Phun Physiology: Branched-Chain Amino Acids

What are branched-chain amino acids (BCAAs), the claims being made about them as they relate to endurance athletic performance, and the reality?

If the following commercial claims do not provide sufficient incentive for you to want to try BCAAs, arouse a little intellectual curiosity, or make you wonder whether your cycling buddies haven’t intentionally withheld vital information from you in order to retard your development and prevent you from assuming your rightful place on the podium, don’t worry. There’s more!

Ever since the 1970s, when it was first thought that BCAAs were legal performance enhancers, there has been much interest in these three naturally occurring essential amino acids on the part of nutritionists and athletes, not to mention commercial purveyors.

In fact, just this week, I received two different e-mails from a well-known nutrition-supplement chain store touting the alleged benefits of BCAAs. Here is a portion of the claim for one of the products.

Branch Chain Amino Acids (BCAA) – leucine, isoleucine and valine – are critically important for stimulating muscle protein synthesis, reducing protein breakdown and preserving muscle glycogen stores.* The body uses these essential amino acids as fuel during exercise.*

The asterisks of course alert us to the following claim . . .

*These statements have not been evaluated by the Food and Drug Administration.
Another BCAA product marketed by the same retailer comes with a slightly different set of claims, also accompanied with an asterisk:

BCAA's [sic] enter your bloodstream and attach directly to muscle where they repair damaged muscle tissue. This process helps ensure maximum muscle recovery and growth!*
I then searched for the claims by a competitor regarding its BCAA formulation:

· Decreases perception of fatigue and increase cognitive performance.
· Helps build, maintain, and repair lean muscle mass.
In all, I’ve been able to compile a list of ten (10) alleged benefits of BCAAs by commercial vendors that I wish to discuss. The claims include: 1) performance enhancement as a third energy source after carbohydrates and lipids, 2) glycogen preservation, 3) faster muscle recovery, 4) reduced muscle damage, 5) reduced muscle soreness post-exercise, 6) reduced mental fatigue, 7) increased cognitive capacity post-exercise, 8) decreased muscle wasting, 9) performance enhancement in heat, at altitude, and in other situations, and 10) immune system support.

Before getting down to the business of addressing each of these claims vis-à-vis the scientific literature, I shall provide some background information on BCAAs and amino acids in general.

No doubt humans require protein. Dietary protein is digested enzymatically by our body into smaller molecules known as amino acids, which in turn are immediately absorbed by the small intestine. After entering each of our body’s cells, the amino acids are then used as building blocks for the manufacture of tens of thousands of different human proteins, including muscle tissue, which is largely protein.

Twenty types of amino acids are required by the body. Our cells have the ability to synthesize all but eight of the 20 amino acids. The eight we cannot synthesize are called “essential” amino acids, because they can only be obtained from our diet. The BCAAs—valine, leucine, and isoleucine—are three of the eight essential amino acids the body requires but cannot synthesize.

Why are BCAAs important to athletes? It is true that BCAAs, particularly leucine, play an important role in muscle synthesis. Not only do BCAAs take their place alongside other amino acids as raw materials in protein synthesis, but leucine acts uniquely as a signaling molecule that initiates post-exercise protein synthesis.

The importance of ingesting leucine post-exercise becomes abundantly clear when we learn that its intracellular concentration diminishes as a direct function of the duration of endurance exercise. The longer we exercise the less leucine there is to trigger post-exercise muscle protein synthesis.

Where do BCAAs get their name? All amino acids have the same general structure:
Note that the central carbon (C) atom has four bonds depicted as lines radiating out like points on a compass to four different groups called “side chains.” Three of the side chains are invariable. The variable fourth side chain (in green) determines the particular amino acid, in this case glycine. The BCAAs get their name from the fact that the variable fourth side chain is branched.
Locate the branched side chain located on the “south” side of each of the three BCAAs. Imagine valine as an aerodynamic Tour rider on a road bike, leucine as an upright randonneur, and isoleucine as a trick rider doing a wheelie.

Now it is time to unveil my list of ten alleged benefits of BCAAs followed by critical commentary:

1. Since the 1970s, assertions keep popping up that BCAAs act as fuel during exercise in addition to carbohydrate and fat. To the contrary, BCAAs are not performance enhancing in the sense that they provide a significant or “third” energy source for endurance athletes. Thus, the commercial claim quoted above that, “The body uses these essential amino acids as fuel during exercise,” is highly misleading. While it is true that amino acid breakdown during exercise may result in some energy production, it is trivially true.

According to one expert, a Tour de France cyclist consumes only twice the amount of BCAAs during competition as a sedentary individual. Compare this to both carbohydrate and fat consumption, where detailed studies have shown that

the oxidation of BCAAs only increases 2- to 3-fold during exercise, whereas the oxidation of carbohydrate and fat increases 10- to 20-fold.

Moreover, the same researcher argues that when athletes fuel primarily with carbohydrates as they normally do, BCAA oxidation slows.

[C]arbohydrate ingestion during exercise can prevent the increase in BCAA oxidation. BCAAs, therefore, do not seem to play a major role as a fuel during exercise, and from this point of view, the supplementation of BCAAs during exercise is unnecessary.
Yet another research team concludes that

Leucine oxidation increases in proportion to energy expenditure, but the total contribution of BCAA to fuel provision during exercise is minor and insufficient to increase dietary protein requirements.
2. The claim made by the first commercial provider above, that BCAAs play a role in “preserving muscle glycogen stores” is also misleading, again, because it is only trivially true.

We just noted above that during exercise the relationship between BCAAs and carbohydrates is the reverse of this last claim; that is, carbohydrate ingestion during exercise slows BCAA oxidation. But since BCAAs are gradually depleted as a function of exercise duration, it is not altogether clear how they could preserve muscle glycogen as claimed.

While it is true that BCAAs do cause glucose synthesis and subsequent gluconeogenesis (glycogen synthesis), it is difficult to understand the significance of these events in the grand scheme of muscle energetics in light of each of the previous comments.

3. There is good evidence that BCAAs aid muscle recovery. In fact, the mechanism by which leucine is thought to initiate post-exercise protein synthesis is well known.

And although researchers tell us that

endurance exercise reduces the rate of muscle protein synthesis in proportion to the duration and intensity of activity,
they remind us that the post-exercise ingestion of a combination of leucine and carbohydrate allows maximum stimulation of protein synthesis.

4. BCAAs do seem to reduce muscle damage that results from exercise, according to many experts, one of which notes:

BCAA supplementation before and after exercise has beneficial effects for decreasing exercise-induced muscle damage and promoting muscle-protein synthesis.
5. In some situations, BCAAs may reduce post-exercise muscle soreness and fatigue. One group of researchers found that delayed-onset muscle soreness (DOMS) that results from resistance exercise is attenuated by BCAA supplementation.

BCAA supplementation prior to squat exercise decreased DOMS and muscle fatigue occurring for a few days after exercise.

6. There seems to be good evidence that BCAAs can reduce mental fatigue, and that

7. BCAAs improve cognitive function post-exercise. First a note on what is meant by physical fatigue.

There are two components of physical fatigue that affect endurance athletic performance, that is, the ability to maintain power output. These are the peripheral and central (mental) components.

Peripheral factors affecting physical fatigue include failure in neuromuscular signaling, waste buildup, and muscle energy store depletion, for example.

Central (mental) factors of physical fatigue are less well known. Central fatigue however can be demonstrated by the fact that willful maximal muscle effort is less than that which can be achieved when the motor nerve to the same muscle is electrically stimulated.

According to a well respected study by a Swedish researcher,

[W]hen BCAAs were supplied to human subjects during a standardized cycle ergometer exercise their ratings of perceived exertion and mental fatigue were reduced, and, during a competitive 30-km cross country race, their performance on different cognitive tasks was improved after the race.
Part of the reason researchers believe that BCAAs reduce mental fatigue is that they seem to have cornered a mechanism for its action. In short, it is thought that BCAAs block the uptake of a particular serotonin precursor (the amino acid tryptophan) in the brain. Serotonin (5-HT) is thought to play a role in causing central fatigue.

8. BCAAs are currently being studied as a way to slow muscle wasting in the elderly. Although this has little to do with endurance athletes in their prime, muscle wasting known as sarcopenia is a direct function of aging processes. Medical researchers assert:

[L]ong-term essential amino acid supplementation may be a useful tool for the prevention and treatment of sarcopenia, particularly if excess leucine is provided in the supplement.
9. Finally, the claim that arises from time to time regarding the alleged performance enhancing nature of BCAAs. Although some have suggested that BCAAs help athletes perform at moderate levels (40% VO2max) in the heat (34° C/ 93.2° F), the evidence is equivocal. So too is the suggestion that athletes perform better at altitude after ingesting BCAAs.

Generally speaking, no study so far suggesting a performance enhancing effect of BCAAs for endurance athletes has been able to withstand criticisms concerning methodology. One team of researchers reports that

A number of research groups examined whether BCAA supplementation might have a beneficial effect on endurance performance, but the results are inconsistent.

10. Although controversial, at least one study suggests that BCAAs decrease the negative effects of long-term strenuous exercise on the immune response. While exercise generally bolsters the immune response, long-term strenuous exercise as a stressor can have the opposite effect. Whether BCAAs benefit those who over train is currently debatable.

In sum, there are indeed great benefits that BCAAs provide for endurance athletes, in particular, benefits relating to muscle recovery, maintenance, and growth, but also cognitive function and reduced physical fatigue caused by mental factors. On the other hand, claims regarding muscle energetics and increased performance are generally viewed by most experts as dubious at best.

What about the toxicity of BCAAs? Most athletes are aware of the problems associated with ingesting too much protein: kidney damage, arteriosclerosis, and dehydration, for example. The same prohibitions apply to BCAAs, although there don’t seem to be any problems inherent to them. One researcher notes that

Acute intake of BCAA supplements of about 10-30 g/d seem to be without ill effect.
Finally, one group of researchers assures us that

There are no reports concerning BCAA toxicity in relation to exercise and sports.

What about cost and availability of BCAAs? One can easily compare costs of supplements on the one hand with natural foods on the other. One researcher notes
a typical BCAA supplement sold in tablet form contains 100 mg of valine, 50 mg isoleucine, and 100 mg leucine. A chicken breast (100 g) contains ~470 mg valine, 375 mg isoleucine, and 656 mg leucine, the equivalent of about 7 BCAA tablets.

Plant products also contain BCAAs. The previous researcher reports that in fact,

One quarter cup of peanuts (60 g) contains even more BCAA and is equivalent to 11 tablets.

Endurance athletes need more protein than non-athletes but not as much as body builders. Most of us probably get as much protein as we need. The amount of protein we consume on a daily basis can easily be calculated and compared with recommended amounts.

The best time to consume complete proteins (along with carbohydrates to ensure maximal protein synthesis) is immediately after exercise, when the machinery of protein synthesis is literally waiting in the wings for raw material, a pool of exogenous amino acids. Complete proteins contain all 20 amino acids, including ample amounts of BCAAs. Examples include beans and rice or food of animal origin.

A little protein ingested during exercise may also help with sundry physiological processes like hydration given the fact that there are amino acid transporters whose activity aids intestinal water absorption, thereby aiding quick and maximal hydration.

Regarding the purchase of BCAAs, one can obtain powders of complete proteins that contain sufficient amounts of BCAAs at grocery stores or online and which generally cost much less than commercially supplied BCAA caplets.

The amounts of BCAAs in everyday foods, including plants material, can be found at web sites like this. Some of the highest levels of leucine are found, for example, in soy protein and spirulina, a sea weed. The optimal ratios of BCAAs occur naturally in food.

As parting advice, I wouldn’t become too anxious about BCAAs as long as I knew that I was getting sufficient amounts of (complete) protein in my diet. Researchers note that BCAAs consumed in excess are simply excreted by the body.

We’ll get a meal as soon as we finish, but for now just grab a burger or a pack of peanuts and . . .

Let’s ride!