Muscle hypertrophy

Probably the reason most often cited for consuming a high-protein diet is to increase muscle mass, particularly among body builders, power lifters and other strength athletes. Strength and power athletes commonly eat well over 2 g protein/kg body mass per d and many well over 3 g protein/kg body mass per d⁽Reference Phillips^2, Reference Phillips⁵⁾. It is commonly accepted by many strength athletes that high protein intake is necessary for optimal muscle building. This belief is based on the metabolic response to protein ingestion following resistance exercise. The following discussion will be predicated on the assumption that appropriate training is included in any regime dedicated to increasing muscle mass and strength.

The metabolic mechanism for changes in muscle protein is net muscle protein balance (NBAL)⁽Reference Phillips^2, Reference Phillips^5–Reference Tipton and Witard⁸⁾. Muscle proteins, in fact all body proteins, are constantly being synthesised and degraded. Changes in muscle mass result from changes in the balance of the synthesis and breakdown of muscle myofibrillar, i.e. structural proteins, such as actin, myosin, troponin etc. The magnitude and duration of the positive periods of NBAL will determine the extent of muscle hypertrophy⁽Reference Phillips^5, Reference Burd, Tang and Moore^6, Reference Phillips^9, Reference Phillips, Moore and Tang¹⁰⁾. Nutritional intake and exercise are both potent modulators of the duration and magnitude of these periods of positive and negative NBAL. An in-depth discussion of these processes is beyond the scope of this review; interested readers should see one of the many excellent recent reviews on this topic⁽Reference Phillips^2, Reference Phillips^5–Reference Tipton and Witard^8, Reference Rennie, Selby and Atherton^11, Reference Rennie, Wackerhage and Spangenburg¹²⁾, including one from this symposium⁽Reference Phillips¹³⁾.

The rationale for the importance of a high-protein diet for muscle hypertrophy with training stems from the desire to provide amino acids following exercise to build more muscle proteins, thus increasing muscle hypertrophy⁽Reference Phillips²⁾. Resistance exercise increases the synthesis of muscle myofibrillar proteins⁽Reference Wilkinson, Phillips and Atherton¹⁴⁾. Sufficient amino acids must be consumed to support increased synthesis. The impact of resistance exercise on muscle protein synthesis and NBAL lasts for 48 h or more⁽Reference Phillips, Tipton and Aarsland¹⁵⁾. Thus, ingestion of protein within that time period results in the interactive effect on muscle protein synthesis and NBAL⁽Reference Burd, West and Staples¹⁶⁾ contributing to increased myofibrillar protein accretion. Any protein containing meals consumed within 24–48 h following a resistance exercise session will contribute to muscle hypertrophy. Therefore, the notion that ingesting greater amounts of protein will provide more substrate for synthesis of myofibrillar proteins leading to greater hypertrophy stems from the interactive response to resistance exercise and protein ingestion.

Another aspect of the rationale for high protein intake and muscle hypertrophy stems from the impact of exercise on muscle proteins. Resistance exercise, particularly with a strong eccentric component, leads to damaged muscle tissues and disruption of the myofibres⁽Reference Gibala, Interisano and Tarnopolsky^17, Reference Stupka, Tarnopolsky and Yardley¹⁸⁾. The increased rate of muscle protein breakdown following resistance exercise⁽Reference Phillips, Tipton and Aarsland^15, Reference Biolo, Maggi and Williams¹⁹⁾ likely results, at least in part, from the need to degrade these damaged proteins. Whereas the amino acids from these proteins are then reutilised for the increased rate of muscle protein synthesis, transport of amino acids from the blood into the muscle also is increased⁽Reference Biolo, Maggi and Williams^19, Reference Biolo, Tipton and Klein²⁰⁾. Therefore, a source of exogenous amino acids would enhance the ability of the muscle to rebuild and remodel the damaged proteins. Increasing the amount of ingested protein is thought to contribute the amino acids necessary, not only to build new protein, but also to rebuild and remodel damaged proteins such that the muscle function is enhanced⁽Reference Hather, Tesch and Buchanan^21, Reference Roig, O'Brien and Kirk²²⁾. However, there is some controversy as to the necessity for consumption of very large amounts of dietary protein as is often practised⁽Reference Phillips^2, Reference Tipton and Witard^8, Reference Rennie and Tipton²³⁾.

Elevated protein requirements for strength and power athletes, for the reasons discussed above, are often cited as a rationale for recommendations for high-protein diets. Protein requirements for strength and power athletes are usually said to be in the range of 1·5–2·0 g protein/kg body mass per d⁽Reference Phillips^2, Reference Phillips, Moore and Tang^10, Reference Campbell, Kreider and Ziegenfuss^24–Reference Rodriguez, Di Marco and Langley²⁶⁾. These numbers are derived primarily from carefully controlled N balance studies⁽Reference Lemon, Tarnopolsky and MacDougall^27–Reference Tarnopolsky, MacDougall and Atkinson²⁹⁾. Thus, the methodology used to establish the RDA for protein suggests that strength athletes need approximately double that amount of protein just to stay in N balance. Of course, if the athlete simply achieves N balance there will be no muscle hypertrophy. So, the requirements must be, and often are, considered to be the bare minimum and much more protein should be consumed to increase muscle mass. In fact, N balance studies on athletes suggest that the greater the protein intake, the greater the positive N balance, at least up to levels investigated to date⁽Reference Phillips²⁾. It follows that, assuming positive N balance leads to muscle hypertrophy, very-high-protein diets would lead to very large gains in muscle mass.

It should be noted that there seems to be a disconnect, both quantitative and qualitative, between positive N balance and muscle hypertrophy. An examination of the available data suggests that an intake of about 2·5 g protein/kg body mass per d may result in N retention of about 15 g N/d⁽Reference Phillips²⁾. If we assume that all of that N is converted into protein for increased muscle mass, admittedly an overestimation, then about 375 g of muscle would be deposited each day or about 137 kg in a year. An increase in lean body mass of even 10% of that number would be remarkable in already well-trained athletes. It is clear that short-term N balance data may be unreliable as an estimator of long-term gains in muscle mass, particularly at the high end of protein intake⁽Reference Phillips^2, Reference Tipton and Witard^8, Reference Young³⁰⁾. That theoretical argument is supported by data from a study conducted in the early 1990s. N balance was greater when bodybuilders consumed 2·62 g protein/kg body mass per d compared to 1·35 g protein/kg body mass per d⁽Reference Lemon, Tarnopolsky and MacDougall²⁷⁾. However, differences in muscle mass and strength were lacking. Thus, recommendations of very-high protein intakes based solely on N balance data should be considered somewhat problematic.

N balance data alone are not the only data used to support the necessity for high protein intakes. Recent, chronic feeding studies can be found to support both enhanced muscle hypertrophy with increased protein intake⁽Reference Burke, Chilibeck and Davidson³¹⁾ or to refute that notion⁽Reference Lemon, Tarnopolsky and MacDougall²⁷⁾. However, the equivocation is likely due to the inherent difficulties in conducting longitudinal training and feeding studies. The control of these studies is extremely difficult and small differences may not be detected⁽Reference Tipton and Witard⁸⁾. Moreover, a recent meta-analysis determined that increased protein intake does not enhance muscle hypertrophy with training⁽Reference Nissen and Sharp³²⁾. Moreover, there is evidence that increased protein intake increases basal levels of muscle protein synthesis⁽Reference Harber, Schenk and Barkan³³⁾. However, this study was not conducted in the context of training and the results may not apply. Moreover, the high protein intake was accompanied by decreased carbohydrate intake to maintain energy balance. Given the potential detrimental impact of suboptimal carbohydrate intake on resistance training⁽Reference Phillips, Moore and Tang¹⁰⁾, this situation may be untenable for maximal muscle hypertrophy (see later for a more extensive discussion of this topic). Therefore, it is difficult to utilise data from these types of studies to make firm conclusions, either in support of or to refute, concerning the efficacy of high protein intake for optimising muscle hypertrophy.

A logical extension to the argument that very-high protein intakes are necessary for muscle hypertrophy is that no hypertrophy will occur on lower protein intakes. This argument is patently false. Over 100 years ago, Chittenden⁽Reference Chittenden³⁴⁾ showed that muscle mass can be gained on protein intakes as low as 1 g protein/kg per d. There are more recent studies showing that protein intakes as low as 1·2 g protein/kg body mass per d are sufficient for increased muscle mass⁽Reference Moore, Del Bel and Nizi^35, Reference Hartman, Moore and Phillips³⁶⁾. Thus, it seems that support for high protein intakes in the scientific literature must be considered, at the very least, equivocal.

The argument contrary to recommendations that high protein intakes are not necessary for maximal hypertrophy may be made considering the evidence that exercise training increases the efficiency of utilisation of amino acids from ingested protein. Elegant studies from the laboratory of the late Gail Butterfield in the 1980s showed that exercise increased amino acid utilisation⁽Reference Butterfield and Calloway^37, Reference Todd, Butterfield and Calloway³⁸⁾. However, the applicability of these results to intense resistance exercise training may be criticised on the basis of the low-exercise intensities used in those studies⁽Reference Tarnopolsky³⁹⁾. Recently, two investigations utilised a 12-week, longitudinal training design to investigate this question. Both demonstrated that not only was lean body mass gained on 1·2–1·4 g protein/kg body mass per d, but that retention of ingested N was greater following training than before training⁽Reference Moore, Del Bel and Nizi^35, Reference Hartman, Moore and Phillips³⁶⁾. The authors suggested that the anabolic stimulus of the resistance exercise results in greater disposal of the amino acids due to the increase in muscle protein synthesis following each exercise bout⁽Reference Phillips, Tipton and Aarsland^15, Reference Biolo, Maggi and Williams¹⁹⁾. Thus, greater efficiency of N utilisation with training may actually decrease the need for protein making very-high-protein diets quite unnecessary for weightlifters.

Another consideration that may impact a decision on the efficacy of high-protein diets for muscle hypertrophy is the amount of ingested protein that provides the maximal anabolic response after an exercise bout. Since the acute interactive response to protein ingestion and exercise ultimately leads to accumulation of muscle during training⁽Reference Tipton and Witard^8, Reference Phillips^9, Reference Hawley, Tipton and Millard-Stafford⁴⁰⁾, the optimal acute response is critical. Recent data suggest that ingestion of 20–25 g protein results in maximal rates of muscle protein synthesis following resistance exercise⁽Reference Moore, Robinson and Fry⁴¹⁾. This amount of protein provides about 8·5 g of essential amino acids, roughly the amount that maximally stimulated protein synthesis at rest in both young and elderly subjects⁽Reference Cuthbertson, Smith and Babraj⁴²⁾. Thus, it seems that the amount of protein necessary to maximally stimulate muscle protein synthesis is similar, if not slightly less, following exercise than at rest. Moreover, at higher doses of ingested protein, amino acid oxidation was increased⁽Reference Moore, Robinson and Fry⁴¹⁾ suggesting that the excess amino acids are not used for other amino acid requiring processes in the body. If we assume that this response is similar for each meal, then it seems that about 100 g of high-quality protein would be what is required to optimise the anabolic response⁽Reference Phillips, Moore and Tang¹⁰⁾. If those feedings were solely responsible for muscle hypertrophy, these data could be interpreted to suggest that about 1·25 g protein/kg body mass per d is optimal; roughly the amount determined in N balance studies. Then again, this supposition does not include any protein ingested at other times. It is clear that the anabolic response is not the same at all times of day and at all times in relation to exercise⁽Reference Tipton and Witard^8, Reference Phillips, Moore and Tang¹⁰⁾. Thus, the timing of protein intake will influence the overall anabolic response such that hypertrophy may be quite different despite identical total protein intakes. Moreover, it is known that the anabolic response is refractory to continuous hyperaminoacidemia, at least at rest⁽Reference Bohe, Low and Wolfe⁴³⁾. However, muscle protein synthesis does respond to successive bolus ingestions of amino acids after resistance exercise⁽Reference Borsheim, Tipton and Wolf^44, Reference Miller, Tipton and Chinkes⁴⁵⁾. Therefore, it could be argued that the optimal amount of protein ingested per day is greater than 100 g, but the optimal amount is certainly unknown. Moreover, the type of protein⁽Reference Tang, Moore and Kujbida^46–Reference Tipton, Elliott and Cree⁴⁹⁾, timing of protein ingestion and the combination of the type and timing⁽Reference Tipton, Elliott and Cree^49, Reference Tipton, Rasmussen and Miller⁵⁰⁾, as well as other factors such as type and intensity of exercise⁽Reference Burd, West and Staples¹⁶⁾ determine the actual anabolic response. Thus, determination of the optimal amount of dietary protein to maximise muscle hypertrophy is much more complex than simply naming an amount of protein for all situations⁽Reference Tipton and Witard⁸⁾. More research must be done to determine, definitively, the optimal amount of protein ingested to maximise muscle hypertrophy in various training and nutritional situations.

In summary, it is clear that ingestion of protein following exercise stimulates muscle myofibrillar protein synthesis resulting in positive NBAL. This response is the metabolic mechanism for increased muscle mass with training. Evidence from short-term N balance studies suggests that increasing amounts of protein result in continually increasing positive N balance. These results are often interpreted to mean that very high levels of protein in the diet result in superior muscle gains with training. However, issues with N balance measurements, particularly at high protein intakes, increased efficiency of protein utilisation with training and limits to the acute anabolic response to each protein ingestion can be used to argue that very high amounts of protein are unnecessary and the extra protein is oxidised for energy. Properly controlled longitudinal, endpoint studies showing that increased protein intake results in increased muscle growth are rare and, at best, the results of such studies can only be considered equivocal. Additionally, it is probably overly simplistic to assume that a given amount of protein would be optimal in all training and feeding situations and for all individuals. There are many factors, e.g. timing of protein intake, quality of protein source, co-ingestion of other nutrients that influence the anabolic response of muscle to protein. Thus, the amount of protein in the diet may not be the most important factor to consider⁽Reference Phillips^5–Reference Tipton and Witard⁸⁾. On the other hand, given that there may be some advantage to higher protein intakes, what is the down side? If there are no negative repercussions, then erring on the side of elevated protein intake may be advisable. This determination should be made for each individual given that person's particular training situation and exercise goals. The risks of high protein intakes will be discussed below.

Hypoenergetic weight loss

Another situation in which high protein intakes (usually in the range of 30% of total energy or greater) are widely recommended is during energy-restricted weight loss. There is mounting evidence that diets high in protein result in increased weight and fat loss during hypoenergetic dieting in overweight and obese individuals⁽Reference Phillips^5, Reference Layman, Boileau and Erickson^51–Reference Wycherley, Noakes and Clifton⁵⁶⁾. In these studies, dietary protein was typically increased at the expense of carbohydrate. Often, energy intake was reduced simply by reducing carbohydrate intake resulting in relatively greater protein intake. Moreover, addition of exercise enhanced the weight loss with high-protein diets⁽Reference Phillips^5, Reference Layman, Boileau and Erickson^51–Reference Wycherley, Noakes and Clifton⁵⁶⁾. Perhaps more importantly, the loss of lean mass during hypoenergetic weight loss is reduced with higher protein intakes⁽Reference Phillips^5, Reference Layman, Boileau and Erickson^51–Reference Wycherley, Noakes and Clifton⁵⁶⁾. Given the increase in NBAL following exercise⁽Reference Phillips, Tipton and Aarsland^15, Reference Biolo, Maggi and Williams¹⁹⁾ and the known response of muscle protein metabolism to protein intake following exercise⁽Reference Biolo, Tipton and Klein^20, Reference Tipton, Ferrando and Phillips^47–Reference Tipton, Elliott and Cree⁴⁹⁾, the decrease in muscle loss should not be surprising.

The mechanisms for the increased fat and weight loss and decreased muscle loss with higher-protein diets are not entirely clear. Ultimately, the first law of thermodynamics means that energy is energy. However, it should be remembered that gross energy is not the same as metabolisable energy⁽Reference Buchholz and Schoeller⁵⁷⁾. Increased thermic effect of food, increased gluconeogenesis, substrate cycling and increased protein synthesis may all be involved in increased energy expenditure with high-protein v. high-carbohydrate intake⁽Reference Phillips^5, Reference Layman^58–Reference Feinman and Fine⁶⁰⁾. However, Buchholz and Schoeller⁽Reference Buchholz and Schoeller⁵⁷⁾ suggest that, whereas these processes may contribute to greater fat loss, it cannot be explained solely by these increases⁽Reference Buchholz and Schoeller⁵⁷⁾. More recently, Feinman and Fine⁽Reference Feinman and Fine⁶⁰⁾ put forward the argument that these explanations are based on equilibrium thermodynamics, but living organisms are far from equilibrium⁽Reference Feinman and Fine⁶⁰⁾. Thus, non-equilibrium thermodynamics due to variable substrate fluxes has been suggested to help explain the differences.

The leucine content of the protein may be important for the increased fat loss and retention of lean body mass during weight loss dieting⁽Reference Layman⁵⁸⁾. The notion that muscle protein synthesis is increased by the leucine in the protein is behind this proposed mechanism⁽Reference Anthony, Anthony and Layman⁶¹⁾. Muscle protein synthesis is an energetically expensive process. It is estimated that the addition of each amino acid during elongation requires 4 m ATP⁽Reference Browne and Proud⁶²⁾. Given the limited energy available inherent to hypoenergetic dieting, the energy must come from endogenous stores. Since these diets are low-carbohydrate, the energy likely comes from fat oxidation. Moreover, there is evidence that protein intake stimulates fat oxidation to a greater extent than other nutrients⁽Reference Labayen, Diez and Parra⁶³⁾ and fat oxidation is greater following high protein intakes during energy restricted weight loss⁽Reference Labayen, Diez and Gonzalez^64, Reference Labayen, Diez and Parra⁶⁵⁾. Moreover, exercise-induced fat oxidation is greater following high protein intake⁽Reference Soenen, Plasqui and Smeets⁶⁶⁾. Thus, fat oxidation is increased by the ingestion of protein, per se, and to supply energy for protein synthesis. Furthermore, as the diet is successful in preserving muscle mass, more energy will be necessary, further separating the muscle loss from a lower-protein situation.

In addition to increased weight and fat loss with high protein intakes, it seems that elevated protein helps reduce weight regain following weight loss⁽Reference Lejeune, Kovacs and Westerterp-Plantenga^67, Reference Westerterp-Plantenga, Lejeune and Nijs⁶⁸⁾. Moreover, and perhaps more importantly, the weight gained after 6 months of high protein intake was entirely lean body mass, whereas weight regain on habitual dietary intake was both lean and fat mass⁽Reference Lejeune, Kovacs and Westerterp-Plantenga^67, Reference Westerterp-Plantenga, Lejeune and Nijs⁶⁸⁾. Interestingly, these differences were apparent despite a relatively modest difference in protein intake. The difference in protein intake between groups was only about 3% of total energy⁽Reference Lejeune, Kovacs and Westerterp-Plantenga^67, Reference Westerterp-Plantenga, Lejeune and Nijs⁶⁸⁾. Furthermore, these results suggest another mechanism unlikely to be apparent in the studies mentioned earlier involving clamped energy intake. Satiety was greater in the protein group than the habitual energy group⁽Reference Lejeune, Kovacs and Westerterp-Plantenga⁶⁷⁾. Increased satiety has been shown to result from acute protein consumption and high-protein diets⁽Reference Veldhorst, Smeets and Soenen^69–Reference Veldhorst, Westerterp and van Vught⁷²⁾. Satiety measured in the post-absorptive state may be attributed to increased resting energy expenditure⁽Reference Westerterp-Plantenga, Lejeune and Nijs^68–Reference Westerterp-Plantenga, Nieuwenhuizen and Tome⁷⁰⁾. Interestingly, satiety may be influenced by the type of protein ingested⁽Reference Veldhorst, Smeets and Soenen⁶⁹⁾, as well as the sex of the individual⁽Reference Westerterp-Plantenga, Lejeune and Smeets⁷³⁾. Thus, higher protein intake not only leads to increased weight loss during hypoenergetic dieting but also contributes to better maintenance of weight loss when otherwise habitual energy levels are consumed in obese and overweight individuals.

Whereas there is ample evidence for amelioration of lean body mass loss during hypoenergetic weight loss in overweight and obese populations consuming high-protein diets, there is relatively little information available on healthy athletes and exercisers. The metabolic and training status of athletic individuals differs from that of obese and overweight, particularly sedentary, individuals. Thus, the metabolic situation is different and may impact the response to high-protein hypoenergetic diets. We recently demonstrated that a high-protein diet maintained muscle mass during rather severe energy restriction for 2 weeks in weightlifters⁽Reference Mettler, Mitchell and Tipton⁷⁴⁾. These athletes consumed approximately 60% of their normal energy intake, while maintaining their normal training load. One group consumed their habitual macronutrient composition and the other increased their protein intake (about 2·3 g protein/kg body mass per d or 35% of total energy) during the 2 weeks of hypoenergetic dieting. The high-protein group lost much less lean mass than the control group. However, unlike the situation in obese and overweight individuals⁽Reference Layman, Boileau and Erickson^51, Reference Meckling and Sherfey^53, Reference Kerksick, Thomas and Campbell^75, Reference Layman, Evans and Baum⁷⁶⁾, the total weight loss was greater in the group that maintained their habitual dietary composition⁽Reference Mettler, Mitchell and Tipton⁷⁴⁾. Thus, for athletes desiring more total mass loss without regard for composition of the mass that is lost, high protein intake may not be the best choice. Whereas this situation is unlikely to apply to most athletes, there are scenarios, e.g. a climber in cycling, in which this approach may be considered. Alternately, if preservation of lean body mass is the main goal, increased protein intake seems advisable. Moreover, the amount of fat lost by the high protein and control groups was almost identical. Similar results were reported previously⁽Reference Walberg, Leidy and Sturgill⁷⁷⁾. It seems that the pattern of body composition changes during hyopenergetic weight loss may be different for lean exercisers than for overweight or obese individuals even when the latter are exercising⁽Reference Layman, Boileau and Erickson^51, Reference Meckling and Sherfey^53, Reference Kerksick, Thomas and Campbell^75, Reference Layman, Evans and Baum⁷⁶⁾.

The differences in body composition changes between overweight and obese individuals and exercisers may be due to inherent metabolic differences between the two groups or due to methodological differences in studies. Dietary fat oxidation is inversely proportional to body fatness⁽Reference Forbes^78, Reference Hall⁷⁹⁾. Another factor may be the composition of the hypoenergetic diets. In studies with overweight populations, protein intake often is increased at the expense of carbohydrate intake⁽Reference Layman, Boileau and Erickson^51, Reference Meckling and Sherfey^53, Reference Kerksick, Thomas and Campbell^75, Reference Layman, Evans and Baum⁷⁶⁾. This switch would be untenable to an athlete desiring to maintain training quality and quantity⁽Reference Phillips, Moore and Tang^10, Reference Burke, Kiens and Ivy⁸⁰⁾. Therefore, in our study, protein intake was increased at the expense of fat intake⁽Reference Mettler, Mitchell and Tipton⁷⁴⁾. It is conceivable that the relatively greater carbohydrate intake in the obese subjects decreased fat oxidation in those studies⁽Reference Layman, Boileau and Erickson^51, Reference Meckling and Sherfey^53, Reference Kerksick, Thomas and Campbell^75, Reference Layman, Evans and Baum⁷⁶⁾, but since carbohydrate intake was similar in both groups no such difference was noted in our study⁽Reference Mettler, Mitchell and Tipton⁷⁴⁾. Thus, macronutrient composition may explain greater fat and weight loss with high-protein diets in sedentary, overweight v. lean, athletic individuals.

Otherwise or in addition to dietary differences, differences in the exercise performed by the subjects also may contribute to differences noted between athletes and overweight individuals during weight loss. Whereas the studies of overweight individuals often included some fairly limited resistance exercise⁽Reference Meckling and Sherfey^53, Reference Kerksick, Thomas and Campbell^75, Reference Layman, Evans and Baum⁷⁶⁾, the weightlifters in our study maintained a fairly intense training load, including a high volume of resistance exercise training, during the 2 weeks of weight loss⁽Reference Mettler, Mitchell and Tipton⁷⁴⁾. Given the inherent stimulation of muscle protein synthesis by resistance exercise⁽Reference Phillips, Tipton and Aarsland^15, Reference Biolo, Maggi and Williams¹⁹⁾ and the interactive effect of protein ingestion with the resistance exercise for up to 24 h after the exercise⁽Reference Burd, West and Staples^16, Reference Biolo, Tipton and Klein²⁰⁾, the greater volume of exercise⁽Reference Burd, Holwerda and Selby⁸¹⁾ likely resulted in more periods of protein deposition in the trained individuals contributing to the maintenance of muscle mass. Moreover, it is clear that the intensity of the resistance exercise must be high to fully stimulate muscle protein synthesis⁽Reference Burd, West and Staples^16, Reference Kumar, Selby and Rankin⁸²⁾. Thus, the differences in the distribution of weight loss between overweight and athletic populations may be due to any of several factors and is almost certainly multifactorial.

Recovery from intense exercise

Another situation in which high-protein diets have been advocated is for recovery from intense exercise. Intense exercise, particularly exercise with a high-eccentric component, may lead to decrements in the ability to perform subsequent exercise sessions, muscle soreness and damage to muscle proteins⁽Reference Howatson and van Someren⁸³⁾. High protein intake during recovery has been advocated to ameliorate these detrimental consequences of intense exercise.

The most common model used to investigate impact of dietary manipulations on the response to intense exercise is an eccentric exercise, muscle damage model. Recently, we investigated the efficacy of short-term, i.e. 72 h, high protein intake on indices of recovery from intense eccentric exercise. In these studies, we found that an increase in protein intake from about 1·5 to about 2·0 g protein/kg body mass per d had no impact on recovery of muscle function, soreness or plasma creatine kinase concentration, often touted as an indicator of muscle damage (OC Witard, SR Jackman and KD Tipton unpublished results). However, when protein intake was increased to 3 g protein/kg body mass per d, muscle soreness was decreased in the immediate aftermath, i.e. during the first 24 h, of the exercise bout (SR Jackman, OC Witard and KD Tipton, unpublished results). The amelioration of muscle soreness may be due to the branched-chain amino acid content of the extra protein⁽Reference Jackman, Witard and Jeukendrup⁸⁴⁾; however, the physiological mechanism remains to be elucidated.

The practical implications of these data are not clear. Muscle soreness over the entire 72 h period was no better with high protein intake. Furthermore, there was no impact on creatine kinase or muscle function, arguably the most important aspect of recovery. Finally, the subjects in those studies were untrained. It is not clear if the impact of the protein would have been similar in trained individuals, more accustomed to high-intensity exercise. Thus, the efficacy of high protein intake for recovery from intense eccentric exercise seems rather limited, even at a rather high level of protein intake.

Intense training sessions repeated over a prolonged period of time may result in overreaching or overtraining⁽Reference Jeukendrup, Hesselink and Snyder^85, Reference Halson and Jeukendrup⁸⁶⁾. This situation may result in detriments in the ability to perform exercise and thus may impair the effectiveness of training or competitive performance. We recently investigated the impact of a high-protein diet on emotional responses, immune function and decrements in performance with a week of intensified training⁽Reference Witard, Jackman and Kies⁸⁷⁾. Cycling time-trial performance, immune markers and mood state deteriorated in trained cyclists after a week of training at approximately double the volume and intensity of normal training. Doubling the protein intake to 3 g protein/kg body mass per d had a possibly beneficial effect on time trial performance as assessed by inferential statistics⁽Reference Batterham and Hopkins^88, Reference Hopkins, Marshall and Batterham⁸⁹⁾. However, the effect could not be explained by any physiological indices we measured. Thus, it seems, it may be due to a central effect. Whereas, the mood state of the cyclists deteriorated in both trials, the high protein intake clearly ameliorated this effect. Moreover, there were indications that immune impairment with intense training was somewhat alleviated by the high protein intake. Our data suggest that high protein intake may help athletes tolerate intense training.

The improvements in tolerance of increased training loads noted in our study⁽Reference Witard, Jackman and Kies⁸⁷⁾ must be considered in context before these data are used to make recommendations. First, the amount of protein consumed was very high and, anecdotally, was difficult to tolerate for many of the cyclists. Very few endurance athletes consume such large amounts of protein⁽Reference Tarnopolsky^39, Reference Burke, Millet and Tarnopolsky⁹⁰⁾. Moreover, in order to maintain energy balance, it was necessary to clamp carbohydrate intake at 6 g carbohydrate/kg body mass per d. That level of carbohydrate intake is below the recommendation for that level of training⁽Reference Burke, Millet and Tarnopolsky^90, Reference Burke, Loucks and Broad⁹¹⁾. Thus, our results must be considered in the context of a relatively low carbohydrate intake for that situation. It remains to be seen if a lower, more practical, level of protein intake would engender similar results and/or if those results would be similar with sufficient carbohydrate intake. Nevertheless, these studies offer the suggestion that perhaps high protein intake is supportive of intense training. The mechanisms for any impact of protein remain to be elucidated.