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In recent times, there has been a renewed interest in the role that protein can play in enhancing training adaptations and ultimately performance in endurance athletes. Traditionally, the realm of the strength athlete, newer studies on the role of protein intake in endurance athletes have focused not so much on overall protein requirements, but rather on how modulating the time and type of protein might help maximise recovery from, and adaptation to, endurance training1. This article will take a look at the latest research on protein intake in endurance athletes and delve into some of the finer points of protein supplementation.

Protein/Amino Acid Usage During Endurance Exercise

One of the first things one must understand when trying to estimate protein intake requirements for endurance athletes is that during exercise, muscle tissue serves as an important energy reservoir2. That is to say that muscle tissue (and the amino acids that make up muscle tissue) actually supply part of the energy requirements during endurance exercise. How much you ask? It turns out the amount of amino acid oxidation during exercise is dictated in part by factors such as exercise intensity and (or) duration, muscle glycogen availability, habitual level of protein intake (i.e. high vs low) as well as sex1. While the exact level varies from individual to individual studies have suggested that total amino acid oxidation during exercise can amount to up to 10% of total energy requirements during endurance exercise2.

Leucine Oxidation During Endurance Exercise

Leucine is the amino acid oxidised at the highest rate during exercise and there have been a number of studies measuring its specific rate of oxidation during exercise. One such study showed leucine was oxidised at a rate of 8mg/kg/h in endurance athletes performing moderate-intensity (i.e. 60% VO2max) continuous exercise3. This lead to an overall body leucine loss of 1.2g over 2 hours3. When extrapolated to total body protein, this equates to roughly 13g of protein over the two hours3, which by anyone’s standards is a pretty decent whack.

Protein Requirements for Endurance Athletes

Given the amount of protein that can be oxidised during endurance exercise, it’s obvious that endurance athletes require higher protein intakes than sedentary individuals. But just how high is up for debate a bit. While many studies have indicated a protein intake of between 1.5-2.0g/kg bodyweight per day should be adequate to keep most endurance athletes in positive protein balance, there remains the question of what additional benefits extra protein may provide (either in an acute or chronic context) when it comes to factors such as recovery, endurance, immunity and strength.

Protein Requirements During Intensive Training

One particular situation that may warrant very high protein intake in endurance athletes is when they are engaged in periods or blocks of high-intensity training. A recent study in elite high performance cyclists found that consuming 20g of a hydrolysed casein supplement before, during and after exercise totally prevented the decreased in immune function seen in the equally matched group who did not consume the extra protein as part of their high intensity training block. The group taking the protein supplement consumed on average 3.0g/kg protein per day, which is considered very high, even for elite bodybuilders4. But this study is a graphic example of just how high protein intake requirements can be in elite endurance athletes.

Protein Requirements for Ultra Endurance Athletes

Triathletes are the most common example of ultra endurance athletes, where events/races can last anywhere from 6-12 hours. With the information discussed above on protein utilisation during exercise, it’s natural to expect that during such events, protein intake requirements will be higher. This has also been proven in studies, with one study showing that trained endurance athletes receiving 0.7g/kg/hour plus 0.25g/kg/hour protein hydrolysate over the course of 6 hours of exercise (which included running and cycling) had higher protein synthesis rates than the subjects who only received carbohydrate at a rate of 0.7g/kg/hour5.

Timing of Protein Intake in Endurance Athletes

Immediate (<3 hours Postexercise)

One of the unique differences between resistance exercise and endurance exercise is that following resistance exercise, muscles have a heightened anabolic sensitivity for up to 24 hours6. That is to say that muscles still display a heightened anabolic response (i.e. protein synthesis) to ingested protein up to 24 hours after resistance exercise6. This is not the case when it comes to endurance exercise.

For example, one study showed that delaying the ingestion of 10g of a dairy-based protein by 3h markedly attenuated the anabolic effect of the dietary protein and failed to enhance post exercise mixed muscle protein synthesis7. So as an endurance athlete, it’s important that protein be consumed immediately following exercise, preferably in the 15-30 minutes following cessation.

Protein Ingestion During or Before Exercise

The other pivotal question regarding the timing of protein intake for endurance athletes is whether its beneficial to ingest protein during exercise. During exercise, there is an increased blood flow to muscle tissue and scientists have theorised that providing protein/amino acids during this time may provide a powerful stimulus to muscle protein synthesis by virtue of this heightened blood flow and high amino acid availability8. While there are studies with individuals engaged in weight training suggesting that ingesting protein before and/or during resistance exercise results in greater muscle protein synthesis9-12, it remains to be seen if the same is true in endurance exercise.

Protein Ingestion During Exercise & Performance

While there are several studies that have examined the effect of protein co-ingestion with carbohydrate during endurance exercise on performance, they have yielded conflicting results13. Making the matter more complicated is the fact that most studies have used different protein sources, with some of the proteins used including casein hydrolysate, whey concentrate, rice hydrolysate and whey hydrolysate. With at least three studies14-16 showing an improvement in endurance exercise performance when co-ingesting protein with carbohydrate during exercise, it’s likely that the practice may benefit the average endurance athlete performance. But there are some additional benefits other than improved performance, which make the option of consuming protein during exercise particularly appealing.

Protein Intake & Recovery in Endurance Athletes

These effects extend to muscle recovery and associated muscle soreness. Most of the studies showing a benefit with protein supplementation during exercise have shown reduced levels of creatine kinase (CK) in athletes; a popular and validated marker of muscle damage13, 17, 18. Naturally, reductions in post-exercise muscle-damage markers may have important implications for subsequent exercise performance. Endurance athletes commonly train more than once a day and often have less than 12 hours between training sessions. As such, any nutritional strategy to reduce muscle soreness and damage is valuable.

In summary, consuming protein together with carbohydrate during exercise may be beneficial for both performance and recovery in endurance athletes. This is particularly true for endurance athletes engaged in multiple training sessions a day with limited recovery times.The co-ingestion of protein with carbohydrate can serve as a means to optimize muscle glycogen concentrations as well as to provide amino acids for the repair of muscle proteins.

Best Type of Protein for Endurance Athletes

This is another hotly contested topic in the area of sports nutrition. As we have discussed in depth in another article, whey protein hydrolysate has a number of proven benefits for endurance athletes, especially when consumed immediately following exercise. In short, these include improved glycogen storage19, improved endurance capacity (when used daily over a period of weeks)20, 21, more rapid glycogen replenishment22, reduced muscle soreness23 and improved performance23. So when it comes to protein type and quality, its seems its worth spending a bit extra for a good quality whey hydrolysate if you can afford it.

Protein for the Endurance Athlete

In conclusion, it’s important to emphasise that potential benefits of protein supplementation in endurance athletes rests largely on their ability to repair and regenerate damage proteins as well as synthesise a variety of new proteins involved in energy production pathways of the mitochondria and associated enzyme complexes. This is in contrast with bodybuilding, where dietary protein primarily serves to repair and regenerate contractile muscle proteins (called myofibrillar proteins) in muscle tissue. It is this repair and remodelling of muscle proteins involved in energy production in endurance athletes that provides the basis for many of the training-induced adaptations that underpin the increases in muscle quality (i.e. mitochondrial density) that ultimately lead to improved performance1.

Factors such as the type and timing of protein intake in endurance athletes are likely to play more important roles in maximising recovery from, and adaptation to, endurance training than merely meeting or exceeding current protein recommendations on a daily basis.

References
1. Moore DR, et al. Beyond muscle hypertrophy: why dietary protein is important for endurance athletes. Appl Physiol Nutr Metab. 2014;39:1-11.
2. Tarnopolsky, M. Protein requirements for endurance athletes. Nutrition. 2004;20(7–8):662–668.
3. Bowtell JL, et al. Modulation of whole body protein metabolism, during and after exercise, by variation of dietary protein. J Appl Physiol. 1998;85(5):1744–1752.
4. Witard OC, et al. High dietary protein restores overreaching induced impairments in leukocyte trafficking and reduces the incidence of upper respiratory tract infection in elite cyclists. Brain, Behavior, and Immunity. 2014;39:211-219.
5. Koopman R, et al. Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise. Am J Physiol Endocrinol Metab. 2004;287(4):E712-20.
6. Burd NA, et al. Enhanced amino acid sensitivity of myofibrillar protein synthesis persists for up to 24 h after resistance exercise in young men. J Nutr. 2011;141(4):568–73.
7. Levenhagen DK, et al. Postexercise nutrient intake timing in humans is critical to recovery of leg glucose and protein homeostasis. Am J Physiol Endocrinol Metab. 2001;280(6):E982–E993.
8. van Loon LJ. Is there a need for protein ingestion during exercise? Sports Med. 2014;44(Suppl 1):S105–S111.
9. Beelen M, et al. Protein coingestion stimulates muscle protein synthesis during resistance-type exercise. Am J Physiol. 2008;295(1):E70–7.
10. Beelen M, et al. Impact of protein coingestion on muscle protein synthesis during continuous endurance type exercise. Am J Physiol. 2011;300(6):E945–54.
11. Koopman R, et al. Combined ingestion of protein and carbohydrate improves protein balance during ultra-endurance exercise. Am J Physiol. 2004;287(4):E712–20.
12. Tipton KD, et al. Timing of amino acid–carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol. 2001;281(2):E197–206.
13. Saunders MJ, et al. Coingestion of carbohydrate-protein during endurance exercise:influence on performance and recovery. International Journal of Sport Nutrition and Exercise Metabolism. 2007;17:S87-S103.
14. Ivy JL, et al. Effect of a carbohydrate-protein supplement on endurance performance during exercise of varying intensity. Int J Sport Nutr Exerc Metab. 2003;13:388-401.
15. Saunders MJ, et al. Effects of a carbohydrate-protein beverage on cycling endurance and muscle damage. Med Sci Sports Exerc. 2004;36:1233-1238.
16. Saunders MJ, et al. Consumption of an oral carbohydrate-protein gel improves cycling endurance and prevents post-exercise muscle damage. J Strength Cond Res. 2007;21(3):678-84.
17. Romano-Ely BC, et al. Laurent. Effects of an isocaloric carbohydrate-protein-antioxidant drink on cycling performance. Med Sci Sports Exerc. 2006;38:1608-1616.
18. Luden ND, et al. Post-exercise carbohydrate-protein-antioxidant ingestion decreases CK and muscle soreness in cross-country runners. Int J Sports Nutr Exerc Metab. 2007;17:109-122.
19. Morato PM, et al. Whey protein hydrolysate increases translocation of GLUT-4 to the plasma membrane independent of insulin in wistar rats. PLoS One. 2013;8(8):e71134.
20. Morifuji M, et al. Preexercise ingestion of carbohydrate plus whey protein hydrolysates attenuates skeletal muscle glycogen depletion during exercise in rats. Nutrition. 2011;27(7-8):833-7.
21. Liu J, et al. Effect of whey protein hydrolysates with different molecular weight on fatigue induced by swimming exercise in mice. J Sci Food Agric. 2014;94(1):126-30.
22. Morifuji M, et al. Post-exercise carbohydrate plus whey protein hydrolysates supplementation increases skeletal muscle glycogenlevel in rats. Amino Acids. 2010;38(4):1109-15.
23. Hansen M, et al. Effect of whey protein hydrolysate on performance and recovery of top-class orienteering runners. Int J Sport Nutr Exerc Metab. 2014 Jul 14. [Epub ahead of print].

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