Protein is an essential macronutrient required for various physiological processes, including muscle growth, repair, enzyme production, and immune function. While dietary guidelines provide recommendations for protein intake, not all proteins are created equal. The quality of a protein source is determined by its ability to provide essential amino acids in amounts sufficient to meet human needs. 

Over the years, several methods have been developed to assess protein quality, with the Digestible Indispensable Amino Acid Score (DIAAS) emerging as the most accurate and comprehensive. It is a method endorsed by the Food and Agriculture Organization (FAO) of the United Nations in 2013. It is designed to evaluate the quality of dietary proteins based on their digestibility and ability to meet human amino acid requirements. DIAAS replaces the older Protein Digestibility Corrected Amino Acid Score (PDCAAS) and addresses some of its limitations by providing a more precise measurement of protein quality.

The core of DIAAS is: How much of the protein consumed will end up at the very end of the small intestine. DIAAS is a ratio, with the denominator being standard protein/amino acid. Therefore, when DIAAS is greater than 100, the food is considered high-quality protein; when DIAAS is less than 75, it is considered non-high-quality protein. We can find that the DIAAS scores of animal protein are all superior to those of plant protein.

Determining DIAAS requires precise methodologies to evaluate the true ileal digestibility of individual indispensable amino acids in a protein source. Below are the primary methods used for this purpose:

Animal Models (Pig Ileal Digestibility Studies)

Pigs are commonly used as models for human digestion due to the physiological similarities of their gastrointestinal tract to humans. In these studies, pigs are surgically fitted with a cannula at the ileum to collect digesta after consuming a protein-containing diet. The amino acid content in the diet and ileal digesta is analyzed to calculate true ileal digestibility. While accurate, this method raises animal welfare concerns. Therefore, we choose to use INFOGEST, one of the in vitro methods which has been validated.

In Vitro Digestion Models

Laboratory-based in vitro methods replicate human digestion using digestive enzymes (e.g., pepsin, trypsin, chymotrypsin) under controlled conditions.  Protein samples are subjected to enzymatic hydrolysis, and the release of amino acids is measured to estimate digestibility.  There are two different in vitro methods, INFOGEST and TIM.

1. INFOGEST Model

INFOGEST is a standardized static in vitro digestion model designed to simulate human gastrointestinal digestion. It follows an international consensus protocol, ensuring consistency and comparability across studies. The model includes three sequential phases: oral (salivary enzymes), gastric (pepsin at acidic pH), and intestinal (pancreatic enzymes and bile salts). It is particularly useful for comparing the digestibility of different protein sources under controlled conditions. 

2. TIM Model (TNO Gastro-Intestinal Model)

TIM is a dynamic, computer-controlled in vitro digestion system that mimics the human gastrointestinal tract. It simulates physiological conditions, including pH changes, enzyme secretion, and peristaltic movements. TIM is highly advanced and provides detailed insights into protein digestion and amino acid release over time. The model is particularly valuable for studying the impact of food matrix and processing on protein digestibility. 

DIAAS represents a significant advancement in the assessment of protein quality, offering a more accurate and nuanced understanding of how proteins meet human nutritional needs. By accounting for true ileal digestibility and age-specific amino acid requirements, DIAAS provides a reliable tool for improving dietary planning, food labeling, and nutrition policies. As we continue to strive for healthier, more sustainable diets, DIAAS will remain a vital framework for evaluating and enhancing protein sources worldwide.

References

• Food and Agriculture Organization of the United Nations (FAO). (2013). "Dietary protein quality evaluation in human nutrition."
• Rutherfurd, S. M., & Moughan, P. J. (2012). "Digestible indispensable amino acid score (DIAAS) is a superior method for assessing protein quality." Nutrition Research Reviews.
• Minekus, M., et al. (2014). "A standardised static in vitro digestion method suitable for food–an international consensus." Food & Function.
• Brodkorb, A., et al. (2019). "INFOGEST static in vitro simulation of gastrointestinal food digestion." Nature Protocols.

Editorial by Dr. Seiji Aoyagi

Dr. Seiji is the Chief Scientific Officer of NiHTEK & the owner and partner of GPNi Japan. He brings over 35 years of experience in clinical and sports nutrition, with publications in both the U.S. and Japan. He holds 4 nutrition patents, led the approval of HMB in Japan, and founded Total Nutrition Therapy to train physicians in clinical nutrition.