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Protein and Endurance Athletes: Timing it Right

Will Brinks explains the many benefits of protein for endurance athletes.

Endurance athletes are all too familiar with the message that carbohydrates are the most important food to eat. Carbohydrates fuel muscles, like petrol in your car. Muscle is comprised of many components: connective, nerves, blood capillaries, and muscle cells. All these have a common building block: proteins. There are thousands of proteins in your body with many different structures and functions. The building blocks for these proteins come from our diet.

Protein Turnover

Proteins in your body are continuously degraded and synthesized by processes that require energy. These processes are tightly linked to energy consumption by your whole body. Whole-body protein synthesis represents the combined synthesis rates by all the tissues and organs in your body.

Nutrition, ageing, exercise and other factors can affect protein synthesis in individual tissues while having little effect on others. Accordingly, athletes are concerned with protein synthesis and degradation rate in muscle tissue. Yet how does endurance exercise affect muscle protein turnover?

Most short-duration exercise, such as sprinting and weightlifting, relies on carbohydrate for fuel. Glycogen, the stored form of carbohydrate in muscle tissue, and blood glucose provide the preferred substrate for chemical energy that fuels muscle power. Although fat and protein also supply fuel for muscles, glycogen is the most efficient fuel source for long-duration exercise.

During prolonged exercise, such as long-distance running or cycling, proteins can be broken down to provide 3 to 5% of the total energy expenditure. When muscle glycogen becomes low, or if initial glycogen stores are low, energy contribution from protein can be up to 10%.

One way to prevent the breakdown of muscle protein for fuel is to provide adequate carbohydrate. Many endurance athletes think that eating a lot of carbohydrates is enough to protect precious muscle protein. Indeed, insulin, the hormone secreted in response to ingesting carbohydrates, helps protect muscle from protein degradation. Still, insulin alone does not increase protein synthesis. Insulin combined with amino acids is optimal for a positive protein balance.

How Much Protein?

For many years the issue of daily protein intake requirements by athletes has been an ongoing controversy. The typical recommendation by most nutritionists corresponds with the Recommended Daily Allowance (RDA): 0.8 grams of protein per kilogram (0.36 g/lb) of body weight per day.

Because the typical athlete consumption is estimated at 0.7 to 1.6 grams per kilogram (0.3 to 0.7g/lb) per day, exercise physiologists do not consider protein supplementation for the average athlete justified. However, several studies suggest that athletes should consume more than RDA depending on their training demands and overall diet. Several studies have determined that athletes performing low to moderate-intensity endurance exercise maintain a nitrogen balance (an indicator that protein intake equals protein degradation) with a protein intake of 0.9 to 1.1 g/kg per day. On the other hand, to maintain nitrogen balance in elite endurance athletes, estimated protein requirements of 1.5 to 1.8 g/kg (0.7 to 0.8 g/lb) per day may be required.

Protein Intake Timing

We know that dietary protein and carbohydrate intake is important for athletes and that the typical athlete's daily diet is sufficient in both. Nevertheless, recent studies are now suggesting that the timing of protein ingestion may be just as important as the total intake.

Studies support the universal acceptance of increased performance and recovery in endurance athletes using carbohydrate beverages. Yet two recent studies demonstrate that adding protein to a carbohydrate beverage further enhances performance and protein accumulation in trained endurance athletes, especially when ingested during prolonged exercise.

Performance Enhancement

A research group at James Madison University, Virginia, showed that highly-trained cyclists enhanced their athletic performance during prolonged bouts of endurance exercise by consumption of a protein and carbohydrate drink. The cyclists that ingested the carbohydrate and protein drink improved their time to fatigue at 75% maximal effort compared to a carbohydrate only drink.

To determine how the recovery drinks affected a subsequent bout of endurance exercise, the same group cycled again to fatigue, this time at 85% of their maximum. The cyclists performed the second bout twelve to fifteen hours after the first bout and in a partially glycogen depleted state. The authors hypothesized that differences in performance would be greater during the second ride.

In the carbohydrate plus protein group, cyclists ingested 0.52 g/kg/hour of carbohydrates and 0.14 g/kg/hour of protein every 15 minutes of exercise (Table 1). Additionally, they drank 0.73 g/kg/hour carbohydrates and 0.2 g/kg/hour of protein within 30 minutes after the exercise. The carbohydrate only group consumed 0.52 g/kg/hour every 15 minutes during exercise and 0.73 g/kg/hour during the subsequent 30-minute recovery period.

During the first bout, cyclists ingesting the carbohydrate plus protein drink performed 29% longer than the group with carbohydrate only drink. During the second bout, they rode 40% longer than the group consuming the carbohydrate only drink.

These results add data to a similar study conducted at the University of Texas at Austin, where nine trained men cycled to fatigue in three separate sessions separated by seven days. Cyclists drank an experimental beverage (200 ml every 20 minutes) during each cycling session. The beverage contained carbohydrates and protein (7.75 and 1.94 grams per 200 ml, respectively), only carbohydrates (7.75 g/200 ml), or a placebo containing no carbohydrates and protein. After 180 minutes of cycling at variable intensities, the subjects cycled to fatigue at 85% VO2 max during each session. Compared to the carbohydrate only group, the addition of protein to a carbohydrate beverage increased the time to exhaustion by 36%.

What is Going On?

Why would adding protein to carbohydrates during an endurance workout have such a profound effect? One explanation offered for the increased performance seen is a larger sparing of muscle glycogen than seen with carbohydrates alone. Thus, a larger muscle glycogen reserve would be available for prolonged endurance performance.

Earlier studies showed that added protein to a carbohydrate supplement enhanced muscle glycogen storage within the first hour of recovery. The addition of protein to a carbohydrate meal after training enhanced glycogen stores twice as fast as carbohydrate only meals containing the same caloric content.

On the other hand, not all studies have reported improved glycogen storage from a combined protein and carbohydrate feeding. Nonetheless, these studies examined supplementation during recovery from exercise or competition, not during. Consequently, they do not provide convincing evidence that sparing muscle glycogen is the only explanation for the ergogenic effect of protein and carbohydrate intake. Another explanation proposed is the addition of extra calories provided by the protein.

Cyclists in the protein and carbohydrate only trial in the Madison University study consumed 190 more calories than during the carbohydrates trial. "However, "the added calories consumed do not match the additional calories expended during the longer [carbohydrate and protein] trial.

Thus, it is unlikely that the additional calories consumed played a major role in the differences in performance between the trials", write the authors. Regardless, a comparison of isocaloric beverages (containing the same number of calories) needs to be examined to establish that adding protein to a carbohydrate drink is more beneficial than the added calories per se during exercise.

On the other hand, further mechanisms may be involved. John Ivy, professor of kinesiology and principal author of the Texas study, comments: "I do not think that the protein is providing additional calories in the sense of using more protein and sparing carbohydrate. It may be reducing the use of endogenous protein and therefore preventing a decrease in Krebs cycle intermediates or preventing tissue damage (muscle breakdown)." (Ivy et al. 2003)[1]

Alternative mechanisms are being investigated to explain the ergogenic enhancement of added protein to an endurance sports beverage. One is the preservation of muscle protein which enhances recovery. In addition to measuring performance in the two groups, Saunders estimated muscle damage by measuring blood levels of creatine phosphokinase (CPK). (Saunders et al. 2004)[2]

Often used as an index of muscle damage, CPK is an enzyme in muscle cells released into the circulation as a consequence of cell damage. Levels were measured at baseline and 12 to 15 hours after the second exercise bout. CPK levels in the cyclists that drank the carbohydrate plus protein beverage were reduced by 83% compared to the carbohydrate only group. This significant difference indicates that muscle damage was minimized by ingesting a carbohydrate plus protein sports beverage.

Protein Balance

How protein ingested during exercise reduces muscle damage is speculative. Yet one proposed mechanism is improved protein balance but what exactly does this mean?

Keep in mind that skeletal muscle is the major deposit of protein in the body and has a slow turnover rate of about 2% per day. Muscle contains several fractions, or components (contractile, mitochondrial and cytoplasmic), of proteins with each fraction containing a mixture of hundreds of specific proteins.

Of particular interest to endurance athletes are mitochondrial proteins. They are mostly involved in aerobic energy production, and their synthetic rate is two-thirds higher than the rate for mixed muscle proteins.

Thus, the proteins that help provide energy to your muscles have a faster turnover rate than other muscle proteins. So, providing an adequate pool of the basic protein building blocks, amino acids are imperative to replace degraded mitochondrial and other proteins.

The most common method used to measure whole-body protein turnover is by measuring the rate of metabolism or incorporation of labelled amino acid. A solution containing a stable, isotopically labelled amino acid is given to subjects by intravenous injection or orally. A specific by-product from the metabolism of proteins containing the labelled amino acid is then analysed to determine rates of protein degradation and synthesis.

Most protein metabolism studies have utilized one of several possible labelled amino acids administered to subjects under various conditions. Additionally, most of these studies measured protein turnover during exercise after an overnight fast, which is not representative of the normal practice of competitive athletes.

Other studies use nutritional interventions applied after the exercise bout. Consequently, little information exists relating to the effects of carbohydrates and protein during prolonged endurance exercise.

A research team from Maastricht University in the Netherlands measured whole-body protein turnover rates at rest, during prolonged endurance exercise and recovery. They used multiple tracer methods to determine differences in results depending on the tracer method, but all were subjected to the same exercise protocol.

Eight highly-trained men (elite triathletes) exercised for a total of six continuous hours with a combination of cycling and running at moderate intensity (50% VO2 max). Each subject was infused with a mixture of two labelled amino acids and urea starting four hours before exercise, during the exercise and for four hours during the subsequent recovery.

Subjects were studied on two different days when they received a beverage containing protein plus carbohydrates or carbohydrates only. The carbohydrate plus protein group ingested 0.7 g/kg/hour of carbohydrates and 0.25 g/kg/hour of protein; the carbohydrate only group received 0.7 g/kg/hour of carbohydrates. Both groups drank from their beverages every 30 minutes of exercise and during recovery afterwards. Breath and blood were sampled frequently during the entire protocol and analysed to determine net protein balance.

Two of the three methods using the labelled amino acids and urea showed that whole-body protein balance was improved during exercise and recovery with the addition of protein to a carbohydrate beverage. Ingestion of the carbohydrate only beverage resulted in a negative protein balance. These and other recent studies suggest that ingesting protein during exhaustive endurance exercise may maintain plasma amino acid pools within and in between muscle cells, enabling increased protein synthesis and repair as well as providing precursors for metabolic systems in muscle cells.

However, no studies have yet revealed which fraction of the hundreds of muscle proteins are replaced following protein and carbohydrates intake by endurance athletes. This may be an important issue for both strength and endurance athletes. "One thing to keep in mind, it may not be beneficial for an endurance athlete to stimulate myofibril [contractile] synthesis by ingesting an amino acid source. For example, if a cyclist's arms gain mass, the extra weight may be considered a bad thing.

Sports Beverages and Business

Despite increasing evidence of the ergogenic effects of adding protein to endurance sports carbohydrate beverages, a prominent sports supplement company is campaigning against adding protein to carbohydrate sports drinks. They explain that adding protein will impair gastric emptying and thus be ineffective in preventing dehydration when exercising in the heat. However, no evidence in the scientific literature demonstrates that adding the small amount of protein that is shown to be effective (1.5 to 2%) will slow gastric emptying. On the contrary, authors (Shi & Gisolfi 1995)[3] of a published study examining hydration in subjects drinking carbohydrate beverages theorize that "an amino acid should activate yet another transport mechanism to enhance solute and therefore water absorption."

Also, a study comparing a beverage containing glucose with another containing the same calories but derived from protein and carbohydrates demonstrated that gastric emptying rate was similar. Water flux into and out of the intestines was also comparable between the two beverages. While this has limited extrapolation to a carbohydrate and protein beverage during exercise, it does suggest that adding a small amount of protein to a 6% carbohydrate drink will not slow gastric emptying and hinder water absorption.


The protein requirement by an endurance athlete may or may not exceed that suggested by the RDA for the normal population. Depending on their overall diet, athletes training at frequent high intensities may require more protein, but how much remains to be demonstrated.

Athletes who for one reason or another, do not derive enough energy to maintain their energy balance may supplement their protein intake by adding it to their traditional sports drinks that contain carbohydrates. Now research demonstrates that the timing of protein intake may be as critical as how much. Drinking a beverage containing both carbohydrates and protein during their demanding training sessions or competitions may not only enhance their recovery but also increase their performance. Whey proteins can be added (1.5 to 2%) to a 6% carbohydrate beverage. For example, add 6 to 10 grams of protein and 30 to 60 grams of carbohydrates to 32 ounces of liquid. This can be drunk during training or competition to increase performance and reduce muscle damage.

Despite some claims that the addition of protein to a sports beverage will reduce gastric emptying, there is no evidence to support that it does. In addition to pumping fuel into your muscle engines, you can even maintain the integrity of that engine to make you go further with less wear and tear.


  1. IVY, J.L. et al. (2003) Effect of a carbohydrate-protein supplement endurance performance during exercise of varying intensity. Int J Sport Nutr Exerc Metab, 13, p. 388.
  2. SAUNDERS, M. et al. (2004) Effects of a carbohydrate-protein beverage on cycling endurance and muscle performance. Med Sci in Sports Exerc, 36, p. 1233.
  3. SHI, X. and GISOLFI, C. (1995) Effects of carbohydrate type and concentration and solution osmolality on water absorption. Med Sci Sports Exerc, 27, p. 1607

Article Reference

This article first appeared in:

  • BRINKS, W. (2006) Protein and Endurance Athletes: Timing it Right. Brian Mackenzie's Successful Coaching, (ISSN 1745-7513/ 29 / February), p. 6-9

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • BRINKS, W. (2006) Protein and Endurance Athletes: Timing it Right [WWW] Available from: [Accessed

About the Author

William Brinks is a columnist, contributing consultant, and writer for various health/fitness, medical, and bodybuilding publications in the USA. He lectures on the benefits of weight training and nutrition around the U.S. and Canada and has worked with athletes ranging from professional bodybuilders, golfers and fitness contestants.