Dietary Protein Demands for Muscle and Fitness

For information purposes only. Exercise at your own risk

by Huw Williams

‘You have to Eat Muscle to Make Muscle’

From all areas of sport and exercise, there are many factors that contribute to a successful overall performance. According to Williams (1994), ‘physical performance is mainly a function of an individual’s size, shape, sex, and age’. Although these factors stated by Williams play a large part in the overall performance of an athlete, success in sport at a performance level depends on other controllable aspects.

Athletes at all competitive levels speak about embracing the ideal of sport; success through hard work, or simply doing the best that you physically can. However, this ideal does not match the current reality of sport at a performance level. With the genetics endowed to you aside, athletes are turning to more extrinsic methods to enhance their performance, found in the form of ergogenic aids.

With the current high levels of media interest being given to the area of performance enhancement techniques, athletes are searching for ergogenic aids that do not have side effects or illegal properties present. This is where nutritional ergogenic aids, including carbohydrate, creatine, and dietary antioxidants, have provided promising alternatives. The ergogenic benefits from an increased carbohydrate diet, or an increase in creatine intake, are widely supported by a plethora of research; but what about protein?

The theory on the recommended dietary intake of protein for an athlete has changed within the past decade with the increase in sport- specific nutritional knowledge. Past theory has suggested that the normal Recommended Daily Intake (RDA) of protein is adequate for persons undertaking different sport and exercises. This was indicated by Butterfield (1986), who stated that ‘physical activity does not significantly alter dietary protein requirements’. This opinion has been challenged by current research which has shown benefit to athletes in ingesting increased amounts of protein in their diet. On this issue, Lemon (1992) stated that ‘regular exercise leads to an increased dietary requirement for protein’.

This paper will look at this area in detail, and identify the current protein requirements for muscle fitness. It will show the current theory regarding the physiological adaptations of muscle that may result from an increased amount of protein ingested in athlete’s diets, and discuss whether this will prove beneficial to the athlete. As we know, different sports and exercise events have differing nutritional requirements, so a section of this paper will identify differing sport categories and their relationship between protein intake and muscular performance


1. The Use of Dietary Protein in the Body
Often, the need and importance of protein in the diet gets overlooked, and more attention is given to more ‘fashionable’ and more documented parts of the athlete’s diet such as carbohydrates. Its use and benefits in the diet are often overshadowed by those of carbohydrates or creatine. Protein’s once unfashionable status as a performance enhancer is rapidly changing with increasing opinion on its potential benefits. Its importance in normal everyday functions is undisputed, and whether you’re an athlete or not, it is a vital part of the diet. Protein is primarily composed of highly complex chains of amino acids of which there are approximately 20 different types. These amino acid chains are made up of oxygen, carbon, hydrogen, nitrogen, and occasionally sulphur, A good intake of protein is essential in the diet as the essential amino acids are vital to many bodily functions, but have to be consumed due to the body’s inability to produce them. In most countries, the recommended daily allowance (RDA) for protein is currently 0.8g/kg/day for adults. This value is increased slightly for woman who are pregnant (1.0g/kg/day), adolescents (0.9 – 1.0g/kg/day), and even more for children (1.0 – 1.2g/kg/day). As the majority of early studies carried out in the area of dietary protein requirements for exercise have used sedentary or untrained subjects, the protein requirements for athletes undertaking intense exercise programmes cannot be based on these findings. This has led the theory that ‘a normal protein intake is adequate for an athletes diet’ to be challenged by health professionals.

Protein Quality: A Problem for Vegetarians & Vegans?
Common theory had once stated that a diet not consisting of meat would be harmful due to the lack of protein that would be consumed in the diet. As we know, there are numerous sources of protein that can be consumed by a vegan or vegetarian, but is this statement totally without truth? To an endurance or resistance athlete, the amount of protein that they consume is important. The quality of this protein is also a factor that needs to be addressed. Protein sources have been classed under two titles; High quality, and low quality proteins. Protein quality is usually defined according to the amino acid pattern of egg protein, which is regarded as the ideal. As such, it is not surprising that animal proteins, such as meat, milk and cheese tend to be of a higher protein quality than plant proteins. This is why plant proteins are sometimes referred to as low quality proteins. Many plant proteins are lacking in one or another of the essential amino acids. For instance, grains tend to be short of lysine whilst pulses are short of methionine. This does not necessarily mean that vegetarians and vegans go short on essential amino acids. Combining plant proteins, such as a grain with a pulse, leads to a high quality protein which is just as good, and in some cases better, than protein from animal foods. It is the knowledge to combine food groups, know as protein complementing, that will give the athlete an adequate protein intake. So to address the opening line of the paper, to gain an adequate protein intake, you don’t have to eat muscle to make muscle.

As stated previously, protein pays a large part in many functions in the human body. They include growth, repair, muscle contraction, immune protection, and transmission of nerve pulses. Its importance in the areas of growth and muscle contraction are focussed on by performance athletes, and are going to form the basis for the rest of the essay.

2. Muscle Growth & Protein.
Due to the rise in nutritional awareness of the relationship between muscle growth and protein intake, and the influx of protein rich supplements, performance athletes are now more aware of the benefits that the correct amount of protein gives them. For some sporting events, this may give them a vital edge over their opponents.

According to Tipton et al (2001), ‘the metabolic basis for skeletal muscle growth lies in the relationship between the rates of muscle protein synthesis and muscle protein breakdown’. If we are to understand the process of muscle growth, the responses of muscle to exercise and nutrition, and their interactions need to be clarified. It is clear that exercise has an affect on protein metabolism, often resulting in muscle growth, but the plethora of research in the area is still showing uncertainty regarding its influencing factors. On this issue, Tipton et al (2001) stated that ‘although the responses of muscle protein metabolism to nutrition and exercise have been studied for over a century, there are still significant gaps in our understanding of the metabolic mechanisms behind exercise-induced muscle hypertrophy’. Although most adults are in a net state of muscle protein balance, it is now common knowledge that excessive exercise training over a period of time will result in muscle hypertrophy. This increase in muscle hypertrophy will only occur if the net balance of protein is positive. Therefore, levels of muscle protein will fluctuate from positive to negative, but muscle hypertrophy will only occur if the overall summation of protein balance is positive. This raises issues with regard to increased dietary protein intake in an athlete’s diet, but I will address this in a later chapter.

So to what extent does the fluctuation of the basal level of muscle protein effect muscle hypertrophy? Are the affecting factors different between endurance and resistance athletes?

Dietary Protein Requirements: Resistance Exercise
On this issue, Tipton et al (2001) stated that ‘the exact response seems to be dictated by the particular stimulus provided by the bout of exercise, namely, the type of exercise and/or the intensity at which it is performed’. This was supported by Maughn (2002), who stated that ‘it has been reported that a stimulation of protein synthesis in samples of muscle containing mixtures or various proteins results from an acute bout of resistance exercise’. However, similar studies carried out by Roy et al (1997), and Tipton et al (1996) found ‘no increase in muscle protein synthesis following resistance exercise’. One possible reason for the discrepancy found between the conclusions is the training state of the subjects. The subjects used in the Roy and Tipton studies were resistance trained, whereas the subjects in the Maughn study were untrained. It seems that muscle protein synthesis will occur in athletes, but only if the intensity of the exercise is adequate. This was highlighted by the increase in protein synthesis in the untrained subjects, but not in the trained subjects. The training status of the participants is likely to be a major factor in the majority of studies carried out.

This has been addressed to some degree by Tarnopolsky et al (1995) who investigated the effects of dietary intake of protein on strength and body composition in untrained and trained men in a randomised study. Six sedentary and 7 resistance trained athletes ingested 0.86, 1.4 and 2.4g/kg/day of protein for 13 days separated by an 8 day washout period. Resistance athletes were found to have a greater daily requirement for protein of 1.4g/kg/day. However, increased protein intake did not influence changes in fat-free mass in either group. These findings suggest that resistance-trained athletes may need between 1.7 and 1.8g/kg/day of protein in order to ensure nitrogen balance, but ingesting protein above this level does not promote muscle growth. These findings are supported by Lemon (1992) who stated that ‘it has been shown that the optimal protein intake for strength athletes is greater than 1.4 but less than 2.4g/kg/day’.
It is a commonly held belief amongst resistance trained athletes that in order to improve muscle mass, their diet must include a large amount of protein. This theory amongst athletes has resulted in an influx of protein supplements, with the majority being composed of low quality amino acids. Although above studies have shown that athletes who undertake resistance exercise may have greater protein requirements than their untrained counterparts, most athletes who eat enough to maintain energy balance would be expected to achieve these recommendations (Kreider 1999). These findings were not supported by an earlier study carried out by Lemon (Lemon 1992). During that study which included 12 weeks of strength training in untrained, elderly men, greater gains in thigh muscle area and greater total urinary creatinine were observed in the men who consumed a dietary supplement containing 23g of protein than in the men who consumed their normal diet (Lemon 1992). In summary of the study, the author stated that ‘it is interesting to note that despite these greater muscle size gains, there appeared to be no greater gains in strength and it is unclear whether these individuals actually benefited from the additional protein or from the additional energy in the supplement’. The author gave no possible reason or explanation for a lack of muscle strength in the increased muscle mass.

So far, studies have shown that increased dietary protein ingestion can lead to increased muscle hypertrophy. The possibility of increased muscle hypertrophy with the ingestion of amino acid proteins after a resistance session has also been studied.

Amino Acid Ingestion after Resistance Exercise
Tipton et al (1999) tested the hypothesis that oral ingestion of an amino acid solution would augment net muscle protein synthesis following an acute bout of resistance exercise in subjects. Three men and three woman performed heavy leg resistance exercises on three occasions. Starting 45 minutes after exercise, subjects ingested 1 litre of a drink containing either a placebo, 40g of essential amino acids, or 40g of mixed amino acids at a rate of 100ml every 18 – 20 minutes. In fact, the amino acid trial caused a significant increase in the levels of all three amino acids measured in arterial blood; however, only the essential amino acid mixture resulted in a significant increase in their intramuscular concentrations. One possible reason for only one of the amino acid solutions resulting in an increase in the three intramuscular amino acids could be hyperinsulinaemia. This, according to Biolo (1995), has been shown to stimulate protein anabolism in humans. This was dismissed by Tipton et al (1999) who stated that ‘plasma insulin concentrations were low and not significantly different among the three treatments during a 45 minute period following beverage ingestion’. The author concluded the study by stating that ‘the results showed no significant differences between the three conditions in muscle protein synthesis or protein breakdown following exercise’. However, the study showed that net protein balance was significantly higher during both amino acid trials compared to the placebo. During the placebo trial, net balance was negative but was anabolic in the individuals ingesting amino acid solutions.

As stated previously, muscle hypertrophy can only occur when protein net balance is positive in an anabolic state, so the ingestion of essential amino acids following resistance exercise can lead to muscle anabolism.

Dietary Protein Requirements: Endurance Exercise
According to Tipton et al (2001), ‘the response of mixed muscle protein synthesis to a bout of endurance exercise may not be the same as to resistance exercise’. Currently, there is only limited research available in the area of protein synthesis following endurance exercise with two large studies again showing differing results and conclusions. The study by Carraro et al (1990) found that ‘muscle protein synthesis in untrained volunteers was increased following walking on a treadmill for 4 hours’. This was compared to ‘an inability to find a significant increase in muscle protein synthesis in the deltoid muscle following an intense, interval swim workout’ (Tipton et al, 1996). This could also be attributed to training status, in a similar way to that seen with resistance training but these results may be influenced by the intensity of the exercise. In the two studies carried out the subjects performed at differing intensities.

The intensity at which the activity is performed has been found to affect the rate at which the muscle protein is synthesised. According to Kreider (1999), ‘the longer the exercise bout or the more intense the exercise, the longer the rate of muscle protein synthesis is reduced’. This was highlighted in an early study carried out by Dohm et al (1980), who stated that ‘muscle protein synthesis was reduced by 71% in rats run at 28m/min to exhaustion, but when run for only 1 hour, muscle protein synthesis was only reduced by 30%’,

These studies highlight the phenomenon that muscle hypertrophy is affected by the rate at which the protein is synthesised by the body. Although the physiological adaptations incurred by the body through undertaking these two types of exercise differ, it seems that in both cases intensity and training status of the athletes involved determined the rate at which muscle protein is synthesised. The evidence from the studies highlighted above has shown that the training status and intensity of the activity undertaken are major factors in possible muscle hypertrophy.

Conclusion
The importance of protein is vital in any person’s diet., The vital amino acids that are found in protein molecules are important to a persons everyday health, whether they are an athlete or not. So do athletes need more protein in their diet to ensure muscle fitness? Past theory stating that a normal dietary intake of protein is adequate for a resistance training athlete has been found to be incorrect. Increased protein intake has been shown to increase muscle hypertrophy in resistance trained athletes. This increased amount of dietary protein required for an acceleration in muscle hypertrophy has been found to be only up to 2.4g/kg/day, as opposed to 0.8 – 1.2g/kg/day for untrained individuals. An increase in dietary protein intake above 2.4kg/day does not increase the rate of muscle hypertrophy further. For resistance athletes, whose aim is not to increase their muscle mass just to maintain what they have, a normal dietary intake of protein should keep the net balance of protein positive. Another important factor to consider is the effect of intake of amino acids after resistance exercise.which has been shown to effect the net protein balance, resulting in an anabolic state in the muscle. This muscle state also results in muscle hypertrophy. In the area of endurance exercise, it seems that the intensity of the exercise undertaken determines the rate of protein synthesis, which in turn, effects muscle hypertrophy. The training state of the athlete also has been shown to effect dietary protein requirements with above mentioned studies indicating that the improvement in protein synthesis with increasing exercise intensity is greater in untrained athletes than in their trained counterparts.

An area of particular interest that has been debated regarding its adequacies in providing enough protein in the diet is vegan and vegetarianism. It has been shown that a diet without meat or dairy products can give the essential amino acids, but only if the athlete undertakes ‘protein complementing’ to combine essential amino acids found in different foods.

Overall, the influence of protein on performance is generally underestimated by modern athletes. This may be due to a lack of education of athletes and trainers in this area, and is not assisted by a lack of research carried out on athletes and not sedentary individuals. I feel that further research needs to be carried out using subjects from different sporting backgrounds, and not solely using untrained individuals as the subject groups.