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2.2G: Protein Quality

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  • 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 2.2G.1 Limiting amino acids in some common plant foods1
    Food Amino Acid(s)
    Beans 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.

    Figure 2.271 Complementary proteins are kind of like puzzle pieces

    Two examples of complementary proteins are shown below.

    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 measures for some common foods.

    Table 2.2G.2 Measures of protein quality5
    Protein PER Digestibility AA(%) PDCAAS
    Egg 3.8 98 121 100*
    Milk 3.1 95 127 100*
    Beef 2.9 98 94 92
    Soy 2.1 95 96 91
    Wheat 1.5 91 47 42

    *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)].” Ileal digestibility should be utilized to determine the digestibility in DIAAS; ideally in humans, but if not possible in growing pigs or rats6.

    The main differences between DIAAS and PDCAAS are:

    1. DIAAS take into account individual amino acids digestibility rather than protein digestibility.
    2. It focus on ileal instead of fecal (total) digestibility.
    3. Has three different reference patterns (different age groups, 0-6 months, 6 months- 3 years, 3-10 years old) instead of a single pattern
    4. DIAAS scores will not be truncated7

    How do I find out the protein quality of what I'm eating and identify complementary proteins?

    Nutrition Data is a useful resource for determining protein quality and identifying complementary proteins. To use the site, go to, type in the name of the food you would like to know about in the search bar and hit ‘Enter’. When you have selected your food from the list of possibilities, you will be given information about this food. Included in this information is the Protein Quality section. This will give you an amino acid score and a figure that illustrates which amino acid(s) is limiting. If your food is an incomplete protein, you can click "Find foods with a complementary profile". This will take you to a list of dietary choices that will provide complementary proteins for your food.

    References & Links

    1. Wardlaw GM, Hampl J. (2006) Perspectives in nutrition. New York, NY: McGraw-Hill.
    4. Young VR, Pellett PL. (1994) Plant proteins in relation to human protein and amino acid nutrition. Am J Clin Nutr. May; 59 (5 Suppl): 1203S-1212S.
    5. Schaafsma G. (2000) The protein digestibility-corrected amino acid score. J Nutr 130(7): 1865S-1867S.
    7. Rutherford SM, Fanning AC, Miller BJ, Moughan PJ. Protein Digestibility-Corrected Amino Acid Scores and Digestible Indispensable Amino Acid Scores Differentially Describe Protein Quality in Growing Male Rats. J Nutr. 145(2): 372-379.


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