Mixed Muscle Protein Research Articles

Disuse Impairs the Muscle Protein Synthetic Response to Protein Ingestion in Healthy Men

Disuse leads to rapid skeletal muscle atrophy, which brings about numerous negative health consequences. Muscle disuse atrophy is, at least in part, attributed to a decline in basal (postabsorptive) muscle protein synthesis rates. However, it remains to be determined whether muscle disuse also impairs the muscle protein synthetic response to dietary protein ingestion. We assessed muscle protein synthesis rates after protein ingestion before and after a period of disuse in humans. Twelve healthy young (24 ± 1 year) men underwent a 14-day period of one-legged knee immobilization by way of a full leg cast. Before and after the immobilization period, quadriceps cross-sectional area, muscle strength, skeletal muscle protein synthesis rates, and associated im (intramuscular) molecular signaling were assessed. Continuous infusions of l-[ring-²H₅]phenylalanine were applied to assess mixed-muscle protein fractional synthetic rates after the ingestion of 20 g dietary protein. Immobilization led to an 8.4% ± 2.8% (P < .001) and 22.9% ± 2.6% (P < .001) decrease in quadriceps muscle cross-sectional area and strength, respectively. Immobilization resulted in a 31% ± 12% reduction in postprandial muscle protein synthesis rates (from 0.046% ± 0.004% to 0.032% ± 0.006% per hour; P < .05). These findings were observed without any discernible changes in the skeletal muscle phosphorylation status of mammalian target of rapamycin or p70 ribosomal protein S6 kinase. A short period of muscle disuse impairs the muscle protein synthetic response to dietary protein intake in vivo in healthy young men. Thus, anabolic resistance to protein ingestion contributes significantly to the loss of muscle mass that is observed during disuse.

Refining dietary protein recommendations for the athlete

Refining dietary protein recommendations for the athlete

Protein Blend Ingestion Following Resistance Exercise Promotes Human Muscle Protein Synthesis

High-quality proteins such as soy, whey, and casein are all capable of promoting muscle protein synthesis postexercise by activating the mammalian target of rapamycin (mTORC1) signaling pathway. We hypothesized that a protein blend of soy and dairy proteins would capitalize on the unique properties of each individual protein and allow for optimal delivery of amino acids to prolong the fractional synthetic rate (FSR) following resistance exercise (RE). In this double-blind, randomized, clinical trial, 19 young adults were studied before and after ingestion of ∼19 g of protein blend (PB) or ∼18 g whey protein (WP) consumed 1 h after high-intensity leg RE. We examined mixed-muscle protein FSR by stable isotopic methods and mTORC1 signaling with western blotting. Muscle biopsies from the vastus lateralis were collected at rest (before RE) and at 3 postexercise time points during an early (0–2 h) and late (2–4 h) postingestion period. WP ingestion resulted in higher and earlier amplitude of blood branched-chain amino acid (BCAA) concentrations. PB ingestion created a lower initial rise in blood BCAA but sustained elevated levels of blood amino acids later into recovery (P < 0.05). Postexercise FSR increased equivalently in both groups during the early period (WP, 0.078 ± 0.009%; PB, 0.088 ± 0.007%); however, FSR remained elevated only in the PB group during the late period (WP, 0.074 ± 0.010%; PB, 0.087 ± 0.003%) (P < 0.05). mTORC1 signaling similarly increased between groups, except for no increase in S6K1 phosphorylation in the WP group at 5 h postexercise (P < 0.05). We conclude that a soy-dairy PB ingested following exercise is capable of prolonging blood aminoacidemia, mTORC1 signaling, and protein synthesis in human skeletal muscle and is an effective postexercise nutritional supplement.

Comparison of different mass spectrometry techniques in the measurement of L‐[ring‐13C6]phenylalanine incorporation into mixed muscle proteins

Precise measurement of low enrichment of stable isotope labeled amino-acid tracers in tissue samples is a prerequisite in measuring tissue protein synthesis rates. The challenge of this analysis is augmented when small sample size is a critical factor. Muscle samples from human participants following an 8 h intravenous infusion of L-[ring-(13)C(6)]phenylalanine and a bolus dose of L-[ring-(13)C(6)]phenylalanine in a mouse were utilized. Liquid chromatography tandem mass spectrometry (LC/MS/MS), gas chromatography (GC) MS/MS and GC/MS were compared to the GC-combustion-isotope ratio MS (GC/C/IRMS), to measure mixed muscle protein enrichment of [ring-(13)C(6)]phenylalanine enrichment. The sample isotope enrichment ranged from 0.0091 to 0.1312 molar percent excess. As compared with GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS showed coefficients of determination of R(2)= 0.9962 and R(2) = 0.9942, and 0.9217 respectively. However, the precision of measurements (coefficients of variation) for intra-assay are 13.0%, 1.7%, 6.3% and 13.5% and for inter-assay are 9.2%, 3.2%, 10.2% and 25% for GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS, respectively. The muscle sample sizes required to obtain these results were 8 µg, 0.8 µg, 3 µg and 3 µg for GC/C/IRMS, LC/MS/MS, GC/MS/MS and GC/MS, respectively. We conclude that LC/MS/MS is optimally suited for precise measurements of L-[ring-(13)C(6)]phenylalanine tracer enrichment in low abundance and in small quantity samples.

A soy, whey and caseinate blend extends postprandial skeletal muscle protein synthesis in rats

Blends of dairy and soy protein are used in commercial sports nutrition products; however, no studies have systematically compared blends to isolated protein sources and their effects on muscle protein synthesis (MPS). Dairy whey protein (WP), soy protein isolate (SP), and two blends (Blend 1 and Blend 2) consisting of ratios of 50:25:25 and 25:50:25 for whey:caseinate:soy, respectively, were evaluated for their ability to affect MPS. Male Sprague-Dawley rats were trained to eat 3 meals/day: a 4 g meal at 0700-0720 hours followed by ad lib feeding at 1300-1400 hours and 1800-1900 hours. After ~5 days of training, fasted rats were administered their respective 4 g meal at 0700-0720 hours and an intravenous flooding dose of (2)H5-phenylalanine 10 min prior to euthanasia. Individual rats were euthanized at designated postprandial time points. Blood and gastrocnemius samples were collected and the latter was used to measure mixed muscle protein fractional synthetic rates (FSR). Plasma leucine concentrations peaked in all groups at 90 min and were still above baseline at 300 min post-meal. FSR tended to increase in all groups post-meal but initial peaks of FSR were different times (45, 90 and 135 min for WP or SP, Blend 1 and Blend 2, respectively). Blend 2 had a significantly higher FSR compared to WP alone at 135 min (P < 0.05). Single source proteins and protein blends all enhance skeletal MPS after a meal, however, Blend 2 had a delayed FSR peak which was significantly higher than whey protein at 135 min.

Insulin fails to enhance mTOR phosphorylation, mitochondrial protein synthesis, and ATP production in human skeletal muscle without amino acid replacement

Systemic insulin administration causes hypoaminoacidemia by inhibiting protein degradation, which may in turn inhibit muscle protein synthesis (PS). Insulin enhances muscle mitochondrial PS and ATP production when hypoaminoacidemia is prevented by exogenous amino acid (AA) replacement. We determined whether insulin would stimulate mitochondrial PS and ATP production in the absence of AA replacement. Using l-[1,2-¹³C]leucine as a tracer, we measured the fractional synthetic rate of mitochondrial as well as sarcoplasmic and mixed muscle proteins in 18 participants during sustained (7-h) insulin or saline infusion (n = 9 each). We also measured muscle ATP production, mitochondrial enzyme activities, mRNA levels of mitochondrial genes, and phosphorylation of signaling proteins regulating protein synthesis. The concentration of circulating essential AA decreased during insulin infusion. Mitochondrial, sarcoplasmic, and mixed muscle PS rates were also lower during insulin (2-7 h) than during saline infusions despite increased mRNA levels of selected mitochondrial genes. Under these conditions, insulin did not alter mitochondrial enzyme activities and ATP production. These effects were associated with enhanced phosphorylation of Akt but not of protein synthesis activators mTOR, p70(S6K), and 4EBP1. In conclusion, sustained physiological hyperinsulinemia without AA replacement did not stimulate PS of mixed muscle or protein subfractions and did not alter muscle mitochondrial ATP production in healthy humans. These results support that insulin and AA act in conjunction to stimulate muscle mitochondrial function and mitochondrial protein synthesis.

Protein Ingestion before Sleep Improves Postexercise Overnight Recovery

The role of nutrition in modulating postexercise overnight recovery remains to be elucidated. We assessed the effect of protein ingestion immediately before sleep on digestion and absorption kinetics and protein metabolism during overnight recovery from a single bout of resistance-type exercise. Sixteen healthy young males performed a single bout of resistance-type exercise in the evening (2000 h) after a full day of dietary standardization. All subjects were provided with appropriate recovery nutrition (20 g of protein, 60 g of CHO) immediately after exercise (2100 h). Thereafter, 30 min before sleep (2330 h), subjects ingested a beverage with (PRO) or without (PLA) 40 g of specifically produced intrinsically [1-C]phenylalanine-labeled casein protein. Continuous intravenous infusions with [ring-H5]phenylalanine and [ring-H2]tyrosine were applied with blood and muscle samples collected to assess protein digestion and absorption kinetics, whole-body protein balance and mixed muscle protein synthesis rates throughout the night (7.5 h). During sleep, casein protein was effectively digested and absorbed resulting in a rapid rise in circulating amino acid levels, which were sustained throughout the remainder of the night. Protein ingestion before sleep increased whole-body protein synthesis rates (311 ± 8 vs 246 ± 9 μmol·kg per 7.5 h) and improved net protein balance (61 ± 5 vs -11 ± 6 μmol·kg per 7.5 h) in the PRO vs the PLA experiment (P < 0.01). Mixed muscle protein synthesis rates were ∼22% higher in the PRO vs the PLA experiment, which reached borderline significance (0.059%·h ± 0.005%·h vs 0.048%·h ± 0.004%·h, P = 0.05). This is the first study to show that protein ingested immediately before sleep is effectively digested and absorbed, thereby stimulating muscle protein synthesis and improving whole-body protein balance during postexercise overnight recovery.

The single biopsy approach is reliable for the measurement of muscle protein synthesis rates in vivo in older men

We aimed to assess the reliability of the single biopsy approach for calculating muscle protein synthesis rates compared with the well described sequential muscle biopsy approach following a primed continuous infusion of L-[ring-(2)H(5)]phenylalanine and GC-MS analysis in older men. Two separate experimental infusion protocols, with differing stable isotope amino acid incorporation times, were employed consisting of n = 27 (experiment 1) or n = 9 (experiment 2). Specifically, mixed muscle protein FSR were calculated from baseline plasma protein enrichments and muscle protein enrichments obtained at 90 min or 50 min (1BX SHORT), 210 min or 170 min (1BX LONG), and between the muscle protein enrichments obtained at 90 and 210 min or 50 min and 170 min (2BX) of the infusion for experiments 1 and 2, respectively. In experiment 2, we also assessed the error that is introduced to the single muscle biopsy approach when nontracer naive subjects are recruited for participation in a primed continuous infusion of isotope-labeled amino acids. In experiment 1, applying the individual plasma protein enrichment values to the single muscle biopsy approach resulted in no differences in muscle protein FSR between the 1BX SHORT (0.031 ± 0.003%·h(-1)), 1BX LONG (0.032 ± 0.002%·h(-1)), or the 2BX approach (0.034 ± 0.002%·h(-1)). A significant correlation in muscle protein FSR was observed only between the 1BX LONG and 2BX approach (r = 0.8; P < 0.001). Similar results were observed in experiment 2. In addition, using the single biopsy approach in nontracer naïve state results in a muscle protein FSR that is negative for both the 1BX SHORT (-0.67 ± 0.051%·h(-1)) and 1BX LONG (-0.19 ± 0.051%·h(-1)) approaches. This is the first study to demonstrate that the single biopsy approach, coupled with the background enrichment of L-[ring-(2)H(5)]-phenylalanine of mixed plasma proteins, generates data that are similar to using the sequential muscle biopsy approach in the elderly population.

Cancer cachexia and anabolic interventions: a case report

BackgroundStandard-of-care (SOC) cancer treatments are primarily aimed at reducing size and progression of a tumor. There is a need for successful supplemental anabolic therapies to combat cancer cachexia in addition to these SOC treatment modalities. Anabolic interventions, including testosterone and amino acid supplements, may be beneficial in reducing and/or reversing muscle wasting in these patient populations.MethodsA 48-year-old Caucasian female with recurrent cervical cancer was scheduled to receive three 21-day cycles of cisplatin and topetecan chemotherapy. She qualified, consented, and enrolled into a blinded interventional pilot study where she received daily whey protein (10 g, three times per day with meals) and a weekly injection of testosterone enanthate (100 mg intramuscular) before and during the SOC chemotherapy treatment period. Body composition, serum inflammatory markers, mixed muscle protein synthesis and breakdown rates, physical function, fatigue, and quality of life were assessed before and after the intervention period.ResultsBody composition, as assessed by an increase in body weight and lean body mass and reduction in fat mass; physical function; fatigue; and quality of life each improved across the entire intervention period despite general increases in inflammatory markers and no improvements in muscle protein turnover towards the end of the intervention.ConclusionsConcomitant treatment of oral amino acids and testosterone may be a viable therapeutic option for fighting cachexia and improving body composition and quality of life during chemotherapeutic treatment of recurrent cervical cancer. These positive outcomes may be attainable over time despite overall poor inflammatory status.

Endurance (ET), Resistance (RT), and Combined (CT) Training Increase Mixed Muscle Protein Synthesis Independently of Age

The effects of different modes of exercise on mixed muscle protein synthesis (MMPS) in young (Y) and older (O) adults are not fully understood. We determined the effects of ET, RT, and CT on MMPS in 33Y (25±1y) and 31O (70±1y) adults who were randomized to one of three 8wk interventions: (i) Control: no exercise, (ii) ET: cycling 1h @ 65% VO2 peak, 5x/wk, (iii) RT: RT 4x/wk. After completing the control period, the controls completed an 8wk (iv) CT program (ET &amp; RT). Subjects were studied at baseline and 48h following the last training bout. [13C6]Phe was infused for 8h, with serial v. lateralis biopsies. Isotopic enrichments (IE) were measured by LC/MS for free Phe from tissue fluid (TF) and from that incorporated into MMP. MMPS (%/h) = (ΔIEMMP)/(IETF)/t*100. A rank sum test was used to test baseline differences between Y and O. 2‐way ANCOVA was used to test for differences between groups by treatment, with baseline values serving as the covariate. MMPS (%/h) tended to be higher in Y (0.060±0.003) compared to O (0.053±0.003) (P=0.06). ET [57%, P&lt;0.0001], RT [21%, P=0.042] and CT [42%, P&lt;0.0001] increased MMPS independently of age (P&gt;0.4). The ET‐induced increase in MMPS was greater than the RT‐induced increase (P&lt;0.05), although ET unlike RT does not increase muscle mass suggesting that muscle protein turnover was increased. In conclusion, ET, RT and CT were effective at increasing MMPS independently of age. R01AG09531, KL2RR024151

Effect of protein blend vs whey protein ingestion on muscle protein synthesis following resistance exercise

Resistance exercise followed by the ingestion of amino acids or protein causes an additive stimulation of muscle protein synthesis (MPS). Proteins such as whey are digested at a fast rate whereas soy and casein are digested at an intermediate and slow rate, respectively. To determine if ingestion of a protein blend (soy and dairy protein) extends the post‐exercise MPS response compared to whey protein we conducted a double‐blind, randomized clinical trial in 20 young adults before and after ingestion of ~20g of the protein blend or whey (N=10 per group) given 1h after a bout of high‐intensity leg resistance exercise. We utilized stable isotopic methods to measure the mixed muscle protein fractional synthetic rate (FSR) and western blotting to examine mTORC1 signaling. Muscle biopsies (vastus lateralis) were collected at baseline and at two time points during post‐exercise (Early: 1–3h &amp; Late: 3–5h post‐ex). We report differences between the two beverages as Protein A and B (data will be unblinded at the EB meeting). FSR increased by ~40–70% in both groups during Early and remained elevated into Late. Protein A and B also activated mTORC1 signaling to a similar extent following exercise, yet Protein B tended to be greater. We conclude, from these preliminary data that following resistance exercise, the ingestion of a protein blend enhances mTORC1 signaling and MPS similarly to that of whey protein.Grant Funding Source : Solae LLC

Neuromuscular electrical stimulation increases muscle protein synthesis rates in type 2 diabetic men

A loss of skeletal muscle proteins ultimately lowers the capacity for blood glucose disposal. Thus, the development of paradigms that can increase muscle protein accretion will have implications for the treatment of type 2 diabetes at a more advanced age. We aimed to determine the effect of neuromuscular electrical stimulation (NMES) on mixed muscle protein synthesis rates in type 2 diabetic men. Six sedentary, type 2 diabetic men (Age: 70±1 y, BMI: 26.5±1.0 kg·m−2) received a primed, constant infusion of L‐[ring‐13C6]phenylalanine and 60 min of one legged NMES. Biopsies of the vastus lateralis were obtained at 0, 2, and 4 h after NMES to measure mixed muscle protein synthesis rates in the unstimulated (CON) and stimulated (STIM) leg. The electrical current of the NMES protocol consisted of a pulse duration of 500 μs, pulse frequency of 60 Hz, and a 3 sec contraction – 3 sec relaxation time. Muscle protein synthesis rates were increased by 21% and 64% above CON at 0–2 h and 2–4 h in the STIM leg, respectively (P&lt;0.05). Similarly, the cumulative MPS response (0–4 h) was greater in the STIM (0.049±0.008 %·h−1) compared with the CON leg (0.039±0.008 %·h−1). Our data demonstrate that NMES stimulates mixed muscle protein synthesis rates in vivo in humans. This work lends credence to the use of NMES to attenuate muscle protein loss during periods of muscle disuse or to support an increase in skeletal muscle hypertrophy.

The reliability of using the single-biopsy approach to assess basal muscle protein synthesis rates in vivo in humans

It has recently been proposed that basal muscle protein synthesis can be effectively assessed by measuring the background enrichment in total plasma protein, thereby omitting the initial biopsy, and determining the difference in enrichment from a single muscle biopsy obtained during a primed continuous infusion of isotope-labeled amino acids. We determined the reliability of calculating basal mixed muscle protein fractional synthetic rates (FSRs) from mixed plasma proteins and a single muscle biopsy compared against the sequential muscle biopsy approach. Ten men (age, 23 ± 1 years; body mass index, 22 ± 1 kg∙m−2) received muscle biopsies of the vastus lateralis after 2 and 4 hours of a primed continuous infusion of l-[ring-13C6]phenylalanine. Mixed muscle protein FSR was calculated from baseline plasma enrichments and muscle protein enrichments determined from the biopsy at 2 hours (1BX SHORT) or 4 hours (1BX LONG), or between muscle protein enrichments at 2 and 4 hours (2BX) of the infusion. No differences (P = .50) were observed in mixed muscle protein FSR, using plasma [ring-13C6]phenylalanine enrichments as the precursor, between the 1BX SHORT (0.031% ± 0.010%∙h−1), 1BX LONG (0.032% ± 0.007%∙h−1), or 2BX (0.035% ± 0.011%∙h−1) approach. A significant correlation was observed between the calculated muscle protein FSR assessed using the 1BX LONG and 2BX approach (r = 0.7, P = .02). Our data demonstrate that the single-biopsy approach, irrespective of whether the biopsy is obtained at 2 or 4 hours, can be used as a surrogate for the sequential-biopsy approach to determine basal muscle protein synthesis in a group.

The curious case of anabolic resistance: old wives' tales or new fables?

the progressive loss of muscle mass, commonly termed sarcopenia, accompanies the process of healthy aging ([27][1]). The underlying basis of this condition, in a very simplistic view, would be at a more advanced age skeletal muscle proteins are being lost because of an imbalance between muscle

A Randomized Pilot Study of Monthly Cycled Testosterone Replacement or Continuous Testosterone ReplacementVersusPlacebo in Older Men

Cycling androgens has been reported by athletes to improve physical performance by enhancing muscle mass and strength, a paradigm that has not been studied, and may have clinical value in older men being treated with testosterone. We investigated the efficacy of a monthly cycled testosterone regimen that uses half the testosterone dose as the current standard of care continuous therapy on body composition and muscle strength in older men. Twenty-four community-dwelling older men 70 ± 2 yr of age with total testosterone levels below 500 ng/dl were randomized at the Institute for Translational Sciences-Clinical Research Center into a 5-month double-blind placebo-controlled trial. Subjects were dosed weekly for 5 months, receiving continuous testosterone (TE, n = 8; 100 mg testosterone enanthate, im injection), monthly cycled testosterone (MO, n = 8; alternating months of testosterone and placebo), or placebo (PL, n = 8). Main outcomes included body composition by dual-energy x-ray absorptiometry and upper and lower body muscle strength. Secondary outcomes included body weight, serum hormones, and mixed-muscle protein fractional synthesis rate (FSR). Total lean body mass was increased and percent fat was reduced after 5 months in TE and MO (P < 0.05). Upper body muscle strength increased in TE, and lower body muscle strength increased in TE and MO (P < 0.05). FSR increased in TE and MO (P < 0.05) but not in PL. Cycled testosterone improved body composition and increased muscle strength compared with placebo and increased FSR similarly to continuous testosterone.

Acute resistance exercise augments integrative myofibrillar protein synthesis

The purpose of this study was to determine whether an acute bout of high-intensity resistance exercise (RE) would augment integrative mixed muscle and myofibrillar protein fractional synthesis rates (FSRs) when total energy and macronutrient intake was controlled. Twelve healthy young men were studied over 24 hours and performed an acute bout of exhaustive (5 sets until volitional failure of their 85% 1-repetition maximum) unilateral leg press and knee extension exercise, such that one leg was exercised (EX) and the other served as a control (CON). 2H 2O (70%) was provided to measure mixed muscle and myofibrillar FSR, and muscle biopsies (vastus lateralis) were collected from the EX and CON legs 16 hours following the RE session. 2H-labeling of body water over the course of the experiment was 0.32 ± 0.01 mole percent excess. Interestingly, integrative mixed muscle FSR (percent per hour) was similar between the CON (0.76% ± 0.08%) and EX (0.69% ± 0.06%) legs. In contrast, upon determination of myofibrillar FSR, there was an RE effect (EX, 0.94% ± 0.16% vs CON, 0.75% ± 0.08%; P < .05). High-intensity RE without prior training impacts integrative myofibrillar 24-hour FSR, perhaps without altering total responses.

Endocrine Responses During Overnight Recovery From Exercise: Impact of Nutrition and Relationships With Muscle Protein Synthesis

Nocturnal endocrine responses to exercise performed in the evening and the potential role of nutrition are poorly understood. To gain novel insight, 10 healthy men ingested carbohydrate with (C+P) and without (C) protein in a randomized order and double-blind manner during 2 hr of interval cycling followed by resistance-type exercise and into early postexercise recovery. Blood samples were obtained hourly throughout 9 hr of postexercise overnight recovery for analysis of key hormones. Muscle samples were taken from the vastus lateralis before and after exercise and then again the next morning (7 a.m.) to calculate mixed-muscle protein fractional synthetic rate (FSR). Overnight plasma hormone concentrations were converted into overall responses (expressed as area under the concentration curve) and did not differ between treatments for either growth hormone (1,464 ± 257 vs. 1,432 ± 164 pg/ml · 540 min) or total testosterone (18.3 ± 1.2 vs. 17.9 ± 1.2 nmol/L · 540 min, C and C+P, respectively). In contrast, the overnight cortisol response was higher with C+P (102 ± 11 nmol/L · 540 min) than with C (81 ± 8 nmol/L · 540 min; p = .02). Mixed-muscle FSR did not differ between C and C+P during overnight recovery (0.062% ± 0.006% and 0.062% ± 0.009%/hr, respectively) and correlated significantly with the plasma total testosterone response (r = .7, p < .01). No correlations with FSR were apparent for the response of growth hormone (r = -.2, p = .4), cortisol (r = .1, p = .6), or the ratio of testosterone to cortisol (r = .2, p = .5). In conclusion, protein ingestion during and shortly after exercise does not modulate the endocrine response or muscle protein synthesis during overnight recovery.

Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men

Sarcopenia has been attributed to a diminished muscle protein synthetic response to food intake. Differences in digestion and absorption kinetics of dietary protein, its amino acid composition, or both have been suggested to modulate postprandial muscle protein accretion. The objective was to compare protein digestion and absorption kinetics and subsequent postprandial muscle protein accretion after ingestion of whey, casein, and casein hydrolysate in healthy older adults. A total of 48 older men aged 74 ± 1 y (mean ± SEM) were randomly assigned to ingest a meal-like amount (20 g) of intrinsically l-[1-(13)C]phenylalanine-labeled whey, casein, or casein hydrolysate. Protein ingestion was combined with continuous intravenous l-[ring-(2)H(5)]phenylalanine infusion to assess in vivo digestion and absorption kinetics of dietary protein. Postprandial mixed muscle protein fractional synthetic rates (FSRs) were calculated from the ingested tracer. The peak appearance rate of dietary protein-derived phenylalanine in the circulation was greater with whey and casein hydrolysate than with casein (P < 0.05). FSR values were higher after whey (0.15 ± 0.02%/h) than after casein (0.08 ± 0.01%/h; P < 0.01) and casein hydrolysate (0.10 ± 0.01%/h; P < 0.05) ingestion. A strong positive correlation (r = 0.66, P < 0.01) was observed between peak plasma leucine concentrations and postprandial FSR values. Whey protein stimulates postprandial muscle protein accretion more effectively than do casein and casein hydrolysate in older men. This effect is attributed to a combination of whey's faster digestion and absorption kinetics and higher leucine content. This trial was registered at clinicaltrials.gov as NCT00557388.

Skeletal Muscle DNA and Mixed Muscle Protein Synthesis Rates with Endurance Training in Aging Humans

Decreased rates of synthesis in skeletal muscle may contribute to loss of muscle mass with age. Acute studies show that resistance exercise can increase mixed muscle protein synthesis, an effect that is enhanced with protein consumption. However, less is known about the ability of endurance exercise training and protein consumption to promote synthetic processes. Deuterium oxide (D2O) consumption allows measurement of multiple synthetic processes and provides insight into skeletal muscle adaptations over extended periods. PURPOSE: The purpose of our study was to use D2O to determine long-term rates of protein, DNA and membrane phospholipid synthesis in skeletal muscle during endurance exercise training. METHODS: ixteen healthy adult males and females (Age: 50±8 yrs, BMI: 25.3±4.1 kg/m2, VO2 max: 28.3±6.4 ml/kg/min) participated in 6-weeks of endurance exercise training and were randomized to consume either carbohydrate (CHO) or an iso-caloric drink with 20-grams protein (PRO) following each session. Each participant also consumed D2O daily to determine the synthesis rate of proteins (MPS), DNA (DNA%F) and membrane phospholipid glycerol (GLY). VO2 max and muscle biopsies were performed pre and post training. Total muscle proteins, DNA and GLY were extracted from muscle biopsy samples and analyzed for deuterium enrichment using gas or liquid chromatography and mass spectrometry. RESULTS: he increase in VO2 max with training was greater in PRO compared to CHO (PRO=+12.8±6.2% vs. CHO=+2.8±8.7%, p=0.033). MPS was not different between PRO and CHO (PRO=0.05±0.01 vs. CHO=0.06±0.01 %/hr). DNA%F was not different between PRO and CHO (PRO=3.3±2.6 vs. CHO=5.3±2.8 % new in 6-weeks). MPS and DNA%F were correlated (r2=0.26, p=0.055). The synthesis of GLY was not different between PRO and CHO (PRO=37.6±11.9 vs. CHO=42.2±11.2 % new in 6-weeks). CONCLUSIONS: Our results suggest that protein consumption following each session of endurance exercise training can lead to long-term changes in aerobic capacity. MPS, DNA%F, or GLY had similar responses to training between PRO and CHO groups. The mean DNA%F of 4% is a novel measure of skeletal muscle DNA synthesis and cannot be accounted for by mitochondrial DNA turnover. These results may indicate satellite cell activation during endurance exercise training in aging adults.

Leucine supplementation during endurance exercise enhances post‐exercise muscle protein synthesis

The benefit of leucine-enriched amino acid supplementation on muscle protein synthesis following resistance exercise has been well established. However, the effects after sustained aerobic-type exercise are not well described and may differ, given that muscle contractile forces generated are normally lower. This study examined how essential amino acid (EAA) supplementation, of varying leucine content, affected mixed muscle protein synthesis (MPS), whole body leucine oxidation (Ox), and non-oxidative leucine disposal (NOLD) following sustained aerobic-type exercise. Ten adults completed two separate 60 min bouts of cycle ergometry (60% VO2peak), during which isonitrogenous (10 g EAA) drinks containing leucine-enriched EAA (L-EAA; 34% leucine) or EAA (18% leucine) were consumed. MPS and leucine kinetics were determined using primed, continuous infusions of [2H5]-phenylalanine and [13C]-leucine. Plasma leucine levels were higher (P < 0.05) post-exercise for L-EAA relative to EAA for up to 3.5 h into recovery. Post-exercise MPS was higher (P < 0.05) for L-EAA compared to EAA (0.078 ± 0.01 vs. 0.056 ± 0.01 %/h). Post-exercise leucine Ox was 65% higher (P < 0.05) and NOLD was 19% lower (P < 0.05) for L-EAA than for EAA. Leucine supplementation during sustained steady-state exercise appears to have similar beneficial effects on MPS as during resistance-type exercise.