The intake of whey, compared with casein and soy protein intakes, stimulates a greater acute response of muscle protein synthesis (MPS) to protein ingestion in rested and exercised muscle.
We ...characterized the dose-response relation of postabsorptive rates of myofibrillar MPS to increasing amounts of whey protein at rest and after exercise in resistance-trained, young men.
Volunteers (n = 48) consumed a standardized, high-protein (0.54 g/kg body mass) breakfast. Three hours later, a bout of unilateral exercise (8 × 10 leg presses and leg extensions; 80% one-repetition maximum) was performed. Volunteers ingested 0, 10, 20, or 40 g whey protein isolate immediately (~10 min) after exercise. Postabsorptive rates of myofibrillar MPS and whole-body rates of phenylalanine oxidation and urea production were measured over a 4-h postdrink period by continuous tracer infusion of labeled (13)C6 phenylalanine and (15)N2 urea.
Myofibrillar MPS (mean ± SD) increased (P < 0.05) above 0 g whey protein (0.041 ± 0.015%/h) by 49% and 56% with the ingestion of 20 and 40 g whey protein, respectively, whereas no additional stimulation was observed with 10 g whey protein (P > 0.05). Rates of phenylalanine oxidation and urea production increased with the ingestion of 40 g whey protein.
A 20-g dose of whey protein is sufficient for the maximal stimulation of postabsorptive rates of myofibrillar MPS in rested and exercised muscle of ~80-kg resistance-trained, young men. A dose of whey protein >20 g stimulates amino acid oxidation and ureagenesis. This trial was registered at http://www.isrctn.org/ as ISRCTN92528122.
Summary Maximizing anabolic responses to feeding and exercise is crucial for muscle maintenance and adaptation to exercise training. We hypothesized that enriching a protein drink with leucine would ...improve anabolic responses to resistance exercise (RE: 6 × 8 knee-extension repetitions at 75% of 1-RM) in both young and older adults. Groups (n = 9) of young (24 ± 6 y, BMI 23 ± 2 kg m−2 ) and older men (70 ± 5 y, BMI 25 ± 2 kg m−2 ) were randomized to either: (i) RE followed by SlimFast Optima (SFO 10 g PRO; 24 g CHO) with 4.2 g of leucine (LEU) or, (ii) RE + SFO with 4.2 g of alanine (ALA; isonitrogenous control). Muscle biopsies were taken before, immediately after, and 1, 2 and 4 h after RE and feeding. Muscle protein synthesis (MPS) was measured by incorporation of 1, 2–13 C2 leucine into myofibrillar proteins and the phosphorylation of p70S6K1 by immunoblotting. In young men, both area under the curve (AUC; FSR 0–4 h P < 0.05) and peak FSR (0.11 vs . 0.08%.h.−1 ; P < 0.05) were greater in the SFO + LEU than in the SFO + ALA group, after RE. Similarly, in older men, AUC analysis revealed that post-exercise anabolic responses were greater in the SFO + LEU than SFO + ALA group, after RE (AUC; FSR 0–4 h P < 0.05). Irrespective of age, increases in p70S6K1 phosphorylation were evident in response to both SFO + LEU and SFO + ALA, although greater with leucine supplementation than alanine (fold-change 2.2 vs . 3.2; P < 0.05), specifically in the older men. We conclude that addition of Leucine to a sub-maximal PRO bolus improves anabolic responses to RE in young and older men.
BACKGROUND: We previously showed that human muscle protein synthesis (MPS) increased during infusion of amino acids (AAs) and peaked at almost equal to120 min before returning to baseline rates, ...despite elevated plasma AA concentrations. OBJECTIVE: We tested whether a protein meal elicited a similar response and whether signaling responses that regulate messenger RNA translation matched MPS changes. DESIGN: Eight postabsorptive healthy men (almost equal to21 y of age) were studied during 8.5 h of primed continuous infusion of 1,2-¹³C₂leucine with intermittent quadriceps biopsies for determination of MPS and anabolic signaling. After 2.5 h, subjects consumed 48 g whey protein. RESULTS: At 45-90 min after oral protein bolus, mean (±SEM) myofibrillar protein synthesis increased from 0.03 ± 0.003% to 0.10 ± 0.01%/h; thereafter, myofibrillar protein synthesis returned to baseline rates even though plasma essential AA (EAA) concentrations remained elevated (+130% at 120 min, +80% at 180 min). The activity of protein kinase B (PKB) and phosphorylation of eukaryotic initiation factor 4G preceded the rise of MPS and increases in phosphorylation of ribosomal protein kinase S6 (S6K1), and 4E-binding protein 1 (4EBP1) was superimposable with MPS responses until 90 min. However, although MPS decreased thereafter, all signals, with the exception of PKB activity (which mirrored insulin responses), remained elevated, which echoed the slowly declining plasma EAA profile. The phosphorylation of eukaryotic initiation factor 2α increased only at 180 min. Thus, discordance existed between MPS and the mammalian target of rapamycin complex 1 (mTORC1) and signaling (ie, S6K1 and 4EBP1 phosphorylation). CONCLUSIONS: We confirm our previous findings that MPS responses to AAs are transient, even with oral protein bolus. However, changes in MPS only reflect elevated mTORC1 signaling during the upswing in MPS.
BACKGROUND: Reduced postprandial muscle proteolysis is mainly due to increased insulin availability. Whether rates of proteolysis in response to low physiologic doses of insulin are affected by aging ...is unknown. OBJECTIVES: We tested the hypothesis that suppression of leg protein breakdown (LPB) by insulin is blunted in older subjects, together with blunted activation of Akt-protein kinase B (PKB). DESIGN: Groups of 8 young mean (±SD) age: 24.5 ± 1.8 y and older (65.0 ± 1.3 y) participants were studied during euglycemic (5 mmol/L), isoaminoacidemic (blood leucine almost equal to 120 μmol/L) clamp procedures at plasma insulin concentrations of almost equal to5 and almost equal to15 μIU/mL for 1.5 h. Leg amino acid balance, whole-leg protein turnover (as dilution of amino acid tracers), and muscle protein synthesis were measured with D₅-phenylalanine and 1,2-¹³C₂leucine. The kinase activity of muscle Akt-PKB and the extent of phosphorylation of signaling proteins associated with the mTOR (mammalian target of rapamycin) pathway were measured before and after the clamp procedures. RESULTS: Basal LPB rates were not different between groups (66 ± 11 compared with 51 ± 10 nmol leucine · 100 mL leg⁻¹ · min⁻¹ and 30 ± 5 compared with 24 ± 4 nmol phenylalanine · 100 mL leg⁻¹ · min⁻¹ in young and older groups, respectively). However, although insulin at almost equal to15 μIU/mL lowered LPB by 47% in the young subjects (P < 0.05) and abolished the negative leg amino acid balance, this caused only a 12% fall (P > 0.05) in the older group. Akt-PKB activity mirrored decreases in LPB. No differences were seen in muscle protein synthesis or associated anabolic signaling phosphoproteins. CONCLUSIONS: At moderate availability, the effect of insulin on LPB is diminished in older human beings, and this effect may be mediated through blunted Akt-PKB activation.
Background: Cachexia is a consequence of tumor burden caused by ill-defined catabolic alterations in muscle protein turnover.Objective: We aimed to explore the effect of tumor burden and resection on ...muscle protein turnover in patients with nonmetastatic colorectal cancer (CRC), which is a surgically curable tumor that induces cachexia.Design: We recruited the following 2 groups: patients with CRC n = 13; mean ± SEM age: 66 ± 3 y; BMI (in kg/m2): 27.6 ± 1.1 and matched healthy controls (n = 8; age: 71 ± 2 y; BMI: 26.2 ± 1). Control subjects underwent a single study, whereas CRC patients were studied twice before and ∼6 wk after surgical resection to assess muscle protein synthesis (MPS), muscle protein breakdown (MPB), and muscle mass by using dual-energy X-ray absorptiometry.Results: Leg muscle mass was lower in CRC patients than in control subjects (6290 ± 456 compared with 7839 ± 617 g; P < 0.05) and had an additional decline after surgery (5840 ± 456 g; P < 0.001). Although postabsorptive MPS was unaffected, catabolic changes with tumor burden included the complete blunting of postprandial MPS (0.038 ± 0.004%/h in the CRC group compared with 0.065 ± 0.006%/h in the control group; P < 0.01) and a trend toward increased MPB under postabsorptive conditions (P = 0.09). Although surgical resection exacerbated muscle atrophy (−7.2%), catabolic changes in protein metabolism had normalized 6 wk after surgery. The recovery in postprandial MPS after surgery was inversely related to the degree of muscle atrophy (r = 0.65, P < 0.01).Conclusions: CRC patients display reduced postprandial MPS and a trend toward increased MPB, and tumor resection reverses these derangements. With no effective treatment of cancer cachexia, future therapies directed at preserving muscle mass should concentrate on alleviating proteolysis and enhancing anabolic responses to nutrition before surgery while augmenting muscle anabolism after resection.
Intramyocellular lipid (IMCL) utilization is impaired in older individuals, and IMCL accumulation is associated with insulin resistance. We hypothesized that increasing muscle total carnitine content ...in older men would increase fat oxidation and IMCL utilization during exercise, and improve insulin sensitivity. Fourteen healthy older men (69 ± 1 year, BMI 26.5 ± 0.8 kg/m2) performed 1 h of cycling at 50% VO2max and, on a separate occasion, underwent a 60 mU/m2/min euglycaemic hyperinsulinaemic clamp before and after 25 weeks of daily ingestion of a 220 ml insulinogenic beverage (44.4 g carbohydrate, 13.8 g protein) containing 4.5 g placebo (n = 7) or L‐carnitine L‐tartrate (n = 7). During supplementation, participants performed twice‐weekly cycling for 1 h at 50% VO2max. Placebo ingestion had no effect on muscle carnitine content or total fat oxidation during exercise at 50% VO2max. L‐carnitine supplementation resulted in a 20% increase in muscle total carnitine content (20.1 ± 1.2 to 23.9 ± 1.7 mmol/kg/dm; p < 0.01) and a 20% increase in total fat oxidation (181.1 ± 15.0 to 220.4 ± 19.6 J/kg lbm/min; p < 0.01), predominantly due to increased IMCL utilization. These changes were associated with increased expression of genes involved in fat metabolism (ACAT1, DGKD & PLIN2; p < 0.05). There was no change in resting insulin‐stimulated whole‐body or skeletal muscle glucose disposal after supplementation. This is the first study to demonstrate that a carnitine‐mediated increase in fat oxidation is achievable in older individuals. This warrants further investigation given reduced lipid turnover is associated with poor metabolic health in older adults.
High Intramyocellular lipid (IMCL) content in older individuals, likely due to reduced fat oxidation, is associated with insulin resistance. The present study demonstrated that increasing muscle carnitine content in healthy older males resulted in a 20% increase in fat oxidation during exercise, likely due to an increase in IMCL utilization. This was supported by changes in expression of genes associated with IMCL turnover and oxidation, but no improvement in insulin sensitivity was observed.
Obesity is increasing, yet despite the necessity of maintaining muscle mass and function with age, the effect of obesity on muscle protein turnover in older adults remains unknown. Eleven obese (BMI ...31.9 ± 1.1 kg · m(-2)) and 15 healthy-weight (BMI 23.4 ± 0.3 kg · m(-2)) older men (55-75 years old) participated in a study that determined muscle protein synthesis (MPS) and leg protein breakdown (LPB) under postabsorptive (hypoinsulinemic-euglycemic clamp) and postprandial (hyperinsulinemic hyperaminoacidemic-euglycemic clamp) conditions. Obesity was associated with systemic inflammation, greater leg fat mass, and patterns of mRNA expression consistent with muscle deconditioning, whereas leg lean mass, strength, and work done during maximal exercise were no different. Under postabsorptive conditions, MPS and LPB were equivalent between groups, whereas insulin and amino acid administration increased MPS in only healthy-weight subjects and was associated with lower leg glucose disposal (LGD) (63%) in obese men. Blunting of MPS in the obese men was offset by an apparent decline in LPB, which was absent in healthy-weight subjects. Lower postprandial LGD in obese subjects and blunting of MPS responses to amino acids suggest that obesity in older adults is associated with diminished muscle metabolic quality. This does not, however, appear to be associated with lower leg lean mass or strength.
We investigated how myofibrillar protein synthesis (MPS) and muscle anabolic signalling were affected by resistance exercise at 20–90% of 1 repetition maximum (1 RM) in two groups (25 each) of ...post‐absorptive, healthy, young (24 ± 6 years) and old (70 ± 5 years) men with identical body mass indices (24 ± 2 kg m−2). We hypothesized that, in response to exercise, anabolic signalling molecule phosphorylation and MPS would be modified in a dose‐dependant fashion, but to a lesser extent in older men. Vastus lateralis muscle was sampled before, immediately after, and 1, 2 and 4 h post‐exercise. MPS was measured by incorporation of 1,2‐13C leucine (gas chromatography–combustion–mass spectrometry using plasma 1,2‐13Cα‐ketoisocaparoate as surrogate precursor); the phosphorylation of p70 ribosomal S6 kinase (p70s6K) and eukaryotic initiation factor 4E binding protein 1 (4EBP1) was measured using Western analysis with anti‐phosphoantibodies. In each group, there was a sigmoidal dose–response relationship between MPS at 1–2 h post‐exercise and exercise intensity, which was blunted (P < 0.05) in the older men. At all intensities, MPS fell in both groups to near‐basal values by 2–4 h post‐exercise. The phosphorylation of p70s6K and 4EBP1 at 60–90% 1 RM was blunted in older men. At 1 h post‐exercise at 60–90% 1 RM, p70s6K phosphorylation predicted the rate of MPS at 1–2 h post‐exercise in the young but not in the old. The results suggest that in the post‐absorptive state: (i) MPS is dose dependant on intensity rising to a plateau at 60–90% 1 RM; (ii) older men show anabolic resistance of signalling and MPS to resistance exercise.
Insulin resistance is closely related to intramyocellular lipid (IMCL) accumulation, and both are associated with increasing age. It remains to be determined to what extent perturbations in IMCL ...metabolism are related to the aging process per se. On two separate occasions, whole-body and muscle insulin sensitivity (euglycemic-hyperinsulinemic clamp with 2-deoxyglucose) and fat utilization during 1 h of exercise at 50% VO2max (U-(13)Cpalmitate infusion combined with electron microscopy of IMCL) were determined in young lean (YL), old lean (OL), and old overweight (OO) males. OL displayed IMCL content and insulin sensitivity comparable with those in YL, whereas OO were markedly insulin resistant and had more than twofold greater IMCL in the subsarcolemmal (SSL) region. Indeed, whereas the plasma free fatty acid Ra and Rd were twice those of YL in both OL and OO, SSL area only increased during exercise in OO. Thus, skeletal muscle insulin resistance and lipid accumulation often observed in older individuals are likely due to lifestyle factors rather than inherent aging of skeletal muscle as usually reported. However, age per se appears to cause exacerbated adipose tissue lipolysis, suggesting that strategies to reduce muscle lipid delivery and improve adipose tissue function may be warranted in older overweight individuals.
We tested the hypothesis that increasing blood amino acid (AA) availability would counter the physical inactivity-induced
reduction in muscle protein synthesis. We determined how 14 days of ...unilateral knee immobilization affected quadriceps myofibrillar
protein synthesis (MPS) in young healthy subjects (10 men, 2 women, 21 ± 1 years; 80.2 ± 4.0 kg, mean ± s.e.m. ) in the post-absorptive state and after infusing AA (10% Primene) at low or high doses (43 and 261 mg kg â1 h â1 ). Muscle cross-sectional area (MRI) and peak isometric torque declined in the immobilized leg (â5.0 ± 1.2% and â25 ± 3%,
respectively, both P < 0.005), but were unchanged (all P > 0.6) in the non-immobilized leg. Immobilization induced a 27% decline in the rate of post-absorptive MPS (immobilized,
0.027 ± 0.003: non-immobilized, 0.037 ± 0.003% h â1 ; P < 0.001). Regardless of dose, AA infusion stimulated a greater rise in MPS in the non-immobilized legs; at 4 h MPS was greater
by +54 ± 12% with low dose and +68 ± 17% with high dose AA infusion (both P < 0.001). There was some evidence of delayed responsiveness of phosphorylation of Akt to high doses of AA and p70S6k at both
doses but no marked differences in that of mTOR, GSK3β or eEF2. Phosphorylation of focal adhesion kinase (Tyr 576/577 ) was reduced ( P < 0.05) with immobilization. We observed no change in polyubiquitinated protein content after immobilization. We confirm
that 14 days of immobilization reduces MPS in the post-absorptive state and this diminution is reduced but not abolished by
increased provision of AA, even at high rates. The immobilization-induced decline in post-absorptive MPS with the âanabolic
resistanceâ to amino acids can account for much of immobilization-induced muscle atrophy.
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