Leucine – The Anabolic Amino


by Josh Hodnik

Amino acids play central roles as building blocks and as intermediates in metabolism. Amino acids also contribute and are also considered crucial for the health of the human nervous system and vital organs, hormone production, and cellular structure. There are over 20 amino acids present in the dietary protein that we consume. Amino acids are categorized as non-essential and essential. Non-essential amino acids can be synthesized by the body and don’t have to taken in through diet or supplement form. Essential amino acids can’t be produced in the body and must be taken in through foods or supplements that contain them.

While it’s known that amino acids are the building blocks of the body, specifically muscle tissue, all amino acids play a different role, and some have more influence on muscle tissue growth than others. Amino acid supplements have been a top seller among sports supplements for years. Today, branched-chained amino acids (leucine, isoleucine, and valine) top the list as the best selling amino acid supplement on the market. BCAAs are often considered a staple supplement among bodybuilders and top competitive athletes. Supplementing with BCAAs has been shown to increase protein synthesis and decrease muscle breakdown. This is a plus when someone is trying to increase muscle mass or limit muscle loss while in a caloric deficit. While branched-chain amino acids exhibit effects that bodybuilders can definitely benefit from, leucine may be the component doing most of the work.

Leucine is a branched- chain amino acid and one of nine essential amino acids, so it must be ingested. It’s utilized in the liver, adipose tissue, and muscle tissue. In adipose and muscle tissue, leucine is used to make sterols. While many amino acids play a role in building muscle tissue, leucine is the only amino acid that is proven to be anabolic by directly increasing protein synthesis.

Leucine is unique in its ability to stimulate protein synthesis. Leucine has an impact on protein synthesis that is up to ten times greater than any other amino acid! Leucine has a direct impact on the muscle building pathway call mTor (mammalian target of rapamycin). We will get into how leucine influences mTor after mTor is explained. mTor is one of the body’s protein synthesis regulators and energy sensors. mTor is activated when ATP levels are elevated and blocked when ATP levels are low. mTor is the most important signal calling complex for muscle growth. mTor can be activated in three different ways:

1.) Lifting heavy loads (weight training)
2.) IGF, GH, Insulin, and other growth factors
3.) Amino Acids (specifically leucine)

mTor increases protein synthesis in two different ways. mTor activates the protein known as ribosomal protein S6. This protein increases the synthesis of components of the protein synthesis pathway. So, not only does mTor increase protein synthesis, it also increases the capacity for protein synthesis. mTor also inactivates the binding protein 4E-BP1, and this allows the eIF4E*eIF4G complex to form, which is crucial for allowing protein synthesis to take place.


mTor is sensitive to leucine, and low leucine concentrations signal to mTor that there is not enough dietary protein available to build new muscle tissue. Increased leucine levels signal to mTor that there is enough dietary protein available, and mTor is turned on to start the process of building more muscle tissue. Though it’s not known how exactly leucine activates mTor, it’s known that mTor is very sensitive to leucine concentrations, and mTor is critical for an increase in protein synthesis, which equates to more muscle growth.
Ingesting leucine at particular times is critical to increase protein synthesis and to slow muscle degradation.

Pre-workout: ATP is used during training to aid in muscle contractions, and when this happens AMP kinase increase. High levels of ATP activate mTor, and AMP kinase halts the activation of mTor and protein synthesis. When mTor gets the signal that ATP levels decrease and AMP kinase increases, muscle building comes to a screeching halt. This is where leucine comes in handy. When ample amounts of leucine are present during training, it keeps the protein synthesis pathway from being shutdown.

Post-Workout: The hour following a training session is known as the “Anabolic Window”. This is a period of time when muscle cells are primed for protein synthesis, but only if the right nutrition is consumed. When there isn’t enough leucine present, mTor is signaled that there isn’t enough building blocks to build muscle tissue and protein synthesis it shut off. mTor uses leucine as the standard amino acid to turn protein synthesis on or off, so this amino acid should be a staple post workout. Ingesting all essential amino acids is important for increasing muscle mass, but leucine should be the amino acid that should be in every post workout meal or shake.

Leucine can be consumed through food sources and powder supplement sources. Leucine makes up 10 percent of whey protein, so there is an average of 10 grams per 100 grams of whey. Leucine also is found in a BCAA supplement along with isoleucine and valine. It can also be found as a stand-alone powder as well. Below is a list of foods that contain high levels of leucine.

Soybeans 2.97g per 100g
Beef 1.76g per 100g
Peanuts 1.67g per 100g
Salmon 1.62g per 100g
Chicken Egg 1.40g per 100g
Oats 1.28g per 100g

While it’s possible to get enough leucine through foods alone to increase protein synthesis, it isn’t possible to do without consuming a large amount of calories. When an individual is attempting to add lean muscle while limiting fat storage, or if someone is in a caloric deficit and trying to just maintain muscle mass, food only is not an option. Taking powders to increase leucine intake is viable option for someone that has a limit on caloric intake.

Recommended leucine intake:
Pre-workout: 30-40g whey protein, or 20g BCAA, or 5g leucine
Post-workout: 40-50g whey protein, or 20g BCAA, or 5g leucine
In between meals: 30g whey protein, or 10g BCAA, 3-5g leucine

Protein is a large focus among bodybuilders and competitive athletes. There is no doubt that protein is important, but the amino acids that make up protein are the actual building blocks of muscle tissue, and not all amino acids are created equal. Leucine is the only amino acid that has been shown to directly increase protein synthesis, making it the anabolic amino acid.