Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use

Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O2 uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg−1 h−1) or without (CON placebo trial; water only). Continuous infusions with [U-13C] palmitate and [6,6-2H2] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (Ra) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA Ra and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men

Protein synthesis rates of muscle, tendon, ligament, cartilage, and bone tissue in vivo in humans

Skeletal muscle plasticity is reflected by a dynamic balance between protein synthesis and breakdown, with basal muscle tissue protein synthesis rates ranging between 0.02 and 0.09%/h. Though it is evident that other musculoskeletal tissues should also express some level of plasticity, data on protein synthesis rates of most of these tissues in vivo in humans is limited. Six otherwise healthy patients (62±3 y), scheduled to undergo unilateral total knee arthroplasty, were subjected to primed continuous intravenous infusions with L-[ring-13C6]-Phenylalanine throughout the surgical procedure. Tissue samples obtained during surgery included muscle, tendon, cruciate ligaments, cartilage, bone, menisci, fat, and synovium. Tissue-specific fractional protein synthesis rates (%/h) were assessed by measuring the incorporation of L-[ring-13C6]-Phenylalanine in tissue protein and were compared with muscle tissue protein synthesis rates using a paired t test. Tendon, bone, cartilage, Hoffa’s fat pad, anterior and posterior cruciate ligament, and menisci tissue protein synthesis rates averaged 0.06±0.01, 0.03±0.01, 0.04±0.01, 0.11±0.03, 0.07±0.02, 0.04±0.01, and 0.04±0.01%/h, respectively, and did not significantly differ from skeletal muscle protein synthesis rates (0.04±0.01%/h; P>0.05). Synovium derived protein (0.13±0.03%/h) and intercondylar notch bone tissue protein synthesis rates (0.03±0.01%/h) were respectively higher and lower compared to skeletal muscle protein synthesis rates (P<0.05 and P<0.01, respectively). Basal protein synthesis rates in various musculoskeletal tissues are within the same range of skeletal muscle protein synthesis rates, with fractional muscle, tendon, bone, cartilage, ligament, menisci, fat, and synovium protein synthesis rates ranging between 0.02 and 0.13% per hour in vivo in humans

Carbohydrate supplementation during prolonged cycling exercise spares muscle glycogen but does not affect intramyocellular lipid use

Using contemporary stable-isotope methodology and fluorescence microscopy, we assessed the impact of carbohydrate supplementation on whole-body and fiber-type-specific intramyocellular triacylglycerol (IMTG) and glycogen use during prolonged endurance exercise. Ten endurance-trained male subjects were studied twice during 3 h of cycling at 63 ± 4% of maximal O2 uptake with either glucose ingestion (CHO trial; 0.7 g CHO kg−1 h−1) or without (CON placebo trial; water only). Continuous infusions with [U-13C] palmitate and [6,6-2H2] glucose were applied to quantify plasma free fatty acids (FFA) and glucose oxidation rates and to estimate intramyocellular lipid and glycogen use. Before and after exercise, muscle biopsy samples were taken to quantify fiber-type-specific IMTG and glycogen content. Plasma glucose rate of appearance (Ra) and carbohydrate oxidation rates were substantially greater in the CHO vs CON trial. Carbohydrate supplementation resulted in a lower muscle glycogen use during the first hour of exercise in the CHO vs CON trial, resulting in a 38 ± 19 and 57 ± 22% decreased utilization in type I and II muscle-fiber glycogen content, respectively. In the CHO trial, both plasma FFA Ra and subsequent plasma FFA concentrations were lower, resulting in a 34 ± 12% reduction in plasma FFA oxidation rates during exercise (P < 0.05). Carbohydrate intake did not augment IMTG utilization, as fluorescence microscopy revealed a 76 ± 21 and 78 ± 22% reduction in type I muscle-fiber lipid content in the CHO and CON trial, respectively. We conclude that carbohydrate supplementation during prolonged cycling exercise does not modulate IMTG use but spares muscle glycogen use during the initial stages of exercise in endurance-trained men

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-2H5]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-2H5]-phenylalanine of mixed plasma proteins, generates data that are similar to using the sequential muscle biopsy approach in the elderly population

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 (r(2) = 0.57). In conclusion, this study shows that severe muscle wasting following nerve damage is accompanied by increased as opposed to decreased MPS

Amino acid absorption and subsequent muscle protein accretion following graded intakes of whey protein in elderly men

Whey protein ingestion has been shown to effectively stimulate postprandial muscle protein accretion in older adults. However, the impact of the amount of whey protein ingested on protein digestion and absorption kinetics, whole body protein balance, and postprandial muscle protein accretion remains to be established. We aimed to fill this gap by including 33 healthy, older men ( 73 ± 2 yr ) who were randomly assigned to ingest 10, 20, or 35 g of intrinsically l-[1-13C]phenylalanine-labeled whey protein ( n = 11/treatment ). Ingestion of labeled whey protein was combined with continuous intravenous l-[ring-2H5]phenylalanine and l-[ring-2H2]tyrosine infusion to assess the metabolic fate of whey protein-derived amino acids. Dietary protein digestion and absorption rapidly increased following ingestion of 10, 20, and 35 g whey protein, with the lowest and highest ( peak ) values observed following 10 and 35 g, respectively ( P < 0.05 ). Whole body net protein balance was positive in all groups ( 19 ± 1, 37 ± 2, and 58 ± 2 μmol/kg ), with the lowest and highest values observed following ingestion of 10 and 35 g, respectively ( P < 0.05 ). Postprandial muscle protein accretion, assessed by l-[1-13C]phenylalanine incorporation in muscle protein, was higher following ingestion of 35 g when compared with 10 ( P < 0.01 ) or 20 ( P < 0.05 ) g. We conclude that ingestion of 35 g whey protein results in greater amino acid absorption and subsequent stimulation of de novo muscle protein synthesis compared with the ingestion of 10 or 20 g whey protein in healthy, older men

Co-ingesting milk fat with micellar casein does not affect postprandial protein handling in healthy older men

Background & aim: Dietary protein digestion and absorption plays an important role in modulating postprandial muscle protein synthesis. The impact of co-ingesting other macronutrients with dietary protein on protein digestion and absorption and the subsequent muscle protein synthetic response remains largely unexplored. This study investigated the impact of co-ingesting milk fat with micellar casein on dietary protein-derived amino acid appearance in the circulation and the subsequent postprandial muscle protein synthetic response in healthy older men. Methods: Twenty-four healthy, older males ( age: 65 ± 1 y, BMI: 25.7 ± 0.5 kg/m2 ) received a primed continuous infusion of L-[ring-2H5]-phenylalanine and L-[1–13C]-leucine and ingested 20 g intrinsically L-[1–13C]-phenylalanine and L-[1–13C]-leucine-labeled casein with ( PRO + FAT; n = 12 ) or without ( PRO; n = 12 ) 26.7 g milk fat. Plasma samples and muscle biopsies were collected in both the postabsorptive and postprandial state. Results: Release of dietary protein-derived phenylalanine into the circulation increased following protein ingestion ( P < 0.001 ) and tended to be higher in PRO compared with PRO + FAT ( Time × Treatment P = 0.076 ). No differences were observed in dietary protein-derived plasma phenylalanine availability ( 52 ± 2 vs 52 ± 3% in PRO vs PRO + FAT, respectively; P = 0.868 ). Myofibrillar protein synthesis rates did not differ between treatments, calculated using either the L-[ring-2H5]-phenylalanine ( 0.036 ± 0.003 vs 0.036 ± 0.004 %/h after PRO vs PRO + FAT, respectively; P = 0.933 ) or L-[1–13C]-leucine ( 0.051 ± 0.004 vs 0.046 ± 0.004 %/h, respectively; P = 0.480 ) tracer. In accordance, no differences were observed in myofibrillar protein-bound L-[1–13C]-phenylalanine enrichments between treatments ( 0.018 ± 0.002 vs 0.014 ± 0.001 MPE, respectively; P = 0.173 ). Conclusion: Co-ingesting milk fat with micellar casein does not impair protein-derived phenylalanine appearance in the circulation and does not modulate postprandial myofibrillar protein synthesis rates. Clinical Trial Registration Number: NCT01680146 ( http://www.clinicaltrials.gov/

Short-term muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates

Short-term muscle disuse has been reported to lower both postabsorptive and postprandial myofibrillar protein synthesis rates. This study assessed the impact of disuse on daily myofibrillar protein synthesis rates following short-term (2 and 7 days) muscle disuse under free living conditions. Thirteen healthy young men (age: 20 ± 1 yr; BMI: 23 ± 1 kg/m−2) underwent 7 days of unilateral leg immobilization via a knee brace, with the nonimmobilized leg acting as a control. Four days before immobilization participants ingested 400 mL of 70% deuterated water, with 50-mL doses consumed daily thereafter. Upper leg bilateral MRI scans and muscle biopsies were collected before and after 2 and 7 days of immobilization to determine quadriceps volume and daily myofibrillar protein synthesis rates. Immobilization reduced quadriceps volume in the immobilized leg by 1.7 ± 0.3 and 6.7 ± 0.6% after 2 and 7 days, respectively, with no changes in the control leg. Over the 1-wk immobilization period, myofibrillar protein synthesis rates were 36 ± 4% lower in the immobilized (0.81 ± 0.04%/day) compared with the control (1.26 ± 0.04%/day) leg (P < 0.001). Myofibrillar protein synthesis rates in the control leg did not change over time (P = 0.775), but in the immobilized leg they were numerically lower during the 0- to 2-day period (16 ± 6%, 1.11 ± 0.09%/day, P = 0.153) and were significantly lower during the 2- to 7-day period (44 ± 5%, 0.70 ± 0.06%/day, P < 0.001) when compared with the control leg. We conclude that 1 wk of muscle disuse induces a rapid and sustained decline in daily myofibrillar protein synthesis rates in healthy young men