Digestibility Issues of Vegetable versus Animal Proteins: Protein and Amino Acid Requirements—Functional Aspects

Food and Nutrition Bulletin, vol. 34, no. 2 © 2013, The United Nations University. Protein digestibility predicts the proportion of ingested nitrogen or amino acid made available to the organism after digestion and absorption. It is determined from the proportion that is not absorbed in the intestine following protein consumption in either human subjects or animal models (mainly growing rats or pigs). This is a complex process due to the continuous movements and exchange of protein, amino acids, and urea between the gut lumen and the systemic pools. For determination of true digestibility, additional discrimination is made between exogenous (food) and endogenous (secretions, desquamations) nitrogen losses. Individual amino acid digestibility is usually related to whole protein digestibility, but individual amino acid digestibility can be determined. The digestibility of protein has largely been determined from fecal digestibility, the difference between nitrogen ingested and excreted in the feces, which does not take into account colonic metabolism. As unabsorbed amino acids are mostly metabolized by colonic bacteria and then converted to ammonia and other compounds that can be absorbed, fecal digestibility can be overestimated. Ileal digestibility, measured at the terminal ileum, is considered more accurate for determination of dietary amino acid digestibility. From these measurements, a high (> 95%) digestibility is usually observed with animal proteins (egg, milk, meat) as well as concentrated or purified plant protein once plant cell wall constituents are removed (wheat gluten, wheat flour, soy protein isolate). A lower digestibility (around 80% to 90%) is observed with some less purified plant products (cereals, peas, soybean flour), and a digestibility of 50% to 80% can be associated with higher levels of plant cell walls, the presence of antinutritional factors, or food processing and heat treatment. Table 1 shows the effect of processing on the digestibility of protein from wheat. Differences between fecal and ileal digestibility are particularly important for plant protein sources, which are poorly digested in the upper intestine and increase the quantity of protein-derived nitrogen to be fermented in the colon. Table 2 compares fecal and ileal digestibility of animal and vegetable protein sources in humans. This is also complicated by the hypothesized recycling of bacterial-derived amino acids to the host and the potential for colonic nitrogen metabolism to either remove or add amino acids to the dietary supply. The recent (and still unpublished) Food and Agriculture Organization (FAO) expert consultation on protein quality evaluation in human nutrition concluded that proteins should first be described on the basis of their digestible amino acid contents, with each amino acid being treated as an individual nutrient, and that the protein digestibility corrected amino acid score (PDCAAS) should be replaced by a new score, the digestible indispensable amino acid score (DIAAS), where DIAAS % = 100 × [(mg of digestible indispensable amino acid in 1 g of dietary protein)/(mg of the same indispensable amino acid in 1 g of reference protein)] [1]. Ileal protein digestibility values are most often lower than those derived from fecal data. This is the case for individual amino acids as well, with the exception of methionine, which had an ileal digestibility 10 percentage points higher than the fecal value [2]. Differences in amino acid ileal digestibility were found between milk Daniel Tomé