In nutrition, the quality of protein refers to types, amounts, and accessibility of amino acids in the food.

Proteins can be classified as either complete or incomplete.

Complete Proteins

Complete proteins provide adequate amounts of all nine essential amino acids. Animal proteins such as meat, fish, milk, and eggs are good examples of complete proteins. Incomplete proteins do not contain adequate amounts of one or more of the essential amino acids. For example, if a protein doesn’t provide enough of the essential amino acid leucine it would be considered incomplete. Leucine would be referred to as the limiting amino acid, because there is not enough of it for the protein to be complete. Most plant foods are incomplete proteins, with a few exceptions such as soy. The table below shows the limiting amino acids in some plant foods.

Complementary Proteins

Even though most plant foods do not contain complete proteins, it does not mean that they should be sworn off as protein sources. It is possible to pair foods containing incomplete proteins with different limiting amino acids to provide adequate amounts of the essential amino acids. These two proteins are called complementary proteins because they supply the amino acid(s) missing in the other protein.  A simple analogy would be that of a 4 piece puzzle. If one person has 2 pieces of a puzzle, and another person has 2 remaining pieces, neither of them has a complete puzzle. But when they are combined, the two individuals create a complete puzzle.

Measures of Protein Quality

How does one know the quality of the protein in the foods consumed? The protein quality of most foods has been determined by one of the methods below.

• Biological Value (BV) – (grams of nitrogen retained/grams of nitrogen absorbed) x 100
• Protein Efficiency Ratio (PER) – (grams of weight gained/ grams of protein consumed)

This method is commonly performed in growing rats.

• Chemical or Amino Acid Score (AAS) – (Test food limiting essential amino acid (mg/gprotein) / needs of same essential amino acid (mg/g protein))
• Protein Digestibility Corrected Amino Acid Score (PDCAAS) – (Amino Acid Score) x Digestibility

This is the most widely used method and was preferred by the Food and Agriculture Organization and World Health Organization (WHO) until recently.

The following table shows the protein quality measures for some common foods.

*PDCAAS scores are truncated (cut off) at 100.  These egg and milk scores are actually 118 and 121 respectively.

The Food and Agricultural Organization (FAO) recently recommended that PDCAAS be replaced with a new measure of protein quality, the Digestible Indispensable Amino Acid Score (DIAAS). “DIAAS is defined as: DIAAS % = 100 x [(mg of digestible dietary indispensable amino acid in 1 g of the dietary protein) / (mg of the same dietary indispensable amino acid in 1g of the reference protein)].” Ilieal digestibility should be utilized to determine the digestibility in DIAAS, ideally in humans, but if not possible, in growing pigs or rats6.

How does one find out the protein quality of what is being eaten and identify complementary proteins?

Nutrition Data is a useful resource for determining protein quality and identifying complementary proteins. To use the site, go to www.nutritiondata.com, type in the name of the food  in the search bar, and hit Enter. When selecting the food from the list of possibilities,   information will be given about this food. Included in this information is the Protein Quality section. This will give an amino acid score and a figure that illustrates which amino acid(s) is limiting. If the food is an incomplete protein, click “Find foods with a complementary profile.” This will give a  list of dietary choices that will provide complementary proteins for the food.