Protein Quality and Protein Combining:
Visual tools for the PDCAAS method

1. Introduction to Protein Quality Evaluation and Protein Combining

This article discusses the quality of protein in the human diet, methods for evaluating protein quality, and the concept of protein combining. The goal of the article is not to persuade readers to adopt any specific dietary practices, but rather to

  • Present the current theory on the topic
  • Make it easier to understand with the use of two interactive visual tools: The Protein Quality Calculator and The Protein Combining Calculator
  • Offer these tools to the public for studying the protein quality of different foods
  • Add value to the discussion by providing additional information that has not been widely discussed in scientific debate.

DISCLAIMER: Please note that the views expressed in this article are those of the author and do not necessarily represent the views of the Food and Agriculture Organization of the United Nations (FAO). The mention of specific companies or products in this article does not imply endorsement or recommendation by FAO or the author.

2. What Are Proteins and What Is Their Function

Proteins are large molecules made up of smaller molecules called amino acids. They are essential for:

  • The growth and development of the body
  • Maintenance and repair of tissue
  • Building and preserving muscle mass
  • Producing enzymes and hormones that are involved in metabolism and digestion.

One gram of protein provides about 4 calories (kcal) of energy.

3. The Recommended Intake of Protein

The recommended intake of protein varies depending on an individual's total energy intake, body weight, body composition, age, lifestyle, and specific needs for nitrogen and amino acids. Protein requirements are typically higher for pregnant or breastfeeding women, as well as for physically active people or those with increased muscle mass. It is generally advised that humans consume a daily intake of protein that accounts for 12-35% of their caloric intake.

4. Protein Quality Evaluation

While the quantity of protein in a person's diet is important, the quality of the protein is just as important. Proper immune function, mental development, linear growth and numerous other body functions all depend on the quality of protein in the diet.

There are several ways to evaluate protein quality, but the most current method, the Digestible Indispensable Amino Acid Score (DIAAS), is not practical for most people due to its complexity and lack of foundational data. As a result, the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), a method recommended by the Food and Agricultural Organization and World Health Organization over 30 years ago, is still the standard. The Protein Quality Evaluation Calculator and The Protein Combining Calculator on this page both use the PDCAAS method.

5. Understanding the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) Method

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is a method for evaluating protein quality that ranges from 0 to 1, with 1 being the highest score. The score is calculated using two indexes: the Amino Acid Score (AAS) and Protein Digestibility (PD).

  • Amino Acid Score - AAS. The AAS rates the ability of a food's select amino acids to meet the human metabolic needs. It is calculated by comparing the actual amount of each amino acid in the tested food (column 3 in the table below) to the amount prescribed by the appropriate 'reference pattern' (column 4). If one or more amino acids are below the ideal amount, the protein is considered to be of lower quality due to the negative impact of the least represented amino acid (called "limiting amino acid") on the utilization of the entire protein. NOTE: The FAO reference patterns are 'minimum amino-acid values' that a food must have in order to be called 'good quality protein'. The reference patterns are determined by years of scientific study of human metabolic needs, and they are age-specific, meaning they are specific to different ages because the digestive system and metabolic needs change as a person ages. For example, the reference pattern for new-borns is based on the protein composition of breast milk, which has been shown to be best at meeting infant's metabolic needs.
  • Protein digestibility - PD. The PD is a value obtained by measuring the absorption rate of a food's amino acid nitrogen at the end of the digestive tract. A higher PD value (with 100 being the maximum) indicates better digestion by the human body.

To understand how the final Protein Digestibility-Corrected Amino Acid Score (PDCAAS) value is determined, we can use the Protein Quality Evaluation Calculator tool below. We will use basmati rice as the tested food.

Example 1: PDCAAS Visual Evaluation of the Protein in Basmati Rice (for Persons Aged 3 Years and Older): The polar area diagram below shows the protein composition of basmati rice. (The amino acids that have no bearing on protein quality have been intentionally omitted.) Each of the nine sectors in the diagram represents one (or two, in the case of sulfur and aromatic amino acids) relevant amino acid. The further a sector extends from the center of the circle, the higher the content of that amino acid, relative to the reference pattern. If the content of all amino acids in the diagram exceeds 100% of the amounts prescribed by the reference pattern, the protein is said to be of high quality. The worst performing amino acid is called "limiting," as it limits, or "brings down," the usability of the rest of the amino acids, and therefore the entire protein (think about it in terms of a chain being only as strong as its weakest link). In our example, the first-limiting amino acid is lysine, as it is most poorly represented - only 77% of the ideal amount prescribed by the reference pattern. Lysine in the example below brings down the utilization of other amino acids in the protein, and thus lowers the overall PDCAAS score of the food. PDCAAS of the basmati rice protein is calculated as follows: We first identify the limiting amino acid (the shortest sector in the diagram). As indicated, it is lysine. We see in the table (red column) that 1 g of basmati rice protein contains 37 mg of lysine (3rd column), which is 77% of the ideal amount prescribed by the reference pattern (48 mg, 4th column). The AAS value of the limiting amino acid lysine, therefore, is 77 (5th column). This value is further corrected downward by the fact that rice protein has a 91% digestibility (PD, 6th column). The result is 70, which is expressed as PDCAAS 0.7. It should be noted that this result applies to people 3 years of age and older. For younger people, the PDCAAS of basmati rice will be even lower, because the younger a human is, the more protein-demanding their body (to see this in action, click the appropriate radio button below the diagram).

EAA g of EAA in 100g of tested food mg of EAA in 1g of tested protein mg of EAA in 1g of ideal protein AAS PD

Due to the lack of data the PD value has been either estimated or assumed to be 85.

You can use the PDCAAS Calculator Visual Tool above to evaluate the protein quality of thousands of different foods. It's worth noting that there is practically no naturally occurring food protein source that is completely lacking in one or more amino acids. Therefore, the classification of proteins as "complete" and "incomplete" is misleading and should be avoided. Instead, it is more accurate to recognize that some proteins are better utilized by the human body than others, depending on their amino acid structure.

As mentioned previously, the maximum PDCAAS value that a food can obtain is 1. This is achieved when the food's protein contains all relevant amino acids in amounts that equal or exceed those prescribed by the reference pattern. For individuals aged 6 months and older, the highest quality protein sources (PDCAAS 1) include egg whites, whey protein, most types of meat, milk, and a few others. For infants, human milk is the highest quality protein source (in fact, relative to infants, hardly any other naturally occurring protein source achieves this value). However, human milk is an inadequate protein source for adults due to its low overall protein content (although it may contain high-quality protein, it is present in insufficient quantities for adults).

6. Protein Combining: A Method for Improving Protein Quality

As was shown above, some protein sources contain less than the ideal amount of one or more essential amino acids (EAA). These proteins are considered inferior because a large portion of their other amino acids end up being used as fuel rather than being utilized as a protein source with all its functions. The goal of protein combining is to prevent this waste by combining foods that make up for their respective weaknesses. For example, rice and beans - each rather inferior proteins - can be combined to create a protein with an ideal balance of all essential amino acids, allowing the body to more efficiently use the protein for its intended functions.

EXAMPLE 2: Combining Rice and Beans for Improved Protein Quality (For Individuals 3 Years and Older): In the first example, we mentioned that basmati rice is low in the amino acid lysine. What we didn't point out, but is obvious from the diagram, is that basmati rice is rich in two sulfur amino acids: methionine and cysteine. With kidney beans the opposite is the case - they are relatively low in sulfur amino acids, but have lysine to spare. As a result, these two foodstuffs complement each other and create a more "complete" protein when paired. To see this effect, you can gradually increase the portion size of kidney beans using the numeric stepper in the table below (If you do not see the up arrow, keep increasing the portion size manually by 2-gram increments.) You will see that a mixture of 50 grams of rice and 50 grams of beans yields a protein with a PDCAAS value of 1, indicating a high quality protein with all relevant amino acids present in sufficient amounts according to the reference pattern.

food item g protein (g) His Ile Leu Lys Met Phe Cys Tyr Thr Trp Val digest. pdcaas
total: -

The Protein Combining Visual Tool can be used to analyze protein combinations for thousands of different foods.

7. Is Protein Combining Necessary?

For individuals living in developed countries with access to a varied diet, protein combining may not be necessary. While it may lead to improved protein utilization, there is limited evidence to support this claim. It is generally believed that a balanced diet with a variety of protein sources and adequate energy intake provides enough essential amino acids to meet human needs.

However, protein combining may be useful for certain groups, including:

  • People with limited access to varied protein sources (e.g. due to poverty, crisis situations, geographical limitations, vegetarianism, veganism)
  • Athletes whose performance depends on high-quality protein intake (e.g. bodybuilders, powerlifters)
  • People who can easily incorporate protein combining into their diet and those who follow a low-protein diet (to ensure that the lower overall protein intake comes from the best sources)
  • Individuals who have been advised to combine proteins by their healthcare provider or trainer

8. Is Protein Combining Complicated?

The debate surrounding protein combining often centers on the potential benefits and complexity of the practice. While more research is needed to fully understand the value of protein combining, especially for individuals living in areas with access to a varied diet, and the most effective way to implement it, it is important to note that protein combining should not be dismissed based on its perceived complexity. Doing so is not only flawed reasoning, but it also ignores the reality of how easy it can be to combine proteins.

The interactive visual tools on this website make protein combining easy by eliminating the need to memorize suitable pairs or groups of complementary food items. The visual tools can also be used to identify universal food items that can be added in small amounts to any protein source to increase its biological value. One such "golden enhancer" is Weider Gold Whey Protein 2000 (EAN: 4044782312599), and to a varying degree, other protein supplements on the market. Using the visual tools, it can be seen that just a teaspoon of Weider Gold Whey Protein 2000 can bring a serving of an inferior protein up to a PDCAAS value of 1. For example, 50 grams of basmati rice (PDCAAS .7) will require 2 grams of this protein supplement; 50 grams of wheat bread (PDCAAS .4) will need about 3-4 grams. As you can see, the practice of protein combining is only as complicated as the effort required to put this knowledge into practice.

9. Final Thoughts on Protein Quality Evaluation and Protein Combining

It is worth noting that the PDCAAS method for evaluating protein quality has some limitations. Firstly, the PDCAAS rating works with a maximum value of 1, so all protein sources that exceed this value are assigned a rating of 1. This means that proteins like egg whites, whey protein, most meats, cheese, and milk are all rated equally, even though some may be more efficiently utilized by the body than others. Secondly, the PDCAAS method uses the PD index to assess the digestibility of the entire protein at the end of the digestive tract (faecal digestibility). This may not be as precise as determining the PD for each amino acid separately at the terminal ileum (ileal digestibility), as is done with the DIAAS method. It is also important to note that digestibility is often determined in growing pigs or rats, rather than humans, which may introduce further discrepancies.

These and other limitations should be considered when evaluating protein quality using the PDCAAS method or any other method. While we have a good understanding of the hierarchy of protein quality at the cluster level, it is important not to adopt an uncritical view of these methods. Further advances in research may reveal new insights into the best ways to evaluate protein quality as well as to protein combining. For the time being, it is reassuring to know that most, if not all, current methods for evaluating protein quality yield similar results.


NOTE: Due to the variability of protein digestibility (PD) measurements the PD index in our visual tools is approximate. Where there is no credible PD data available, our tools use a fictional value of 85 (you will always be informed about this within the tool). As a result, certain inaccuracies are to be expected in the final values.


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Superb info! Thank you!


You're welcome, Abila. I'm glad it helps. Jan.


there. add a little whey into meals and protein combining is dead as a science.


Perhaps reading the article again might help.


Incredibly helpful. Thank you for this.