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Feeling Tired? BCAAs and the Central Fatigue Hypothesis

By Bob Seebohar

Fatigue during prolonged exercise has traditionally been associated with mechanisms that result in dysfunction of the contractile process with muscle. The branched chain amino acids (BCAAs), leucine, isoleucine and valine, have gained much attention as to a possible central mechanism of fatigue originating in the brain. Let’s take a peek into more of the details surrounding the central fatigue hypothesis and the impact on which BCAAs can have.

It is theorized that during endurance exercise, excess serotonin (5-hydroxytryptamine or 5-HT) crosses the blood-brain barrier and causes fatigue. There is a large amount of evidence linking alterations in 5-HT activity to various psychological responses such as arousal, lethargy, sleepiness and mood, all of which may play a role in the central mechanisms of fatigue.

Increased brain 5-HT synthesis occurs in response to an increased delivery to the brain of blood-borne tryptophan (TRP), and amino acid precursor to 5-HT (and by the way in high amounts in the infamous Thanksgiving turkey!). Most of the TRP in blood circulates loosely bound to albumin, but it is the unbound of free tryptophan (f-TRP) that is transported across the blood-brain barrier.

This transport occurs via a specific mechanism that TRP shares with other large neutral amino acids, most notably the BCAAs. Thus, brain 5-HT synthesis will increase when there is an increase in the ration of the concentration of f-TRP in blood to the total concentration of BCAAs. It has been proposed that this would occur during prolonged exercise as BCAAs are taken up from the blood and oxidized for energy in contracting skeletal muscles and plasma free fatty acids (FFAs) increase in the blood, causing an increase in f-TRP because FFAs displace TRP from its usual binding sites on plasma albumin molecules, thus allowing more TRP to be available to the brain.

When the ratio of TRP to BCAAs increases, more serotonin enters the brain (thus causing fatigue). Altering this ratio by increasing BCAAs or carbohydrates allows less TRP into the brain (thus causing less or delayed fatigue).

Studies on the proposed role of BCAA supplementation are limited and are somewhat hampered by a lack of good methodologies (the design of the study was not favorable for deducing any definitive conclusions).  However, it is known that carbohydrate supplementation is associated with large decreases in f-TRP and fatigue is clearly delayed by using this nutritional strategy.
  
Keep in mind that it has not been possible yet to distinguish between the effects of carbohydrate ingestion on central fatigue mechanisms and the well-established beneficial effects of carbohydrate supplementation on the contracting muscles.

The take home message is that it is known (and has been for some time) that carbohydrate feeding delays fatigue, although whether it affects the brain or muscles more so is still in debate. For now, continue to use carbohydrate and experiment with BCAAs by introducing them into your more quality-based training sessions to determine if they do provide benefit for you in reducing your central fatigue.

Nutrition Periodization for Endurance AthletesBob Seebohar, MS, RD, CSSD, CSCS is a sport dietitian and elite triathlon coach.  He traveled to the 2008 Summer Olympics as the U.S. Olympic Committee Sport Dietitian and the personal Sport Dietitian for the 2008 Olympic Triathlon Team.  He is also Sarah Haskins' personal coach and was a performance team member (sport dietitian and strength coach) for Susan Williams, 2004 Olympic Triathlon Bronze Medalist.

Bob's book, Nutrition Periodization for Endurance Athletes: Taking Sports Nutrition to the Next Level will provide triathletes of all levels education on how to structure their nutrition program based on their exercise program. For more information, visit www.fuel4mance.com or contact Bob at coachbob@fuel4mance.com

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