THE SHORT VERSION

  • Protein quality describes the essential amino acids a protein supplies and how well your body can digest it.
  • PDCAAS combines those ideas in a score capped at 1.0. A top score does not make two proteins identical.
  • A lower score does not make a food a poor choice. Other foods in your diet can supply the amino acids it has less of.
  • Collagen is not a complete protein. Its missing essential amino acid matters when you compare it with whey, milk or soy.

Imagine two foods with the same protein number on the label, say 20 grams each. Will your body get the same thing from both?

The grams tell you how much protein is there. They do not tell you its balance of amino acids or how easily your body can digest it. Scientists use the term protein quality to describe those differences.

What does “complete” mean?

Nine amino acids have to come from food. A complete protein supplies all nine in amounts that meet a reference pattern based on human needs. Quality also takes digestion into account: your body has to be able to release and absorb those amino acids.1

Picture building a wall that needs nine kinds of brick. Having plenty of eight kinds cannot make up for too little of the ninth.

In protein science, the essential amino acid in shortest supply relative to the reference pattern is called the limiting amino acid. Other foods can help supply what that protein lacks.1

What does digestibility mean?

Digestibility describes how much of a protein can be broken down and absorbed. A food may contain the amino acids you need, but your body still has to reach them.

Milk and egg proteins are generally highly digestible. In some plant foods, cell walls and compounds that interfere with digestive enzymes can make protein less accessible. Cooking and other preparation methods can improve access.2

From the first bite to your body

After you chew and swallow, protein reaches the stomach. Acid helps unfold it, and a digestive enzyme called pepsin starts cutting the chains into shorter pieces.3

Most digestion and absorption happen next, in the small intestine, the part of your gut after the stomach. More enzymes cut the pieces into amino acids and tiny chains. Cells lining the gut take them up, and amino acids pass into the blood. That uptake is called absorption.3

The path of dietary protein, simplified. Amino acids can be used by the gut and liver as well as muscle and other tissues.3

The path of dietary protein, simplified. Amino acids can be used by the gut and liver as well as muscle and other tissues.3

Blood from the gut travels to the liver before circulating through the rest of the body. The gut and liver use some amino acids; others reach muscle, skin and other tissues. They can be used to make proteins and other compounds, or broken down for energy.3

Some protein reaches the large intestine, where gut microbes can use it. Part 12 takes a closer look at protein fragments and the gut.4

How is quality scored?

One widely used tool is PDCAAS, short for protein digestibility-corrected amino acid score. It compares a protein’s essential amino acids with a reference pattern, identifies the limiting amino acid, then adjusts for digestibility.1,5

The reported score runs from 0 to 1.0. A score of 1.0 means the protein meets the reference amino acid pattern after that adjustment. It does not mean every gram becomes muscle.1,5

Whey, casein and egg can reach 1.0. Soy also scores highly. Many individual grains and legumes score lower, though the result varies with the food and how it is prepared.1

Illustrative, rounded PDCAAS ranges for selected foods. These are not fixed scores for every product or preparation. A score describes protein quality, not the overall value of a food.1,6

Illustrative, rounded PDCAAS ranges for selected foods. These are not fixed scores for every product or preparation. A score describes protein quality, not the overall value of a food.1,6

Grains tend to be relatively low in lysine. Legumes, such as beans and lentils, tend to be lower in the sulfur-containing amino acids, methionine and cysteine.7

That is the logic behind familiar combinations such as beans and rice: one food helps cover what the other has less of. The foods do not have to share a plate. A varied diet with enough total protein can meet essential amino acid needs across the day.8

A lower score does not mean “avoid this food.” It tells you something about one source of protein within the rest of your diet.

Why is collagen different?

Collagen comes from animals, but it is not a complete protein. It lacks tryptophan, one of the nine essential amino acids, giving collagen alone a PDCAAS of zero. That is a result of the scoring method, not a claim that it contains no usable amino acids.6

Other proteins in a mixed diet can supply what collagen lacks. A modeling study showed how collagen could fit within a diet that still met essential amino acid needs. That does not make it nutritionally interchangeable with a complete protein.6

In a small randomized trial, 22 healthy older women took whey or collagen supplements for six days. Whey produced a greater muscle protein synthesis response, both over the hours measured and across the study period.9

Collagen did show a short-term increase in the exercised leg. Across the six-day measurement period, however, its response was not significantly above baseline. The study measured protein synthesis, not long-term muscle gain.9

Whether collagen helps skin or joints is a separate question. For this series, the point is that equal grams of collagen and a complete protein do not supply the same amino acid mix.

What does a top score leave out?

Whey and casein both reach a PDCAAS of 1.0, yet they generally deliver amino acids at different speeds. The score is capped and does not describe that timing. Matching scores therefore do not make two proteins identical.1,3,5

Part 1 follows that question further. It explains what a shared protein score means for the label and introduces another measure, the digestible indispensable amino acid score, or DIAAS. We will take those details one step at a time.

Related reading

References

Educational content. The Fountain is written to explain, not to make health claims about any product. This article summarizes published peer-reviewed research. Nothing here is medical advice.

  1. Boye J, Wijesinha-Bettoni R, Burlingame B. Protein quality evaluation twenty years after the introduction of the protein digestibility corrected amino acid score method. British Journal of Nutrition. 2012;108(Suppl 2):S183–S211. https://doi.org/10.1017/S0007114512002309
  2. Gilani GS, Xiao CW, Cockell KA. Impact of antinutritional factors in food proteins on the digestibility of protein and the bioavailability of amino acids and on protein quality. British Journal of Nutrition. 2012;108(Suppl 2):S315–S332. https://doi.org/10.1017/S0007114512002371
  3. Loveday SM. Protein digestion and absorption: the influence of food processing. Nutrition Research Reviews. 2023;36(2):544–559. https://doi.org/10.1017/S0954422422000245
  4. Diether NE, Willing BP. Microbial fermentation of dietary protein: an important factor in diet–microbe–host interaction. Microorganisms. 2019;7(1):19. https://doi.org/10.3390/microorganisms7010019
  5. Schaafsma G. Advantages and limitations of the protein digestibility-corrected amino acid score (PDCAAS) as a method for evaluating protein quality in human diets. British Journal of Nutrition. 2012;108(Suppl 2):S333–S336. https://doi.org/10.1017/S0007114512002541
  6. Paul C, Leser S, Oesser S. Significant amounts of functional collagen peptides can be incorporated in the diet while maintaining indispensable amino acid balance. Nutrients. 2019;11(5):1079. https://doi.org/10.3390/nu11051079
  7. Hertzler SR, Lieblein-Boff JC, Weiler M, Allgeier C. Plant proteins: assessing their nutritional quality and effects on health and physical function. Nutrients. 2020;12(12):3704. https://doi.org/10.3390/nu12123704
  8. Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: vegetarian diets. Journal of the Academy of Nutrition and Dietetics. 2016;116(12):1970–1980. https://doi.org/10.1016/j.jand.2016.09.025
  9. Oikawa SY, Kamal MJ, Webb EK, McGlory C, Baker SK, Phillips SM. Whey protein but not collagen peptides stimulate acute and longer-term muscle protein synthesis with and without resistance exercise in healthy older women: a randomized controlled trial. The American Journal of Clinical Nutrition. 2020;111(3):708–718. https://doi.org/10.1093/ajcn/nqz332