The branched chain amino acids (BCAAs for short) are the most abundant of the essential amino acids.
There are three branched chained amino acids: Leucine, IsoLeucine and Valine, and they are part of the essential amino acids which make them indispensable for life.
Your body cannot make these amino acids out of other compounds the way it can with the non essential amino acids, therefore you must consume them on a semi regular basis in your diet.
The BCAAs are well known for their important role in muscle growth by signaling protein synthesis through the mTOR pathway. This is part of the reasoning behind using a High Protein Diet. During the digestion process the proteins we consume are broken down into individual amino acids and these amino acids then enter circulation. They can either be used to build new proteins or be burned as fuel to produce energy.
Dietary sources of the branched chain amino acids include most high protein foods, such as dairy, red meat, poultry, eggs and protein supplements (whey protein being the most popular).
As an example, one glass of milk contains about 1 gram of Leucine and about 3.5 grams of total BCAAs. One egg contains about 0.7 grams of leucine and about 1.7 grams of total Branched chain amino acids.
If your diet is adequate in nutritional needs (proteins and calories) it is believed that the branched chain amino acids will be used for protein synthesis which is considered optimal by people looking to build muscle.
This is the reason why BCAAs and Amino Acid supplements continue to sell consistently as sports-supplements and maintain their popularity among bodybuilders.
It is also why amino acids are recommended as a part of most Post Workout Protein plans.
BCAAs are unique because they are poorly metabolized by the liver compared to other amino acids when they’re first ingested. This means that BCAAs can act as a signal to the skeletal muscle and the brain letting them know the amino acid content of the incoming meal.
By acting as a nutrient signal the BCAAs let the body know the nutrient status of the food you just ate. If the levels are high enough, they can signal for the initiation of protein synthesis. This signaling occurs through the activation of the mTOR signaling pathway.
The BCAAs (specifically Leucine) do in fact initiate protein synthesis. This is true - Leucine stimulates protein synthesis. Most people only think of muscle growth when they read the words ’protein synthesis’, but it doesn’t stop there. A little known fact is that they ALSO stimulate protein synthesis in your body fat through the same mTOR pathway we already talked about.
So leucine stimulates protein synthesis in your body fat too.
You may be wondering what the heck is mTOR doing in bodyfat and why does fat need protein synthesis?
Well, in adipose tissue the mTOR signaling pathways play a specific role in the differentiation of pre-adiposities, adipose tissue morphogenesis, hypertrophic growth and leptin secretion. Also downstream molecules from mTOR are novel regulators of adipogenesis and metabolism.
Really this is all just a fancy way of saying that Leucine signals for growth in muscle AND adipose (fat) tissue (and probably a whole host of other tissues too).
In this way, you can think of Leucine and the other BCAAs as nutrient signals in the body - telling both muscle and fat that there is an abundance of energy, and acting as some sort of signal of the amount of amino acids that were contained in the last meal you ate.
Now obviously a little bit of leucine can’t cause you to gain fat (you need a caloric surplus to do that) but it can signal for your bodyfat to prepare to increase in size. In this way, it could be considered a ’first step’ in the fat gaining process.
Interestingly, in obesity there is an elevated Leucine and branch chain amino acid concentration in the blood.
In the body, Leucine also acts to signal for an increase in insulin secretion (Yes, Amino Acid can stimulate Insulin). This means Leucine acts on adipose (fat) via the mTOR pathway, but also through increasing the insulin levels within the body (signaling for nutrient storage).
In fact, Leucine may affect adipose tissue growth through two different mechanisms:
1) mTOR leading to increased protein synthesis
2) Through insulin mediated pathways
There are at least two lines of research that link branched chain amino acids to adipose tissue metabolism.
1) Adipose tissue is capable of metabolizing significant quantities of branched chain amino acids
2) Adipose tissue has been hypothesized to be the major site where excess BCAAs are stored in the form of lipids (transformed into body fat).
So not only does Leucine signal for an increase in fat mass hypertrophy, but it’s also oxidized by fat mass (burned as a fuel source) and is converted into fat and stored during times of excess intake.
Based on this research it seems clear that protein has its own unique way of signaling to the body. When there is an excess intake higher that it needs at the moment, it can signal for the changes in adipose tissue needed to support the storage of protein as body fat, and itself can be used as fuel by body fat. This seems to happen when plasma Branched Chain Amino acid levels are elevated. This could be considered a safety measure to prevent long-term exposure to elevated branch chain amino acids in circulation.
Lastly it seems as though these changes can occur in the absence of insulin, suggesting that these effects are specific to protein, and do not require the presence of carbohydrates.
These findings seem to dispute the old body-builder belief that extra protein intake cannot be turned into body fat, when in fact, not only can it be turned into body fat, it can also signal multiple pathways leading to the growth of body fat stores.
Interestingly it seems that fasting can cause a rapid decease in the ability of adipose to degrade BCAAs. This has been suggested as a way that fasting preserves BCAAs for other uses in the body, including the maintenance and preservation of skeletal muscle mass.
While this research definitely increases our understanding that Leucine is a WHOLE BODY signal for growth, and that leucine acts as a indictor of amino-acid status, it does not suggest that we should AVOID leucine.
Rather, it shows that leucine plays a very important role as part of our overall diet and nutrition, but may not need to be supplemented in excess.
I’m still undecided if there is any benefit to leucine supplementation during a short period of intermittent fasting, such as Eat Stop Eat, especially considering the role that Amino Acid and Insulin may play in decreasing the fasting response
For this reason, I still recommend an average daily protein intake in the same range as I suggest in "How Much Protein?"
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