The quality and the quantity of nutritional intake play a critical role in response to training and in athletic performance. It is well established that CHO ingestion during moderate to high-intensity exercise can prolong endurance capacity (6). Evidence has since emerged to suggest that the ingestion of multiple transportable CHO can increase exogenous CHO oxidation and improve endurance performance over and above the effect of a single CHO (7,12). Recently, the addition of protein to a CHO containing beverage (CHO + Pro) during exercise has been reported to prolong endurance time to exhaustion in the range of 13%-36% (11,26,27). In contrast, other well-controlled studies did not report such an ergogenic effect of CHO + Pro on time-trial performance (22,31). Recently, Saunders et al. (28) suggested the addition of protein to CHO specifically improved late-exercise time-trial performance. Thus, the efficacy of adding protein to CHO for endurance performance remains unclear.
Several methodological differences between studies make it difficult to discern whether any benefits observed for CHO + Pro are the result of a protein-mediated effect. First, there are discrepancies in the way exercise performance is assessed for CHO and CHO + Pro treatments. The ecological validity of time-to-exhaustion protocols is limited because endurance athletes do not compete in events that require sustaining a fixed power output for as long as possible. Moreover, claims that CHO + Pro improves late-exercise performance were based on a time-trial protocol that was not specifically designed to record power output and measure late-stage exercise performance (28). Second, additional energy consumed when protein is added to a CHO-containing beverage may explain the performance benefits observed by others. If CHO is consumed at a rate considered optimal for exogenous CHO oxidation (14), the role of protein for improving endurance performance may be negligible (31). Finally, the control of external variables (or in some cases lack thereof) has varied greatly between previous studies. Investigations attempting to implement a strictly controlled exercise environment reported no effect of protein coingestion for time-trial performance (22,31).
The majority of studies examining the effect of CHO-protein coingestion have focused on the impact on endurance performance. Several authors suggest that consuming CHO + Pro during and after endurance exercise improves recovery, defined as the recovery of endurance capacity (26,33) and muscle function (29,30). Reductions in plasma creatine kinase (CK) concentrations (17,18,25-27,29,30) and muscle soreness (17,18,25,29) are thought to play a key role in aiding the recovery process. However, evidence to support an effect of additional protein for improving recovery is inconsistent. Others have found no difference in the postexercise rise in plasma CK (4,9), muscle soreness (4,18), or recovery of endurance capacity (3) for CHO + Pro. Thus, the effectiveness of additional protein for enhancing recovery is unclear, perhaps because of methodological and design differences among studies, including variable amounts of control. These issues require further investigation.
The primary aim of the present study was to investigate the efficacy of adding protein to CHO for improvement of late-exercise performance. Unlike previous studies, we aimed to supply a recommended dose of CHO during cycle exercise to determine whether the addition of protein improved cycle time-trial performance in a controlled exercise environment when preceded by a standardized steady-state ride. Second, we aimed to determine the effect CHO-protein beverages on indices of recovery, 24 h after time-trial cycle exercise by measuring plasma CK concentration, muscle soreness, and muscle function. Participant diet and physical activity were stringently controlled for the 48-h trial duration to ensure participants were in a similar state of energy balance and fuel repletion.
Twelve trained male cyclists were recruited through advertisements from local clubs (subject characteristics are shown in Table 1). Only individuals who undertook two or more training sessions per week of 1- to 5-h duration were eligible to participate. Data were collected over a 4-month period, such that all participants were in a similar phase of their training cycle. All trials were completed within a 3-wk period with no more than 14 d between trials, and the purpose and the methodology of the study were clearly explained to participants. All participants signed an informed consent and a completed a general health questionnaire before taking part in the study. The experimental protocol was approved by the School of Sport and Exercise Sciences Safety and Ethics Subcommittee.
The study design was counterbalanced, crossover, and double blind. Maximal oxygen uptake and muscle function were assessed at baseline. After baseline testing, participants performed two trials, during which they completed a 120-min steady-state cycle, consuming either a CHO or a CHO-protein (CHO + Pro) treatment beverage. After the 120-min steady-state ride at 50% maximal power output (W˙max) participants were then asked to complete a time trial lasting approximately 1 h (Fig. 1). Participants returned after 7-14 d and repeated the protocol while consuming the alternate treatment beverage. For example, if the participant consumed CHO + Pro during trial 1, he consumed the CHO treatment during trial 2. Markers of sarcolemmal disruption were obtained immediately before and 24 h postexercise, and the recovery of muscle function was assessed 24 h postexercise. All tests were conducted in a fasted state at the same time of day to minimize any circadian variance.
Body mass (BM).
A digital scale was used to determine body weight to the nearest 0.1 kg. Each participant was weighed in cycling clothing without shoes. This was repeated before each of the two testing visits to ensure body weight remained similar throughout.
Maximal power output.
Before each treatment trial, maximal oxygen uptake (V˙O2max) and W˙max were determined using an incremental cycle test to exhaustion on an electrically braked cycle ergometer (Lode Excalibur Sport Version 2.0; Lode BV, Groningen, The Netherlands). The test consisted of a 3-min warm-up at 95 W followed by an increase of 35 W every 3 min until exhaustion at a self-selected cadence. The seat position, the handlebar height, and the orientation used during testing were recorded and replicated on subsequent visits to the laboratory. All cycle tests were conducted in thermoneutral conditions (23°C, 40% relative humidity). W˙max values were used to determine the workload for the preloaded time trial. HR was measured continuously by using telemetry using HR monitor (Polar S625X; Polar Electro Oy, Kempele, Finland). V˙O2 was considered maximal if three of the four following conditions were met: 1) a plateau in V˙O2 with further increasing workloads (an increase of <2 mL·kg−1·min−1), 2) an HR within 10 bpm of the age-predicted maximum (220 bpm − age), 3) an RER of >1.05, and 4) an RPE greater than 17.
Metabolic and physiological measures.
Expired O2, CO2, and RER were measured during the last 60 s of each stage and approximately 60 s before the cessation of exercise to determine V˙O2 and V˙CO2. Participants indicated to the investigator the point at which they felt they were only able to exercise for one more minute. These data were obtained using the Douglas gas collection bags (Plysu, Milton Keynes, UK), with air expired through a two-way breathing valve connected to the bag. Bags were analyzed using a gas analyzer (Servomex 1440, East Sussex, UK), which was calibrated before each analyses using medical-grade gasses of known concentrations (British Oxygen Company, Windlesham, UK). The RPE was obtained using the 6-20 Borg scale.
Assessment of maximal isometric strength.
Twenty-four hours after each time trial a Tornvall chair (19) was used to determine maximal isometric strength (MIST). These values were compared with a single rested MIST test conducted at baseline to determine the postexercise loss of isometric strength. Both voluntary and stimulated isometric contractions of the quadriceps femoris were determined in the nondominant leg. A series of known weights was applied to a strain gauge, and the resulting voltage output was recorded daily for conversion into force. Participants were seated with the knee at 90o, which was fixed in place by a cord fastened round the ankle. The cord was attached to a strain gauge interfaced with a computer, on which the resultant force was displayed and recorded. Two arm straps held the participant in place in the chair and minimize any extraneous movement. Damp electrodes (13 × 8 cm) were secured proximally and distally to the anterolateral thigh. At baseline, maximal electrical stimulation was determined using a twitch overlay program; progressively, larger voltages (mA) were applied until two produced the same force. Once this was determined, participants received a percutaneous electrical stimulation three times before being encouraged to extend their knee as hard as possible. Three electrical stimulations were superimposed while contraction was occurring to ensure each effort was maximal. This procedure occurred three times during each trial, with participants resting for 90 s between efforts. The highest value was recorded as the participant's maximal voluntary contraction.
Two-hour preload cycle.
No less than 7 d after baseline testing, participants underwent familiarization of the time trial while consuming water only. For each trial, participants reported to the human performance laboratory in a fasted state (0600-0700 h), having undergone 24 h of dietary control before this visit. Participants completed 120 min of constant preload ergometer cycling at a workload 50% of their previously determined W˙max (175 ± 5 W). Participants were instructed to maintain a comfortable cadence above 60 rpm.
One-hour time trial.
Upon completion of the 120-min preload cycle, the ergometer was set in the linear mode, and participants were asked to complete an energy-based target amount of work at 70% of their predetermined W˙max. The total amount of work for a 1-h time trial (880 ± 27 kJ) was calculated according to the formula derived from Jeukendrup et al. (13). A linear factor, calculated as work rate at 70% W˙max divided by (90 rpm)2, was entered into the ergometer controller. During the time trial, participants received no verbal or visual feedback regarding performance time or physiological measures. Participants had access to a computer screen indicating the amount of the total energy target they had completed. Trials were performed in a small testing laboratory with a screen placed behind each participant to minimize distraction. An air conditioner and a fan were used when requested by participants, for which the settings were recorded and replicated during the second trial.
Metabolic and physiological measures.
Oxygen uptake, CO2 production, and RER data were collected for 2-min intervals every 30 min of the 120-min preload cycle to determine the intensity of the workload. HR and RPE were obtained also to confirm that a steady state was maintained before the time trial. During the time-trial HR, RPE and average power output were recorded at 25%, 50%, and 75% of the total energy target (kJ). These same measures were also obtained approximately 10 s before completion of the trial.
Assessments of muscle function and force production.
Twenty-four hours after the start of the preloaded cycle trial (0600-0700 h), participants were asked to return to the laboratory in a fasted state to undergo a MIST as described previously.
Visual analog scale of muscle soreness.
Using a 50-mm line with word descriptors at each end reading "no soreness" and "extreme soreness," participants marked on the line the point that they felt represented their current degree of general muscle soreness in their nondominant leg. The distance to the mark was then measured in millimeters. Visual analog scale (VAS) soreness was also rated when the knee was both flexed and extended. Average VAS soreness is reported here because it closely represents general, flexed, and extended soreness values. A VAS was completed in the morning before each time trial and 24 h after the onset of the preload cycle upon return to the laboratory before the MIST. The VAS scale has previously been shown to be a reliable device in measuring acute pain (5).
Upon arrival for each time trial, a cannula (IV Venflon 20 G × 32 mm; BD, Franklin Lakes, NJ) was inserted into the antecubital vein of the forearm. A two-way stopcock (Medex, Monsey, NY) was applied and opened to collect a 10-mL sample at rest. A further blood sample was taken upon each participant's return to the laboratory for the MIST on day 2 (24 h after the onset of the preload cycle). Samples were centrifuged at 3000 rpm at 4°C to allow the extraction of blood plasma for further analysis. All plasma samples were processed and stored frozen at −80°C. The exact time points of all sample collections were recorded and repeated during the second blinded treatment. Enzymatic analysis of CK concentration (CK NAC CP; ABX Diagnostics, UK) was performed in duplicate at each time point using a semiautomated analyzer (COBAS MIRA S-plus; ABX Diagnostics).
The two experimental trials were a 6% maltodextrin CHO beverage (65 g·h−1; CHO) and a CHO + protein (CHO + Pro) beverage containing 6% maltodextrin (65 g·h−1) and 1.8% protein hydrolysate (19 g·h−1). Treatment beverages were counterbalanced throughout the study to minimize any influence of order, with six participants receiving CHO first and six receiving CHO + Pro first. Beverages were served in non-see-through bottles, with a different flavor used in each trial to minimize the risk of taste comparisons between beverages. During pilot testing, a group of cyclists was able to successfully identify the beverage composition when flavors were the same. Flavors were added using 3 g of nonenergetic sweetened drink mix. Participants consumed 270 mL every 15 min of the 120-min steady-state preload, beginning at the onset of the ride (1080 mL·h−1). An investigator handed the participant one container at a time and verbally encouraged them to drink at an appropriate rate. Water was consumed ad libitum during the time trial, with the amount recorded (∼430 ± 26 mL) and administered in the same volume during the second trial. Beverages were prepared by an independent investigator who had no part in the data collection of the study.
Diet and Exercise Control
Participant diet was standardized for the 48-h duration of each treatment. This ensured that nutrient intake was controlled for 24 h before each time trial and over the 24-h course of each time-trial day before the assessment of muscle function the following morning. In between baseline testing and each blinded trial, participants completed a 3-d food diary, representative of their average week. A questionnaire of food preferences was also completed by participants. Using an online diet planner, each of the 3 d was logged, and the energy and the nutrient intake were estimated. An average of the 3 d was combined with the participant's weight to calculate the total energy content of the diet. The standardized diet was matched to each participants habitual daily energy intake and contained 8 g·kg−1 BM CHO and 1.6 g·kg−1 BM protein with the remainder of energy derived from fat. The macronutrient composition of the standardized diet was not significantly different from participants' habitual diet (Table 2). Participants were instructed to consume only the food provided for them over the 2-d testing period. The same 2-d food parcel was provided to each participant before the second trial. Participants were instructed to maintain normal volume and intensity of training throughout the course of the study but to refrain from training for 48 h before each testing phase.
Data are reported as mean ± SE, unless otherwise stated. All data were analyzed by two-way ANOVA for repeated measures (group × time). Significant differences between means were determined using the Statistical Package for the Social Sciences for Windows (Version 15; SPSS Inc., Chicago, IL). Bonferroni adjustment was applied to post hoc means-comparison tests. Differences were considered significant at P < 0.05. In keeping with recent trends in inferential statistics, we made magnitude-based inferences about the "true" values of the effect of treatment on outcomes by expressing the uncertainty as 90% confidence intervals and by calculating and interpreting chances that the true effect was beneficial or detrimental. For time-trial performance data, a substantial effect was derived from 0.5 times (8) the average of estimates for the typical error of measurement for time-trial performance and power output. For all other biochemical and psychometric variables, the smallest standardized (Cohen) difference in the mean (0.20 times the between-subjects standard deviation for the control group) was used to identify the magnitude of the smallest substantive effect. On the basis of an analysis of the confidence intervals and P values using a published spreadsheet (8), the likelihood of a substantial benefit or detriment, increase or decrease, for an outcome was qualified as follows: <1%, almost certainly not; 1%-5%, very unlikely; 5%-25%, unlikely; 25%-75%, possible; 75%-95%, likely; 95%-99%, very likely; and >99%, almost certain (9); otherwise, an effect was deemed unclear or inconclusive if the confidence interval overlapped the thresholds for positive and negative substantial effects by >5%. When the mean and >50% of the confidence interval lie within the threshold for a substantial effect, the effect is qualified as trivial. P values were provided for an inferential comparison for the main experimental outcomes only.
Mean power output during the time trial was not significantly different between treatments (247 ± 11 W for CHO and 247 ± 13 W for CHO + Pro, respectively). Power output was reduced at 75% compared with 25% and 50% of time-trial completion (Fig. 2); however, average power outputs at these time points were not different between treatments. As a result, there was also no significant difference in the time to complete the time trial for CHO or CHO + Pro (Fig. 3). Inference-based statistics determined no clear effect of CHO + Pro compared with CHO on the time to complete the trial (P = 0.81). HR and RPE were significantly elevated at completion of time-trial target compared with 25%, 50%, and 75% completion (P < 0.001), with no between-group differences at any time point between CHO and CHO + Pro (Table 2).
There was no between-group difference in heart rate and V˙O2 at 0, 30, 60, 90, and 120 min of the 120-min preload cycle. Average HR for CHO and CHO + Pro was 141 ± 12 and 142 ± 12 bpm, respectively (Table 2). Average V˙O2 (34.9 ± 3.7 mL·kg−1·min−1 for CHO and 34.3 ± 4.2 mL·kg−1·min−1 for CHO + Pro) and RER (0.88 ± 0.01 for both) were not different between treatments at any time point. Metabolic data indicated that participants were cycling at approximately 55% V˙O2max. Throughout the steady-state ride, RPE was not significantly different between treatments (between-subject range = 10-15) (Table 3).
Indices of Recovery
Isometric knee extensor strength.
A significant time effect revealed that knee extensor MIST declined 24 h after the onset of each trial compared with baseline MIST (15% ± 3% for CHO and 11% ± 3% for CHO + Pro; P < 0.002), with no significant between-group difference (Fig. 4). Inference-based statistics revealed that the effect of CHO + Pro over CHO on the postexercise recovery of knee extensor MIST was most likely trivial (P = 0.28).
VAS of muscle soreness.
A significant time effect (P < 0.05) revealed VAS soreness increased from 4.8 ± 1.2 to 14.3 ± 4.3 mm for CHO and from 5.9 ± 1.9 to 14.3 ± 4.8 mm for CHO + Pro 24 h after the onset of each trial (Fig. 5). No between-group difference in VAS soreness was evident at 24 h postexercise. Inference-based statistics indicated no clear difference in the extent of VAS muscle soreness between groups (P = 0.91).
Plasma CK concentration.
A significant time interaction (P < 0.002) revealed that plasma CK was significantly increased for CHO and CHO + Pro at 24 h postexercise (Fig. 6). No between-group difference in plasma CK concentrations was evident at 24 h postexercise (P > 0.05). Inference-based statistics revealed the effect of CHO + Pro compared with CHO on postexercise CK concentrations was most likely trivial (P = 0.53).
Seven of the 12 participants completed the time trial more quickly during their first treatment visit, suggesting that there was no learning effect between trials. Five of the 12 participants correctly identified the beverage order during testing. Of the five participants who identified the beverages correctly, only two performed faster with CHO + Pro, suggesting that our attempts to minimize treatment bias were successful. Seven of the 12 participants performed better when consuming a CHO beverage, with the remaining 5 completing the task more quickly with CHO + Pro.
The present study is the first to our knowledge to determine the effect of a CHO + Pro beverage on late-stage cycle time-trial performance and postexercise indices of recovery. Here we show that the addition of protein to a moderate CHO dose did not improve late-exercise time-trial performance when beverages were consumed during a steady-state preload cycle. Furthermore, there were no differences in power output between treatments at any stage of the time trial. Finally, the addition of protein to a CHO containing beverage did not improve any of the measured indices of postexercise recovery. Thus, our results do not support the notion that CHO-protein coingestion during exercise enhances late-exercise performance or postexercise recovery over to CHO alone.
In contrast to the present findings, several investigations suggest that the addition of protein to CHO improves endurance performance (11,26-28). In many cases, the discrepancies between studies may be due to differences in the exercise test used to assess performance. Studies suggesting improved performance actually measured time to exhaustion (11,26,27) not performance per se. Currell and Jeukendrup (8) highlighted the fact that the ecological validity of exhaustive exercise protocols is limited because endurance athletes do not compete in events that require sustaining a fixed power output for as long as possible. Furthermore, the precision of time to exhaustion is reportedly in the range of 26% (13). On the contrary, time-trial cycle tests have been found to be highly reproducible and sensitive to small changes in exercise performance (13,15,23). Our observation of no improvement in time-trial performance for CHO + Pro is consistent with other studies measuring overall time-trial performance (22,31). It should be noted that the final measurement of performance is influenced not only by the exercise test but also by the control measures used and the subject feedback during exercise. Knowledge of parameters such as time elapsed, distanced traveled, and heart rate during exercise may compromise the blinding of treatments, creating a placebo effect (20), particularly when additional protein is consumed, which is often difficult to mask. Early studies did not report the control of these conditions (11,25-28,30). Further, to the well-controlled study protocol we used, participant diet was assessed and standardized, whereas physical activity was minimized to ensure participants were in a similar state of energy balance and fuel repletion before and during each treatment trial. Thus, when performance rather than endurance capacity is measured in a controlled environment, additional protein does not seem to be advantageous for performance.
Recently, the importance of additional protein on more specific aspects of endurance performance has been touted. In particular, the notion that additional protein is important for enhancement of performance late in exercise has been put forward (28). Despite no difference in the 60-km time-trial performance between CHO and CHO + Pro treatments, ingestion of additional protein resulted in improved performance during the final stages of the time trial (28). Although this result can be interpreted as important for endurance athletes, it should be emphasized that overall performance was not improved. Although this result can be interpreted as important for endurance athletes, it should be emphasized that overall performance was not improved, thus minimizing the importance of any late-exercise improvement. Given that overall performance was not different, the shorter time taken to complete late-exercise stages with CHO + Pro may have been due to a slower completion of earlier exercise stages or greater variability in the earlier section of the time trial relative to more reliable late-exercise performance. Although these results contrast with our findings, the differences may be because Saunders et al. (28) did not design their study to specifically examine the impact of additional protein on late-stage performance. The preloaded cycle time trial used in the present study was designed to record power output and to determine late-exercise time-trial performance and has been validated in previous studies from our laboratory (8,13). In support of our findings, a recent study by Osterberg et al. (22) found no enhancement of late-exercise time-trial performance preceded by a 120-min steady-state ride, when protein was added to CHO. Accurate determination of late-exercise performance requires that the exercise intensity of the prior bout should be standardized and power output reported. Thus, it seems that when properly controlled, late-stage exercise performance is not improved by the addition of protein to CHO during exercise.
Furthermore, lack of a definitive physiological mechanism also contributes to doubt of the importance of adding protein to CHO during exercise. Several mechanisms have been theorized (27), but only one has any support in studies. Laursen et al. (15) showed that ingesting CHO + Pro during ultraendurance exercise resulted in twofold greater protein oxidation compared with CHO. Thus, studies in which extended time to exhaustion was found suggest that the addition of protein may alter substrate utilization, potentially sparing muscle glycogen (10,28,29). However, CHO intake in these studies was too low (<50 g·h−1) to attain peak exogenous oxidation rates (13), and protein plus CHO provided more energy than CHO alone. Investigations in which the total energy content of CHO and CHO + Pro beverages was matched have found no difference in time to exhaustion (25,30). Thus, the benefit of adding protein to CHO likely is due to the additional energy delivered by protein as opposed to a protein per se. We show that the addition of protein (19 g·h−1) to a CHO dose (65 g·h−1) that meets the recommended upper limit to attain peak exogenous CHO oxidation rates did not improve late-exercise time-trial performance despite increasing the total beverage energy load by 29%. To our knowledge, no other purported mechanism for the enhancement of performance by additional protein has been demonstrated at this time.
In addition to the enhancement of endurance performance, adding protein to CHO has been promoted to enhance recovery. Previous studies have shown that CHO + Pro beverages prolong subsequent time to exhaustion in the range of 40%-55% (26,29,33). Others have found that the addition of protein to CHO results in small increases in the recovery of muscle function in the order of one to two knee extension lifts (30) and 1-2 cm in vertical jump height (30). Although these data suggest that the addition of protein to CHO improves the recovery of performance and function, there are many factors that differ between studies; thus, the picture is less than clear. It is difficult to determine whether these benefits were influenced by the timing of beverage ingestion (before, during, or after exercise), by the way in which recovery was measured, or by the time given between the exercise and the assessment of recovery. We chose to measure MIST at 24 h postexercise because of its practical relevance (1). Athletes often repeat their training or compete 24 h after an exercise bout, and therefore recovery of muscle function and exercise performance would be important in this time frame. In concert with two recent studies (4,9), we showed that the coingestion of CHO and protein does not improve the recovery of isometric strength. Others have failed to show improvements in recovery as based on improved subsequent performance assessed in various methods (3,9,17,18,25). Thus, the impact of adding protein to CHO on recovery from intense exercise must still be considered to be somewhat equivocal.
Studies that report improvements in the recovery of endurance capacity typically measured these changes several hours after protein ingestion (26,33). The ingestion of protein is known to stimulate muscle protein synthesis (10), resulting in positive net whole-body protein balance after endurance exercise (16), which has been suggested to enhance recovery through the repair and remodeling of damaged proteins (17,26,27). However, the turnover of myofibrillar proteins is relatively slow, in the order of days to weeks (24). Thus, it is difficult to comprehend how this process may influence changes noted in just a few hours (26,33). Hence, if recovery is enhanced by adding protein to CHO, it is unlikely to be due to the improved net muscle protein balance from the protein.
Although recovery may be defined by the ability to reproduce an optimal level of performance, several investigations have sought to measure the effect of CHO + Pro on the other markers thought to be important to the recovery process. Studies have shown that CHO + Pro beverages reduce plasma CK, a putative marker of sarcolemmal disruption, and muscle soreness (17,18,25,27,28). Others have found that lower CK concentrations and lower ratings of muscle soreness accompany improvements in the recovery of time to exhaustion (26,29) and vertical jump height (29). Consequently, previous studies suggest that protein coingestion may play an important role in reducing plasma CK and muscle soreness, thus improving recovery. However, the changes in plasma CK when protein is added to CHO are often minimal (17,25,27,28,30), and the interindividual variation is very large, making it difficult to determine the physiological significance. Moreover, the plasma CK and the muscle soreness values found in our study are in a similar range to those reported previously, yet we show that CHO + Pro did not ameliorate postexercise markers of sarcolemmal disruption. A recent study from Betts et al. (4) used a strenuous bout of intermittent shuttle running to induce a greater degree of sarcolemmal disruption than the cycle time trial used in our study. In support of our findings, the authors showed that the addition of protein to a CHO beverage did not ameliorate postexercise plasma CK and muscle soreness. Thus, it is not clear what impact, if any, ingestion of protein has on recovery by ameliorating CK and soreness.
It is likely that the importance of plasma CK and muscle soreness for enhancement of recovery has been overemphasized by studies that suggest the addition of protein to CHO attenuates these responses (17,18,25,27,28). At best, plasma CK and muscle soreness should be considered putative markers of muscle damage. Studies have shown that these indirect markers have a poor relationship with the loss of muscle function (21) and direct markers of sarcolemmal disruption (2,32). Moreover, "true" treatment effects may be masked by the inherent variability of the CK response (4,21,32). Further to this suggestion, three participants in our study exhibited threefold greater resting plasma CK response compared with the group average when protein was coingested. Resting CK concentrations for these same individuals were vastly reduced during the CHO-only trial. Thus, our results suggest a clear need to further investigate the impact of protein ingestion on recovery from endurance exercise. It would seem that studying direct markers of sarcolemmal disruption (biopsy or MRI techniques) in concert with postexercise tests of muscle function may provide clearer answers regarding the efficacy of CHO + Pro for improving recovery.
In conclusion, when energy intake is controlled and CHO is ingested at rates considered optimal for peak exogenous CHO oxidation, the addition of protein does not improve late-exercise cycle time-trial performance or indices of recovery at 24 h postexercise. The metabolic and the physiological response to CHO + Pro may be dependent on the mode, duration, and intensity of the exercise bout and warrants further study. On the basis of our findings and other carefully controlled studies (22,31), there is currently no basis to recommend CHO + Pro beverages to endurance athletes for performance enhancement or improved recovery.
First and foremost, the authors thank the participants for their time and effort. The authors also thank the experimental support of Matthew Cocks and Hollie Williams.
No funding was received for this study.
There are no conflicts of interests for any of the authors.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:©2010The American College of Sports Medicine
TIME TRIAL; ENDURANCE EXERCISE; SARCOLEMMAL DISRUPTION; MUSCLE FUNCTION