How Essential Amino Acids Improve Protein Quality

Close-Up Of Protein Powder In Scoop On Table

Jordan Beal/EyeEm/Getty Images

Protein intake is well-known as important for muscle growth and development among active adults and athletes. What improves the quality of protein making it more effective for muscle protein synthesis (MPS) is the presence of amino acids. 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 are unable to be produced by the body 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.

The following describes the function of each essential amino acid:

  • Methionine — Supports bone cartilage production, creatine formation, and muscle growth.
  • Valine — Prevents muscle breakdown during exercise, supports daily body function, muscle metabolism and growth, 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, and regulates blood sugar before and after exercise. Considered the ‘main’ branched chain amino acid (BCAA) responsible for muscle protein synthesis.
  • Iso-leucine — A form of leucine helping with energy production and increases nitrogen growth in muscle cells. Part of the branch-chained amino acid (BCAA) group.
  • Threonine — Helps maintain a healthy heart, liver, immunity, and central nervous system.
  • Lysine — Maintains proper carnitine levels which help lower cholesterol.
  • Tryptophan — Acts as a neurotransmitter, regulates certain hormones, and promotes the nervous system and brain health. It is known as relaxing amino acid.
  • Phenylalanine — Converts into tyrosine after ingestion and helps with brain chemistry and making thyroid hormone.
  • Histidine — Helps with the development and maintenance of healthy body tissue and the nervous system. Important for children and early childhood development.

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 usually lack one or more essential amino acids creating an incomplete protein. Plant protein is shown to be limiting in specific amino acids including lysine, methionine, and tryptophan which limits the functioning of the protein 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.

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 by how much is present in that protein
  • Protein Efficiency Ratio — A measure of weight gain of a test participant divided by food protein intake during a trial period
  • Biological value — A measure of protein in the bloodstream and the body can use for muscle growth, etc.
  • 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 how effective it is 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 largest role in consuming a quality protein source. Research also indicates there are three essential amino acids that 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.

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. Research indicates consuming leucine alone between meals extends protein synthesis by increasing energy levels in our muscle tissue. Several sports nutrition studies recommend athletes consume adequate leucine from quality protein sources in each of their meals.

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 (EEAs) are indicated to be 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 important to stimulate muscle protein synthesis.
  • Branched-chain amino acids (BCAAs) isoleucine, leucine, and valine appear to function alone or collectively to stimulate the making of protein 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 (EEAs) promote the greatest 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 have been widely researched and are shown to 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 but remains elevated for a longer period of time. 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 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 shown to significantly increase protein synthesis in muscle tissue and in 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 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. Chronic 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 are usually in powder form combining whey and casein proteins. Some blends 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 greatest increase in muscle mass over a 10-week period. Similar studies lasting 12 weeks indicated improved strength gains and body composition. 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.

The following is a summary of evidence-based key points on protein sources:

  • Many protein sources are available for athletes, with 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.
  • Chronic research indicates leucine content and rate of digestion is important 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 ideal composition for maximal stimulate 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.

Was this page helpful?
Article Sources
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.