Co-authored by Nick Price

Take a look at nutritional labels from different foods and you will see %Daily Values for most of the nutrients, but rarely for protein. This is because the declared amount of protein in products needs to be adjusted for its ability to provide us with sufficient amino acids.

In general, the %DVs declared on nutritional labels can either involve simple math or be complicated. For example, a serving of fluid milk with 300 mg of calcium supplies 30% of its DV, while peanuts with 35 mg calcium provide 4% of its DV. Both use 1,000 mg/day as the Daily Value. For protein, you must also factor in how well it is used by our bodies. Fluid milk containing 8g protein in a serving supplies 16% of its DV; however, peanuts containing the same amount of protein in a serving supplies only 8% of its DV. PDCAAS is the way to measure the nutritional quality of a protein.

Proteins are made of many amino acids and PDCAAS evaluates a food’s protein quality by comparing its amino acid composition to what our bodies can use. PDCAAS compares the amount of the essential amino acids in the food to a reference (scoring) pattern based on the essential amino acid requirements of a 2 to 5-year old child to determine its most limiting amino acid (amino acid score). This approach is recommended by the Food and Drug Administration (FDA) and is described in the 1991 FAO/WHO Protein Quality Report.

Part of calculating a PDCAAS involves referencing a protein digestibility value. Extensive evaluation of existing in vitro and in vivo methods of determination of the protein digestibility in foods led to the conclusion that the rat balance method is the most suitable practical method for predicting protein digestibility by humans. Using this method, digestibility values for many common ingredients have been established and made available for reference. For food mixtures, when data for the protein digestibility of the individual ingredients are well established, the protein digestibility of the mixture can be calculated by means of a weighted average procedure. This can be used if the distribution of ingredients in the product is known.

The other part required for PDCAAS calculation is laboratory analysis. Proximates and amino acid composition must be measured in order to determine the amino acid score. Protein digestibility-corrected amino acid score of a test food is then calculated by multiplying the amino acid score times the protein digestibility value. PDCAAS is typically expressed as a decimal, but sometimes as a percentage. PDCAAS can be used to calculate the nutritional quality adjusted amount of protein in the product and its % Daily Value.

In January 1993, FDA suggested the use of PDCAAS in food labeling for evaluating protein quality. Pre-existing regulations, at CFR§ 101.9(c)(7), required the use of PDCAAS for determining whether a food contained a significant amount of protein per serving and for calculating the percent DV for protein.

Consequently, the PDCAAS and the %DV of a food must be known before a protein content claim is made. In the U.S., one qualification for a “Good Source of Protein” claim is the food containing 10-19 %DV per serving. An “Excellent Source” claim can be made when 20 %DV of protein is present.

The highest PDCAAS value that any protein can achieve is 1.0. Generally casein, whey, soy, and egg are considered good quality proteins and have PDCAAS scores of 1.00, while those of tree nuts are a bit under 0.50 and wheat gluten even lower. The graph below compares some of the common protein sources and range out there.

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