How Essential Amino Acids Improve Protein Quality

Close-Up Of Protein Powder In Scoop On Table

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Protein intake is well-known as necessary for muscle growth and development among active adults and athletes.

The presence of amino acids improves the quality of protein, making it more effective for muscle protein synthesis (MPS). Both plants and animal food sources contain protein but differ in the type and proportion of amino acid makeup.

Protein and Amino Acids

Amino acids are the building blocks of protein and help determine protein quality. There are 20 total amino acids comprised of nine essential amino acids (EAAs) and 11 non-essential amino acids (NEAAs). The body requires all 20, but the EAAs cannot be produced and must come from the food we eat.

The essential amino acids obtained from our diet include methionine, valine, leucine, isoleucine, threonine, lysine, tryptophan, and phenylalanine. Histidine is included as an additional essential amino acid required for early childhood development.

Functions of Amino Acids

The following describes the function of each essential amino acid:

  • Methionine — A sulfur-containing amino acid responsible for the growth and tissue repair. This amino acid also maintains hair, skin, and the strength of nails. Also, methionine protects the body from pollutants, slows cell aging, and is essential for the absorption and bioavailability of zinc and selenium.
  • Valine — Prevents muscle breakdown during exercise, supports daily body function, muscle metabolism, growth, metabolism, helps the nervous system including cognitive function, and maintains nitrogen balance. Part of the branch-chained amino acid (BCAA) group.
  • Leucine — Stimulates muscle growth and strength, regulates blood sugar, contributes to growth hormone production, and helps with wound healing. Considered the ‘main’ branched-chain amino acid (BCAA) responsible for muscle protein synthesis.
  • Iso-leucine — A form of leucine that helps with energy production, assists with wound healing, detoxifies nitrogen waste, stimulates immune function, is necessary for hemoglobin formation, and assists with blood sugar regulation. Part of the branch-chained amino acid (BCAA) group.
  • Threonine — Helps maintain the structure of tooth enamel, collagen, and elastin. This amino acid is also vital for the nervous system, fat metabolism, and preventing fat build-up in the liver. Also, this amino acid may improve anxiety and mild depression.
  • Lysine —Maintains proper carnitine levels, which help lower cholesterol—required for growth and tissue repair. Also responsible for supporting the immune system, calcium uptake, and production of carnitine and collagen.
  • Tryptophan — Acts as a neurotransmitter, regulates certain hormones, and promotes the nervous system and brain health. It is the precursor for serotonin. Serotonin is responsible for regulating sleep, appetite, mood, and pain.
  • Phenylalanine —Responsible for the structure and function of many proteins and enzymes. Converts into tyrosine, responsible for dopamine and norepinephrine (neurotransmitter).
  • Histidine — A semi-essential amino acid. Helps with the development and maintenance of healthy body tissue and the nervous system. Essential for children and early childhood development. It also plays a role in the immune system, gastric secretion, and sexual function. Histidine protects the cell from radiation and heavy metal damage in blood cell formation.

Proteins from most animal food sources contain all the essential amino acids (EAAs) in the right amounts. These are also referred to as complete proteins.

Foods from plant sources tend to lack one or more essential amino acids creating an incomplete protein.Plant protein is limited in specific amino acids, including lysine, methionine, and tryptophan, limiting the protein's functioning in the body.

According to research, animal and dairy-based proteins contain the highest amounts of EAAs for protein synthesis and muscle growth post workout.

How Protein Quality is Measured

Protein quality is measured using several methods, including:

  • Chemical Score: Refers to the amino acid profile of a protein, and each amino acid is ranked against the ideal or referenced protein.
  • Protein Efficiency Ratio: The first method adopted for assessing food's protein quality. A measure of weight gain of a test participant divided by food protein intake during a trial period.
  • Biological value: A measure of protein retained and then utilized in the body.
  • Net protein utilization (NPU): A ratio of amino acids used by the body compared to amino acids supplied in the diet. 
  • Protein Digestibility-Corrected Amino Acid Score (PDCAAS): Considered the preferred best method to measure protein quality; evaluates amino acid requirements and our ability to digest it.
  • Indicator Amino Acid Oxidation (IAAO) technique: The newest successful method used to determine metabolic availability of amino acids from dietary proteins and total protein requirements.

Overall, protein quality refers to its effectiveness at stimulating muscle protein synthesis (MPS) and promoting muscle growth. This is a concern for many active adults, athletes, and fitness-minded people who want the best from their protein intake.

It appears the amino acid profile plays the most prominent role in consuming a quality protein source. Research also indicates three essential amino acids are primarily responsible for regulating protein balance.

The Top 3 Essential Amino Acids for Muscle Growth

Amino acids provide the ability for protein to repair and rebuild skeletal muscle and connective tissues. While all essential amino acids (EAAs) are important for this function, three are indicated to play a primary role.

The EAAs leucine, isoleucine, and valine are uniquely identified to regulate protein metabolism, neural function, and blood glucose and insulin regulation.

In a 2017 study involving 11 healthy men, leucine, isoleucine, and valine are also branched-chain amino acids (BCAAs) shown to be key components of muscle protein synthesis (MPS).

Evidently, BCAAs enter the bloodstream rapidly when taken orally and provide muscle tissue with high concentrations of these amino acids for muscle repair and growth. This is why many active adults and athletes elect to supplement with BCAAs.

While the top three essential amino acids have been identified, it appears leucine is superior for muscle growth and strength. Several sports nutrition studies recommend athletes consume adequate leucine from quality protein sources in each of their meals to suppress muscle damage, aid with recovery, and activate protein synthesis.

Essential Amino Acids and Protein Quality

The Journal of the International Society Sports Nutrition has provided the following key points on essential amino acids (EAAs) and protein quality:

  • Protein sources with higher levels of essential amino acids are considered higher quality.
  • The body uses 20 amino acids to make proteins, but the nine essential amino acids are supplied only by the food we eat to meet our daily needs.
  • Essential amino acids (EAAs) are responsible for increased muscle protein synthesis at doses ranging from 6 to 15 grams.
  • Leucine doses of 1 to 3 grams per meal appear to be essential to stimulate muscle protein synthesis.
  • Branched-chain amino acids (BCAAs) isoleucine, leucine, and valine appear to function alone or collectively to stimulate protein-making for muscle growth and repair.
  • Although greater doses of leucine alone are shown to stimulate muscle growth, it’s indicated that a balanced intake of all essential amino acids (EAAs) promotes the most significant increases.
  • Consuming quality protein sources at the right time with adequate levels of leucine/BCAAs will best promote increases in muscle protein synthesis (MPS).

Protein Source Comparisons

The best protein sources are those that can positively affect protein balance upon consumption and stimulate muscle growth along with fat loss over the long term. In addition and according to research, the ability for a protein to enhance immune function and promote an antioxidant environment should also be considered. 

What appears to accomplish this goal and important factors in protein selection is leucine content and the rate the protein can be digested. Understanding how the following protein sources differ in quality and effectiveness will help you select the right protein for you:

Milk Proteins 

Milk proteins have been widely researched and significantly improve post-workout muscle recovery. They are also a great way to replenish glycogen stores and improve protein balance to stimulate muscle protein synthesis (MPS).

Milk proteins are also indicated to increase skeletal and neuromuscular strength. They possess the greatest density of leucine content and the highest score on the Protein Digestibility-Corrected Amino Acid (PDCAAS) scale. Milk proteins are broken down into two classes:

  • Casein: Highest leucine content, water-soluble but gels in the gut slowing down the digestion rate. The slow increase in amino acid concentration remains elevated for a more extended period. Casein is shown to stimulate muscle protein synthesis and growth.
  • Whey: Highest leucine content, water-soluble, mixes easily and rapidly digested. Chronic research indicates faster digesting whey protein beneficial for lean mass gains in bodybuilders.

Egg Proteins 

Egg proteins are considered the ideal protein source with an amino acid profile that has been used as the standard to compare other dietary proteins. Eggs are a high-quality protein source rich in leucine. They are easily digestible, a favored protein food for athletes, and significantly increase protein synthesis in muscle tissue and the bloodstream.

Egg protein is cost-effective and also considered a functional food for fitness-minded individuals. According to research, functional foods contain a nutrient profile with health benefits beyond what is supplied through basic nutrition.

Meat Proteins 

Meat proteins are well-known to be rich sources of essential amino acids (EAAs). Beef contains a full balance of EAAs and considered to have a high biological value. Meat proteins contain a high concentration of leucine and a 30g serving of beef protein is shown to stimulate muscle protein synthesis (MPS) in both young and elderly individuals.

Meat proteins also contain quality micronutrients and minerals including iron, B12, and folic acid. Research shows meat proteins help increase muscle mass and decrease fat mass. Meat proteins are also a rich source of a molecule called carnitine indicated to help decrease muscle damage caused by physical exercise.

Protein Blends 

Protein blends are usually in powder form combining whey and casein proteins. Some combinations also include branch-chained amino acids (BCAAs), glutamine, and other added nutrients. Research indicates combining protein sources may provide additional benefits for athletes.

A resistance training study demonstrated those participants consuming a whey and casein blend to have the most significant increase in muscle mass over 10 weeks. Similar studies lasting 12 weeks indicated improved strength gains and body composition compared to just protein or a protein/calorie combination.

Protein blends were also shown to have a positive and prolonged effect on amino acid balance. It appears protein blends may be a beneficial supplement to ensure adequate dietary protein intake for muscle growth.

Summary of Evidence on Protein Sources

  • Many protein sources are available for athletes, each having pros and cons.
  • Protein sources are evaluated based on amino acid content, especially the concentration of essential amino acids (EAAs). Other nutrients and chemical compounds also contribute to protein quality.
  • Leucine content and rate of digestion are essential for athletic performance, muscle growth, and recovery.
  • Protein blends appear to provide a combination of beneficial nutrients, including leucine, EAAs, bioactive peptides, and antioxidants. Further research is required to determine the ideal composition for maximal stimulation rates of MPS at rest and following training.

A Word From Verywell

Consuming the right protein source is important for building muscle and losing fat. It appears not all protein is the same, and more attention to the essential amino acids (EAAs) profile is recommended to ensure quality and effectiveness.

Branch chained amino acids (BCAAs), especially a high concentration of leucine in our protein source is shown to be primarily responsible for muscle growth, strength, and recovery. The good news is there are multiple protein source options to accommodate an active or competitive lifestyle.

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Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Darla Leal
Darla Leal is a Master Fitness Trainer, freelance writer, and the creator of Stay Healthy Fitness, where she embraces a "fit-over-55" lifestyle.