In the United States and Canada, where nutritious foods are abundant, most people easily obtain enough protein to receive all the amino acids that they need. In countries where food is scarce and people eat only marginal amounts of protein-rich foods, however, the quality of the protein becomes crucial.
The protein quality of the diet determines, in large part, how well children grow and how well adults maintain their health. Put simply, high-quality proteins provide enough of all the essential amino acids needed to support the body’s work, and low-quality proteins don’t. Two factors influence protein quality: the protein’s digestibility and its amino acid composition.
What Affects Protein Quality?
Proteins must be digested before they can provide amino acids. Protein digestibility depends on such factors as the protein’s source and the other foods eaten with it. The digestibility of most animal proteins is high (90 to 99 percent); plant proteins are less digestible (70 to 90 percent for most, but more than 90 percent for soy).
2. Amino Acid Composition
To make proteins, cells must have all the needed amino acids available simultaneously. The liver can produce any nonessential amino acid that may be in short supply so that the cells can continue linking amino acids into protein strands. If an essential amino acid is missing, however, a cell must dismantle its own proteins to obtain it. Therefore, to prevent protein breakdown, dietary protein must supply at least the nine essential amino acids plus enough nitrogen-containing amino groups and energy for the synthesis of the others.
If the diet supplies too little of any essential amino acid, protein synthesis will be limited. The body makes whole proteins only; if one amino acid is missing, the others cannot form a “partial” protein. An essential amino acid that is available in the shortest supply relative to the amount needed to support protein synthesis is called a limiting amino acid.
Sources of High-quality Proteins
A high-quality protein contains all the essential amino acids in amounts adequate for human use; it may or may not contain all the others. Generally, proteins derived from animal foods (meat, seafood, poultry, cheese, eggs, and milk and milk products) are high quality, although gelatin is an exception. Proteins derived from plant foods (legumes, grains, nuts, seeds, and vegetables) tend to be limited to one or more essential amino acids. Some plant proteins, such as corn protein, are notoriously low quality. Others, such as soy protein, are high quality.
Below is a list of the protein contents of foods based on the food groups of the USDA Food Patterns:
Milk and Milk Products
Each of the following provides about 8 grams of protein:
- 1 c milk, buttermilk, or yogurt (choose low-fat or fat-free)
- 1 oz regular cheese (for example, cheddar or Swiss; choose low-fat)
- 1⁄4 c cottage cheese (choose low-fat or fat-free)
Each of the following provides about 7 grams of protein:
- 1 oz meat, poultry, or fish (choose lean meats to limit saturated fat intake)
- 1⁄2 c legumes (navy beans, pinto beans, black beans, lentils, soybeans, and other dried beans and peas)
- 1 egg
- 1⁄2 c tofu (soybean curd)
- 2 tbs peanut butter
- 1 to 2 oz nuts or seeds
Each of the following provides about 3 grams of protein:
- 1 slice of bread
- 1⁄2 c cooked rice, pasta, cereals, or other grain foods
Each of the following provides about 2 grams of protein:
- 1⁄2 c cooked vegetables
- 1 c raw vegetables
If the body does not receive all the essential amino acids it needs, the supply of essential amino acids will dwindle until body organs are compromised. Obtaining enough essential amino acids presents no problem to people who regularly eat foods containing high-quality proteins, such as meat, seafood, poultry, cheese, eggs, milk, and many soybean products. The proteins of these foods contain ample amounts of all the essential amino acids.
An equally sound choice is to eat two different plant-based protein foods so that each supplies the amino acids limited in the other. In this strategy, the two protein-rich foods are combined to yield complementary proteins—proteins containing all the essential amino acids in amounts sufficient to support health. The two proteins need not even be eaten together, as long as the day’s meals supply them both, and the diet provides enough energy and total protein from a variety of sources.
Protein on Food Labels
All food labels must state the quantity of protein in grams. The “percent Daily Value” for protein is not mandatory on all labels, but it is required whenever a food makes a protein claim or is intended for consumption by children younger than age four. Whenever the Daily Value percentage is declared, researchers must determine the quality of the protein. Thus, when a % Daily Value is stated for protein, it reflects both quantity and quality.
- A diet inadequate in any of the essential amino acids limits protein synthesis.
- The best guarantee of amino acid adequacy is to eat foods containing high-quality proteins or combinations of foods containing complementary proteins so that each can supply the amino acids missing in the other.
- Vegetarians who consume no foods of animal origin can meet their protein needs by eating a variety of whole grains, legumes, seeds, nuts, and vegetables.
- Ample carbohydrate and fat in the diet allow amino acids to be used to build body proteins.
- All food labels must state the quantity of protein in grams.