29 2.27 Protein Quality
Proteins can be classified as either complete or incomplete. 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.
Table 2.271 Limiting amino acids in some common plant foods1
| Food | Limiting Amino Acid(s) |
| Bean and Most Legumes | Methionine, Tryptophan |
| Tree Nuts and Seeds | Methionine, Lysine |
| Grains | Lysine |
| Vegetables | Methionine, Lysine |
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 have a complete puzzle. But when they are combined, the two individuals create a complete puzzle.
Two examples of complementary proteins are shown below.
| Peanut Butter and Jelly Sandwich | Red Beans and Rice |
Figure 2.272 Two complementary protein examples2,3
It should be noted that complementary proteins do not need to be consumed at the same time or meal. It is currently recommended that essential amino acids be met on a daily basis, meaning that if a grain is consumed at one meal, a legume could be consumed at a later meal, and the proteins would still complement one another4.
Measures of Protein Quality
How do you know the quality of the protein in the foods you consume? 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/g protein) / 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 recently5,6.
The following table shows the protein quality me