Amino acids are organic compounds known as the building blocks of protein, that perform diverse biological functions in the human body.
The body uses them for a wide range of biochemical processes, including tissue repair and growth, hormone synthesis, neurotransmitter production, and muscle development .
Even though universal dietary allowances are expressed in protein, the human body has an essential biological need for amino acids.
In total, there are about 500 non-proteinogenic (protein-creating) amino acids identified on earth, with only 20 being present in the human genome .
From the 20 amino acids, 9 are considered “essential”, which means they can not get synthesized by the body, and need to come through diet.
They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The best dietary sources of essential amino acids (EAAs) include eggs, meat, dairy, fish, seafood, nuts, seeds, and legumes.
The rest 11 amino acids are considered non-essential because they can be synthesized by the liver from glucose and other metabolites, including other amino acids.
They include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, proline, serine, and tyrosine.
Branched-chain amino acids (BCAAs) are 3 of the 9 essential amino acids our body needs to obtain from food, namely, leucine, isoleucine and valine.
The term “branched-chain” refers to their chemical structure which has a side chain with extra carbons.
While most amino acids are metabolized in the liver, BCAAs along with asparagine, aspartate and glutamate are metabolized mainly in muscle tissue.
Muscle mass in humans equates to about 40% of total body weight, meaning that the muscle protein pool represents a very big reservoir of BCAAs for the body.
When muscles get rapidly broken down in cases of severe catabolism, as happens in certain diseases or big caloric deficits, BCAAs are the first amino acids to be oxidized .
For this reason, they have a solid reputation as an effective, anti-catabolic agent and ergogenic aid.
Leucine: The Anabolic Amino Acid
The branched‐chain amino acid leucine is a master regulator of muscle protein synthesis, insulin signaling, and glucose sparing, through the stimulation of the glucose-alanine cycle.
Leucine acts both as a ‘trigger’ for the initiation of protein synthesis, as well as a building-block for newly synthesized muscle tissue.
Muscle Protein Synthesis (MPS) is a set of biochemical processes through which the body synthesizes new proteins, including new muscle tissue.
Muscle growth occurs only if muscle protein synthesis (MPS) is higher than the catabolic process of muscle protein breakdown (MPB).
Muscle protein synthesis is essentially the driving force behind all exercise-induced adaptations .
Leucine, as with all BCAAs, is mainly metabolized within skeletal muscles, leaving open the possibility that its metabolites possess anabolic properties.
One metabolite specifically, called HMB (β‐hydroxy‐β‐methyl butyrate) has shown to exert positive effects on lean body mass and strength following exercise, as well as in disease‐related muscle wasting .
However, it’s effects on acute muscle protein breakdown remain unclear.
Now, let’s have a look at how leucine affects muscle growth.
During training, muscle protein synthesis (MPS) decreases, while protein turnover and BCAAs oxidation increases.
The decrease in muscle protein synthesis (MPS) is associated with the inhibition of specific proteins or translation factors, called 4E, 4G, and ribosomal protein S6 .
Leucine stops that inhibition through the activation of a protein called mTOR (mammalian target of rapamycin), allowing the upregulation of muscle protein synthesis.
Leucine is a powerful stimulator of mTOR, a signaling molecule for muscle anabolism.
Why Are BCAAs Considered More Anabolic Than Other Amino Acids?
BCAAs and especially leucine, are said to be powerful promoters of a protein called mTOR (mammalian target of rapamycin) .
The main property of mTOR is the initiation of various anabolic, intracellular processes that stimulate muscle protein synthesis (MPS).
Indeed, amongst the properties of androgenic anabolic steroids (AAS) is the elevated activation of mTOR .
While BCAAs may promote mTOR, they may also decrease the rate of muscle protein breakdown (MPB), something of great importance, since muscle growth occurs only when MPS is upregulated and MPS is downregulated.
Muscle growth is a very expensive metabolic process for the body and requires a net anabolic environment, where organic molecules, such as muscle proteins, are being synthesized from smaller ones, like amino acids.
The anti-catabolic effects of BCAAs, most likely result from their leucine content.
Leucine also stimulates the secretion of insulin, a glucose transporting anabolic hormone, which stops proteolysis- the process of protein breakdown .
They both act synergistically on mTOR via the phosphoinositol 3-kinase signaling pathway, constituting potent anabolic agents .
Insulin and leucine allow skeletal muscles to coordinate the process of muscle protein synthesis along with other factors, such as the body’s metabolic status and nutritional intake.
Do BCAAs Increase Athletic Performance?
BCAAs consumption when training for longer than 2 hours per day is said to assist in sparing already existing glycogen stores in the muscles and liver.
Glycogen is a form of stored carbohydrates and its retention indirectly promotes greater body fat utilization as an energy source, especially the kind found inside the muscle (intramuscular fat).
Using BCAAs during prolonged training sessions may result in reduced fatigue and increased muscular endurance.
The proposed mechanism for this effect was suggested some years ago by some studies showing that BCAAs supplementation interferes with the uptake of tryptophan in the brain.
L- tryptophan is an amino acid converted by the enzyme 5-HTP decarboxylase to serotonin, known as the feel-good neurotransmitter.
Serotonin provides a feeling of relaxation and tiredness, which is ideal when someone wants to go to sleep, but counterproductive during exercise.
Exercise increases serotonin synthesis in the brain, which induces central fatigue.
BCAAs compete with tryptophan, and due to their chemically larger structure can block its entry to the brain, preventing serotonin synthesis .
Another interesting biochemical property of BCAAs is that leucine is an exclusively ketogenic amino acid, which means that it can be converted only to ketones and not glucose.
On the other hand, isoleucine can be converted to both ketones and glucose, while valine can be converted only to glucose (exclusively glucogenic).
Ketones are metabolic acids, by-products of fat metabolism, particularly well-known to those who prefer low carbohydrate diets.
Low carb diets tend to increase ketone levels in the blood, leading to the state of ketosis.
NOTE: Some people confuse ketosis with diabetic ketoacidosis, which are two completely different metabolic states. Ketoacidosis is a pathological condition of abnormally high ketone levels due to insulin insufficiency, occurring mainly as a result of uncontrolled diabetes.
Ketone production aids in weight-loss, because ketones tend to suppress appetite.
They also can be used as an alternative energy source to glucose by the brain and muscles.
In practice, however, if someone consumes BCAAs and consumes adequate calories each day, the likelihood of them being converted to ketones or glucose is very low.
Additionally, many people disagree with the use of BCAAs as a dietary supplement.
Personally, I have been suggesting for years to all my clients the use of essential amino acids (EAAs) instead of BCAAs when it comes to maximizing athletic performance.
Recently, a study came to validate my point.
In a meta-analysis conducted in 2017, it was shown that BCAAs alone cannot support an increased rate of muscle protein synthesis, due to a shortage of the other 6 essential amino acids .
When ingested, BCAAs prompted the body to acquire the missing amino acids by breaking down its own muscle tissue.
Put another way, BCAAs not only do not increase MPS but in actuality promote muscle catabolism.
BCAAs alone cannot increase muscle protein synthesis due to the shortage of the other 6 essential amino acids.
BCAAs or Whey Protein?
One very common question about BCAAs is whether they can replace the use of whey protein as a dietary supplement.
This essentially translates to “Are three essential amino acids (leucine, isoleucine, valine) able to induce the same levels of muscle protein synthesis as a complete protein supplement, such as whey?”
The answer is no.
First of all, whey is a whole protein source that contains all the 9 essential amino acids, alongside other milk-derived anabolic factors that benefit muscle growth.
Also, whey protein is the richest natural source of BCAAs among all other dietary protein sources, even eggs.
Secondly, the aforementioned study proved that BCAAs alone actually decrease MPS, which renders them useless, if not harmful, in the promotion of muscle growth.
The body needs all the 9 essential amino acids to build new muscle tissue.
For that reason, consuming whey protein or even better- eating whole, animal foods, will supply your body with the building blocks required to create lean muscle mass.
When it comes to protein sources for building muscle, your priority should go like this: BCAAs < Plant-Based Proteins < Whey protein < Wholesome, animal products.
So if time is on your side, you’d much better off scrambling 4 whole eggs than drinking a scoop of whey.
In this way, you’ll also be certain that you are in fact ingesting 24 gr. of protein (about 6gr. per egg), since supplement companies are notorious for claiming bigger protein percentages than those actually present in their product (amino spiking) .
On the other hand, if you can’t tolerate or don’t like dairy and want to invest in a protein supplement, go for a high-quality formula that includes multiple sources of plant protein (preferably not soy).
PS. If you haven’t already, you may check out our Recommendations List for high-quality supplements, health products and services you can trust. There is probably nothing health-related you won’t find there + special discount codes are waiting for you.
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About George Kelly
George Kelly M.Sc. is a Registered Dietitian Nutritionist that specializes in chronic and autoimmune conditions. He is the CEO and lead author of Metabolic Body.
This content is for informational and educational purposes only. It is not intended to provide medical advice or to take the place of such advice or treatment from a personal physician. All readers/viewers of this content are advised to consult their doctors or qualified health professionals regarding specific health questions. Neither Metabolic Body nor the publisher of this content takes responsibility for possible health consequences of any person or persons reading or following the information in this educational content. All viewers of this content, especially those taking prescription or over-the-counter medications, should consult their physicians before beginning any nutrition, supplement or lifestyle program.