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Protein Digestibility – Definition & Importance

Protein digestibility describes how efficiently the body breaks down and absorbs amino acids from dietary protein. It is a key measure of nutritional protein quality.

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Things worth knowing about "Protein digestibility"

Protein digestibility describes how efficiently the body breaks down and absorbs amino acids from dietary protein. It is a key measure of nutritional protein quality.

What Is Protein Digestibility?

Protein digestibility refers to the extent to which amino acids from dietary protein are broken down in the gastrointestinal tract and absorbed through the small intestine into the bloodstream. It is a fundamental criterion for assessing protein quality and determines how effectively a food protein can meet the body's requirements for essential amino acids.

Animal-based proteins – such as those found in meat, fish, eggs, and dairy products – typically exhibit very high digestibility values above 90 %, while plant-based proteins often range between 60 and 90 %, depending on the food source and preparation method.

Measurement Methods and Scoring Systems

Several established methods are used to scientifically evaluate protein digestibility:

PDCAAS (Protein Digestibility Corrected Amino Acid Score): Recommended by the WHO and FAO, this score evaluates protein quality based on the amino acid composition and fecal digestibility. The maximum score is 1.0.
DIAAS (Digestible Indispensable Amino Acid Score): A more recent and more precise method that measures the digestibility of individual essential amino acids in the small intestine and is considered superior to the PDCAAS.
Apparent vs. true digestibility: Apparent digestibility measures the difference between ingested and excreted nitrogen. True digestibility additionally corrects for endogenous nitrogen losses from the digestive tract.

Factors Affecting Protein Digestibility

A variety of factors can influence how well a protein is digested:

Food-Specific Factors

Antinutritional substances: Compounds such as trypsin inhibitors, phytic acid, and lectins found in legumes, cereals, and some vegetables can impair protein digestion.
Protein structure: Highly cross-linked proteins, such as collagen, are less digestible than proteins that unfold more easily during digestion.
Dietary fiber content: High levels of dietary fiber can reduce digestive enzyme activity and limit amino acid absorption.

Processing and Preparation Factors

Heating and cooking: Thermal treatment denatures proteins and largely inactivates antinutritional factors, generally improving digestibility.
Fermentation and germination: These processes break down antinutritional compounds and significantly increase the bioavailability of amino acids from plant-based foods.
Mechanical processing: Milling and grinding increase the surface area of the food, facilitating enzymatic access to the protein.

Individual Factors

Age (reduced digestive capacity in older adults)
Health status of the digestive system (e.g., inflammatory bowel disease)
Composition of the gut microbiota
Adequate production of digestive enzymes (proteases) by the stomach, pancreas, and small intestine

Protein Digestibility of Selected Foods

The following overview provides approximate digestibility values for common protein sources:

Eggs: approx. 97 % (PDCAAS: 1.0)
Lean meat and fish: 94–97 %
Whey protein: approx. 99 % (PDCAAS: 1.0)
Soy protein: approx. 91 % (PDCAAS: 0.91–1.0 depending on processing)
Legumes (e.g., lentils, beans): 70–85 %
Wheat and grain products: 60–80 %

Relevance for Nutrition and Health

Adequate protein digestibility is essential for meeting daily protein requirements, particularly for:

Children and adolescents during growth phases
Older adults at increased risk of muscle loss (sarcopenia)
Pregnant and breastfeeding women
Athletes with elevated protein needs
Individuals following predominantly plant-based diets (vegan or vegetarian)

Those eating a predominantly plant-based diet can improve overall protein quality by combining different plant protein sources, as their amino acid profiles complement one another (e.g., legumes paired with grains).

References

FAO/WHO: Dietary Protein Quality Evaluation in Human Nutrition. FAO Food and Nutrition Paper 92. Rome, 2013.
Rutherfurd S.M. et al.: Protein Digestibility-Corrected Amino Acid Scores and Digestible Indispensable Amino Acid Scores Differentially Describe Protein Quality in Growing Male Rats. Journal of Nutrition, 2015; 145(2):372–379.
Schaafsma G.: The Protein Digestibility-Corrected Amino Acid Score. Journal of Nutrition, 2000; 130(7):1865S–1867S.

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