Protein comes from a Greek word ''proteios'' meaning ''of first rank'' as its responsible for most bodily functions. Around 40% of protein is found in muscles, while over 25% if found in body organs. The remaining is in the skin or blood. Proteins are made of amino acids, and these amino acids determine the quality of the protein as different proteins have different amino acids. For muscle growth we want to focus on the protein rich in branched chain amino acids (BCAA), particularly Leucine, and essential amino acids (EAA). On top of this, the more complete amino acid profile of the protein is, the better it is at stimulating muscle protein synthesis.
Leucine and BCAA
Leucine is an essential amino acid and is part of the BCAA. It is particularly important amino acid as it directly stimulates mTOR (master enzyme responsible for muscle growth), which is why it is often said that it is an anabolic amino acid that is responsible for muscle protein synthesis.
While this is completely true, consuming Leucine or BCAA in isolation will not help you build more muscle growth as your body needs other amino acids that will actually support muscle protein synthesis after it has been initially triggered by Leucine. In this case, consuming other 6 essential amino acids together with Leucine is just as important, otherwise it would be like flipping a switch on without electricity, nothing would happen.
Basically when it comes to making sure you are consuming highest quality protein, it boils down to two most important things:
1. Triggering muscle protein synthesis
2. Having enough building blocks (amino acids) that support muscle growth
Because of this, consuming BCAA in isolation is definitely not recommended, and I suggest you to completely avoid those type of supplements in the first place. Simply consuming the whole source of protein, like dairy, poultry, meat or fish, will supply you with enough leucine to trigger muscle protein synthesis and supply you with additional amino acids needed to support growth.
The leucine threshold
The leucine threshold is simply the minimal amount of leucine that you have to consume in an each meal to stimulate muscle protein synthesis. Below this amount, the stimulus for muscle growth from nutrition is not optimal, and pretty much nothing happens. To be safe, you should be consuming a minimum of 0.3 g / kg of body-weight per meal to make sure your leucine intake is high enough for maximal effects.
The food matrix
Amino acid content is not the only factor that determines the quality of a protein source. What is also important is interaction with the food matrix in which the protein is consumed. Without going into too much biochemical details, research has found that bound form of amino acids (the ones that we find in whole source of protein like dairy, poultry, meat or fish) stimulate more muscle protein synthesis compared to its free form (like BCAA or EAA).
In general, the more whole the protein source is the better and research supports this. For example, whole milk stimulates more muscle protein synthesis than skimmed milk. Probably because of food processing.
Mixtures of different protein types tend to stimulate MPS more than pure whey.
Vegetarian protein
Unfortunately, all high quality protein sources are animal foods. Plants have a food matrix that negatively affect protein quality, as they contain many anti-nutrients (a protective mechanism). As a result, these anti-nutrients make amino acids difficult for human body to absorb and use. The worst candidates for this are wheat and soy. The isoflavons found in soy may even antagonize mTOR (master enzyme that triggers muscle growth).
Another issue is the amino acid content even when we do not take into consideration anti nutrient problem. BCAA content is on average 21% lower than that of animal sources. When is comes to EAA (essential amino acids), plant sources contain 16% less on average.
On the other hand, lysine and sulfur-containing amino acids like methionine and cysteine can be even more difficult to find in plant based protein. And what is often the case with plant based protein is that different sources provide different amounts of amino acids. For example legumes contain a good amount of amino acid lysine, but they have very little sulfur-containing acids. To make sure you consume enough sulfur amino acids you need to include protein sources from seeds and grains, however they lack in lysine. By the time you combine all of these sources to cover everything, portions are so big that they become impractical and can potentially cause some digestive issues due to high amount of fiber.
So where do you go from here if you are a vegetarian who is looking to build lean muscle and get in shape? Well it depends on what are your preferences. If your preference is to eat your protein and include it in your meals, then my recommendation would be to divide your protein intake per meal like this: 50% from legumes, 25% from seeds, 25% from grains. On top of this you also want to add the digestibility correction of 24% and increase your protein intake to 2.7 g / kg of body-weight. If you are lacto-ovo-vegetarian, then you can simply include dairy or eggs in each meal.
Another alternative is to supplement your protein intake with protein powder. For vegetarians rice and pea protein is ideal as they have complementary amino acid profile. In case you go for this option, a total protein intake of 2.2 g / kg is sufficient.
Practical application
Now that we know more about the amino acids and how they affect protein quality together with the food matrix, we can look at some examples in terms of high quality and low quality protein.
When it comes to the total protein intake, minimal protein intake is 1.8 g / kg per day for non vegetarians, and 2.7 g / kg per day for vegetarians. At least 50% of the total intake should come from high quality foods (listed below). This means that you need to take into consideration even the protein from your carbohydrate sources, like rice, pasta, and even some vegetables to calculate it towards your total daily protein intake.
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