Branched Chain Amino Acids (BCAA)

Branched Chain Amino Acids (BCAA) are three specific amino acids, leucine, isoleucine and valine. Their name comes from their structure; they all have a carbon side chain, or a branch. Their importance is due partly to the fact that they are essential amino acids, i.e. the body can’t synthesise them, and they must be obtained from the diet. Like other amino acids, they are included in the primary sequence of many proteins, and are therefore essential to the synthesis of these proteins. However, they also have other functions very specific roles in the control of metabolism, that make them of great interest to athletes, bodybuilders, and people interested in weight control.

BCAA Metabolism in Muscle Cells

Normally, amino acids must be transported to the liver for metabolism. BCAAs are different; they can be used as fuel directly by muscle cells. This has very significant implications for muscle function. The reasons the work done my your muscles is limited is because they run out of fuel in the form of ATP. Normally we think of the oxidation of glucose as the source of ATP, but when glucose runs out the body uses other methods, such as converting glycogen and amino acids into glucose, which can then be used to produce more ATP. These methods are relatively slow, however, because they take place in the liver, and the glucose produced has then to be transported back to the muscle cells. The great advantage of metabolising BCAA amino acids for energy is that it happens right in the muscle cells where the extra ATP is needed.

Loading up the muscles with BCAA amino acids before exercise therefore increases the amount of fuel available to muscle cells for contraction. Scientific studies have shown that a side effect of BCAA supplementation is increased endurance during a workout.

BCAAs Stimulate Protein Synthesis

It was shown some time ago that BCAA amino acids, especially leucine, activate the mTOR pathway. mTOR (mammalian Target of Rapamycin) is a protein, and an essential component of the a complex that acts as a sensor of nutrient levels. When cells are supplied with a lot of nutrients, including amino acids, protein synthesis proceeds at a high rate, cells proliferate, and the body grows. On the other hand, during periods of malnutrition when nutrient levels are low, growth is minimal and the rate of protein synthesis and cell proliferation is very slow.

You might wonder how cells ‘know’ what the nutrient levels are. The answer is that the mTOR complex acts as a sensor. However, it does not measure the levels of all amino acids, just leucine. The idea is that if leucine levels are high, then presumably other amino acids are also abundant.

When leucine levels are high, the mTOR protein is phosphorylated, which activates it. When active it phosphorylates the proteins, called initiation factors, which are necessary for protein synthesis to start at the ribosomes. mTOR also activates transcription factors which stimulate the expression of genes of the components of ribosomes, the tiny organelles where translation (the conversion of nucleotide sequence of messenger RNA into amino acid sequence of proteins) occurs. This increases the number of ribosomes, and therefore the cells’ capacity for building protein.

A supplement that encourages protein synthesis and results in muscle gain is obviously important for bodybuilders and athletes. However BCAAs are also important for people who are trying to lose weight. One of the bad things that happen when you go on a calorie-restricted diet, is that you lose muscle tissue as well as fat. Since BCAAs stimulate protein synthesis, they prevent muscle loss, in fact they are given in hospitals to long-term bedridden patients to slow down muscle wasting. Clearly when you are trying to lose weight, you want to lose fat rather than muscle mass. This is particularly important since muscle has a higher metabolic rate than fat. Maintaining your muscle not only looks better, but it also aids in further weight control. Because of their role in stimulating protein synthesis and preventing muscle wasting branched chain amino acids are of interest to people trying to achieve their ideal weight, as well as body builders.

BCAA Help Control Blood Sugar Levels and appetite

Another problem faced by people trying to reduce weight using low carbohydrate diets and/or exercise, is the drop in blood sugar levels, which results in hunger hormone release, which induce cravings, especially for sugary foods. It is very difficult to diet based on willpower alone, the physiological urge to eat food that will cause a blood glucose spike is very difficult to resist. Exercise, by using up glucose and glycogen only makes things worse, the increase in appetite after exercising is something we have all experienced.

Luckily researchers now think that BCAAs can help with these problems. Studies show that branched chain amino acids can help with glycaemic control, preventing blood sugar level from falling too low. They also appear to increase the beta oxidation of fats, as well as directly dampening appetite through their actions on the mTOR and cAMP pathways. A population study of over 4000 subjects, found that there was a good inverse correlation between obesity and BCAA consumption.

The Effects of BCAA on Hormone Balance

There is also evidence that BCAA supplementation inhibits the release of the stress hormone cortisol after exercise. Cortisol is a catabolic hormone, which acts to stimulate the breakdown of muscle proteins into free amino acids, and inhibits protein synthesis. Cortisol also inhibits the release of the anabolic hormone testosterone. BCAA supplementation therefore balances the body’s hormones towards an anabolic state, in which protein synthesis and muscle gain can proceed uninhibited.

It is also possible that the appetite suppressing functions of BCAAs might be due, in part to their inhibition of cortisol release. Cortisol does appear to act on the hypothalamus in the brain to induce cravings, especially for sweet or starchy foods. A study of middle- aged women found that there was a significant correlation between cortisol levels and central obesity.

The unique properties of BCAAs result in multiple positive side effects. They increase the amount of fuel available for muscle cells, and therefore improve endurance during a workout. They stimulate protein synthesis, which results in better muscle gain after exercise, and less muscle loss when on a reduced calorie diet. They also increase fat oxidation, help regulate blood glucose levels, and act on the brain to repress appetite, making dieting easier.


Further Reading

The role of leucine on glucose homeostasis:

The role of leucine in appetite control:

Correlation between BCAA supplementation and Weight:

BCAA and protein synthesis:

Branched chain amino acids and exercise endurance: