Intradialytic Protein Ingestion and Exercise do Not Compromise Uremic Toxin Removal Throughout Hemodialysis.

OBJECTIVE: Dietary protein and physical activity interventions are increasingly implemented during hemodialysis to support muscle maintenance in patients with end-stage renal disease (ESRD). Although muscle maintenance is important, adequate removal of uremic toxins throughout hemodialysis is the primary concern for patients. It remains to be established whether intradialytic protein ingestion and/or exercise modulate uremic toxin removal during hemodialysis. METHODS: We recruited 10 patients with ESRD (age: 65 ± 16 y, BMI: 24.2 ± 4.8 kg/m2) on chronic hemodialysis treatment to participate in this randomized cross-over trial. During hemodialysis, patients were assigned to ingest 40 g protein or a nonprotein placebo both at rest (protein [PRO] and placebo [PLA], respectively) and following 30 min of exercise (PRO + exercise [EX] and PLA + EX, respectively). Blood and spent dialysate samples were collected throughout hemodialysis to assess reduction ratios and removal of urea, creatinine, phosphate, cystatin C, and indoxyl sulfate. RESULTS: The reduction ratios of urea and indoxyl sulfate were higher during PLA (76 ± 6% and 46 ± 9%, respectively) and PLA + EX interventions (77 ± 5% and 45 ± 10%, respectively) when compared to PRO (72 ± 4% and 40 ± 8%, respectively) and PRO + EX interventions (73 ± 4% and 43 ± 7%, respectively; protein effect: P = .001 and P = .023, respectively; exercise effect: P = .25 and P = .52, respectively). Nonetheless, protein ingestion resulted in greater urea removal (P = .046) during hemodialysis. Reduction ratios and removal of creatinine, phosphate, and cystatin C during hemodialysis did not differ following intradialytic protein ingestion or exercise (protein effect: P > .05; exercise effect: P>.05). Urea, creatinine, and phosphate removal were greater throughout the period with intradialytic exercise during PLA + EX and PRO + EX interventions when compared to the same period during PLA and PRO interventions (exercise effect: P = .034, P = .039, and P = .022, respectively). CONCLUSION: The removal of uremic toxins is not compromised by protein feeding and/or exercise implementation during hemodialysis in patients with ESRD.

The association between gastrointestinal injury, complaints, and food intake in 60-km ultramarathon runners.

We aimed to assess the association between gastrointestinal (GI) injury, complaints, and food intake in 60-km ultramarathon runners. Thirty-three ultramarathon runners provided pre- and post-race blood samples for assessment of GI injury by intestinal fatty-acid binding protein (I-FABP), and inflammatory response by interleukin (IL)-6, IL-8, tumour necrosis factor alpha (TNF-α), and C-reactive protein (CRP). GI complaints and nutritional intake were reported by a post-race questionnaire. GI complaints were reported by 73% of the runners, of which 20% reported 1 or 2 severe complaints. IL-6, IL8, TNF-α, and CRP increased significantly from pre- to post-race (P < 0.001 for all biomarkers), while I-FABP did not (1375 [IQR: 1264-2073] to 1726 [IQR: 985-3287] pg/mL; P = 0.330). The 'GI complaints score', as the integral of the number and severity of GI complaints, did not correlate with ΔI-FABP (rs: -0.050, P = 0.790) or energy intake (rs: 0.211, P = 0.260). However, there was a significant negative correlation between energy intake and ΔI-FABP (rs: -0.388, P = 0.031). In conclusion, GI complaints were neither associated with food intake nor GI injury as assessed by plasma I-FABP response. Energy intake, however, was inversely related to the I-FABP response to exercise. This finding suggests that substantial energy intakes during exercise may prevent exercise-induced GI injury as assessed by the I-FABP response. Novelty: No association between gastrointestinal complaints and gastrointestinal injury (I-FABP response) or food intake was present. There was an inverse correlation between energy intake and plasma I-FABP response, suggesting that higher energy intakes may prevent gastrointestinal injury as assessed by the I-FABP response.

L-arabinose co-ingestion delays glucose absorption derived from sucrose in healthy men and women: a double-blind, randomised crossover trial.

Dietary interventions to delay carbohydrate digestion or absorption can effectively prevent hyperglycaemia in the early postprandial phase. L-arabinose can specifically inhibit sucrase. It remains to be assessed whether co-ingestion of L-arabinose with sucrose delays sucrose digestion, attenuates subsequent glucose absorption and impacts hepatic glucose output. In this double-blind, randomised crossover study, we assessed blood glucose kinetics following ingestion of a 200-ml drink containing 50 g of sucrose with 7·5 g of L-arabinose (L-ARA) or without L-arabinose (CONT) in twelve young, healthy participants (24 ± 1 years; BMI: 22·2 ± 0·5 kg/m2). Plasma glucose kinetics were determined by a dual stable isotope methodology involving ingestion of (U-13C6)-glucose-enriched sucrose, and continuous intravenous infusion of (6,6-2H2)-glucose. Peak glucose concentrations reached 8·18 ± 0·29 mmol/l for CONT 30 min after ingestion. In contrast, the postprandial rise in plasma glucose was attenuated for L-ARA, because peak glucose concentrations reached 6·62 ± 0·18 mmol/l only 60 min after ingestion. The rate of exogenous glucose appearance for L-ARA was 67 and 57 % lower compared with CONT at t = 15 min and 30 min, respectively, whereas it was 214 % higher at t = 150 min, indicating a more stable absorption of exogenous glucose for L-ARA compared with CONT. Total glucose disappearance during the first hour was lower for L-ARA compared with CONT (11 ± 1 v. 17 ± 1 g, P < 0·0001). Endogenous glucose production was not differentially affected at any time point (P = 0·27). Co-ingestion of L-arabinose with sucrose delays sucrose digestion, resulting in a slower absorption of sucrose-derived glucose without causing adverse effects in young, healthy adults.

Does supplementation with leucine-enriched protein alone and in combination with fish-oil-derived n-3 PUFA affect muscle mass, strength, physical performance, and muscle protein synthesis in well-nourished older adults? A randomized, double-blind, placebo-controlled trial.

BACKGROUND: Leucine-enriched protein (LEU-PRO) and long-chain (LC) n-3 (ω-3) PUFAs have each been proposed to improve muscle mass and function in older adults, whereas their combination may be more effective than either alone. OBJECTIVE: The impact of LEU-PRO supplementation alone and combined with LC n-3 PUFAs on appendicular lean mass, strength, physical performance and myofibrillar protein synthesis (MyoPS) was investigated in older adults at risk of sarcopenia. METHODS: This 24-wk, 3-arm parallel, randomized, double-blind, placebo-controlled trial was conducted in 107 men and women aged ≥65 y with low muscle mass and/or strength. Twice daily, participants consumed a supplement containing either LEU-PRO (3 g leucine, 10 g protein; n = 38), LEU-PRO plus LC n-3 PUFAs (0.8 g EPA, 1.1 g DHA; LEU-PRO+n-3; n = 38), or an isoenergetic control (CON; n = 31). Appendicular lean mass, handgrip strength, leg strength, physical performance, and circulating metabolic and renal function markers were measured pre-, mid-, and postintervention. Integrated rates of MyoPS were assessed in a subcohort (n = 28). RESULTS: Neither LEU-PRO nor LEU-PRO+n-3 supplementation affected appendicular lean mass, handgrip strength, knee extension strength, physical performance or MyoPS. However, isometric knee flexion peak torque (treatment effect: -7.1 Nm; 95% CI: -12.5, -1.8 Nm; P < 0.01) was lower postsupplementation in LEU-PRO+n-3 compared with CON. Serum triacylglycerol and total adiponectin concentrations were lower, and HOMA-IR was higher, in LEU-PRO+n-3 compared with CON postsupplementation (all P < 0.05). Estimated glomerular filtration rate was higher and cystatin c was lower in LEU-PRO and LEU-PRO+n-3 postsupplementation compared with CON (all P < 0.05). CONCLUSIONS: Contrary to our hypothesis, we did not observe a beneficial effect of LEU-PRO supplementation alone or combined with LC n-3 PUFA supplementation on appendicular lean mass, strength, physical performance or MyoPS in older adults at risk of sarcopenia. This trial was registered at clinicaltrials.gov as NCT03429491.

Protein Type, Protein Dose, and Age Modulate Dietary Protein Digestion and Phenylalanine Absorption Kinetics and Plasma Phenylalanine Availability in Humans.

BACKGROUND: Dietary protein ingestion stimulates muscle protein synthesis by providing amino acids to the muscle. The magnitude and duration of the postprandial increase in muscle protein synthesis rates are largely determined by dietary protein digestion and amino acid absorption kinetics. OBJECTIVE: We assessed the impact of protein type, protein dose, and age on dietary protein digestion and amino acid absorption kinetics in vivo in humans. METHODS: We included data from 18 randomized controlled trials with a total of 602 participants [age: 53 ± 23 y; BMI (kg/m2): 24.8 ± 3.3] who consumed various quantities of intrinsically l-[1-13C]-phenylalanine-labeled whey (n = 137), casein (n = 393), or milk (n = 72) protein and received intravenous infusions of l-[ring-2H5]-phenylalanine, which allowed us to assess protein digestion and phenylalanine absorption kinetics and the postprandial release of dietary protein-derived phenylalanine into the circulation. The effect of aging on these processes was assessed in a subset of 82 young (aged 22 ± 3 y) and 83 older (aged 71 ± 5 y) individuals. RESULTS: A total of 50% ± 14% of dietary protein-derived phenylalanine appeared in the circulation over a 5-h postprandial period. Casein ingestion resulted in a smaller (45% ± 11%), whey protein ingestion in an intermediate (57% ± 10%), and milk protein ingestion in a greater (65% ± 13%) fraction of dietary protein-derived phenylalanine appearing in the circulation (P < 0.001). The postprandial availability of dietary protein-derived phenylalanine in the circulation increased with the ingestion of greater protein doses (P < 0.05). Protein digestion and phenylalanine absorption kinetics were attenuated in older when compared with young individuals, with 45% ± 10% vs. 51% ± 14% of dietary protein-derived phenylalanine appearing in the circulation, respectively (P = 0.001). CONCLUSIONS: Protein type, protein dose, and age modulate dietary protein digestion and amino acid absorption kinetics and subsequent postprandial plasma amino acid availability in vivo in humans. These trials were registered at clinicaltrials.gov as NCT00557388, NCT00936039, NCT00991523, NCT01317511, NCT01473576, NCT01576848, NCT01578590, NCT01615276, NCT01680146, NCT01820975, NCT01986842, and NCT02596542, and at http://www.trialregister.nl as NTR3638, NTR3885, NTR4060, NTR4429, and NTR4492.

Protein content and amino acid composition of commercially available plant-based protein isolates.

The postprandial rise in essential amino acid (EAA) concentrations modulates the increase in muscle protein synthesis rates after protein ingestion. The EAA content and AA composition of the dietary protein source contribute to the differential muscle protein synthetic response to the ingestion of different proteins. Lower EAA contents and specific lack of sufficient leucine, lysine, and/or methionine may be responsible for the lower anabolic capacity of plant-based compared with animal-based proteins. We compared EAA contents and AA composition of a large selection of plant-based protein sources with animal-based proteins and human skeletal muscle protein. AA composition of oat, lupin, wheat, hemp, microalgae, soy, brown rice, pea, corn, potato, milk, whey, caseinate, casein, egg, and human skeletal muscle protein were assessed using UPLC-MS/MS. EAA contents of plant-based protein isolates such as oat (21%), lupin (21%), and wheat (22%) were lower than animal-based proteins (whey 43%, milk 39%, casein 34%, and egg 32%) and muscle protein (38%). AA profiles largely differed among plant-based proteins with leucine contents ranging from 5.1% for hemp to 13.5% for corn protein, compared to 9.0% for milk, 7.0% for egg, and 7.6% for muscle protein. Methionine and lysine were typically lower in plant-based proteins (1.0 ± 0.3 and 3.6 ± 0.6%) compared with animal-based proteins (2.5 ± 0.1 and 7.0 ± 0.6%) and muscle protein (2.0 and 7.8%, respectively). In conclusion, there are large differences in EAA contents and AA composition between various plant-based protein isolates. Combinations of various plant-based protein isolates or blends of animal and plant-based proteins can provide protein characteristics that closely reflect the typical characteristics of animal-based proteins.

Muscle Atrophy Due to Nerve Damage Is Accompanied by Elevated Myofibrillar Protein Synthesis Rates.

Muscle loss is a severe complication of many medical conditions such as cancer, cardiac failure, muscular dystrophies, and nerve damage. The contribution of myofibrillar protein synthesis (MPS) to the loss of muscle mass after nerve damage is not clear. Using deuterium oxide (D2O) labeling, we demonstrate that MPS is significantly increased in rat m.tibialis anterior (TA) compared to control (3.23 ± 0.72 [damaged] to 2.09 ± 0.26%∗day-1 [control]) after 4 weeks of nerve constriction injury. This is the case despite substantial loss of mass of the TA (350 ± 96 mg [damaged] to 946 ± 361 mg [control]). We also show that expression of regulatory proteins involved with MPS (p70s6k1: 2.4 ± 0.3 AU [damaged] to 1.8 ± 0.2 AU [control]) and muscle protein breakdown (MPB) (MAFbx: 5.3 ± 1.2 AU [damaged] to 1.4 ± 0.4 AU [control]) are increased in nerve damaged muscle. Furthermore, the expression of p70s6k1 correlates with MPS rates (r2 = 0.57). In conclusion, this study shows that severe muscle wasting following nerve damage is accompanied by increased as opposed to decreased MPS.

The Effect of Beetroot Juice Supplementation on Dynamic Apnea and Intermittent Sprint Performance in Elite Female Water Polo Players.

Nitrate-rich beetroot juice is thought to have ergogenic effects, particularly in conditions where oxygen availability is limited. Whether these effects also apply to elite athletes is currently unknown. The aim of this study was to assess the effects of beetroot juice supplementation on dynamic apnea and intermittent sprint performance in elite female water polo players. In a double-blinded, randomized, crossover manner, the Dutch National female water polo team (N = 14) was subjected to two 6-day supplementation periods (1 and 2), with either 140 ml/day of nitrate-rich (BR; ∼800 mg/day nitrate) or nitrate-depleted (PLA) beetroot juice. Following blood sampling on Day 6, the athletes performed a maximal-distance front crawl swimming test without breathing (dynamic apnea test). In addition, intermittent sprint performance was assessed by performing 16 swim sprints of 15 m, in a 4 × 4 block with 30-s recovery between blocks (intermittent test). Distance covered during the dynamic apnea test did not differ between BR (49.5 ± 7.8 m) and PLA (46.9 ± 9.1 m, p = .178). However, when correcting for test order, the distance covered was significantly larger in BR versus PLA when BR was ingested in Period 2 (50.1 ± 8.5 vs. 42.8 ± 5.7 m, p = .002), whereas no difference was observed when BR was ingested in Period 1 (48.8 ± 7.4 vs. 52.3 ± 10.4 m, p = .10). The time to complete the intermittent test was not different between BR and PLA (316.0 ± 7.9 vs. 316.3 ± 6.9 s, p = .73). In conclusion, beetroot juice supplementation does not improve intermittent performance in elite female water polo players, but there may be a potential for ergogenic effects during dynamic apnea.

The effect of prolonged dietary nitrate supplementation on atherosclerosis development.

BACKGROUND: Short term dietary nitrate or nitrite supplementation has nitric oxide (NO)-mediated beneficial effects on blood pressure and inflammation and reduces mitochondrial oxygen consumption, possibly preventing hypoxia. As these processes are implicated in atherogenesis, dietary nitrate was hypothesized to prevent plaque initiation, hypoxia and inflammation. AIMS: Study prolonged nitrate supplementation on atherogenesis, hypoxia and inflammation in low density lipoprotein receptor knockout mice (LDLr(-/-)). METHODS: LDLr(-/-) mice were administered sodium-nitrate or equimolar sodium-chloride in drinking water alongside a western-type diet for 14 weeks to induce atherosclerosis. Plasma nitrate, nitrite and hemoglobin-bound nitric oxide were measured by chemiluminescence and electron parametric resonance, respectively. RESULTS: Plasma nitrate levels were elevated after 14 weeks of nitrate supplementation (NaCl: 40.29 ± 2.985, NaNO3: 78.19 ± 6.837, p < 0.0001). However, prolonged dietary nitrate did not affect systemic inflammation, hematopoiesis, erythropoiesis and plasma cholesterol levels, suggesting no severe side effects. Surprisingly, neither blood pressure, nor atherogenesis were altered. Mechanistically, plasma nitrate and nitrite were elevated after two weeks (NaCl: 1.0 ± 0.2114, NaNO3: 3.977 ± 0.7371, p < 0.0001), but decreased over time (6, 10 and 14 weeks). Plasma nitrite levels even reached baseline levels at 14 weeks (NaCl: 0.7188 ± 0.1072, NaNO3: 0.9723 ± 0.1279 p = 0.12). Also hemoglobin-bound NO levels were unaltered after 14 weeks. This compensation was not due to altered eNOS activity or conversion into peroxynitrite and other RNI, suggesting reduced nitrite formation or enhanced nitrate/nitrite clearance. CONCLUSION: Prolonged dietary nitrate supplementation resulted in compensation of nitrite and NO levels and did not affect atherogenesis or exert systemic side effects.

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

BACKGROUND: Based 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. OBJECTIVE: To 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. METHODS: A 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. RESULTS: After 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). CONCLUSIONS: In 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 REGISTRATION: Clinicaltrials.gov NCT01551238.

Short-term muscle disuse atrophy is not associated with increased intramuscular lipid deposition or a decline in the maximal activity of key mitochondrial enzymes in young and older males.

Aging is generally accompanied by a progressive loss of skeletal muscle mass and impairments in metabolic function. Even a few days of muscle disuse (such as that occurring during injury or illness) leads to considerable loss of muscle mass and strength. It has been speculated that short, successive periods of muscle disuse throughout the lifespan may be largely responsible for the age-related loss of muscle mass. However, it remains unknown whether such short periods of disuse also induce impairments in metabolic function within skeletal muscle. Here, we investigated the effects of a five day period of muscle disuse on intramyocellular triacylglycerol (IMTG) content, muscle oxidative capacity, and the expression of key genes that regulate oxidative metabolism in healthy young and elderly men. Muscle biopsies were collected from healthy, young (n=12; 23±1y) and elderly (n=12; 70±1y) men prior to and immediately after a five day period of one-legged knee immobilization by way of a full leg cast. At baseline, elderly men had a greater IMTG content when compared with the young (56.2±5.1 and 34.8±7.3µmol·g(-1), respectively; P<0.05) with no changes in either group following immobilization (53.4±5.0 and 35.7±5.0µmol·g(-1), respectively; P>0.05). In line, five days of disuse did not lower citrate synthase, ß-HAD or cytochrome C oxidase activity in skeletal muscle tissue. Pyruvate dehydrogenase activity increased following immobilization in the older subjects only, from 0.39±0.06 to 0.55 0.05µmol·g(-1)·min(-1) (71±33%; P<0.01). The skeletal muscle mRNA expression of PGC1α and citrate synthase both declined following immobilization in both the young and elderly subjects. We conclude that five days of muscle disuse does not increase intramuscular lipid deposition or reduce the maximal activity of key mitochondrial enzymes within the skeletal muscle of young or older men.

The skeletal muscle satellite cell response to a single bout of resistance-type exercise is delayed with aging in men.

Skeletal muscle satellite cells (SCs) have been shown to be instrumental in the muscle adaptive response to exercise. The present study determines age-related differences in SC content and activation status following a single bout of exercise. Ten young (22 ± 1 years) and 10 elderly (73 ± 1 years) men performed a single bout of resistance-type exercise. Muscle biopsies were collected before and 12, 24, 48, and 72 h after exercise. SC content and activation status were assessed in type I and type II muscle fibers by immunohistochemistry. Myostatin and MyoD protein and messenger RNA (mRNA) expression were determined by Western blotting and rtPCR, respectively. In response to exercise, it took 48 h (young) and 72 h (elderly) for type II muscle fiber SC content to exceed baseline values (P < 0.01). The number of myostatin + SC in type I and II muscle fibers was significantly reduced after 12, 24, and 48 h of post-exercise recovery in both groups (P < 0.01), with a greater reduction observed at 24 and 48 h in the young compared with that in the elderly men (P < 0.01). In conclusion, the increase in type II muscle fiber SC content during post-exercise recovery is delayed with aging and is accompanied by a blunted SC activation response.

Muscle disuse atrophy is not accompanied by changes in skeletal muscle satellite cell content.

Muscle disuse leads to a considerable loss in skeletal muscle mass and strength. However, the cellular mechanisms underlying disuse-induced muscle fibre atrophy remain to be elucidated. Therefore we assessed the effect of muscle disuse on the CSA (cross-sectional area), muscle fibre size, satellite cell content and associated myocellular signalling pathways of the quadriceps muscle. A total of 12 healthy young (24±1 years of age) men were subjected to 2 weeks of one-legged knee immobilization via a full-leg cast. Before and immediately after the immobilization period and after 6 weeks of natural rehabilitation, muscle strength [1RM (one-repetition maximum)], muscle CSA [single slice CT (computed tomography) scan] and muscle fibre type characteristics (muscle biopsies) were assessed. Protein and/or mRNA expression of key genes [i.e. MYOD (myogenic differentiation), MYOG (myogenin) and MSTN (myostatin)] in the satellite cell regulatory pathways were determined using Western blotting and RT-PCR (real-time PCR) analyses respectively. The present study found that quadriceps CSA declined following immobilization by 8±2% (P<0.05). In agreement, both type I and type II muscle fibre size decreased 7±3% and 13±4% respectively (P<0.05). No changes were observed in satellite cell content following immobilization in either type I or type II muscle fibres. Muscle MYOG mRNA expression doubled (P<0.05), whereas MSTN protein expression decreased 30±9% (P<0.05) following immobilization. Muscle mass and strength returned to the baseline values within 6 weeks of recovery without any specific rehabilitative programme. In conclusion, 2 weeks of muscle disuse leads to considerable loss in skeletal muscle mass and strength. The loss in muscle mass was attributed to both type I and type II muscle fibre atrophy, and was not accompanied by a decline in satellite cell content.

Disuse impairs the muscle protein synthetic response to protein ingestion in healthy men.

BACKGROUND: 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. PURPOSE: We assessed muscle protein synthesis rates after protein ingestion before and after a period of disuse in humans. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Substantial Differences between Organ and Muscle Specific Tracer Incorporation Rates in a Lactating Dairy Cow.

We aimed to produce intrinsically L-[1-(13)C]phenylalanine labeled milk and beef for subsequent use in human nutrition research. The collection of the various organ tissues after slaughter allowed for us to gain insight into the dynamics of tissue protein turnover in vivo in a lactating dairy cow. One lactating dairy cow received a constant infusion of L-[1-(13)C]phenylalanine (450 µmol/min) for 96 h. Plasma and milk were collected prior to, during, and after the stable isotope infusion. Twenty-four hours after cessation of the infusion the cow was slaughtered. The meat and samples of the various organ tissues (liver, heart, lung, udder, kidney, rumen, small intestine, and colon) were collected and stored. Approximately 210 kg of intrinsically labeled beef (bone and fat free) with an average L-[1-(13)C]phenylalanine enrichment of 1.8±0.1 mole percent excess (MPE) was obtained. The various organ tissues differed substantially in L-[1-(13)C]phenylalanine enrichments in the tissue protein bound pool, the highest enrichment levels were achieved in the kidney (11.7 MPE) and the lowest enrichment levels in the skeletal muscle tissue protein of the cow (between 1.5-2.4 MPE). The estimated protein synthesis rates of the various organ tissues should be regarded as underestimates, particularly for the organs with the higher turnover rates and high secretory activity, due to the lengthened (96 h) measurement period necessary for the production of the intrinsically labeled beef. Our data demonstrates that there are relatively small differences in L-[1-(13)C]phenylalanine enrichments between the various meat cuts, but substantial higher enrichment values are observed in the various organ tissues. We conclude that protein turnover rates of various organs are much higher when compared to skeletal muscle protein turnover rates in large lactating ruminants.

Minced beef is more rapidly digested and absorbed than beef steak, resulting in greater postprandial protein retention in older men.

BACKGROUND: Older individuals generally experience a reduced food-chewing efficiency. As a consequence, food texture may represent an important factor that modulates dietary protein digestion and absorption kinetics and the subsequent postprandial protein balance. OBJECTIVE: We assessed the effect of meat texture on the dietary protein digestion rate, amino acid availability, and subsequent postprandial protein balance in vivo in older men. DESIGN: Ten older men (mean ± SEM age: 74 ± 2 y) were randomly assigned to a crossover experiment that involved 2 treatments in which they consumed 135 g of specifically produced intrinsically L-[1-(13)C]phenylalanine-labeled beef, which was provided as beef steak or minced beef. Meat consumption was combined with continuous intravenous L-[ring-(2)H5]phenylalanine and L-[ring-(2)H2]tyrosine infusion to assess beef protein digestion and absorption kinetics as well as whole-body protein balance and skeletal muscle protein synthesis rates. RESULTS: Meat protein-derived phenylalanine appeared more rapidly in the circulation after minced beef than after beef steak consumption (P < 0.05). Also, its availability in the circulation during the 6-h postprandial period was greater after minced beef than after beef steak consumption (61 ± 3% compared with 49 ± 3%, respectively; P < 0.01). The whole-body protein balance was more positive after minced beef than after beef steak consumption (29 ± 2 compared with 19 ± 3 µmol phenylalanine/kg, respectively; P < 0.01). Skeletal muscle protein synthesis rates did not differ between treatments when assessed over a 6-h postprandial period. CONCLUSIONS: Minced beef is more rapidly digested and absorbed than beef steak, which results in increased amino acid availability and greater postprandial protein retention. However, this does not result in greater postprandial muscle protein synthesis rates. This trial was registered at clinicaltrials.gov as NCT01145131.

The muscle protein synthetic response to the combined ingestion of protein and carbohydrate is not impaired in healthy older men.

Aging is associated with a progressive decline in skeletal muscle mass. It has been hypothesized that an attenuated muscle protein synthetic response to the main anabolic stimuli may contribute to the age-related loss of muscle tissue. The aim of the present study was to compare the muscle protein synthetic response following ingestion of a meal-like amount of dietary protein plus carbohydrate between healthy young and older men. Twelve young (21 ± 1 years) and 12 older (75 ± 1 years) men consumed 20 g of intrinsically L-[1-(13)C]phenylalanine-labeled protein with 40 g of carbohydrate. Ingestion of specifically produced intrinsically L-[1-(13)C]phenylalanine-labeled protein allowed us to assess the subsequent incorporation of casein-derived amino acids into muscle protein. Blood samples were collected at regular intervals, with muscle biopsies obtained prior to and 2 and 6 h after protein plus carbohydrate ingestion. The acute post-prandial rise in plasma glucose and insulin concentrations was significantly greater in the older compared with the younger males. Plasma amino acid concentrations increased rapidly following drink ingestion in both groups. However, plasma leucine concentrations were significantly lower at t = 90 min in the older when compared with the young group (P < 0.05). Muscle protein-bound L-[1-(13)C]phenylalanine enrichments increased to 0.0071 ± 0.0016 and 0.0072 ± 0.0013 mole percent excess (MPE) at 2 h and 0.0229 ± 0.0016 and 0.0213 ± 0.0024 MPE at 6 h following ingestion of the intrinsically labeled protein in the young and older males, respectively, with no differences between groups (P > 0.05). We conclude that the use of dietary protein-derived amino acids for muscle protein synthesis is not impaired in healthy older men following intake of protein plus carbohydrate.

Dietary protein digestion and absorption are impaired during acute postexercise recovery in young men.

Previously, we demonstrated that exercise can cause small intestinal injury, leading to loss of gut barrier function. The functional consequences of such exercise-induced intestinal injury on subsequent food digestion and absorption are unclear. The present study determined the impact of resistance-type exercise on small intestinal integrity and in vivo dietary protein digestion and absorption kinetics. Twenty-four young males ingested 20 g specifically produced intrinsically l-[1-(13)C]phenylalanine-labeled protein at rest or after performing a single bout of resistance-type exercise. Continuous intravenous infusions with l-[ring-(2)H5]phenylalanine were employed, and blood samples were collected regularly to assess in vivo protein digestion and absorption kinetics and to quantify plasma levels of intestinal fatty-acid binding protein (I-FABP) as a measure of small intestinal injury. Plasma I-FABP levels were increased after exercise by 35%, reaching peak values of 344 ± 53 pg/ml compared with baseline 254 ± 31 pg/ml (P < 0.05). In resting conditions, I-FABP levels remained unchanged. Dietary protein digestion and absorption rates were reduced during postexercise recovery when compared with resting conditions (P < 0.001), with average peak exogenous phenylalanine appearance rates of 0.18 ± 0.04 vs. 0.23 ± 0.03 mmol phenylalanine·kg lean body mass(-1)·min(-1), respectively. Plasma I-FABP levels correlated with in vivo rates of dietary protein digestion and absorption (rS = -0.57, P < 0.01). Resistance-type exercise induces small intestinal injury in healthy, young men, causing impairments in dietary protein digestion and absorption kinetics during the acute postexercise recovery phase. To the best of our knowledge, this is first evidence that shows that exercise attenuates dietary protein digestion and absorption kinetics during acute postexercise recovery.

Protein supplementation during resistance-type exercise training in the elderly.

INTRODUCTION: Resistance training has been well established as an effective treatment strategy to increase skeletal muscle mass and strength in the elderly. We assessed whether dietary protein supplementation can further augment the adaptive response to prolonged resistance-type exercise training in healthy elderly men and women. METHODS: Healthy elderly men (n = 31, 70 ± 1 yr) and women (n = 29, 70 ± 1 yr) were randomly assigned to a progressive, 24-wk resistance-type exercise training program with or without additional protein supplementation (15 g·d-1). Muscle hypertrophy was assessed on a whole-body Dual-energy X-ray absorptiometry (DXA), limb (computed tomography), and muscle fiber (biopsy) level. Strength was assessed regularly by 1-repetition maximum (RM) strength testing. Functional capacity was assessed with a sit-to-stand and handgrip test. RESULTS: One-RM strength increased by 45% ± 6% versus 40% ± 3% (women) and 41% ± 4% versus 44% ± 3% (men) in the placebo versus protein group, respectively (P < 0.001), with no differences between groups. Leg muscle mass (women, 4% ± 1% vs 3% ± 1%; men, 3% ± 1% vs 3% ± 1%) and quadriceps cross-sectional area (women, 9% ± 1% vs 9% ± 1%; men, 9% ± 1% vs 10% ± 1%) increased similarly in the placebo versus protein groups (P < 0.001). Type II muscle fiber size increased over time in both placebo and protein groups (25% ± 13% vs 30% ± 9% and 23% ± 12% vs 22% ± 10% in the women and men, respectively). Sit-to-stand improved by 18% ± 2% and 19% ± 2% in women and men, respectively (P < 0.001). CONCLUSION: Prolonged resistance-type exercise training increases skeletal muscle mass and strength, augments functional capacity, improves glycemia and lipidemia, and reduces blood pressure in healthy elderly men and women. Additional protein supplementation (15 g·d-1) does not further increase muscle mass, strength, and/or functional capacity.

Leucine co-ingestion improves post-prandial muscle protein accretion in elderly men.

BACKGROUND & AIMS: It has been speculated that the amount of leucine in a meal largely determines the post-prandial muscle protein synthetic response to food intake. The present study investigates the impact of leucine co-ingestion on subsequent post-prandial muscle protein accretion following the ingestion of a single bolus of dietary protein in elderly males. METHODS: Twenty-four elderly men (74.3±1.0 y) were randomly assigned to ingest 20 g intrinsically L-[1-(13)C]phenylalanine-labeled casein protein with (PRO+LEU) or without (PRO) 2.5 g crystalline leucine. Ingestion of specifically produced intrinsically labeled protein allowed us to create a plasma phenylalanine enrichment pattern similar to the absorption pattern of phenylalanine from the ingested protein and assess the subsequent post-prandial incorporation of L-[1-(13)C] phenylalanine into muscle protein. RESULTS: Plasma amino acid concentrations increased rapidly following protein ingestion in both groups, with higher leucine concentrations observed in the PRO+LEU compared with the PRO group (P<0.01). Plasma L-[1-(13)C]phenylalanine enrichments increased rapidly and to a similar extent in both groups following protein ingestion. Muscle protein-bound L-[1-(13)C]phenylalanine enrichments were significantly greater after PRO+LEU when compared with PRO at 2 h (72%; 0.0078±0.0010 vs. 0.0046±0.00100 MPE, respectively; P<0.05) and 6 h (25%; 0.0232±0.0015 vs. 0.0185±0.0010 MPE, respectively; P<0.05) following protein ingestion. The latter translated into a greater muscle protein synthetic rate following PRO+LEU compared with PRO over the entire 6 h post-prandial period (22%; 0.049±0.003 vs. 0.040±0.003% h(-1), respectively; P<0.05). CONCLUSION: Leucine co-ingestion with a bolus of pure dietary protein further stimulates post-prandial muscle protein synthesis rates in elderly men.