors in assumed values with the bioavailability method can potentially be significant, but are not likely to result in the systematic over- or under-estimations of rates of whole-body protein synthesis and breakdown. The optimal method depends on the degree of uncertainty regarding required assumptions in a particular circumstance. With all methods, it is advisable to calculate upper and lower bounds of whole body protein kinetics, in accord with reasonable maximal and minimal assumed values. Simultaneous use of two methods requiring different assumptions can also serve to confirm the validity of single approach.

Stable isotope approaches to study muscle mass outcomes in clinical populations

Both low muscle mass and muscle loss are associated with reduced physical function, mobility, independence, and quality of life, and are characteristic of a number of clinical conditions including diabetes, cardiovascular disease (CVD), chronic obstructive pulmonary disease (COPD), and critical illness. The accurate measurement of muscle mass is critical to assess the efficacy of an intervention or therapy. Stable isotope amino acid approaches can be used to quantify specific aspects of whole-body and muscle protein turnover, including synthesis and breakdown, which play distinctive roles in muscle mass maintenance in direct response to therapies. This review aims to elucidate whether acute responses measured using stable isotope amino acid tracers relate to changes in muscle mass in vulnerable clinical populations. Experimental studies quantifying whole-body protein synthesis and breakdown rates in clinical populations have been conducted to determine the response to nutritional interventions or to compare disease with health; however, these studies show limited potential to translate to expected muscle mass outcomes. In addition, clinical studies that have assessed both muscle mass and acute changes in whole-body or muscle protein turnover are lacking. We argue that the assessment of both muscle protein synthesis and breakdown rates, or simply limb net balance, obtains the most complete picture in relation to muscle-specific outcomes. While stable isotope amino acid tracer experiments provide meaningful mechanistic insight into the acute response to clinical interventions, they should be combined with, and/or followed-up by, longer-term studies incorporating measurements of muscle mass to ascertain the impact of an intervention on muscle mass maintenance in clinical populations.

Comprehensive assessment of post-prandial protein handling by the application of intrinsically labelled protein in vivo in human subjects.

All human tissues are in a constant state of remodelling, regulated by the balance between tissue protein synthesis and breakdown rates. It has been well-established that protein ingestion stimulates skeletal muscle and whole-body protein synthesis. Stable isotope-labelled amino acid methodologies are commonly applied to assess the various aspects of protein metabolism in vivo in human subjects. However, to achieve a more comprehensive assessment of post-prandial protein handling in vivo in human subjects, intravenous stable isotope-labelled amino acid infusions can be combined with the ingestion of intrinsically labelled protein and the collection of blood and muscle tissue samples. The combined application of ingesting intrinsically labelled protein with continuous intravenous stable isotope-labelled amino acid infusion allows the simultaneous assessment of protein digestion and amino acid absorption kinetics (e.g. release of dietary protein-derived amino acids into the circulation), whole-body protein metabolism (whole-body protein synthesis, breakdown and oxidation rates and net protein balance) and skeletal muscle metabolism (muscle protein fractional synthesis rates and dietary protein-derived amino acid incorporation into muscle protein). The purpose of this review is to provide an overview of the various aspects of post-prandial protein handling and metabolism with a focus on insights obtained from studies that have applied intrinsically labelled protein under a variety of conditions in different populations.

Effects of dietary leucine supplementation in low crude protein diets on performance, nitrogen balance, whole-body protein turnover, carcass characteristics and meat quality of finishing pigs.

Eighteen Duroc × Landrace × Yorkshire barrows, with an average initial body weight (BW) of 75.4 ± 2.0 kg, were randomly allotted to one of three diets with six replicates per treatment for 25 days. The diets comprised a normal protein diet (NP, 14.5% crude protein), a low crude protein diet supplemented with 0.27% alanine (LP + Ala, 10.0% crude protein), or a low crude protein diet supplemented with 0.40% leucine (LP + Leu, 10.0% crude protein). The whole-body protein synthesis rate, whole-body protein breakdown rate and protein deposition rate in pigs fed the LP + Leu diet were similar to the NP diet (P > 0.05), and both were significantly higher than pigs fed the LP + Ala diet (P < 0.05). The Longissimus muscle area (LMA) of pigs fed the LP + Leu diet was larger than those fed the LP + Ala diet (P = 0.05). In addition, drip loss and intramuscular fat of pigs fed the LP + Ala diet were higher than that of the others (P < 0.05). In conclusion, supplementation of leucine in low protein diet could stimulate protein deposition and improve the meat quality of finishing pigs more than an alanine-supplemented one.

Prolonged Adaptation to a Low or High Protein Diet Does Not Modulate Basal Muscle Protein Synthesis Rates - A Substudy.

BackgroundBased on controlled 36 h experiments a higher dietary protein intake causes a positive protein balance and a negative fat balance. A positive net protein balance may support fat free mass accrual. However, few data are available on the impact of more prolonged changes in habitual protein intake on whole-body protein metabolism and basal muscle protein synthesis rates.ObjectiveTo assess changes in whole-body protein turnover and basal muscle protein synthesis rates following 12 weeks of adaptation to a low versus high dietary protein intake.MethodsA randomized parallel study was performed in 40 subjects who followed either a high protein (2.4 g protein/kg/d) or low protein (0.4 g protein/kg/d) energy-balanced diet (30/35/35% or 5/60/35% energy from protein/carbohydrate/fat) for a period of 12 weeks. A subgroup of 7 men and 8 women (body mass index: 22.8±2.3 kg/m2, age: 24.3±4.9 y) were selected to evaluate the impact of prolonged adaptation to either a high or low protein intake on whole body protein metabolism and basal muscle protein synthesis rates. After the diet, subjects received continuous infusions with L-[ring-2H5]phenylalanine and L-[ring-2H2]tyrosine in an overnight fasted state, with blood samples and muscle biopsies being collected to assess post-absorptive whole-body protein turnover and muscle protein synthesis rates in vivo in humans.ResultsAfter 12 weeks of intervention, whole-body protein balance in the fasted state was more negative in the high protein treatment when compared with the low protein treatment (-4.1±0.5 vs -2.7±0.6 μmol phenylalanine/kg/h;P<0.001). Whole-body protein breakdown (43.0±4.4 vs 37.8±3.8 μmol phenylalanine/kg/h;P<0.03), synthesis (38.9±4.2 vs 35.1±3.6 μmol phenylalanine/kg/h;P<0.01) and phenylalanine hydroxylation rates (4.1±0.6 vs 2.7±0.6 μmol phenylalanine/kg/h;P<0.001) were significantly higher in the high vs low protein group. Basal muscle protein synthesis rates were maintained on a low vs high protein diet (0.042±0.01 vs 0.045±0.01%/h;P = 0.620).ConclusionsIn the overnight fasted state, adaptation to a low-protein intake (0.4 g/kg/d) does not result in a more negative whole-body protein balance and does not lower basal muscle protein synthesis rates when compared to a high-protein intake.Trial RegistrationClinicaltrials.gov NCT01551238.

Effects of Type 2 Diabetes and Insulin on Whole-Body, Splanchnic, and Leg Protein Metabolism

Type 2 diabetes (T2D) is characterized by insulin resistance to glucose metabolism. Most studies suggest that protein metabolism is unaffected by T2D, but regional protein metabolism and response to multiple doses of insulin have not been examined. Our objective was to determine whether insulin regulation of splanchnic and leg protein metabolism are affected by T2D during hyperglycemia and graded insulin levels. We conducted a cross-sectional study at an academic medical center. T2D and non-T2D adults were matched for age (62 yr) and body mass index (30 kg/m(2)). Glucose was maintained at approximately 9 mmol/liter while insulin was infused at three progressively higher rates, achieving circulating concentrations of approximately 150, 350, and 700 pmol/liter, respectively. Protein kinetics were measured using labeled phenylalanine (Phe) and tyrosine (Tyr). Whole-body protein breakdown and synthesis rates were higher in T2D but declined with increasing insulin in both groups. Leg Phe and Tyr appearance and disappearance and estimates of protein breakdown and synthesis, respectively, were higher in T2D but did not decline significantly with insulin, resulting in similar net balance between groups. Splanchnic response to insulin was blunted in T2D, shown by a smaller reduction in rates of disappearance and net balance of Phe and Tyr as insulin increased. Splanchnic conversion of Phe to Tyr was lower in T2D and less sensitive to insulin, whereas nonsplanchnic Phe to Tyr tended to be higher in T2D. T2D results in higher whole-body, splanchnic, and leg protein turnover and blunts the insulin-mediated suppression of splanchnic protein anabolism under hyperglycemic, hyperinsulinemic conditions.

Arginine, citrulline and nitric oxide metabolism in sepsis

Arginine has vasodilatory effects, via its conversion by NO synthase into NO, and immunomodulatory actions which play important roles in sepsis. Protein breakdown affects arginine availability and the release of asymmetric dimethylarginine, an inhibitor of NO synthase, may therefore affect NO synthesis in patients with sepsis. The objective of the present study was to investigate whole-body in vivo arginine and citrulline metabolism and NO synthesis rates, and their relationship to protein breakdown in patients with sepsis or septic shock and in healthy volunteers. Endogenous leucine flux, an index of whole-body protein breakdown rate, was measured in 13 critically ill patients with sepsis or septic shock and seven healthy controls using an intravenous infusion of [1-13C]leucine. Arginine flux, citrulline flux and the rate of conversion of arginine into citrulline (an index of NO synthesis) were measured with intravenous infusions of [15N2]guanidino-arginine and [5,5-2H2]citrulline. Plasma concentrations of nitrite plus nitrate, arginine, citrulline and asymmetric dimethylarginine were measured. Compared with controls, patients had a higher leucine flux and higher NO metabolites, but arginine flux, plasma asymmetric dimethylarginine concentration and the rate of NO synthesis were not different. Citrulline flux and plasma arginine and citrulline were lower in patients than in controls. Arginine production was positively correlated with the protein breakdown rate. Whole-body arginine production and NO synthesis were similar in patients with sepsis and septic shock and healthy controls. Despite increased proteolysis in sepsis, there is a decreased arginine plasma concentration, suggesting inadequate de novo synthesis secondary to decreased citrulline production.

Application of liquid chromatography-tandem mass spectrometry (LC–MS/MS) for the analysis of stable isotope enrichments of phenylalanine and tyrosine

Whole-body protein synthesis and breakdown are measured by a combined tracer infusion protocol with the stable isotope amino acids l-[ ring- 2H 5]-phenylalanine, l-[ ring- 2H 2]-tyrosine and l-[ ring- 2H 4]-tyrosine that enable the measurement of the phenylalanine to tyrosine conversion rate. We describe a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method for the measurement of very low tracer–tracee ratios (TTR) of the amino acids l-phenylalanine and l-tyrosine in human plasma. TTR calibration curves of the tracers l-[ ring- 2H 5]-phenylalanine, l-[ ring- 2H 2]-tyrosine and l-[ ring- 2H 4]-tyrosine were linear ( r 2 > 0.99) in the range between 0.01% and 5.0% TTR and lowest measurable TTR for the tracers was 0.01% at a physiological concentration of 60 μM. The method was applied successfully to plasma samples from a clinical study reaching a steady state enrichment plateau (mean ± SD) of 3.33 ± 0.19% for l-[ ring- 2H 5]-phenylalanine, 2.40 ± 0.43% for l-[ ring- 2H 2]-tyrosine and 0.29 ± 0.07% for l-[ ring- 2H 4]-tyrosine, respectively. The LC–MS/MS method can be applied for measurement of very low plasma enrichments of phenylalanine and tyrosine for the determination of whole-body protein synthesis and breakdown rates in humans.

Co-ingestion of leucine with protein does not further augment post-exercise muscle protein synthesis rates in elderly men

Leucine has been suggested to have the potential to modulate muscle protein metabolism by increasing muscle protein synthesis. The objective of this study was to investigate the surplus value of the co-ingestion of free leucine with protein hydrolysate and carbohydrate following physical activity in elderly men. Eight elderly men (mean age 73 +/- 1 years) were randomly assigned to two cross-over treatments consuming either carbohydrate and protein hydrolysate (CHO+PRO) or carbohydrate, protein hydrolysate with additional leucine (CHO+PRO+leu) after performing 30 min of standardized physical activity. Primed, continuous infusions with L-[ring-(13)C(6)]phenylalanine and L-[ring-(2)H(2)]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover as well as protein fractional synthetic rate in the vastus lateralis muscle over a 6 h period. Whole-body protein breakdown and synthesis rates were not different between treatments. Phenylalanine oxidation rates were significantly lower in the CHO+PRO+leu v. CHO+PRO treatment. As a result, whole-body protein balance was significantly greater in the CHO+PRO+leu compared to the CHO+PRO treatment (23.8 (SEM 0.3) v. 23.2 (SEM 0.3) micromol/kg per h, respectively; P < 0.05). Mixed muscle fractional synthetic rate averaged 0.081 (SEM 0.003) and 0.082 (SEM 0.006) %/h in the CHO+PRO+leu and CHO+PRO treatment, respectively (NS). Co-ingestion of leucine with carbohydrate and protein following physical activity does not further elevate muscle protein fractional synthetic rate in elderly men when ample protein is ingested.

Co-ingestion Of Carbohydrate With Protein Does Not Stimulate Post-exercise Muscle Protein Synthesis Rates

Endurance athletes maximize post-exercise muscle glycogen synthesis by ingesting ample amounts of carbohydrate following exercise. In contrast, resistance athletes generally focus on protein intake during recovery to augment muscle protein anabolism. It has been suggested that both carbohydrate and protein should be ingested to maximize muscle protein synthesis rates. As many recreational athletes perform resistance type exercise for health and/or esthetic purposes, they generally prefer to restrict their caloric intake during recovery. Therefore, it is of importance to assess the need for co-ingestion of carbohydrate with protein on post-exercise muscle protein synthesis. PURPOSE: To determine the impact of the co-ingestion of different amounts of carbohydrate with protein on post-exercise muscle protein synthesis. METHODS: Ten healthy, untrained men were randomly assigned to 3 randomized cross-over trials. After 60 min of resistance exercise, subjects consumed 0.3 g·kg-1·h-1 protein hydrolysate with 0, 0.15 or 0.6 g·kg-1·h-1 carbohydrate during a 6 h recovery period (PRO, PRO+LCHO, and PRO+HCHO, respectively). Primed, continuous infusions of L-[ring-13C6]phenylalanine, L-[ring-2H2]tyrosine, and [6,6-2H2]glucose were applied, and blood and muscle samples were collected to assess whole-body protein turnover, glucose kinetics and fractional synthetic rate (FSR) in the vastus lateralis muscle over 6 h of recovery. RESULTS: Plasma insulin response was significantly higher in HCHO+PRO compared to LCHO+PRO and PRO (18.4±2.9 vs. 3.7±0.5 and 1.5±0.2 U·6h·L-1, respectively, P<0.001). Plasma glucose rate of appearance (Ra) and disappearance (Rd) increased over time in HCHO+PRO and LCHO+PRO but not in PRO. Plasma glucose Ra and Rd were substantially greater in HCHO+PRO vs. LCHO+PRO (P<0.01). Whole-body protein breakdown, synthesis and oxidation rates, as well as whole-body protein balance did not differ between trials. In accordance mixed muscle FSR did not differ between trials and averaged 0.114±0.010, 0.104±0.009% and 0.100±0.009%·h-1 in the HCHO+PRO, LCHO+PRO and PRO trial, respectively (NS). CONCLUSIONS: Co-ingestion of carbohydrate with an ample amount of protein hydrolysate (0.3 g·kg-1·h-1) does not further augment muscle protein synthesis rate during post-exercise recovery.

Addition of glutamine to essential amino acids and carbohydrate does not enhance anabolism in young human males following exercise

We examined the effect of a post-exercise oral carbohydrate (CHO, 1 g.kg(-1).h(-1)) and essential amino acid (EAA, 9.25 g) solution containing glutamine (0.3 g/kg BW; GLN trial) versus an isoenergetic CHO-EAA solution without glutamine (control, CON trial) on muscle glycogen resynthesis and whole-body protein turnover following 90 min of cycling at 65% VO2 peak. Over the course of 3 h of recovery, muscle biopsies were taken to measure glycogen resynthesis and mixed muscle protein synthesis (MPS), by incorporation of [ring-2H5] phenylalanine. Infusion of [1-13C] leucine was used to measure whole-body protein turnover. Exercise resulted in a significant decrease in muscle glycogen (p < 0.05) with similar declines in each trial. Glycogen resynthesis following 3 h of recovery indicated no difference in total accumulation or rate of repletion. Leucine oxidation increased 2.5 fold (p < 0.05) during exercise, returned to resting levels immediately post-exercise,and was again elevated at 3 h post-exercise (p < 0.05). Leucine flux, an index of whole-body protein breakdown rate, was reduced during exercise, but increased to resting levels immediately post-exercise, and was further increased at 3 h post-exercise (p < 0.05), but only during the CON trial. Exercise resulted in a marked suppression of whole-body protein synthesis (50% of rest; p < 0.05), which was restored post-exercise; however, the addition of glutamine did not affect whole-body protein synthesis post-exercise. The rate of MPS was not different between trials. The addition of glutamine to a CHO + EAA beverage had no effect on post-exercise muscle glycogen resynthesis or muscle protein synthesis, but may suppress a rise in whole-body proteolysis during the later stages of recovery.

Protein kinetic differences between children with edematous and nonedematous severe childhood undernutrition in the fed and postabsorptive states

Pathogenic factors that cause a child to develop the edematous instead of the nonedematous form of severe childhood undernutrition (SCU) during food deprivation are not clear. It was hypothesized that, in edematous but not nonedematous SCU, impaired protein breakdown leading to inadequate amino acids for maintenance of important organ systems was a factor. We measured protein kinetics in children with edematous and nonedematous SCU. Endogenous leucine flux, an index of whole-body protein breakdown rate, was determined in 4 groups of children with edematous or nonedematous SCU in the malnourished and recovered states. Two groups were studied in the postabsorptive state, and 2 groups were studied in the fed state. In the postabsorptive state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state, but in the recovered state, it was faster (P < 0.05) in the children who previously had edematous SCU. When compared with the malnourished state value, leucine flux at recovery doubled in the group that previously had edematous SCU, but it did not change in the other group. In the fed state, leucine flux was slower (P < 0.01) in the edematous group than in the nonedematous group in the malnourished state but not in the recovered state. In the recovered state, enteral leucine extraction by splanchnic tissues trended higher in the group that previously had edematous SCU than in the nonedematous group. These findings indicate different protein breakdown responses to food deprivation between children with edematous and nonedematous SCU and inherent differences in protein metabolism when they have recovered.

Short-term Carbohydrate Restriction Increases Both Proteolysis And Protein Synthesis

PURPOSE To assess the influence of a short-term weight-maintaining carbohydrate restricted diet on whole-body proteolysis and skeletal muscle protein synthesis. METHODS Eight subjects (4 women, 4 men) were studied after 2d of a standard diet (60% CHO, 30% fat, 10% protein) and after 2d and 7d of an isocaloric low-carbohydrate (LC) diet (5% CHO, 60% fat, 35% protein) during a 9d hospital stay. Whole-body proteolysis (leucine rate of appearance in plasma [Ra]) and skeletal muscle fractional synthetic rate (FSR) were measured using isotope tracer techniques after an overnight fast before (PRE), and after 2d and 7d of the LC diet. We also measured fat free mass (FFM), using DEXA, before and after the LC diet. RESULTS Leucine Ra was elevated (p < 0.05) 21 ± 2% and 15 ± 1% above PRE after 2d and 7d LC diet, respectively. Compared with PRE, muscle FSR was −2 fold higher (p < 0.05) after 7d of the LC diet (0.049 ± 0.004 v. 0.094 ± 0.013 %/h). However, FFM was not altered by the LC-diet. 24h insulin AUC was 50% lower (p < 0.05) after 2d and 7d LC diet while 24h growth hormone (GH) secretion and plasma insulin-like growth factor 1 (IGF-1) were unchanged with the LC diet. CONCLUSION Despite a lower 24h exposure to insulin and no change in GH or IGF-1 concentrations, whole-body protein breakdown and skeletal muscle protein synthetic rate were both increased in response to short-term isocaloric LC diet. Identical FFM before and after the diet suggests that the enhanced proteolysis during the dietary treatment was offset by a compensatory increase in FSR. This research funded by NIH#MO1RR042 (JFH).

Frailty amplifies the effects of aging on protein metabolism: role of protein intake

We previously showed that muscle contributes less to whole-body protein breakdown with healthy aging. We hypothesized that frailty further compromises protein metabolism and that short-term protein supplementation improves protein status. Protein metabolism was studied with the oral, 60-h [(15)N]glycine and N(tau)-methylhistidine methods in 8 frail and 13 healthy elderly women during a 9-d isoenergetic, isonitrogenous formula diet and then after increased protein intakes in the frail women, to match the intakes of healthy subjects, for 12 d. Compared with healthy women, frail women had higher rates of whole-body protein synthesis and breakdown per kg fat-free mass and lower rates of muscle protein breakdown when expressed as total amounts per day but higher rates when expressed per kg muscle. Because muscle mass was lower in frail women, the contribution of muscle to whole-body protein breakdown was lower and that of nonmuscle lean tissues was higher. The protein-enriched diet had no effect on these variables but resulted in an increase in net endogenous protein balance and a positive nitrogen balance at the end of the diet period. Frailty exacerbates age-related changes in protein metabolism by inducing an increase in muscle protein catabolism and a decrease in muscle mass. At low protein intakes, the increase in muscle catabolism may be a form of protection for both nonmuscle lean tissue mass and function at the expense of muscle mass. Frail women maintained the capacity to retain nitrogen after increased protein intakes, which could convey health benefits if sustained over a long enough period to result in lean tissue accretion.

Response of splanchnic and whole-body leucine kinetics to treatment of children with edematous protein-energy malnutrition accompanied by infection

Although the reduction in whole-body protein turnover and net protein loss induced by protein-energy malnutrition (PEM) has been well documented, it is unclear whether the protein-sparing mechanisms elicited by chronically inadequate intakes of dietary protein and energy are affected by the protein catabolic response to infection. The objective of this study was to determine whether the presence of infection alters the PEM-induced reduction in whole-body protein metabolism. We determined whole-body leucine kinetics in 4 boys and 3 girls aged 6-15 mo with edematous PEM and infection approximately 3 d after admission (study 1), when they were both infected and malnourished; approximately 11 d after admission (study 2), when infection had resolved but they were still anthropometrically malnourished; and at recovery (study 3), when weight-for-length was at least 90% of that expected. The children had significantly less leucine flux in both study 1 and study 2 than they had in study 3. There were no significant differences in the amount of leucine released from protein breakdown or used for protein synthesis between study 1 and study 2. There were no significant differences in leucine balance or in either the amount or percentage of enteral leucine extracted by the splanchnic tissues among the 3 studies. When subjects are in the fed state, severe PEM induces a marked reduction in whole-body protein synthesis and breakdown rates, and the presence of infection does not alter this adaptation and hence the overall protein balance. A corollary is that children with severe PEM do not mount a protein catabolic response to infection.

Glutamine supplementation in cancer patients

OBJECTIVES: Three series of studies investigated whether 1) glutamine deficiency occurs in tumor-bearing rats, 2) glutamine supplementation improves protein metabolism during chemotherapy in tumor-bearing rats, and 3) oral glutamine supplement improves systemic immune and gut-barrier function in patients with esophageal cancer receiving radiochemotherapy.METHODS: In the animal studies, AH109A hepatoma cells or Yoshida sarcoma cells were inoculated into male Donryu rats to induce tumors. Glutamine production was measured by U-14C-glutamine infusion and the conversion of arginine to glutamine was measured by infusion of U-14C-arginine. The effect of glutamine on protein metabolism was investigated by 1-14C-leucine infusion. In the clinical study, 13 patients with esophageal cancer were randomized into two groups, control and glutamine supplemented (30 g/d), for 4 wk.RESULTS: Glutamine levels in plasma and skeletal muscle were decreased in tumor-bearing rats, although glutamine production and the conversion of arginine to glutamine were increased. Glutamine-supplemented total parenteral nutrition reduced whole-body protein breakdown rate during chemotherapy in tumor-bearing rats. Oral supplementation of glutamine to the patients with esophageal cancer enhanced lymphocyte mitogenic function and reduced permeability of the gut during radiochemotherapy.CONCLUSIONS: Glutamine depletion in host tissues occurs in tumor-bearing rats. Glutamine supplementation can attenuate loss of protein in the muscle in tumor-bearing animals and protect immune and gut-barrier function during radiochemotherapy in patients with advanced cancer.

A summary of our work: effects of intravenous ω-6 and ω-3 fat emulsions on nitrogen retention and protein kinetics in burned rats

The effect of w-3 fat emulsion on nitrogen retention and kinetics in relation to fatty acid profile were investigated in burned rats receiving total parenteral nutrition (TPN). A fat emulsion of a structured symmetrical triacylglycerol containing only eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (2:1) was prepared. Sprague-Dawley rats were fed by fat-free chow for 2 wk. Then rats were fed exclusively with one of three types of TPN for 7 d. Animals in group C received fat-free TPN (n = 11). Group ω6 received safflower oil fat emulsion, which accounted for 20% of total caloric intake (n = 11). Group ω3 received fat emulsion containing only EPA and DHA (1 % of total calories, n = 11), in addition to safflower oil emulsion (19% of total calories). On day 5, each rat was subjected to 20% full-thickness scald burns. Rats were sacrificed under ether anesthesia 48 h after burning. The rats in group C became deficient in ω-6 essential fatty acids. Cumulative nitrogen balance was decreased significantly in group ω6. The rates of whole-body protein synthesis were increased significantly in both groups ω6 and ω3. In ω6, however, the rates of whole-body protein breakdown were increased significantly. In conclusion, the rates of whole-body protein breakdown increased and nitrogen retention was aggravated significantly in animals administered the safflower oil emulsion. Significant increases of urinary excretion of total catecholamine were also observed. Prostaglandin E 2 and thromboxane B 2 concentrations were not significantly different among three groups. Supplementation with the new ω-3 fat emulsion, however, improved protein metabolism in burned rats receiving TPN.