Dietary protein is the building block for muscle growth and recovery. It also plays a significant role in not only for our muscles, but in our bones, organs, skin, nails, and enzymes.
Protein is made up of 20 amino acids either made from our body or from food. The nine essential amino acids, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine must come from food; therefore, not only the amount but the quality of protein you eat can impact muscle growth and recovery. What is protein quality? Protein quality is a based on how quickly we can make new proteins (protein synthesis) to repair and build muscle after eating a protein-rich meal. Factors that influence protein quality include:
Essential amino acid content (EAA)
If the food contains leucine
Digestion and absorption of protein
Muscle growth (positive protein balance)
Comparing high quality vs low quality protein
High-quality protein includes the nine EAA, including leucine. Leucine is a building block for forming new muscle and is a signaling protein to switch on protein synthesis. It’s recommended to eat a meal containing 2-3 grams of leucine every 3-5 hours during the day to maximize muscle gains and recovery.
High-quality proteins, including leucine, are also more easily digested and absorbed, which include protein from animals (meat, cheese, milk/yogurt, and eggs). Some plant proteins, such as isolated soy protein are included as high quality, but animal protein is generally of higher quality than plant based since it contains all EAA. For example isolated soy protein is very well digested >95%; whereas the protein from whole soy beans may be digested at a rate of only 75% or 80%.
Plant proteins on average have digestibility values of about 70% to 90%, vs soy, casein, and egg, which have a digestibility value of 97% or above. It’s recommended to eat an additional 10% to 20% more protein when eating a plant protein to compensate for digestibility and the limiting essential amino acids compared to animal protein. This is why you may see plant protein powders containing 30 grams of protein per serving from pea or rice compared to 20 grams of protein from whey to meet leucine recommended amounts.
Eating foods with less leucine may not stimulate enough protein synthesis impacting muscle repair and growth. This is a primary concern for vegans not eating a varied diet. Eating a variety of protein choices, including soy can increase the protein quality and leucine content to meet the recommended daily intake.
Scoring Protein Quality
The Digestible Indispensable Amino Acid Score (DIAAS) is a protein scoring test identifying protein quality based on a percentage of the DIAAS of the food. If a food item has a DIAAS value greater than 100, it can be considered an “excellent” quality protein source for the specific age group. A food item can be considered a “good” quality protein source if the DIAAS value is between 75 and 99.
For example, peas may have a high quantity of protein, but with a DIAAS value of about 64 it has a low quality, whereas milk has both a high quantity of protein and high quality of amino acids with a DIAAS of 122. Therefore, you would have to consume more than twice as much pea protein compared with milk protein to meet the human amino acid requirements.
Take Home Message The protein quality of a single ingredient is less important than the quality of the mixed meal that is eaten. To help calculate the protein quality of meals DIAAS can be used to allow for the calculation of the quality of a meal consisting of a number of protein foods. For example, if you eat rice protein, this may be low in the amino acid lysine, but when eaten with peas, the lysine content increases improving the quality of the protein content. Overall, eating enough protein to help promote muscle protein gains and recovery is important, but the quality needs to be taken into account to make sure you are getting enough of the EAA, including leucine throughout the day.