Foods that contain an abundance of all 9 essential amino acids are considered to be the best sources of protein. This is because such foods can meet all of the body’s protein requirements, allowing it to perform all its designed functions.
Several plant sources contain an insufficient level of protein or are missing one or more of the 9 essential amino acids. Notwithstanding, there are also many good vegan and vegetarian sources of protein.
The list of animal and plant-based foods with good protein content include:
Animal Proteins: milk, curd, cottage cheese (paneer), cheese, whole eggs, red meat, poultry, and seafood
Plant Proteins: quinoa, tofu, tempeh, edamame, spirulina, hemp seeds.
An interesting way to fill in the missing amino acid gaps in one food source is by combining it with a complementary food item that contains high levels of the missing amino acid. This process is called ‘Mutual Supplementation’, and it helps us meet our basic protein requirements. A good example is the combination of rice with beans.
Rice (white and brown) is low in lysine but rich in methionine (both are essential amino acids). Beans, on the other hand, contain high levels of methionine but are low in lysine. When the two are combined, you improve the amino acid profile and, hence, make this rice and beans combination a complete protein.
But why do all amino acids have to be present in a food item to make up a complete protein?
There are 20 different amino acids. Every protein is made up of a unique combination of these 20 amino acids, just like each of the words in any language, long and short, are made up by a unique sequence of the alphabet in that language. Of the 20 amino acids, the 9 that can’t be synthesized by the human body need to be acquired through food, and are referred to as essential amino acids.
Hence, foods that contain at least the 9 essential amino acids are considered to be ‘Complete Protein’ or ‘First Class Protein’, since the remaining 11 amino acids can be synthesized by the human body.
All 20 amino acids need to be available in the muscle's storehouse called the 'Amino Acid Pool' for the body’s growth and repair processes to begin. Even if one amino acid is missing or is not available in sufficient quantity, the body can neither do without it nor replace it with a different amino acid. In such cases, the body’s repair processes stop until the missing amino acid is either made (synthesized) by the body or is supplied through eating the right foods.