During the digestion process, whole-food proteins, such as milk, meat and eggs, are broken down into their elemental parts, which are amino acids. Twenty-two dietary amino acids have been identified. Eight are considered essential, in that they cannot be synthesized in the body but have to be supplied by food. While all essential amino acids are vital for making gains in muscular size and strength—indeed, recent studies show that only the essential aminos are required in muscle protein synthesis—some of them are more vital than others.
Three—valine, isoleucine and leucine—are collectively known as the branched-chain amino acids, so named because of their molecular structure, which features branched chains of carbon extending from their primary carbon structure. The BCAAs are unique because they’re not metabolized in the liver but in muscle instead. That’s why they’re called muscle aminos.
Leucine stands out as having the most anabolic properties. When you see ads for supplements that claim to “overcome genetic limitations” and that feature complicated diagrams of protein synthesis, you’re probably looking at an illustration of BCAA molecules, especially leucine.
BCAAs make up more than a third of the minimum daily requirement for essential aminos. They account for 15 percent of the total amino acid content of food proteins. Some popular protein supplements, such as whey, are effective primarily because of their rich BCAA content, although other aminos in whey, such as cysteine, offer considerable health benefits.
Controversy dogs the BCAAs. Many scientists suggest that endurance athletes need more protein than strength athletes because extended exercise taps into protein as an energy source. The aminos primarily used for energy purposes in muscle are the BCAAs.
Ammonia is a by-product of BCAA metabolism in muscle. While a high level of ammonia production is linked to fatigue, the ammonia produced in muscle by BCAA metabolism converts into two other amino acids, glutamine and alanine, which are energy substrates. Alanine gets transported to the liver, where it’s converted into glucose.
Early studies concluded that the energy-producing effect of BCAAs was so potent that it constituted a third form of muscle energy, after carbohydrate and fat. Later studies, however, showed that the enzymes involved in BCAA oxidation are too limited to affect muscle energy. Using radioactive-tagged leucine, researchers observed that BCAA oxidation increases—at most—by a factor of three. Compare that with the 10-to-20-fold increase in carbohydrate and fat oxidation. Besides, taking in carbs before or during a workout prevents the use of BCAAs for energy, which is good, as BCAAs are more suitable for muscle-building purposes.
BCAAs offer a definite protein-sparing action in muscle during low-carbohydrate dieting. When glycogen stores in muscle and the liver are low as a result of limited carb intake, the body tends to tap into muscle aminos as a source of energy. That’s more likely to happen when total calories are very low in relation to exercise or when bodyfat is below a certain point. The leaner you are, the higher the risk of tapping into muscle amino acid stores, especially during aerobic exercise. Taking BCAAs before cardio spares—that is, doesn’t gobble up—muscle aminos, thus preventing excess muscle protein breakdown during training.
Another controversy related to BCAAs is whether they improve exercise performance or efficiency. In one experiment, presented at the 2004 conference of the National Strength and Conditioning Association, six healthy men took either BCAAs or a placebo, then engaged in weight training. When they took BCAAs, their levels of the stress hormone cortisol and creatine kinase, an enzyme released during muscle breakdown, went down and their testosterone count went up. The authors noted that participants who had more bodyfat needed a bigger dose of BCAAs to experience any anabolic effect.
In a study of endurance exercise that used rats as subjects, researchers found that feeding the rats BCAAs led to a threefold increase in blood BCAA after only five minutes and gave them more stamina.1 The rats also generated more ammonia in their muscles because of BCAA metabolism.
Studies show a relationship between the oxidation, or burning, of fat in muscle and the subsequent oxidation of BCAAs in the body.2 Exercise promotes the activity of an enzyme (BCKDH) that controls BCAA oxidation. That implies that any exercise leading to fat oxidation also raises the requirement for BCAA intake, explaining why those engaged in endurance exercise, which uses fat as an energy source, need more BCAAs. The same may apply to those engaged in extensive aerobic exercise to lose bodyfat.
Too much exercise can depress immune function, exposing hard-training athletes to disease. Some studies have linked the immune suppression to both the increased presence of cortisol and the decreased presence of glutamine. Studies with endurance athletes show that when they supplement with BCAAs, their glutamine count and immune function hold steady.3 Glutamine is important because immune cells use it as a direct fuel source.
The relationship of BCAAs to glutamine synthesis raises the obvious question of whether it’s beneficial to use glutamine supplements if you’re also getting plenty of BCAAs. As with so many health issues, the answer is, it depends. For example, if you overtrain or are under industrial-strength stress, taking extra glutamine would be a sound idea. It counteracts the catabolic properties of cortisol and myostatin, and your body would prefer to conserve BCAAs for anabolic activity.
One great advantage of BCAAs is that they spare muscle under hard-training conditions. In an experiment that used swimmers as subjects, one group received BCAA supplements, while another group got a placebo. Those in the BCAA group experienced decreased muscle breakdown following intensive exercise.4
Elsewhere, researchers who observed the behavior of rat skeletal muscle found that BCAAs block catabolic pathways in muscle by activating ubiquitin lysosome enzymes.5
Some have suggested taking BCAAs before training as an anabolic stimulus, but they don’t come into play as an energy source except under extended low-carb dieting or with people who are way low in bodyfat. Nor do BCAAs contribute to protein synthesis during actual training; in fact, training blunts protein synthesis.6 The main concerns during training are energy production and maintaining muscle function. Protein synthesis kicks in after exercise. On the other hand, recent studies show that taking essential amino acids, including BCAAs, before exercise kick-starts anabolic processes. That’s because the increased blood flow you get from training promotes greater amino acid entry into muscle.
One study showed that BCAAs help diminish muscle damage during exercise and block what is called delayed-onset muscle soreness following intense training.7 Sixteen women and 14 men took five grams of BCAAs prior to doing seven sets of 20-rep squats, resting two minutes between sets. Some of the subjects got a placebo. Those who took the BCAA supplement had significantly less soreness than the placebo group, the effect being more pronounced in the male than female subjects. The authors suggest that the mechanism may be a combination BCAAs’ blunting muscle breakdown and leucine’s stimulation of muscle protein synthesis.
We already know that overtraining releases cortisol, which helps pump your stress up and break your muscle down. Some studies show that BCAAs trigger the metabolic processes that result in muscle protein synthesis. Other studies show that abundant cortisol interferes with BCAA metabolism, which may partially explain why cortisol is linked to muscle loss.8
Also acting against cortisol are anabolic hormones, such as testosterone, growth hormone and insulin. It turns out that BCAAs stimulate the activity of all three anabolic hormones, which may help explain why BCAAs spare muscle protein.9 Leucine in particular potently partners with insulin, which is anticatabolic and which, along with the essentital aminos, encourages muscle protein synthesis.
Still another controversial aspect of BCAA metabolism involves the relationship of BCAA intake to exercise fatigue. The literature lists five possible causes of exercise-related fatigue:
1) Depletion of muscle creatine
2) Increased metabolic acidosis in muscle
3) Depleted muscle glycogen
4) Lowered blood glucose
5) An increase in the ratio of the amino acid tryptophan to BCAA.
Take another look at item 5, which is known as the central fatigue theory, formulated in 1987. It seems that fatty acids and L-tryptophan compete for places to bind with a protein in the blood called albumin. During exercise, when free fatty acids increase in the blood, tryptophan gets bumped from albumin and travels to the brain, where it rapidly converts into serotonin, the brain chemical linked to relaxation, sleep and feelings of fatigue. BCAAs are in the picture because they compete with tryptophan for uptake into the brain. Their molecular structures enable them to get to the brain more easily than tryptophan, thereby blunting the fatigue effect.10
The problem with the central fatigue idea is that it’s unlikely to play a significant role in bodybuilding workouts, the notable exception being working out in the heat. That’s why you tire more when you train in a warm environment. Taking BCAAs could possibly energize you, but you probably couldn’t tell anyway if your workout is shorter than one hour.11 The central fatigue theory seems more relevant to long-distance endurance exercise. So really, you need to take another look at items 1 through 4 above.
Recent scientific findings show that of all amino acids, leucine is the most potent by far in relation to muscle protein synthesis. Some studies even suggest that taking leucine all by itself would be sufficient. It works both with and without insulin to get the muscle protein synthesis ball rolling.12
Several studies point to leucine as a means of maintaining muscle, while dieting to lose bodyfat. If you do aerobics while dieting, taking a BCAA supplement prior to training—a dose of about five grams ought to do it—will prevent muscle loss. To get the most out of the BCAAs, include a source of vitamin B1, or thiamine, which is required for BCAA metabolism. A B-complex vitamin would work.
Studies with older people show that a primary cause of frailty is muscle loss, known as sarcopenia. One reason that condition is so prevalent in older people is that age has blunted muscle protein synthesis. Studies show that such people respond favorably when provided with higher doses of BCAAs, particularly leucine.13, 14
One recent study found that depleting animals of leucine initiated a type of starvation response and led to a complete loss of bodyfat.15 After 17 days on their leucine-deficient diet, normal mice lost 48 percent of their liver fat and 97 percent of their abdominal fat. While that finding could lead to the development of new drugs in the war against obesity, don’t try it at home: Removing leucine from your diet would result in serious health problems. Yes, you would lose plenty of fat—but muscle too.
One thing that does impair leucine’s protein-synthesis effect is alcohol, which produces leucine resistance in muscle and shuts down muscle protein synthesis.15
While leucine is the most anabolic of the three BCAAs—indeed the most anabolic of any amino acids—your body still needs the other essential aminos. The usual suggested proportions for the BCAAs are 50 percent leucine, 25 percent isoleucine and 25 percent valine. Since the BCAAs compete with other amino acids for uptake into the brain, getting too much of one or another could lead to a drop in the synthesis of brain chemicals, such as serotonin, dopamine and norepinephrine. By and large, BCAAs are nontoxic and balanced in dietary intake. The only exception is with a genetic disorder specific to newborns, called maple syrup disease. So named because the urine smells like maple syrup (don’t add to pancakes!), it’s caused by a lack of enzymes that metabolize BCAAs, leading to an unhealthy buildup of the aminos in blood and brain. One result: mental retardation.
So do you need to take BCAAs? BCAA supplements do come in handy just prior to training, but if you’re taking a protein supplement, odds are you’re getting plenty. Consider the content of common protein sources:
Food: leucine, BCAAs
Whey protein isolate: 14%, 26%
Milk protein: 10%, 21%
Muscle protein (meat): 8%, 18%
Soy protein isolate: 8%, 18%
Wheat protein: 7%, 15%
To take or not to take; the decision is up to you.
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2 Shimomura, Y., et al. (2004). Exercise promotes BCAA catabolism: Effects of BCAA supplementation on skeletal muscle during exercise. J Nutr. 134:1583S-1587S.
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10 Blomstrand, E. (2001). Amino acids and fatigue. Amino Acids. 20:25-34.
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12 Anthony, J.C., et al. (2000). Orally administered leucine stimulates protein synthesis in skeletal muscle of postabsorptive rats in association with increased eIF4F formulation. J Nutr. 130:139-45.
13 Rieu, I., et al. (2006). Leucine supplementation improves muscle protein synthesis in elderly men independently of hyperaminoacidemia. J Physiol. 575:305-315.
14 Fujita, S., et al. (2006). Amino acids and muscle loss with age. J Nutr. 136:277S-280S.
15 Guo, F., et al. (2007). The GCN2 elIF2a kinase regulates fatty acid homeostasis in the liver during deprivation of an essential amino acid. Cell Metabol. 5:103-114. IM