There is presently no convincing scientific support for the contention that protein be included in CHO-based sports supplements. Before any such strategy can be truly advocated, published evidence should first reveal consistently positive effects on practically meaningful outcomes (i.e., performance) across multiple independent laboratories, ideally also isolating plausible mechanisms of action. With due consideration of the precise context and nature of these effects, I propose that a "worthwhile" nutritional supplement must fulfill at least one of the following interrelated criteria:
a. true "supplementation" of the diet (i.e., either compensating for deficiencies or meeting increased requirements);
b. supplement formulation facilitates digestion, absorption, and/or metabolism to more effectively deliver active ingredients than possible via whole foods; and/or
c. optimal timing for supplement ingestion renders it impractical to acquire active ingredients via whole foods as a meal (e.g., effects are either short-lived or situation dependent).
A relevant example to illustrate these contextual issues is CHO-based sports supplements per se (i.e., without added protein), the use of which is supported in a recent joint position statement (25) both because of the increased CHO requirements of many athletes beyond the typical diet and the efficacy of specially formulated CHO supplements, which are easily incorporated into athletes' preexercise, midexercise, or postexercise regimen (p. 719-21). In contrast, that same position statement attests that even the slightly elevated protein requirements of athletes are almost always met by diet alone and that "…protein or amino acid supplementation has not been shown to positively impact athletic performance…" (p. 715). The perspective set out below will therefore defend the prevailing view that CHO without protein should be included in sports supplements; initially by presenting the balance of published data that have yet to reveal any ergogenic or mechanistic benefit of added protein or amino acids during exercise, before questioning whether the positive effects evident in recovery can legitimately be interpreted as supporting any practical value of these supplements when considered in a real-world context (e.g., in the fed state and/or when following existing guidelines for CHO intake).
Combined CHO-Protein Ingestion during Exercise
Although some studies have shown ergogenic benefits when combined CHO-protein supplements are ingested during exercise (11,13,28-30), one of these actually reflects an alternative but ultimately no more effective choice of pacing strategy (30), and the majority of evidence on this topic conforms to the prevailing view that there is no benefit (4,9,19,22,26,35-37). Could it then be argued that positive findings emerging from even only a minority of studies nonetheless demonstrate that added protein can enhance performance, albeit if that effect may be specific to the experimental setup in certain laboratories? On the contrary, even those researchers observing performance benefits (13,28,29) have also reported negative outcomes when conducting similar studies (9,19,26,30,36). In fact, the only discernible methodological factor to explain why certain studies have observed a benefit of added protein is that all these studies have compared CHO-protein supplements relative to control supplements providing CHO alone at a lower energy content and/or below the recommended intake for the exercise tests used (11,13,28,29). Whereas these findings are undoubtedly of mechanistic interest, the only logical interpretation from a practical viewpoint is that protein is an unnecessary addition to an otherwise adequate CHO supplementation regimen.
In summary, the case against including protein in CHO supplements to be ingested during exercise is fundamentally grounded in the numerical balance of just four published studies demonstrating any benefit of added protein (11,13,28,29) opposed to nine showing no effects on physical performance (4,9,19,22,26,30,35-37), with the few extant reports of benefit explained by the insufficient quantities of CHO ingested in these studies. Lastly, another major problem facing the challenging view is the absence of any empirically supported mechanism to explain why ingesting protein during exercise would be expected to impart performance gains, with the most plausible proposed mechanisms (e.g., altered skeletal muscle metabolism) having recently been comprehensively examined and largely discounted (6).
Combined CHO-Protein Ingestion in Recovery
Whereas combined CHO and protein simply lacks efficacy when ingested during exercise, this does not negate any possibility that added protein may be of practical value in other situations (e.g., between exercise bouts). The question under debate, however, is not whether ingesting protein can be efficacious under certain circumstances but whether protein should be included in CHO-based sports supplements. These are conceptually very different questions, with valid justification for the inclusion of protein dependent not only on demonstrable efficacy but also on whether benefits persist even when evaluated within the context of the habitual diet. This point has greater relevance to recovery than during exercise, as meals are usually a more viable option in recovery. For this reason, consideration of the criteria identified earlier leads inexorably to the conclusion that protein is an unnecessary addition to supplements in a real-world setting.
The first of these criteria is easiest to address. Athletes may have a slightly increased daily protein requirement (34), so dietary protein supplementation might at first seem appropriate for this population. However, the vast majority of athletes in westernized societies exceed their recommended protein intake even without supplementation such that long-term protein requirements are almost always met by diet alone (20) and even the potential short-term benefit of added protein for muscle glycogen synthesis after exercise does not apply to conditions where the existing diet is already adequate (3). Ingestion of added protein therefore does not supplement the habitual diet in the true sense because there is no chronic protein deficiency to be rectified and also no acute acceleration in the delivery of coingested CHO, provided that sufficient is ingested.
The only remaining issues to consider are whether protein itself might be more effectively delivered to relevant tissues if ingested in the form of a supplement, particularly at time points so proximal to exercise that meals become an impractical option. Indeed, recent studies on this topic indicate that the temporal proximity and precise types of protein or amino acids ingested can mediate both acute protein synthetic rates and chronic accrual of lean tissue (23). However, these studies do not necessarily imply that feeding must occur immediately after exercise, just not delayed by as long as 2 h (8,24), and neither do they rule out the informed selection of certain whole foods such as milk. Informed dietary choices could still therefore be based on emerging knowledge of the more effective constituent proteins (e.g., whey) (23) and such foods could easily be consumed as part of a meal within the first hour after exercise. Ultimately, until combined CHO-protein supplements are shown to be clearly superior to carefully considered mixed meals, it remains an (in)convenient truth (perspective dependent) that whole foods can achieve the desired protein-mediated effects while simultaneously being more likely to provide a variety of other important nutrients (also reducing the risk of contamination with harmful or illegal substances) (10).
REPLY TO THE CHALLENGING VIEW
It is highly conducive to an informative debate that both Dr. Stevenson and myself independently chose to present our initial perspectives with separate consideration of CHO-protein ingestion during exercise and then in postexercise recovery. Moreover, it is intriguing that these purposely contrasting perspectives cite essentially the same body of evidence and even concur that neither during nor after exercise are CHO-protein mixtures any more effective than simply ingesting adequate quantities of CHO without protein.
Where our interpretations differ is in Dr. Stevenson's risk-benefit analysis whereby, in the absence of any likely detriment to performance, added protein can be justified based on the slightest probability of either performance enhancement in certain situations or some "other" benefit. Before responding to the latter possibility of these "other" benefits, this approach can firstly be evaluated in more general terms. Aside from the fact that at least one study has shown a detrimental effect of combined CHO-protein ingestion on performance (35), I feel this risk-benefit analysis does not give due recognition to the majority of studies reporting no ergogenic effects of added protein (i.e., just one example of potential benefit takes precedence over any number of well-controlled studies to the contrary). It also seems unlikely that the enhanced performance apparent in the minority of studies has simply been undetected by all others (i.e., type 2 errors), given that the latter seem to have been conducted with at least equal scientific rigor to those that have detected effects. To reverse this argument, given the existing balance of published evidence and likely publication bias on this topic, perhaps the few examples of benefits may reflect false-positive results within the commonly accepted type 1 error rate (i.e., 5%).
Ultimately, rather than adding protein or anything else to sports supplements "just in case," the approach informing my prevailing view was evidence based, with plausible explanations to reconcile any apparently conflicting findings. In this case, the few documented benefits of added protein for either performance during exercise or recovery thereafter are easily accounted for by the low amounts of CHO ingested in those studies. It therefore remains questionable why anyone would even consider including protein in a sports supplement when the likelihood of attaining a slight performance enhancement is entirely dependent on that supplement already being deficient in another nutrient with far better-established ergogenic properties (i.e., CHO).
Returning to the possibility of the "other" benefits associated with including protein in CHO-based sports supplements, Dr. Stevenson rightly identifies the established roles of supplementary protein for recovery in a broader sense and/or over multiple days of sustained training. Indeed, coingestion of CHO and protein in the immediate postexercise period clearly facilitates processes contributing to both short-term recovery and long-term adaptations (e.g., muscle glycogen and protein synthesis, respectively), with consequent translation into functional/whole-body end points (e.g., improved restoration of performance and training responses) (3,17). However, to reiterate the view expressed in my initial perspective, there is no reason why these same outcomes cannot be achieved using nutritionally balanced whole foods as part of a more holistic approach. In fact, studies cited in the challenging view as evidence of improved rehydration with combined CHO-protein ingestion is a prime example of this reasoning (32,39). This is because the superior supplement in these studies was actually milk, with the greater fluid retention primarily attributed not to the presence of protein but to milk's naturally elevated electrolyte content (32).
Finally, without intending to individually critique particular studies, I feel this reply must address the issue of attenuated exercise-induced muscle damage with protein ingestion given that this forms a recurrent theme in the challenging view. Impaired muscle function is the most practically meaningful and arguably most valid quantitative measure of exercise-induced muscle damage, yet my own measurements of this variable in recovery are consistent with almost every other extant study in showing no practical benefit whatsoever of added protein for the recovery of strength (2). The only consistent finding on which to base the view that protein ingestion can protect against muscle damage is therefore the reduced systemic activity of creatine kinase commonly observed with CHO-protein supplements-but what does this actually mean? The response of systemic creatine kinase to exercise can often be dissociated from most other indices of muscle damage (e.g., soreness and strength impairment), partly due to variability in the rate at which the enzyme is cleared from the circulation (12). Therefore, it is entirely possible that protein ingestion may similarly affect the kinetics of the creatine kinase response without reflecting differences in the state of muscle damage/repair. Even accepting that combined CHO-protein ingestion may reduce creatine kinase efflux from muscle or that the previously discussed improvement in fluid balance by only ∼300 mL may partly be attributable to protein rather than electrolytes, we are still left without any evidence that these factors translate into practically worthwhile outcomes (i.e., improved physical performance) (39).
Athletes, coaches, and supplement manufacturers understandably strive to gain competitive advantage by adopting nutritional strategies in anticipation of future empirical support. However, the scientific data currently available indicate that protein is not an effective addition to CHO-based sports supplements when ingested during exercise, whereas the potential advantages of added protein during recovery can be most comprehensively achieved via whole foods as part of a balanced diet.
James A. Betts, FACSM
Human Physiology Research Group
Department for Health
University of Bath
Dietary CHO intake is fundamental to an athlete's nutritional program to provide fuel to the working muscle and to facilitate muscle glycogen synthesis between exercise sessions. As a consequence of this, a wide range of CHO-based sports supplements are commercially available. During the last 20 yr, evidence has been emerging that the addition of protein to CHO-based supplements may be beneficial; however, this remains a controversial topic. This challenging view will present the evidence that demonstrates the favorable metabolic consequences of consuming protein along with CHO. The majority of studies in this area have focused on CHO-protein intake either during exercise or in recovery from exercise, but of course, nutritional strategies during recovery also become preexercise nutritional strategies in many instances.
Providing CHO-Protein Mixtures During Exercise
Several studies have investigated the effect of consuming protein with CHO compared with CHO only during prolonged exercise. Ivy et al. (13) reported a 36% improvement in time to fatigue after 3 h of variable-intensity cycling when cyclists ingested a mixture of CHO and protein compared with a matched quantity of CHO. Subsequently, several others have reported improvements in time to fatigue when including protein in a CHO supplement during exercise (11,28,29,33).
The studies that have shown no performance improvement after a mixture of CHO and protein over CHO only are those that have provided sufficient CHO to satisfy oxidative requirements during exercise or when the addition of protein does not increase the energy content of the supplement (19,26,36,37). Despite this, a recent study by Ferguson-Stegall et al. (9) reported that adding a moderate amount of protein to a low-CHO drink provided during exercise at least maintained exercise capacity relative to a CHO supplement containing twice as much CHO and over 40% more energy.
Although the evidence for an improvement in performance from adding protein to a CHO supplement during exercise seems to be equivocal, it is unlikely that consuming protein with CHO during prolonged endurance exercise will have a detrimental effect on performance. Indeed, there are other physiological effects that may be of benefit to the exercising individual. Coingestion of protein with CHO during prolonged endurance exercise has been shown to improve protein balance by increasing synthesis and decreasing breakdown, resulting in a positive net protein balance during exercise (1,16) and during recovery (16).
The inclusion of protein in a CHO supplement consumed during exercise has been shown to reduce indices of postexercise muscle disruption compared with CHO supplements matched for CHO content or total calories (36) during 12-48 h. Studies have reported attenuated subjective ratings of perceived soreness (26,27) and creatine kinase (CK) concentrations (26-28) 12-24 h after exhaustive cycling exercise when protein was added to a CHO supplement provided during and immediately after exercise compared with CHO alone.
It is therefore clear that, although the benefits of consuming a CHO-protein mixture during exercise on immediate performance are still uncertain, this nutritional strategy can have a positive effect on the recovery process and therefore may improve exercise performance on subsequent days.
Providing CHO-Protein Mixtures during Recovery from Exercise
Nutritional interventions during recovery from exercise focus on CHO intake to maximize muscle glycogen resynthesis. Studies investigating the effects of adding protein to CHO have very much focused on enhancing muscle glycogen resynthesis during short-term recovery through maximizing the postexercise insulinemic response. Despite some contrasting evidence, the general consensus is similar to that for during exercise; if CHO is provided in adequate quantities (≥1 g·kg−1·h−1), then the addition of protein provides no extra benefit. However, reducing the CHO content of a supplement and adding an appropriate source of protein does not seem to have any detrimental effects on muscle glycogen resynthesis or subsequent performance. Indeed, the addition of protein to a CHO supplement has many other potential benefits, and therefore, in many circumstances, reducing the CHO content and increasing the protein content of a supplement will result in a superior recovery strategy than consuming CHO alone. The majority of studies investigating the performance benefits of adding protein to a CHO recovery supplement have only considered the effect of supplementation on a subsequent exercise bout performed a few hours later. Although some athletes will train twice a day, many more will train hard or compete on consecutive days and therefore require a supplement that will enable them to maintain a high level of performance over multiple days.
The addition of amino acids to a CHO drink consumed regularly during a 2-wk period has been shown to reduce muscle damage (as evidenced by a decrease in plasma CK), decrease subjective fatigue, and maintain exercise performance after consecutive days of exercise compared with when CHO only was consumed in trained male athletes (33). Saunders et al. (28) reported increased time to exhaustion (but no difference in RPE) and decreased muscle damage after two consecutive days of exercise after CHO-protein drinks compared with CHO alone. Supplementation with a CHO-protein drink immediately after training for 6 d resulted in lower CK and muscle soreness in trained cross-country runners compared with when CHO only was consumed after training (18). Despite no differences in overall cross-country performance after the 6-d training and supplementation, there were trends toward significant improvements in race performance in higher-mileage runners (18). Therefore, for those athletes who want to maintain performance over days of heavy training or competition, adding protein to a CHO supplement would be beneficial.
Consuming CHO alone in the recovery period after a muscle-damaging exercise seems to have no beneficial effect on attenuating indirect measures of exercise-induced muscle damage (7). In fact, Cockburn et al. (7) reported CK and Mb concentrations increased when participants consumed a CHO drink after muscle-damaging exercise. On the contrary, consuming a recovery drink containing CHO and protein attenuated CK and Mb for 48 h after exercise and attenuated declines in peak torque and total work of the set.
The addition of protein to a CHO drink may confer advantages in terms of rehydration during recovery from exercise. There is evidence that consuming solutions containing protein after exercise-induced dehydration improves fluid retention over solutions with no added protein (31,32,39). A very recent study from James et al. (14) has reported that a CHO-milk protein solution is better retained than an energy- and electrolyte content-matched CHO solution consumed after intermittent exercise in the heat. In both trials, participants ingested a volume equivalent to 150% of body mass loss; however, the substitution of 25 g·L−1 of CHO for 25 g·L−1 of milk protein reduced total urine volume produced over the trial by 281 mL by the end of the study.
Recent interesting data suggest that supplementation with a mixture of CHO and protein after exercise increases plasma albumin content and plasma volume in both older and younger subjects (21). Postexercise plasma volume increases result in cardiovascular and thermoregulatory adaptation and are therefore an important part of the training process. It has been reported that older individuals have an attenuated postexercise increase in plasma volume potentially due to an insufficient protein intake for albumin synthesis. Okazaki et al. (21) investigated the effect of providing a CHO-protein supplement after exercise during an 8-wk training program on plasma volume and cardiovascular and thermoregulatory responses to exercise in older subjects. Compared with the placebo group, plasma volume, albumin content, thermoregulatory, and aerobic adaptations to training all increased significantly after the CHO-protein supplement. Although further research is warranted in this area, this provides further evidence that consuming CHO alone during the postexercise period may not be the most appropriate nutritional practice to facilitate all aspects of recovery.
In summary, there are several benefits of adding protein to a CHO supplement to be consumed both during and after exercise. The favorable consequences of consuming CHO-protein mixtures in many cases make them a more effective nutritional strategy than consuming CHO only, particularly over the longer term.
REPLY TO THE PREVAILING VIEW
The key point in my challenging perspective is that the addition of protein to a CHO-based sports supplement should be considered in view of the available evidence on all aspects of exercise training and performance as well as adaptations to training. Although the ultimate goal of sports nutrition is to improve exercise performance, we should not solely focus on the effects of a supplement on acute exercise performance but consider the practical application of the supplement and the potential longer-term implications of its use. I believe that the prevailing perspective presented focuses too heavily on the performance results of acute laboratory-based studies, which, at present, still provide conflicting results.
Combined CHO-Protein Ingestion during Exercise
The argument provided by Dr. Betts for not including protein in a CHO supplement to be ingested during exercise is based on 13 laboratory-based studies that have investigated the effects of consuming CHO versus a CHO-protein mixture during an acute bout of exercise on performance. No consideration has been given to the other reported benefits of consuming protein along with CHO during exercise (i.e., the effects on protein balance and indices of postexercise muscle damage), and these factors are likely to affect subsequent exercise performance. In reality, supplements will be taken on a daily basis by athletes, and therefore, it is important to consider the broader practical application of the supplement and the potential longer-term benefits to the athlete. As raised in the contrasting perspective, despite equivocal findings in relation to acute exercise performance, there seems to be no detrimental effect of reducing the CHO content of a supplement to incorporate a small amount of protein. Indeed, the potential additional benefits of consuming some protein may be more beneficial to performance over subsequent days of training and competition.
A very recent applied research study by Cathcart et al. (5) highlights this point. They reported that consuming a CHO-protein supplement during exercise during 8 d of strenuous competition (TransAlp mountain bike race) in a hot environment prevented body mass loss, enhanced thermoregulatory capacity, and improved competitive exercise performance in trained cyclists compared to when CHO only was consumed. This is one of the first studies to investigate the effects of a CHO-protein mixture on exercise performance and recovery in a real sporting context. Although it is essential that we carry out laboratory-based, controlled, randomized crossover studies, we should be cautious about making conclusions and recommendations on nutritional strategies based purely on the results of these studies, particularly if they only investigate responses during a single exercise bout.
Combined CHO-Protein Ingestion during Recovery
According to the prevailing view of Dr. Betts, the addition of protein to a CHO supplement to be consumed during recovery from exercise is unnecessary for two key reasons: 1) protein intake during recovery is sufficiently achieved through carefully timed meals consumed as part of the habitual diet and 2) providing that sufficient CHO is ingested, the addition of protein does not provide any additional benefit to CHO delivery and muscle glycogen resynthesis. In response to the first point, I agree that meals can be a viable option during recovery from exercise but several practical issues often make it very difficult and, in some cases, impossible for an athlete to consume a "carefully considered mixed meal" within the first hour after exercise. Traveling time, other training and work commitments, availability of foods and cooking facilities, and a reduced appetite postexercise are among the reasons why, in a real-life situation, athletes are unlikely to consume adequate protein through their habitual diet within the hour after exercise to maximize the recovery process. Therefore, reliance on the optimal timing and composition of an athlete's postexercise food intake is not a strong-enough reason to exclude protein from CHO-based recovery supplements.
The second point to address is the issue that protein does not provide any additional benefit in terms of CHO delivery and muscle glycogen resynthesis provided that sufficient CHO is ingested. However, as raised in the contrasting perspective, there seem to be no detrimental effects of reducing the amount of CHO provided in a supplement and replacing this with a source of protein or amino acids. From a practical perspective, it is not always feasible for an athlete to consume 1.0-1.2 g CHO·kg−1·h−1 during recovery from exercise. If athletes are competing or training again later that day, such large intakes of CHO may not be achieved, particularly as studies have reported gastrointestinal problems after the ingestion of such large amounts of CHO (15,38). Therefore, the coingestion of protein along with a smaller amount of CHO would be a more practical approach to postexercise feeding because similar rates of muscle glycogen resynthesis can be achieved, and as documented in the contrasting view, there are several other benefits of consuming protein in the postexercise period.
The composition of a sports nutrition supplement needs to be based on scientific evidence and on practical application and use. I believe that the addition of protein to CHO-based supplements results in a more "complete" nutritional product for athletes. Optimal muscle glycogen resynthesis can be achieved without the need for large and frequent CHO feedings during recovery, skeletal muscle growth, and repair is facilitated and rehydration may also be improved. These factors are important to both the endurance and strength athlete in considering the longer-term adaptations to training and not just the short-term performance effects. In conclusion, although the scientific evidence on performance is still conflicting, the addition of protein to a CHO-based supplement provides a more practical approach to optimizing the training and recovery of athletes, particularly those who train or compete in multiple sessions on the same or consecutive days.
Brain, Performance and Nutrition
School of Life Sciences
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