Protein Supplementation in Athletes

By Scott Kaar, MD

What are proteins and their building blocks?

Proteins and their building blocks, amino acids, have been ingested for years as dietary athletic performance enhancers. In fact, supplemental dietary protein intake is most likely the first, or one of the first ever performance enhancers taken. There exist 20 amino acids in total, 9 of which are essential and 11 of which the body can synthesize from other metabolism byproducts. The 9 essential amino acids therefore must be obtained through the diet since the body has no internal source of them. The great majority of the body’s amino acids are contained within proteins, one of the major organic polymeric structures found within the body. They serve many vital functions such as in metabolism, cellular signaling, enzymes and cell structure and function such as in muscle. Only about 1% of amino acids in the body are in the free pool such as the blood stream available for protein synthesis. One way to increase this free pool of amino acids is through an increase in dietary protein intake.

What are the recommended protein requirements?

The US Food and Nutrition Board recommends 0.8 gram protein/kg bodyweight per day for the average American. This number was carefully obtained through scientific research. This is about 65 g of protein per day for a 180 lb person, or the equivalent of about 2½ 4 oz boneless skinless chicken breasts. There has been recent thought however that the protein requirements in an athlete may be greater than this recommendation depending on the specific type of athletic activity and training goals.

What is the role of amino acids and the endurance athlete?

As recommended recently by the American College of Sports Medicine, endurance athletes should maintain an increased dietary protein intake of 1.2 gram protein/kg bodyweight per day, 50% higher than the standard recommended intake amount. In athletes that train at higher intensities or for long periods of time, the required amount of protein may be even slightly higher as suggested in some studies.

The benefit of higher dietary amino acid intake in endurance athletes has been debated beyond simply balancing the amount of nitrogen in the body for protein composition. One theory regarding the benefit of amino acid supplementation is the “Central fatigue theory” which suggests that prolonged exercise lowers amino acid levels and increases free fatty acid levels in the blood. This causes a secondary increase in tryptophan levels that in turn can cause a depressive affect on the central nervous system leading to a decrease in athletic performance. Despite numerous studies regarding the “Central fatigue theory”, there has been no conclusive evidence to support the theory.

Another theory regarding the benefit of increasing dietary protein intake in endurance athletes is their role in exercise recovery. Again no definitive proof exists that demonstrates a benefit in athletic performance. However, there is some evidence that creatine kinase and lactic acid levels decrease with amino acid supplementation both of which have been correlated with delayed onset muscle soreness. There is also a suggested decrease in the rate of infections through a similar mechanism.

What is the role of amino acids and the strength training athlete?

Amino acid intake and protein synthesis has long been a topic in the forefront in the setting of the strength training athlete. Resistance exercise is followed by a 48 hour period when muscle protein synthesis is elevated. Protein synthesis is necessary for an increase in muscle mass and therefore an increased dietary intake of amino acids is suggested. Some studies suggest that the daily dietary protein intake in weight-training athletes is between 1.4 and 1.8 g protein/kg bodyweight per day, or more than double the normal recommended daily intake for non-athletes. While the short term data available does not clearly confer that an increased dietary protein intake improves strength, there has been this suggestion. It is possible that longer term duration studies of strength athletes may prove an increased measured strength with increased dietary protein and amino acid supplementation. It is however clear that there is a limit on the amount of protein synthesis and therefore muscle building potential is based on oral protein intake. Protein or amino acids ingested above this limit will not induce further protein synthesis.

Furthermore, there may be an increased protein requirement during early muscle building periods when an athlete is training to build muscle mass. However, most athletes reach a phase in their training when they are no longer increasing muscle mass and instead are maintaining a high, but stable level of muscle mass. During this stable muscle mass period, protein requirements may be elevated somewhat above normal requirements due to a small increase in resting muscle protein turnover. Research studies have suggested that this level is increased to 0.9 g protein/kg bodyweight per day, only 12% above the daily recommendations for sedentary individuals.

What is the relationship between athletic performance and dietary protein intake?

Despite all the academic debate over the proposed benefits of oral protein supplementation based on protein synthesis and nitrogen balance, the true measuring stick is athletic performance. Demonstrating clear athletic performance benefits is very difficult to achieve because of the many confounding factors involved such as gender, specific sport, varying training routines within a single sport and an athlete’s normal diet to name just a few. Supplemental protein intake is unnecessary for most athletes as long as they consume a healthy diet containing complete protein foods, and it meets their energy needs.

However up to 1/5 of all athletes consume levels of protein below the estimated requirements for sedentary individuals. There are risk factors for athletes that do not have a sufficient dietary protein intake including vegetarians, athletes in weight-class competition sports, those with insufficient energy intake, sudden increases in training intensity and athletes in weight loss programs.

How is a vegetarian athlete affected?

Vegetarian athletes are at a higher risk of protein deficiency than other athletes. A plant-based vegetarian diet can supply all essential and nonessential amino acid requirements for protein synthesis. However careful attention must be paid to the variety of dietary amino acid sources consumed as well as the amount of protein and energy contained in the athlete’s diet. Vegan athletes are at further risk of protein insufficiency because their diets lack animal protein sources altogether. There is also some concern that protein from plant-based sources is used less effectively by the body than protein from animal sources. Although both vegetarian and vegan diets can provide sufficient protein, if this is not the case, then additional dietary or supplemental protein could be considered.

What is the relationship between athletic performance and dietary energy intake?

There is a strong relationship between an athlete’s protein requirements and their dietary energy (caloric) intake. In fact, energy intake may have as significant an affect on protein requirements as does the amount of dietary protein itself. A positive nitrogen balance to increase protein synthesis for muscle building can not occur if there is an energy deficit no matter how much protein is in an athlete’s diet. Athletes can gain strength and maintain muscle mass even when dietary protein intake is low if energy intake is sufficient. With strength training, a positive energy balance is more important than increased protein for stimulating gains in lean body mass. Therefore, athletes that restrict energy intake must be especially conscious of their dietary protein intake. This often includes athletes in weight class sports like wrestling and boxing as well as those in sports at risk for eating disorders like gymnastics, long distance running and figure skating.

There are also potential performance drawbacks for athletes to energy restriction in high protein diets. A recent research study demonstrated that the performance of well-trained cyclists was impaired on a high protein, low carbohydrate diet. Comprised carbohydrate intake with higher protein intake may cause glycogen levels to be reduced. Subsequently athletes whose training involves high intensity or prolonged workouts may suffer.

What about the use of creatine?

Creatine, widely used as an ergonomic supplement since the early 1990s, is a common protein synthesized in the liver from the amino acids glycine and arginine. The great majority of creatine is found in the liver, however some creatine is also seen in the heart, brain and other organs. About 50% of the daily requirement is provided by diet and the remainder is synthesized by the liver. Creatine is abundant in meat and fish and as more creatine is ingested in the diet, the less is necessary to be provided by the liver.

Creatine in muscle becomes creatine kinase by the addition of phosphorous (phosphorylation) and then is a source of ATP (adenosine triphosphate – the main basic energy source used by the body). The ATP located within muscle provides energy during intense, quick repeated bursts of exercise seen in some competitive sports as well as strength training. Dietary supplementation is widely promoted to provide muscle with and increased level of creatine. In theory, higher creatine levels in muscle will allow for improved ability to produce energy during and recover quicker from high intensity exercise.

The performance effects of creatine supplementation have been researched widely. Dietary creatine supplementation enhances an athlete’s ability to produce higher muscle forces and power during short bouts of exercise. There is an increase seen in total body mass along with greater gains in strength, fat-free mass and sprinting performance. No improvement in aerobic performance occurs during endurance training as normal ATP production provides sufficient ATP in this circumstance.

For healthy athletes with no history of kidney disease, creatine is a safe product taken as a short-term supplement. Creatine supplementation is thought to lead to dehydration, however this has not been a problem in healthy athletes. Athletes with a history of kidney disease should be cautioned about possible side effects of excessive oral creatine intake because creatine and its metabolites are processed in the kidneys. Longer term effects of creatine supplementation are not well known. There is some research evidence that longer term use can lead to a blunting of the body’s response to creatine, or in other words creatine resistance.

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