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

1. Introduction

In this article we will be talking about the quality of protein in human diet, methods of protein quality evaluation, and a related topic of protein combining. My aim as an author is not to convince anyone of the utility or necessity of any particular dietary practice. Rather, the objective is to

  • present the existing theory,
  • facilitate its understanding by way of interactive visual tools,
  • offer these interactive tools to the public for the purpose of studying thousands of food items and their protein quality, and
  • add value by presenting implications that have been largely omitted in scientific debate.

DISCLAIMER: The content on this web page does not imply the expression of any opinion on the part of the Food and Agriculture Organization of the United Nations (FAO). The mention of specific companies or products does not imply their endorsement or recommendation - by FAO or the author - in preference to others of a similar nature. Any non-descriptive statements are the views of the author, and they do not necessarily reflect the views of others.

2. What is protein

PROTEINS are large molecules composed of smaller molecules of amino acids. They are necessary for:

  • growth and development of the body,
  • body maintentance and the repair of worn out or damaged tissue,
  • building and preservation of muscle,
  • production of metabolic and digestive enzymes and certain hormones.

One (1) gram of protein provides approx. 4 kcal of energy.

3. Recommended intake of protein

The need for protein varies from person to person and over time, in relation to factors such as total energy intake, body weight, body composition, age, lifestyle, and individual need for nitrogen and amino acids. Requirements for protein are comparatively greater during pregnancy and breastfeeding, and for physically active people and individuals with enhanced muscle mass. It is generally recommended that humans consume about 12-35 % of daily calories from protein.

4. Protein quality and its evaluation in human diet

QUANTITY IS NOT EVERYTHING when it comes to protein. Body functions such as immunity, linear growth and mental development rely on protein quality.

There are several methods of evaluating the quality of protein. The most modern one, the Digestible Indispensable Amino Acid Score (DIAAS), is still quite impractical for the general public due to its complexity and missing foundational data. Consequently, the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), a method recommended by FAO/WHO more than thirty years ago (FAO/WHO Expert Consultation on Protein Quality Evaluation, 1989), remains the standard. The visual tools on this page use the PDCAAS method.

The PDCAAS values range from 0 to 1, with 1 being the highest possible score. The score is calculated from two indexes:

  • Amino Acid Score - AAS. Broadly speaking, the AAS rates the ability of a foodstuff's select amino acids to satisfy the human metabolic demands. It is calculated for each relevant amino acid of the tested food by comparing its actual amounts (column 3 in the table below) to the amounts prescribed by the appropriate 'reference pattern' (col. 4). In a good protein source, each amino acid will be present in at least the prescribed amounts (100 percent of the ideal). If one or more amino acids are below this ideal, such protein is considered to be of a lower quality. This is because the least represented amino acid (the limiting amino acid) has negative effect on the utilization of the entire protein. Note: The FAO reference patterns are 'minimum amino-acid values' that a foodstuff must have in order to be called a good quality protein. The FAO reference patterns have been derived from years of scientific observation of human metabolic needs, and are age-specific (they differ for children and adults) because as humans age, their digestive system evolves and metabolic needs change. The reference pattern prescribed for a new-born child, for instance, corresponds to the protein composition of breast milk, because breast milk proved to be best at satisfying infants' metabolic needs.
  • Protein digestibility - PD. PD is a value obtained by measuring - at the end of the digestive tract - the absorption rate of a foodstuff's amino acid nitrogen. The higher the PD value (PD 100 being the maximum), the better the food is digested by a human body.

To better understand how the final value of PDCAAS is obtained, let's use the Protein Quality Evaluation visual tool to evaluate the protein quality of basmati rice.

Example 1: PDCAAS Visual Evaluation of the protein of basmati rice (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 (percent) 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 downwards 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 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.

The above PDCAAS calculator visual tool can be used to analyze protein quality of thousands of other foods. It should be noted that there is hardly any naturally occurring food protein source that would be completely lacking in one or more amino acids. The classification of proteins as 'complete' and 'incomplete' is thus misleading and should be dropped. A better approach is to realize that, depending on the amino acid structure, some proteins are better utilized by a human body than others.

The highest PDCAAS value any food can obtain is 1. It is achieved when a protein source contains all the relevant amino acids in the amounts equal to or exceeding those prescribed by the reference pattern. For persons 6 months and older, the highest quality protein sources (PDCAAS 1) are egg whites, whey protein, most kinds of meat, milk and a few others. For infants, nothing rivals the protein quality of human milk (in fact, relative to infants hardly any naturally occurring protein source achieves PDCAAS 1). In contrast, human milk is an inadequate protein source for adults because of its low overall protein content (it may contains good quality protein, but in too small amounts for an adult).

5. Protein combining as a means of improving protein quality

As was shown in Example 1, 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 burned as fuel instead of being utilized as a protein source with all its functions outlined at the beginning of this article. The goal of protein combining is to prevent this waste, by combining foods that make up for their respective weaknesses.

EXAMPLE 2: Combining rice with beans for a higher quality protein (persons 3 years of age and older): As was noted above, basmati rice is deficient in the amino acid lysine. What we did not mention, 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 are great complements and when paired, yield a better (a more 'complete', if you must) protein. To see this effect, keep clicking the up arrow in the numeric stepper in the table below to gradually increase the portion size of kidney beans. (If you do not see the arrows, keep increasing the portion size manually, by 2-gram increments.) It will be noted that a mixture of 50 grams of rice and 50 grams of beans yields a protein with the PDCAAS value 1 - a perfect protein in which all relevant amino acids are found in amounts equal to or greater than those prescribed by the relevant reference pattern.

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

Once again, the Protein Combining Visual Tool can be used to analyze protein combinations of thousands of foods.

6. Is it necessary to combine proteins?

For those who live in the developed world and have access to a varied diet, protein combining is likely not necessary. It may be useful as a supplementary dietary practice, as it may indeed lead to a better protein utilization, but no convincing evidence quantifying the added value has been submitted. It is generally held that a balanced diet composed of a variety of protein sources and an adequate energy supply provides enough essential amino acids to cover human needs.

The practice of protein combining, on the other hand, is likely useful to the following groups:

  • people with limited access to varied sources of protein (e.g. poverty, crisis situations, geographical limitations, vegetarianism, veganism),
  • athletes whose performance depends on the intake of quality protein (e.g. bodybuilding, powerlifting etc.),
  • all people for whom the practice won't pose a nuisance complication, particularly those who follow a low-protein diet (the lower overall amount of protein should be compensated by it coming from the best sources),
  • people who have been advised to combine proteins by their doctor, nutritionist, or trainer.

7. Is protein combining complicated?

The debate concerning protein combining is often limited to the questions of benefits and complexity of the practice. While further research is needed to assess the added value of protein combining, particularly in respect of those who live in geographical areas that allow good access to a varied diet, as well as the best method of implementation of the practice, it must be noted that protein combining should not be dismissed on the grounds of its complexity, as sometimes happens. Doing so is not only logically flawed; it also ignores reality.

The interactive visual tools on this website simplify the task of protein combining to such an extent that users no longer need to memorize suitable pairs or groups of complementing food items. Moreover, they can be utilized to identify a few universal food items that can be added in trace amounts to any other protein source to increase its biological value. One such 'golden enhancer of everything' is the Weider Gold Whey Protein 2000 (EAN: 4044782312599), and to a varying degree other protein dietary supplements on the market. As can be validated in the visual tools, a mere teaspoon of the Weider Gold Whey Protein 2000 will bring any typical serving of an inferior protein to the value of PDCAAS 1. For instance, 50 grams of basmati rice (PDCAAS .7) will require 2 grams of this protein supplement; and 50 grams of wheat bread (PDCAAS .4) will need about 3-4 grams of it. It can be concluded that the practice of protein combining is only as complicated as it is to put this knowledge into practice.

8. Final notes

This article would not be complete without mentioning the limitations of the PDCAAS method. Firstly, the rating of protein based on this measure has a ceiling of PDCAAS 1, and all protein sources that exceed this value are assigned PDCAAS 1. As a result, protein sources such as egg whites, whey protein, most meats, cheese or milk are rated equally, despite the fact that some of them are better utilized by the body than others. Second, the PDCAAS method works with the PD index that assesses digestibility of the entire protein at the end of the tract (faecal digestibility). This is arguably less precise than if the PD were determined for each amino acid separately, at the terminal ileum at the end of the small intestine (ileal digestibility), as is the case with the DIAAS method. It should also be noted that digestibility is often determined in growing pigs or rats, and not humans, which may lead to further discrepancies.

These and other limitations should be enough to prevent the adoption of uncritical views towards the PDCAAS, or any other method of protein evaluation. At this moment we should content ourselves with the knowledge that we have a decent understanding of protein quality hierarchy, at least on the level of clusters, and that most if not all methods of estimating dietary 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.


Incredibly helpful. Thank you for this.