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Protein Requirements for Exercise

This is an excerpt from Sport Nutrition 4th Edition With HKPropel Access by Asker Jeukendrup & Michael Gleeson.

The protein requirements and the recommendations for protein intake for athletes have not been without controversy. Generally, scientists seem to be divided into two camps: those who believe that participation in exercise and sport increases the nutritional requirement for protein and those who believe that protein requirements for athletes and exercising people are no different from the requirements for sedentary people. Evidence has been found for both arguments. Although this issue may be scientifically relevant, from a practical perspective, the requirement for protein—as most often defined—may not be relevant to most athletes. The scientists who believe that protein requirements are greater for athletes and exercising people offer two explanations:

1. Amino acids may be oxidized during exercise.
2. Increased (net) protein synthesis is necessary to repair damage incurred during exercise and forms the basis of training adaptations, including increases in muscle mass with regular resistance exercise training.

Acute endurance exercise results in increased oxidation of the BCAAs leucine, isoleucine, and valine. Because these are essential amino acids that cannot be synthesized within the body, the implication is that they come from increased breakdown of proteins. Dietary protein requirements thus increase. Several studies using the nitrogen balance technique confirm that the dietary protein requirements for athletes involved in prolonged endurance training are higher than those for sedentary individuals (Houltham and Rowlands 2014; Tarnopolsky 2004). Based on nitrogen balance, it can be estimated that protein contributes about 5% to 15% to energy expenditure at rest. During exercise, in absolute terms, more amino acids may be oxidized. In relative terms, however, protein as a fuel is not important because of the much greater increase of carbohydrate and fat oxidation. Therefore, during prolonged exercise, the relative contribution of protein to energy expenditure is usually much lower than it is at rest; it is usually well below 5% of total energy expenditure. Only in extreme conditions when carbohydrate availability is limited (e.g., after a few hours of strenuous exercise that results in liver and muscle glycogen depletion) can the contribution of protein increase up to about 10% of total energy expenditure.

Nevertheless, it could be argued that the oxidation of the essential amino acids leucine, isoleucine, and valine is increased and, therefore, the requirements are increased. The counterarguments are that leucine oxidation does not represent overall total protein oxidation, and it grossly overestimates protein oxidation. For example, one study by Koopman and colleagues (2004) found an increase in leucine oxidation during prolonged endurance exercise but no change in phenylalanine oxidation, confirming that not all amino acids are oxidized to the same degree and suggesting that leucine oxidation overestimates protein oxidation. Furthermore, the oxidized amino acids do not appear to be derived from the degradation of myofibrillar proteins (Kasperek and Snider 1989). Finally, several nitrogen balance studies have not found differences or even improved nitrogen and leucine balances in active people (el-Khoury et al. 1997; Phillips et al. 2007).

After resistance exercise, muscle protein turnover increases because of an acceleration of protein synthesis and degradation. Muscle protein breakdown increases after resistance exercise but to a smaller degree than muscle protein synthesis provided that dietary protein intake is adequate. When amino acid availability is limiting (i.e., in the fasted state), the rate of muscle protein breakdown exceeds that of muscle protein synthesis, and net tissue protein gain does not occur. The elevations in protein degradation and synthesis are transient but are still present at 3 and 24 hours after exercise, although protein turnover returns to baseline levels after 48 hours. These results seem to apply to resistance exercise or dynamic exercise at a relatively high intensity. Low-intensity to moderate-intensity dynamic endurance exercise does not seem to have the same effects on muscle protein turnover, although studies have shown that endurance exercise may result in increased protein oxidation, especially during the later stages of very prolonged exercise and in conditions of glycogen depletion (Koopman et al. 2004).

Some studies have shown that the body adapts to training by becoming more efficient with protein (Butterfield and Calloway 1984; Phillips et al. 1999). Protein turnover decreases after training, and less net protein degradation occurs. In other words, after training, athletes become more efficient and waste less protein (Butterfield and Calloway 1984). Another study has demonstrated that BCAA oxidation at the same relative workload is the same in untrained and trained individuals (Lamont, McCullough, and Kalhan 1999). Although the protein requirement may increase initially, after adaptation to the training, this increase seems to disappear. This finding has been used as an argument that protein requirements are not greater in athletes.

Recommendations for Endurance Athletes

Although most researchers agree that exercise increases protein oxidation to some extent and this increased oxidation is accompanied by increased nitrogen losses, controversy persists over whether athletes have to eat more protein than less active people do. Nitrogen balance studies (Houltham and Rowlands 2014; Tarnopolsky 2004) show that endurance athletes need to eat about 1.2 to 1.4 g of protein per kilogram of body weight per day to maintain nitrogen balance. Furthermore, the application of the IAAO technique to endurance athletes (Kato et al. 2016) has indicated that the metabolic demand for protein on a high-volume training day is greater than current recommendations for athletes based primarily on nitrogen balance methodology (1.6-1.8 vs. 1.2-1.4 g protein per kg body weight per day, respectively). Several research groups claim that evidence supports the contention that athletes should eat more protein, whereas others believe that the evidence is insufficient to make such a statement.

One interesting observation is that training seems to have a protein-sparing effect. The better trained a person is, the lower the protein breakdown and oxidation are during exercise. The research groups that advocate increased protein intake for endurance athletes usually recommend an intake of 1.2 to 1.6 g of protein per kilogram of body weight per day (as opposed to the recommended intake of 0.8 g of protein per kilogram of body weight per day for the average person). Protein requirements for endurance athletes to achieve nitrogen balance are likely somewhere around 1.2 g per kilogram of body weight per day (Bolster et al. 2005) but could be as high as 1.6 g in individuals who engage in intense exercise (Houltham and Rowlands 2014; Kato et al. 2016; Tarnopolsky 2004). However, these amounts do not necessarily represent the optimal protein intake, which could

(1) support an athlete’s ability to repair and replace any damaged proteins (due potentially to oxidative stress or mechanical damage); (2) adaptively “remodel” proteins in muscle, bone, tendon, and ligaments to better withstand the mechanical stress imposed by athletic training and competition; (3) maintain optimal function of all metabolic pathways in which amino acids are participatory intermediates (which includes being oxidative fuels); (4) support increments in lean mass, if desired; (5) support an optimally functioning immune system; and (6) support the optimal rate of production of all plasma proteins required for optimal physiological function. If the protein “requirements” of athletes were sufficient to support all of the aforementioned processes, then the intake would not be a requirement to prevent deficiency but rather an intake that is “optimal” and would provide an adaptive advantage for athletes. (Phillips 2012)

Even if protein requirements are increased, omnivorous athletes have no problem achieving the elevated dietary protein intakes needed to maintain nitrogen balance. As an extreme example, we can look at the Tour de France. Cyclists in this event compete for 3 to 7 hours per day, and maintaining energy balance is often problematic (Jeukendrup, Craig, and Hawley 2000). Nevertheless, the athletes seem to have no problems maintaining nitrogen balance (Brouns et al. 1989a). With greater food intake, the intake of protein automatically increases because many food products contain at least some protein. A study by van Erp-Baart et al. (1989a) showed a linear relationship between energy intake and protein intake. Tour de France cyclists consumed 12% of their daily energy intake (26 MJ [6,214 kcal]) in the form of protein, and they easily met the suggested increased requirements (about 2.5 g per kilogram of body weight per day). These results demonstrate that if energy intake matches energy expenditure on a daily basis, endurance athletes do not need to supplement their diets with protein, although this conclusion may have to be modified for athletes who adhere to a vegetarian and particularly a vegan diet, as discussed later in this chapter. In reality, the timing of intake and the quality of protein may be more important factors than the amount.

Recommendations for Strength Athletes

Unlike endurance exercise, resistance exercise does not increase the rate of leucine oxidation to any major degree. The suggested increased dietary protein requirements are related to an increased need for amino acids as precursors for proteins being synthesized, resulting in increased muscle bulk (hypertrophy).

As with endurance exercise, the question of whether strength athletes have increased protein requirements is controversial. Nitrogen balance studies suggest that resistance athletes need about 1.5 g per kilogram of body weight per day, but these studies have been criticized because they were generally of short duration, and a steady-state situation may not be established in such circumstances (Rennie and Tipton 2000). Gontzea, Sutzeescu, and Dumitrache (1975) showed that the negative nitrogen balance used by many to indicate increased protein needs disappears after approximately 12 days of training (see figure 8.5). Note, however, that this study examined cycling exercise training rather than resistance training. The protein requirements, therefore, may be only temporarily elevated, but with a further increase in training load, the protein requirement is likely to increase again. The recommendation for protein intake for strength athletes is often 1.6 to 1.7 g per kilogram of body weight per day, and this is supported by a meta-analysis indicating that this amount of protein was optimal for lean body mass gains with resistance exercise training (Morton et al. 2018). Again, people seem to be able to meet this requirement easily with a normal diet, so they do not need extra protein intake. Protein supplements are often used, but they are not needed to meet the recommended protein intake, with the possible exception of some vegan athletes.

Recommendations for Game Players

Games such as football, rugby, and soccer can be thought of as a series of intermittent sprints separated by periods of less intense running or walking and that contain a substantial number of high-load, lengthening contractions with an eccentric bias when decelerating from fast running and jumping. What this means is that playing these games is similar to a very heavy resistance exercise bout that uses a lot of eccentric or plyometric movements (activities in which the muscle is lengthened during its activation) and generally results in sensations of muscle soreness in the 12- to 72-hour postexercise period (Baar and Heaton 2015; Heaton et al. 2017). This delayed onset muscle soreness (DOMS) results from inflammation of the muscles following exercise-induced damage to some muscle fibers. In other words, there is an injury stimulus that can lead to larger, stronger muscles given optimal recovery time. In the short term, however, there is a need for muscle repair and recovery of function because for several days postexercise, there is typically up to a 30% loss of muscle strength and power. Ingestion of amino acid mixtures or protein is known to facilitate earlier recovery of muscle function and reduce the degree of DOMS following exercise with a high eccentric component (Cockburn et al. 2012). For these reasons, additional protein intake is recommended at around 1.4 to 1.6 g per kilogram of body weight per day with emphasis on dietary protein ingestion in the immediate post-training or postgame period (Baar and Heaton 2015; Collins et al. 2021) for reasons that will be explained later in this chapter. For all athletes, no matter what their sport, the day-to-day protein needs will vary depending on the intensity and duration of training sessions and competition days, just as they do for carbohydrate and overall energy intakes.

Reported Protein Intake by Athletes

The literature contains several reports of protein intake by athletes in a variety of sports. These intakes are usually self-reported but generally give a good indication of nutritional habits and can reveal whether the athlete is achieving the recommended protein intake. In a study by van Erp-Baart et al. (1989b), protein intake in a variety of elite athletes was investigated. The lowest recorded intake was in a group of field hockey players, but their intake was still over 1.0 g per kilogram of body weight per day. The highest intakes were recorded for endurance cyclists, who consumed almost 3 g per kilogram of body weight per day, and bodybuilders, who consumed 2.5 g per kilogram of body weight per day. Some reports describe intakes below 0.8 g per kilogram of body weight per day in gymnasts and runners and well above 3.0 g per kilogram of body weight per day in weightlifters and bodybuilders. Because most athletes have protein intakes that exceed the daily recommendations (0.8-1.6 g per kilogram of body weight per day, depending on their activity level), the whole discussion about how much protein an athlete needs on a daily basis is rather academic. As previously indicated, factors such as the protein timing and type may be more important (this will be discussed in the Timing of Protein Intake and Type of Protein sections).

Athletes at Risk of Insufficient Protein Intake

People with extremely low protein intakes may suffer from protein deficiency, which can compromise function and ultimately lead to loss of body protein (atrophy). Certain groups of athletes are primarily recognized as being at risk from protein and energy deficiency: female runners, male wrestlers, boxers and other athletes in weight category sports, ski jumpers, male and female gymnasts, and female dancers. Although protein intake for these groups may be adequate on average, certain people within these groups may have protein intakes well below the RDA due to low energy intake.

Other groups that have been suggested to be at risk are vegetarian and vegan athletes. Plant food sources typically contain lower-quality proteins that have low levels of one or more essential amino acids (for a definition of high- and low-quality proteins, see chapter 1). In addition, the protein density (i.e., grams protein per 100 g) and digestibility of plant protein can be low compared with animal protein. Although some concern exists that vegetarian and vegan athletes may struggle to meet the protein requirements, the evidence for this is lacking, and adequate protein intake seems possible through a balanced vegetarian diet with the use of whey (if dairy-derived produce is allowed) or soy protein (if strictly vegan) isolate supplements where relatively high protein intakes are desirable. The deliberate avoidance of animal-based products (and in particular, meat) is also seen as a sustainability issue that benefits the environment, and we will revisit the use of animal versus plant and alternative sources of protein for athletes later in this chapter when we discuss the impact of the type of protein ingested on the muscle anabolic response to resistance training.

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