Joseph F. McCaffrey MD, FACS


A Measure of the Quality of Dietary Protein


In humans, especially those with a scientific bent, always trying to find ways to accurately measure things. This is certainly true when it comes to the food we eat. I'm going to describe one currently recommended way of evaluating protein, but before I do I want to cut to the chase: you really don't need to pay too much attention to this metric. Common sense does just about as well as the fairly complicated system I'm going to describe.


For decades, we only talked about rather rough measures such as the total number of calories in a serving of food and how much of it came from protein or fat or carbohydrate. Going somewhat deeper, we also looked at what vitamins and minerals a food might contain. Now we're going even deeper.


One example is the glycemic index, which most people have heard about. This is a measure that evaluates the effect of carbohydrate on our metabolism, specifically on the effect of a given amount of carbohydrate on our blood sugar. While a piece of white bread and the same number of calories of oatmeal are both basically 100% carbohydrate, their effect on our blood sugar levels and thereby our entire metabolic response to eating them is entirely different. The glycemic index is an attempt to measure that difference.


Similarly, we now realize that not all fats are equal in their effect on our bodies. By law fat has approximately 9 calories per gram, we now know that there are "good" and "bad" fats in terms of their effects on our health.


Now there is a scale that attempts to delineate the quality of protein in terms of its nutritional completeness and availability.


Proteins consist of a series of different amino acids in various combinations. The amino acids themselves are generally categorized as essential, nonessential and conditionally essential.


A nonessential amino acid is one that our bodies can produce from other amino acids via some pretty amazing metabolic pathways. An essential amino acid is one that we can produce ourselves. It needs to be in the diet. Finally, a conditionally essential amino acid is one that we can make to some extent but might not be able to make as much as our bodies need under certain specific circumstances.


So one way of evaluating a pro team is to look at the amino acids it contains. However, that's not the only factor we need to consider.


Some proteins are more digestible and easier to absorb than others. So if we eat a protein that has a great amino acid profile but it's indigestible and passes right through us, we really don't get any nutritional value from it.


Over the years, there've been several attempts to develops protein scoring systems. Systems tried in the past limited by the fact that they were either based on animal models or didn't take into account actual digestibility. Since the mid-1990s there's been a new system that, at least for the time being, both the FDA and the Food and Agricultural Organization of the United Nations subcommittee of the World Health Organization recommend as best tool to describe the quality of approaching. This tool is the Protein Digestibility Corrected Amino Acid Score (PDCAAS). That name is certainly a mouthful.


This method evaluates protein based on its amino acid profile versus the amino acid requirements of humans while also considering it's absorbability.


To produce a score, the amino acid profile of the test protein is compared to a reference protein that provides the complete spectrum of amino acids required by human beings. If a protein has an inadequate amount of a given amino acid that amino acid is considered factor. The PDCAAS is affected by the ratio of the amount of the limiting amino acid in the test protein versus the reference protein.


Next, the amount of the protein absorbed is measured by comparing the amount of protein ingested and subtracting the amount of protein measured in the subsequent stool. This is one potential weakness of this scoring system because some protein that reaches the large intestine will be digested and utilized by the bacteria there. This protein is not nutritionally available to the human being, but won't be measured in the store resulting in a falsely high PDCAAS value.


Also, PDCASS is usually only reported for one protein, not for the combination of proteins that we may more typically have in our diets.


For example a protein that is complete except for the absence of one amino acid would have a very low score. The second protein may be complete except for the absence of a different amino acid. This protein would also have a low score. However, if both proteins were eating together they would be nutritionally complete.


Keeping these limitations in mind, this scoring system still has some usefulness. It's reported as the ratio of the tested protein score to the reference protein score. Therefore a perfect score is 1.0, with lower scores suggesting incomplete nutrition.


To give you some idea of how these scores tend to fall out, milk protein, egg white, whey protein all have scores of 1.0. Beef has a score of about 0.9. Pinto beans, kidney beans chickpeas and other legumes tend to have scores of 0.7 to 0.8. Cereals and grains are in the 0.4 to 0.6 range.


Having gone through all that, the results of the PDCASS evaluation pretty much confirm what we've generally thought. For example beef, eggs and dairy products are traditionally popular protein sources. Vegetarians know that they need to combine foods to get a complete protein.


Here's a reference that describes the digestibility corrected score in more detail: 


Schaafsma G (July 2000). "The protein digestibility-corrected amino acid score". The Journal of Nutrition 130 (7): 1865S–7S. PMID   10867064


Just to give you an idea of how “exciting” the original papers are on this stuff, here’s an abstract from PubMed on one of the papers I’ve been looking at on evaluating protein quality:


J Assoc Off Anal Chem. 1990 May-Jun;73(3):347-56.

Evaluation of protein digestibility-corrected amino acid score method for assessing protein quality of foods.

Sarwar G,  McDonough FE.


Health and Welfare Canada, Health Protection Branch, Ottawa, Ontario.


The current concepts of protein quality evaluation were reviewed. A detailed examination of existing animal assays and more promising amino acid scoring methods has been carried out by an Ad Hoc Working Group on Protein Quality Measurement for the Codex Committee on Vegetable Proteins during the last 5 years. Several factors such as inadequacies of protein efficiency ratio (PER, the poorest test) and other animal assays, advancements made in standardizing methods for amino acid analysis and protein digestibility, availability of data on digestibility of protein and individual amino acids in a variety of foods, and reliability of human amino acid requirements and scoring patterns were evaluated. On the basis of this evaluation, amino acid score, corrected for true digestibility of protein, was recommended to be the most suitable routine method for predicting protein quality of foods for humans. Amino acid scores corrected for true digestibility of protein (as determined by rat balance method) were termed "protein digestibility-corrected amino acid scores." A detailed method for the determination of the protein digestibility-corrected amino acid score was proposed, and information about the range of scores to be expected in foods or food products was provided in the present investigation. The protein digestibility-corrected amino acid score method is a simple and scientifically sound approach for routine evaluation of protein quality of foods. Accuracy of the method would, however, be confirmed after validation with growth or metabolic balance studies in humans.


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