Protein serves as a foundational macronutrient, providing the building blocks for structural tissues, enzymes, and hormones. All dietary proteins are composed of amino acids, including indispensable amino acids that the body cannot produce and must obtain from food. The true measure of a protein’s value lies in how much of it the body can actually absorb and utilize, a concept known as bioavailability.
Defining Protein Bioavailability
Protein bioavailability represents the proportion of protein that is digested, absorbed as amino acids, and subsequently utilized by the body for maintenance, repair, and growth. This concept focuses on the efficiency with which the body can access and use the constituent amino acids to support metabolic functions. The process involves breaking down large protein molecules into individual amino acids or small peptides within the digestive tract. Bioavailability is a combined function of the protein’s inherent amino acid profile and its digestibility, as poorly digested protein, even with an excellent profile, will have low bioavailability.
Standardized Methods for Measuring Quality
Scientists use standardized scoring systems to quantify a protein source’s value based on the availability of its indispensable amino acids. Historically, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was widely adopted for assessing protein quality. The PDCAAS calculation compares a protein’s indispensable amino acid profile to a reference pattern and corrects the score for fecal digestibility, measuring total protein digestion.
The PDCAAS has limitations, including using a single digestibility value and truncating scores exceeding 100% down to 100%. This truncation fails to recognize proteins that offer a surplus of indispensable amino acids or their potential to complement lower-quality sources.
The Digestible Indispensable Amino Acid Score (DIAAS) is the current preferred method, recommended by the Food and Agriculture Organization (FAO) to replace the PDCAAS. The DIAAS provides a more accurate measure by determining the digestibility of each individual indispensable amino acid specifically at the end of the small intestine (ileum). This ileal measurement is superior because amino acids passing beyond the small intestine are not available for metabolic use, and the DIAAS does not truncate scores above 100%.
Dietary and Processing Variables
A protein’s bioavailability can be reduced by other components in the food or by preparation methods. Certain compounds in plant foods, known as antinutrients, interfere with the body’s ability to break down and absorb protein. For example, phytic acid in grains and legumes, and tannins in cereals, can negatively affect absorption.
Protease inhibitors, such as trypsin inhibitors in soybeans, block digestive enzymes, substantially reducing protein digestibility. Food processing techniques, such as heating, boiling, and fermentation, can mitigate these effects by deactivating many antinutrients, thereby improving bioavailability.
The speed of digestion also influences utilization, with “fast” proteins like whey being rapidly absorbed and “slow” proteins like casein absorbed over a longer period. This difference in digestion kinetics affects the sustained availability of amino acids for muscle protein synthesis.
Comparing Common Protein Sources
Animal-based proteins generally have higher bioavailability due to their complete amino acid profiles and high digestibility. Highly bioavailable examples include whey protein, which is rapidly digested, and whole egg, often used as a gold standard in quality studies. Casein, another milk protein, is highly bioavailable but digested more slowly than whey, providing a sustained release of amino acids.
Plant proteins, such as those from legumes, grains, and nuts, often have lower inherent bioavailability compared to animal sources. This is due to the presence of antinutrients and the fact that most plant proteins are “incomplete,” meaning they are low in one or more indispensable amino acids (e.g., grains lack lysine; legumes lack methionine).
To optimize plant-based protein bioavailability, complementary protein combining can be employed. This strategy involves pairing two or more incomplete protein sources whose amino acid profiles balance each other out, such as combining grains with legumes. Consuming a variety of plant proteins throughout the day ensures the body receives all necessary indispensable amino acids.