Two protein sources can contain the same number of grams of protein on a nutrition label, yet your body may actually absorb and use meaningfully different amounts from each. This is the concept of protein digestibility, and it’s an important, often overlooked factor in comparing protein sources. Here’s what it means.
What Is Protein Digestibility?
Protein digestibility refers to how efficiently the body can break down a protein source and absorb its constituent amino acids during digestion. Not all of the protein listed on a nutrition label is necessarily absorbed and available for the body to use — some is lost during digestion, influenced by the protein’s source, structure, and the presence of other compounds that can interfere with absorption.
How Protein Digestibility Is Measured
Two common scoring systems are used to evaluate protein quality, incorporating both amino acid completeness and digestibility:
PDCAAS (Protein Digestibility-Corrected Amino Acid Score) evaluates protein based on its essential amino acid content relative to human needs, adjusted for digestibility. Scores range from 0 to 1.0, with animal proteins like whey, egg, and casein typically scoring at or near the maximum.
DIAAS (Digestible Indispensable Amino Acid Score) is a newer, generally considered more accurate scoring method that measures digestibility at the point of amino acid absorption in the small intestine, rather than using fecal measurements as PDCAAS does. DIAAS scores can exceed 1.0 for some very high-quality proteins.
Why Animal Proteins Generally Score Higher
Animal-based proteins (meat, eggs, dairy) typically have higher digestibility scores than plant-based proteins for a few key reasons:
Fewer digestive inhibitors. Plant foods often contain compounds like phytates, tannins, and fiber that can interfere with protein digestion and absorption, whereas animal proteins generally lack these inhibitory compounds.
Protein structure. The structural characteristics of animal proteins tend to be more readily broken down by human digestive enzymes compared to some plant protein structures.
Does Lower Digestibility Mean Plant Protein Is “Bad”?
Not necessarily — it simply means that, gram for gram, you may need to consume somewhat more plant protein to achieve the same amount of actually absorbed and usable amino acids compared to an equivalent amount of animal protein. This is part of why some nutrition guidelines suggest slightly higher total protein intake targets for people following predominantly plant-based diets, to help offset this digestibility difference.
Additionally, certain food preparation methods — such as cooking, soaking, sprouting, or fermenting legumes and grains — can meaningfully improve the digestibility of plant proteins by reducing the concentration of inhibitory compounds like phytates.
Practical Implications
For omnivores: digestibility differences are less of a practical concern, since a varied diet including both animal and plant proteins generally provides more than adequate digestible protein.
For vegans and vegetarians: paying attention to total protein intake (potentially aiming somewhat higher than standard recommendations), using preparation methods that improve digestibility, and incorporating higher-digestibility plant sources like soy can help ensure adequate absorbed protein.
For protein supplement users: whey, casein, and egg-based proteins generally offer very high digestibility, while plant-based protein powders (particularly blends) are formulated to help offset the digestibility limitations of individual plant sources.
The Bottom Line
Protein digestibility highlights an important nuance beyond simply counting grams of protein on a label — how efficiently your body actually absorbs and uses that protein matters too. While animal proteins generally have a digestibility advantage, thoughtful food choices, preparation methods, and modestly higher intake targets can help those on plant-based diets meet their effective protein needs just as effectively.
This article is for informational purposes only and is not a substitute for professional medical or nutritional advice.
