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Protein for Endurance Athletes

Summary Points

  • Protein requirements for endurance athletes are high due to use of muscle tissue as an energy source during exercise
  • Recommendations for protein intake in endurance athletes vary based on training level
  • Protein intake around 2.0g/kg body weight is not uncommon for endurance athletes in heavy training
  • It's unclear whether endurance athletes benefit from very high protein intake, however, some studies have shown benefits for immune function
  • There are no studies to suggest that a protein intake of between 2-3g/kg bodyweight per day are harmful to endurance athletes
  • Some research suggests consumption of hydrolysed protein during endurance exercise can help improve recovery, but not necessarily performance
  • There is slim research suggesting that consumption of carbs and a milk hydrolysate during endurance exercise may improve performance
  • Protein intake shortly after endurance exercise can help improve muscle recovery and replenish glycogen especially when combined with carbohydrate


There has been renewed interest in the role protein can play in enhancing training adaptations to endurance exercise. Researchers are also interested in whether protein can improve performance. 

New studies on protein intake in endurance athletes have focused on modulating the amount, timing and type of protein. Each of these factors are thought to impact recovery from, and adaptation to endurance training (1).

This article will look at the latest research on protein intake and supplementation in endurance athletes.


Protein/Amino Acid Usage During Endurance Exercise

Endurance athletes have high protein requirements largely because muscle serves as an important energy reservoir during endurance exercise (2)

The amount of amino acids burned during exercise is dictated by: 

  • Exercise intensity (and duration);
  • Muscle glycogen;
  • Habitual level of protein intake;
  • Training status, and;
  • Sex (1)

While the exact level varies, research suggests total protein oxidation during exercise can be up to 10% of total energy requirements (2).


Leucine Oxidation During Endurance Exercise

Of all the amino acids, leucine is oxidised at the highest rate during exercise. There are several studies measuring its specific rate of use during exercise. One showed leucine oxidised at a rate of 8mg/kg/h in endurance athletes performing moderate to intense (i.e. 60% VO2max) exercise (3)

This lead to an overall body leucine loss of 1.2g over 2 hours (3). When extrapolated to total body protein, this equates to 13g of protein over the two hours (3). By anyone’s standards that is a pretty decent whack.


Protein Requirements for Endurance Athletes

Given the amount of protein oxidised during endurance exercise, it’s obvious endurance athletes require higher protein intakes than sedentary individuals. But just how high is up for debate.

Several studies suggest a protein intake between 1.5-2.0g/kg bodyweight per day should be adequate to keep most endurance athletes in positive protein balance. But there remains the question of what additional benefits extra protein may provide for recovery, endurance, immunity and strength.


Protein Requirements During Intensive Training

One situation that may warrant very high protein intake in endurance athletes is during blocks of high-intensity training. 

A recent study in elite cyclists gave 20g of a hydrolysed casein supplement before, during and after exercise. This prevented the decrease in immune function seen in the equally matched group who did not consume extra protein.

The group taking protein supplements consumed 3.0g/kg protein per day, which is considered very high; even for bodybuilders (4). 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 common examples of ultra-endurance athletes. Ironman events can last anywhere from 6-12 hours. With protein use related to length of exercise, it’s natural that protein intake requirements will be higher for such athletes.

This has also been proven in studies. One gave trained endurance athletes 0.7g/kg/hour of carbs plus 0.25g/kg/hour of protein hydrolysate over 6 hours of exercise. Compared with subjects who only received carbohydrate, they had higher protein synthesis rates.


Timing of Protein Intake in Endurance Athletes

Immediate (<3 hours Postexercise)

Following resistance exercise, muscles have a heightened anabolic sensitivity. That’s to say muscles display a greater anabolic response to ingested protein up to 24 hours after resistance exercise (6). 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. This decreased the anabolic effect of the protein and failed to enhance post-exercise muscle protein synthesis (7). So, as an endurance athlete, its important protein is consumed immediately following exercise, preferably 15-30 minutes after.


Protein Ingestion During Exercise

The other pivotal question is whether its beneficial to ingest protein during exercise. 

During exercise there is increased blood flow to muscle. Scientists have theorised providing protein/amino acids during this time may stimulate muscle protein synthesis (8)

Studies of traditional weight training suggest protein before and/or during exercise result in greater muscle protein synthesis (9, 10, 11, 12). But it remains to be seen if the same is true in endurance exercise.


Protein Ingestion During Exercise & Performance

Several studies have examined the effect of protein co-ingestion with carbohydrate during endurance exercise on performance. But they have yielded conflicting results (13)

Making the matter more complicated is most studies have used different protein sources. Some of the proteins used include casein hydrolysate, whey concentrate, rice hydrolysate and whey hydrolysate

At least three studies (14, 15, 16) have shown an improvement in performance when co-ingesting protein with carbohydrate during endurance exercise. Therefore it’s likely the practice may benefit performance of the average endurance athlete.

But there are additional benefits other than improved performance, which make the option of consuming protein during exercise particularly appealing.


Protein Intake & Recovery in Endurance Athletes

These benefits extend to muscle recovery and associated muscle soreness. For example, several studies have shown reduced levels of creatine kinase when endurance athletes consumed protein during exercise. Creatine kinase is a well known marker of muscle damage (13, 17, 18)

Naturally, reductions in post-exercise muscle damage have important implications for subsequent exercise performance. Endurance athletes commonly train more than once a day and often have less than 12 hours between 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 time. 

The co-ingestion of protein with carbohydrate can serve to optimize muscle glycogen concentrations and provide amino acids for muscle repair.


Best Type of Protein for Endurance Athletes

This is another hotly contested topic in sports nutrition. As discussed in another article, whey protein hydrolysate has a number of proven benefits for endurance athletes. This is especially the case when consumed immediately following exercise. These benefits include: 

  • Improved glycogen storage (19);
  • Improved endurance capacity (when used daily over a period of weeks) (20, 21);
  • More rapid glycogen replenishment (22);
  • Reduced muscle soreness (23), and;
  • Improved performance (23)

When it comes to protein type and quality, it's worth spending a bit extra for a good quality whey hydrolysate.


Summing Up: Protein for the Endurance Athlete

In conclusion, the benefits of protein supplementation for endurance athletes’ are mediated by its ability to repair proteins damaged during exercise. Benefits are seen through the synthesis of new proteins involved in energy production. 

This is in contrast with bodybuilding, where dietary protein primarily serves to repair and regenerate contractile muscle proteins in muscle tissue. 

It’s this repair and remodeling of muscle proteins involved in energy production that provides the basis for training-induced adaptations. These changes underpin increases in muscle quality and performance in endurance athletes (1). 

Factors such as the amount, type and timing of protein intake in endurance athletes likely play important roles. Each of these factors may help protein optimise recovery from, and adaptation to endurance training.


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