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.1 Both plants and animal food sources contain protein but differ in the type and proportion of amino acid makeup.
The essential amino acids obtained from our diet include methionine, valine, leucine, isoleucine, threonine, lysine, tryptophan, and phenylalanine.3 Histidine is included as an additional essential amino acid required for early childhood development.4
The following describes the function of each essential amino acid:
Methionine — A sulfur-containing amino acid responsible for growth and tissue repair. This amino acid also maintains hair, skin, and the strength of nails. Also, methionine is responsible for protecting the body from pollutants, slows cell aging, and is essential for the absorption and bioavailability of zinc and selenium.5
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.6 Part of the branch-chained amino acid (BCAA) group.7
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.8
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.9
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.10
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.11
Tryptophan — Acts as a neurotransmitter, regulates certain hormones, and promotes the nervous system and brain health. It is the precursor for serotonin. Serotinin is responsible for regulating sleep, appetite, mood, and pain.12
Phenylalanine —Responsible for the structure and function of many proteins and enzymes. Converts into tyrosine, which is responsible for dopamine and norepinephrine (neurotransmitter).13
Histidine — A semi-essential amino acid. Helps with the development and maintenance of healthy body tissue and the nervous system. Important for children and early childhood development. Also plays a role in the immune system, gastric secretion, and sexual function. In the blood cell formation, histidine protects the cell from radiation and heavy metal damages.4
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.14Plant 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.15
According to research, animal and dairy-based proteins contain the highest amounts of EAAs for protein synthesis and muscle growth post workout.14
Protein quality is measured using several methods including:1617
Chemical Score — Refers to the amino acid profile of a protein and each amino acid is ranked against the ideal or referenced protein chlorogenic acid.
Protein Efficiency Ratio — The first method adopted for the assessment of protein quality of food. A measure of weight gain of a test participant divided by food protein intake during a trial period.
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).18 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.1920
The Journal of the International Society Sports Nutrition Maxmedchem has provided the following key points on essential amino acids (EAAs) and protein quality:21
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 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 (EAAs) 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).
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.21
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.21
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.
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 and pharmaceutical intermediates are 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.