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Departments of 1Human Biology and 2Movement Sciences, Nutrition Research Institute Maastricht, Maastricht University, Maastricht; 3Numico Research BV, Wageningen, The Netherlands; and 4School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom
Submitted 1 September 2004 ; accepted in final form 22 November 2004
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ABSTRACT |
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resistance exercise; protein metabolism; supplements; protein intake
Combined ingestion of carbohydrate with protein and/or free amino acids (34, 38, 47–49), as well as intravenous infusion of free amino acids (16, 17), has been shown to further elevate plasma insulin levels. It has been speculated that such elevated insulin concentrations can stimulate the uptake of selected amino acids (6) and muscle protein synthesis rate (6, 20). In addition, insulin has been reported to effectively inhibit proteolysis (9, 18, 19). Nonetheless, the exact mechanisms responsible for the stimulatory effects of carbohydrate and protein/amino acid ingestion on muscle protein synthesis during recovery from resistance exercise have not yet been established in detail. For example, in animal models, free leucine has been shown to stimulate protein synthesis by an insulin-independent mechanism (2, 3). Consequently, it has been speculated that a nutritional supplement containing carbohydrate, protein, and free leucine could represent an effective tool to further increase postexercise muscle protein synthesis and/or to inhibit protein degradation.
The main aim of the present study was to investigate the effect of carbohydrate (CHO), carbohydrate and protein (CHO+PRO), and carbohydrate, protein, and leucine (CHO+PRO+Leu) ingestion on the anabolic response to resistance exercise. By measuring the incorporation rate of labeled amino acids in the skeletal muscle protein pool, mixed muscle protein synthesis rates were directly measured during 6 h of recovery from strenuous resistance exercise. A primed constant infusion of L-[ring-13C6]phenylalanine was combined with plasma and muscle tissue sampling to simultaneously measure mixed muscle fractional protein synthetic rate and whole body protein turnover.
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METHODS |
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Pretesting.
All subjects participated in an orientation trial to become familiarized with the exercise protocol and the equipment. Proper lifting technique was demonstrated and practiced for each of the two lower-limb exercises (leg press and leg extension). Thereafter, maximum strength was estimated using the multiple-repetitions testing procedure (29). In another session, 
1 wk before the first experimental trial, subjects' one-repetition maximum (1-RM) was determined (26). After the subjects warmed up, the load was set at 90–95% of the estimated 1-RM and was increased after each successful lift until failure. A 5-min resting period between the subsequent attempts was allowed. A repetition was valid if the subject was able to complete the entire lift in a controlled manner without assistance. The average 1-RM for the leg press was 185 ± 9 kg, and the mean 1-RM for the leg extension was 106 ± 4 kg.
Diet and activity before testing. All subjects consumed a standardized meal [66.1 ± 2.7 kJ/kg, consisting of 72 energy% (En%) carbohydrate, 23 En% protein, and 5 En% fat] the evening before the trials. All volunteers were instructed to refrain from any sort of heavy physical exercise and to keep their diet as constant as possible 3 days before the trials. In addition, subjects were asked to record their food intake for 48 h before the start of the first experimental trial and to consume the same diet 48 h before the start of the second and third tests.
Experimental trials.
Each subject participated in three trials separated by 7 days, in which recovery drinks containing CHO, CHO+PRO, or CHO+PRO+Leu were tested in a randomized and double-blind fashion. Each trial lasted 
8 h. Repeated boluses of a given test drink were ingested after the exercise protocol to ensure a continuous supply of ample glucose and amino acids to the muscle. Plasma and muscle samples were collected during a 6-h postexercise period. These trials were designed to simultaneously assess whole body amino acid kinetics and fractional synthetic rate (FSR) of mixed muscle protein by the incorporation of L-[ring-13C6]phenylalanine in the vastus lateralis muscle.
Protocol.
At 8:00 AM, after an overnight fast, subjects reported to the laboratory, and a Teflon catheter was inserted into an antecubital vein for stable isotope infusion. A second Teflon catheter was inserted in a heated dorsal hand vein of the contralateral arm placed in a hot box (60°C), for arterialized blood sampling. After collection of a resting blood sample, a single intravenous dose of L-[ring-13C6]phenylalanine (2 µmol/kg) was administered to prime the phenylalanine pool. Thereafter, tracer infusion was started with subjects resting in a supine position. After 75 min of infusion, a second blood sample was collected, and subjects started the resistance exercise protocol. The exercise session consisted of eight sets of eight repetitions on the horizontal leg press machine (Technogym, Rotterdam, The Netherlands) and eight sets of eight repetitions on the leg extension machine (Technogym). Both exercises were performed at 80% of the subjects' individual 1-RM with 2-min rest intervals between the sets and in total required 45 min to complete. All subjects were verbally encouraged during the test to complete the whole protocol. At the end of the resistance exercise protocol (t = 0), subjects rested supine, and an arterialized blood sample from the heated hand vein as well as a muscle biopsy from the vastus lateralis muscle were collected. Subjects then received an initial bolus (3 ml/kg) of a given test drink. Repeated boluses (3 ml/kg) were taken every 30 min until t = 330 min after exercise. Blood samples were subsequently taken from the heated hand vein at t = 15, 30, 45, 60, 75, 90, 120, 150, 180, 210, 240, 270, 300, 330, and 360 min, with an additional muscle biopsy taken at 360 min after exercise.
Muscle biopsies.
Muscle biopsies were obtained from the middle region of the vastus lateralis (15 cm above the patella) and 2 cm below entry through the fascia by means of the percutaneous needle biopsy technique (5). Muscle samples were immediately freed from blood, visible fat, and connective tissue, rapidly frozen in liquid nitrogen, and stored at –80°C for measurement of the amino acid enrichment in the muscle free amino acid pool and in mixed muscle protein.
Beverages.
Subjects received a beverage volume of 3 ml/kg every 30 min to ensure a given dose of 0.3 g carbohydrate/kg (50% as glucose and 50% as maltodextrin) and 0.2 g/kg of a protein hydrolysate every hour, with or without the addition of 0.1 g·kg–1·h–1 leucine. The total amount of protein (0.2 g·kg–1·h–1) provided in both the CHO+PRO and CHO+PRO+Leu trials by far exceeded the calculated amount of protein that was estimated to provide sufficient precursor substrate to sustain maximal protein synthesis rates for 6 h (50). Repeated boluses were administered to enable a continuous ample supply of amino acids in the circulation, preventing perturbations in L-[ring-13C6]phenylalanine enrichments. The whey protein hydrolysate used in the present study contained 9.95% leucine; consequently, the total amount of leucine administered in the CHO+PRO and CHO+PRO+Leu trial was 0.02 vs. 0.12 g·kg–1·h–1, respectively.
The compositions of all test drinks are listed in Table 1. Glucose and maltodextrin were obtained from AVEBE (Veendam, The Netherlands). Whey protein hydrolysate was prepared by Numico Research (Wageningen, The Netherlands). Leucine was purchased from BUFA (Uitgeest, The Netherlands). To make the taste comparable in all trials, beverages were uniformly flavored by adding 0.2 g of sodium saccharinate solution (25% wt/wt), 1.8 g of citric acid solution (50% wt/wt), and 5 g of cream vanilla flavor (Numico Research) for each liter of beverage. Trials were performed in a randomized order, with test drinks provided in a double-blind fashion.
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Analysis. Blood samples were collected in EDTA-containing tubes and centrifuged at 1,000 g and 4°C for 5 min. Aliquots of plasma were frozen in liquid nitrogen and stored at –80°C. Insulin was analyzed by radioimmunoassay (Insulin RIA kit; Linco Research, St. Charles, MO). Plasma (500 µl) for amino acid analysis was deproteonized on ice with 100 µl of 24% (wt/vol) 5-sulfosaliculic acid and mixed (vortexing), and the clear supernatant was collected after centrifugation. Plasma amino acid concentrations were analyzed on an automated, dedicated amino acid analyzer (LC-A10; Shimadzu Benelux, Den Bosch, The Netherlands), using an automated precolumn derivatization procedure and a ternary solvent system (15). The exact phenylalanine concentration in the infusates (8.06 ± 0.21 mmol/l) was measured using the same method. Plasma phenylalanine and tyrosine were derivatized to their tert-butyldimethylsilyl (TBDMS) derivatives and their 13C enrichments determined by electron ionization gas chromatography-mass spectrometry (GC-MS, Finnigan Incos XL) using selected ion monitoring of masses 336 and 342 for unlabeled and labeled phenylalanine, respectively, and masses 466 and 472 for unlabeled and labeled tyrosine (51).
For measurement of L-[ring-13C6]phenylalanine enrichment in the free amino acid pool and mixed muscle protein, 55 mg of wet muscle were freeze-dried. Collagen, blood, and other nonmuscle fiber material were removed from the muscle fibers under a light microscope. The isolated muscle fiber mass (2–3 mg) was weighed, and 2 ml of ice-cold 2% perchloric acid (PCA) were added. The tissue was then homogenized and centrifuged. The supernatant was collected and processed in the same manner as the plasma samples, such that intracellular free L-[ring-13C6]phenylalanine and L-[ring-13C6]tyrosine enrichments could be measured using their TBDMS derivatives on a GC-MS. The protein pellet was washed with three additional 2-ml washes of 2% PCA and dried, and the proteins were hydrolyzed in 6 M HCl at 120°C for 15–18 h. The hydrolyzed protein fraction was dried under a nitrogen stream while heated to 120°C and then dissolved in a 50% acetic acid solution and passed over a Dowex exchange resin (AG 50W-X8, 100–200 mesh hydrogen form; Bio-Rad, Hercules, CA) with 2 M NH4OH. Thereafter, the eluate was dried, and the purified amino acid fraction was derivatized into the N-acetyl-methyl (NAM)-esters to determine the 13C enrichment of protein bound phenylalanine enrichment using GC-IRMS (Finnigan, MAT 252).
Calculations.
Infusion of L-[ring-13C6]phenylalanine with muscle and arterialized blood sampling was used to simultaneously assess whole body amino acid kinetics and FSR of mixed muscle protein. Whole body rates of appearance (Ra) and disappearance (Rd) of phenylalanine were calculated using the non-steady-state Steele equations, adapted for stable isotope methodology (41).
 | (1) |
 | (2) |
 | (3) |
As whole body Rd comprises the phenylalanine Rd from the free amino acid pool in the blood due to protein synthesis (S) and oxidation, whole body protein synthesis can be calculated as Rd minus oxidation. The rate of phenylalanine oxidation was calculated using the phenylalanine balance model from Thompson et al. (42). Whole body phenylalanine oxidation (Qpt) can be determined from the conversion (hydroxylation) of L-[ring-13C6]phenylalanine to L-[ring-13C6]tyrosine without the necessity to measure 13CO2 enrichment in breath gases (42). The rate of hydroxylation can be calculated using the formula
 | (4) |
 | (5) |
 | (6) |
Ep is the increment of protein bound L-[ring-13C6]phenylalanine during incorporation periods. Eprecursor is 1) the average plasma L-[ring-13C6]phenylalanine enrichment during the time period for determination of amino acid incorporation; 2) the free muscle L-[ring-13C6]phenylalanine enrichment in muscle biopsy taken at 6 h after exercise; and 3) the free muscle L-[ring-13C6]phenylalanine enrichment in muscle biopsy taken at 6 h after exercise corrected for the contribution of extracellular water, as previously described (50). t Indicates the time interval (h) between biopsies, and the factor 100 is needed to express the FSR in percent per hour (%/h). The factor 2 arises because, in the NAM-ester of L-[ring-13C6]phenylalanine molecule, 6 of a total of 12 carbon atoms are labeled. Statistics. All data are expressed as means ± SE. The plasma essential amino acid, insulin, and glucose responses were calculated as area under the curve above predrink values. Statistical analysis of the data was performed using a one-factor repeated-measures analysis of variance (ANOVA) for non-time-dependent variables. A two-factor repeated-measures ANOVA, with time and treatment as factors, was used to compare differences between treatments over time. In case of significant difference between trials, Tukey's post hoc test was applied to locate these differences. Statistical significance was set at P < 0.05.
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RESULTS |
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DISCUSSION |
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It has been shown that both muscle protein synthesis (7, 35, 44) and protein breakdown rates (7, 35) are accelerated following resistance exercise. Resistance exercise exerts its effect either via the increased local production of IGF-I in the muscle (21) and/or via other contraction-induced signaling events. Although muscle protein synthesis is stimulated during the postexercise recovery phase (35), net muscle protein balance remains negative in the absence of food intake (7, 35, 36). It has been shown that the administration of amino acids with or without carbohydrate results in a rapid increase in muscle protein synthesis rate, whereas the protein degradation rate is decreased (8, 11, 39, 44, 45). This results in an improved (positive) net muscle protein balance at rest and/or during postexercise recovery.
The combined intake of free leucine and protein with carbohydrate has been shown to increase the plasma insulin response (47–49). Increased amounts of circulating insulin have been reported to increase net muscle protein balance during conditions of hyperaminoacidemia (18, 19, 22), which occurs primarily by inhibition of muscle protein breakdown rather than by stimulating protein synthesis. Furthermore, leucine ingestion has been reported to stimulate muscle protein synthesis, independently of circulating plasma insulin levels, by increasing the phosphorylation (activation) of key proteins involved in the regulation of protein synthesis in rat (1, 4) and human skeletal muscle (23, 28). An insulinotropic mixture containing carbohydrate, protein, and leucine could therefore likely represent an efficient nutritional intervention to optimize net muscle accretion during postexercise recovery.
Using whole body tracer balance methodology, we observed that the protein breakdown rates exceeded the protein synthesis rates in the CHO trial, leading to a negative net protein balance (Fig. 5). The latter is in line with recently published data by Borsheim et al. (10), who showed that protein balance over the (leg) muscle remains negative after resistance exercise when only carbohydrate is ingested. However, measurements over the muscle do not necessarily account for all of the changes observed on a whole body level. Whole body protein breakdown rates were significantly suppressed in the CHO+PRO and CHO+PRO+Leu trials compared with the CHO trial, whereas protein synthesis rates were increased (Fig. 5). Coingestion of protein or protein and leucine with carbohydrate resulted in 50 ± 2 and 62 ± 2% reductions in protein degradation, respectively, compared with carbohydrate ingestion only. In addition, protein synthesis was 54 ± 5 and 45 ± 5% increased in the CHO+PRO and CHO+PRO+Leu trials, respectively, compared with the CHO trial. When whole body protein synthesis rates were expressed as a percentage of the phenylalanine Rd, we found the highest values in the CHO+PRO+Leu trial (95.6 ± 0.1 vs. 92.0 ± 0.4 and 94.2 ± 0.4% in the CHO and CHO+PRO trials respectively, P < 0.05), which could be attributed to a decreased rate of phenylalanine oxidation in the CHO+PRO+Leu compared with the CHO+PRO trial, with intermediate values found in the CHO trial. Such a reduction in amino acid oxidation after administration of leucine has previously been established (32). Our findings are in accord with earlier reports (27, 30, 39) showing that the combined ingestion of a single bolus of protein/amino acids and carbohydrate improves protein balance in the postresistance exercise period. In the present study, we extend on those findings by investigating whether additional leucine could further promote protein anabolism. Therefore, in both the CHO+PRO and CHO+PRO+Leu trials we provided an ample supply of protein (0.2 g·kg–1·h–1). This amount by far exceeds the amount of protein that was estimated to allow sustained maximal protein synthesis rates (50) and was selected to ensure that precursor substrate availability would not be limiting. Repeated boluses were administered, as opposed to a single-bolus approach, to ensure a continuous supply of amino acids. Consequently, we show that the coingestion of leucine even further increases the net protein balance compared with carbohydrate and protein ingestion (Fig. 5).
Most studies have applied the (whole body) tracer balance methodology to investigate the effect of postexercise nutrition on protein metabolism. Unfortunately, whole body tracer methods have some important limitations, as they do not allow the direct measurement of muscle protein synthesis rates. As such, they do not provide information on the contribution of individual tissues to protein metabolism and their response to nutritional and/or exercise intervention and do not enable the discrimination between active and inactive muscle. Therefore, we also measured the incorporation rate of labeled phenylalanine in skeletal muscle tissue after resistance exercise to determine the FSR of mixed muscle protein in the active vastus lateralis muscle. We used plasma L-[ring-13C6]phenylalanine and free muscle L-[ring-13C6]phenylalanine enrichments to provide an estimate of the lower boundary (based on plasma precursor enrichments) and the higher boundary (based on intracellular muscle precursor enrichments) for the true FSR of mixed muscle proteins. Both methods provided similar results. Because we observed a higher variation in intracellular muscle precursor enrichment, we based the FSR values on plasma precursor enrichments. Mixed muscle FSR averaged 0.0605 ± 0.0085%/h during 6 h of postexercise recovery while carbohydrate was ingested. Combined ingestion of leucine and protein with carbohydrate resulted in a significantly increased FSR compared with carbohydrate ingestion alone (Fig. 6). The combined ingestion of protein and carbohydrate without free leucine resulted in an intermediate value and was not significantly different from the lower values in the CHO trial or the higher values reported in the CHO+PRO+Leu trials (Fig. 6). Therefore, our data on the muscle level also show that the combined ingestion of leucine and protein with carbohydrate can effectively stimulate muscle protein synthesis.
The presented FSR data are in line with our observations using whole body tracer methods, showing that coingestion of leucine and protein with carbohydrate accelerates protein synthesis and improves net protein balance compared with carbohydrate ingestion only. With the whole body tracer methodology, our data indicate that the combined ingestion of leucine and protein with carbohydrate can reduce protein breakdown. The latter seems to be in line with earlier reports by Nair et al. (32), who showed that leucine infusion significantly decreases protein breakdown as well as amino acid oxidation rate. In addition, data from the same study showed a decrease in the plasma concentrations of the other EAAs during leucine infusion. In the present study, we also observed a reduced plasma EAA response in the CHO+PRO+Leu trial compared with the CHO+PRO trial, although the same amount of EAA (with the exclusion of the supplemented leucine) was consumed. However, no differences were observed in phenylalanine Rd between the CHO+PRO+Leu and CHO+PRO trials. This diminished plasma EAA response following leucine supplementation could be attributed to a reduced release from the muscle and indicate a reduced protein breakdown and/or reduced protein oxidation rate (Fig. 5).
We recently reported that coingestion of leucine, phenylalanine, and protein with carbohydrate results in an increased plasma insulin response in the postabsorptive resting state (46, 49) and during recovery from endurance exercise (47, 48). In the present study, we demonstrated that combined ingestion of leucine and protein with carbohydrate (without phenylalanine) can substantially increase the insulin response by 250% compared with the ingestion of only carbohydrate. Circulating insulin concentrations play an important role in regulating protein metabolism. The mechanism by which insulin mediates muscle hypertrophy is still a subject of debate, as some reports (9, 18, 19) indicate that insulin infusion reduces protein breakdown whereas others (6, 20) have reported an enhanced muscle protein synthesis rate when sufficient amino acids were made available. However, in the absence of an ample supply of amino acids, insulin does not seem to stimulate muscle protein synthesis (9). In the present study, we found plasma insulin responses to be negatively correlated with whole body protein degradation (r = –0.641, P < 0.01), whereas whole body protein synthesis was positively correlated with plasma insulin response (r = 0.451, P < 0.05). Muscle protein synthesis rates (FSR) did not correlate with the plasma insulin response, whereas mixed muscle protein FSR did correlate with the amount of leucine that was ingested (r = 0.471, P < 0.05). Whether the observed differences can be attributed to the insulin- or non-insulin-dependent stimulatory effects of leucine remains to be established.
The combined ingestion of leucine and protein with carbohydrate in the recovery period from resistance exercise could stimulate muscle protein synthesis in several ways. It provides amino acids as precursors for muscle protein synthesis, and the added leucine further increases plasma insulin concentrations and could also directly stimulate protein synthesis. Leucine has been shown to have the potential to affect muscle protein metabolism by decreasing the rate of protein degradation (32), most likely via increases in circulating insulin, and the phosphorylation of key proteins involved in the regulation of protein synthesis (23, 28, 37). The latter has been shown to occur even in the absence of an increase in circulating insulin concentrations. These studies indicate that leucine has the ability to function as a nutritional signaling molecule that modulates muscle protein synthesis and/or breakdown following food intake. The stimulatory effect of leucine on protein synthesis occurs at the level of translation initiation and involves signaling through mammalian target of rapamycin (mTOR) (24, 37). The protein kinase referred to as mTOR is thought to serve as a convergence point for leucine- and insulin-mediated effects on translation initiation (24, 37). Maximal rates of protein synthesis rates probably require both leucine and insulin signaling and the anabolic signal of resistance exercise. However, under normal in vivo conditions, it is impossible to discriminate between the effects of leucine and insulin and resistance exercise (IGF-I and mechanogrowth factor) on muscle protein synthesis, as all these signals are substantially elevated in the CHO+PRO+Leu trial.
In conclusion, the combined ingestion of protein and leucine with carbohydrate improves whole body protein balance during recovery from resistance exercise compared with the ingestion of carbohydrate or carbohydrate with protein. The combined ingestion of both leucine and protein with carbohydrate augments postexercise mixed muscle protein synthesis compared with the ingestion of only carbohydrate. The present data indicate that the additional ingestion of free leucine in combination with protein and carbohydrate likely represents an effective strategy to increase muscle anabolism following resistance exercise.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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