Protein During Intense Endurance Exercise: Yes or No?

If you consume protein during prolonged endurance exercise, will it improve your performance?

Yes, it will improve your performance.

BUT, only if you are not already consuming an optimal level of carbohydrates.

Let me explain.

The Limiting Factor

Carbohydrate is the limiting factor in moderate to high intensity exercise, as your body stores a limited amount as glycogen in the muscles and liver and glucose in the blood. If your body’s carbohydrate stores run low, you “bonk” and basically have to slow down, as your body must rely more on the slower energy providing processes of breaking down fatty acids (fats) and proteins. In order to slow down the depletion of carbohydrate stores, endurance athletes will consume carbohydrates before exercise to “top off” carbohydrate stores in the body and ingest carbohydrates during exercise to attenuate the loss and sustain a higher level of intensity.

Protein Can Improve Exercise Performance

Researchers have also looked at the effect of ingesting protein in addition to carbohydrates during exercise to see if there was an ergogenic (i.e. “performance enhancing") effect. A few studies showed that protein in addition to a fixed amount of carbohydrate delayed fatigue compared to carbohydrate alone, but the shortcoming of these studies were that the amount of carbohydrate was ingested at a lower rate (0.50-0.75 g/min) than recommended for optimal performance (≥1.0 g/min). Furthermore, the subjects who consumed the additional protein were given the same amount of carbohydrate, which likely means they performed better simply because they were able to consume more calories.

Subsequent studies that examined whether adding protein would improve cycling time trial performance when ingesting carbohydrate at a more optimal rate of 1 g/min did not show any benefit from adding protein.

In other words, if the added protein gives you the additional calories that your body can use, consuming a protein source during exercise can improve performance. HOWEVER, if you are already taking in an optimal level of carbohydrate – the body’s preferred energy source during moderate to intense endurance exercise – then replacing some of the carbohydrate protein does nothing for performance improvement and may in fact be a performance detriment.

Overall it appears that adding protein to a carbohydrate during prolonged exercise can delay fatigue, but only if the amount of carbohydrate is below recommended levels.

The Study

What about the effect of ingesting protein during a shorter endurance event, like a 40 km cycling time trial in which carbohydrate depletion is not a concern?

Rebecca J. Toone and James A. Betts from the University of Bath (UK) recently published a study¹ in the International Journal of Sorts Nutrition and Exercise Metabolism to answer this question. Their hypothesis was that there would be no difference in performance if the total amount of energy consumed was the same.

The researchers tested 12 male trained cyclists and triathletes who regularly performed 11+/-5 hours per week of cycling. After an initial assessment to determine each subject’s VO2 max (average of 64+/- 6.4 mil/kg/min indicating a high caliber of endurance athlete), each athlete performed two main trials (randomly assigned 5-10 days apart) consisting of a 10 minute warm up then a 45-minute variable intensity protocol followed by a 6-km time trial. The total exercise time was approximately 62 minutes with the protocol designed to simulate a competitive road race during which intensity is variable with a fast, hard finish.

Every 15 minutes during each trial, each athlete was given either an energy matched (i.e. same amount of calories) and flavor matched 9% carbohydrate only solution or a 6.8% carbohydrate and 2.2% whey-isolate protein solution (3:1 ratio of carbohydrate to protein).

The researchers measured time, rate of perceived effort, blood lactate and expired gases. During the time trial, the athletes could only see distance remaining and not time. 

The Results

The final result for the time trial was 433+/-21 sec (carbohydrate only) vs. 438+/-22s sec (carbohydrate and protein solution) for a difference of approximately 1%, which equates to about 60 meters in a road cycling race – a significant amount in a sprint to the line finish that is typical of a road race.

The study also showed higher blood glucose levels in the carbohydrate only drink test vs. the carbohydrate-protein drink test at the onset of exercise through the first 45 minutes suggesting that adding protein may have caused a delay in absorption of carbohydrate into the blood stream and therefore it’s availability as an energy source (per the authors).

Some of the subjects also indicated gastrointestinal discomfort while consuming the carbohydrate-protein solution.

My own personal experience with using a drink mix with a 4:1 ratio of carbohydrate to protein during exercise was that it upset my stomach when I raced triathlons at high intensities and caused me to vomit.   Needless to say, I don't race triathlons with any protein in my sports drinks any more.

The Takeaway

The study indicates the reducing the quantity of carbohydrate and replacing with protein does not appear to be a performance enhancing strategy during a variable intensity 60-minute cycling event. Consuming an optimal level of carbohydrate (≥1 g/min) will provide you the ergogenic benefit of carbohydrates without the potential stomach distress of added protein.

I speculate that during an equivalent time sprint triathlon that the athletes would experience even more gastrointestinal discomfort during the running portion of the event.

However, if you can tolerate the added protein during high intensity endurance exercise and believe it can help you then by all means give it a try, but like anything in endurance sports, figure out what works best for you by practicing your nutrition strategy in training and in less important races before using in your key races.

Live strongly and boldly! 

David

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David B. Glover, MS, CSCS 
Triathlon Coach, Athlete and Writer 
Author of Full Time and Sub-Nine: Fitting Iron Distance Training into Every Day Life
Web: enduranceworks.net

¹ Source: Toone, Rebecca J. and James A. Betts, “Isocaloric Carbohydrate Versus Carbohydrate-Protein Ingestion and Cycling Time-Trial Performance,” International Journal of Sport Nutrition and Exercise Metabolism 20 (2010): 34-43.

© 2010 David B. Glover