Significance of Dietary Protein Source in Human Nutrition: Animal and/or Plant Proteins?

Abstract

Within the framework of the interdisciplinary symposium on “Feeding a World Population of More Than Eight Billion People: A Challenge to Science,” the organizers asked us to discuss the role of animal protein foods as a necessary source of the nutritionally indispensable (essential) amino acids for meeting the needs of humans, including children. Specifically, we were directed to consider the recommendations made by a joint FAO/WHO Expert Consultation (FAO/WHO, 1991) concerning estimates of indispensable amino acid requirements in humans and the extent to which these estimates “were realistic in light of the observed adequacy of vegetarian diets; and how these requirements can be fulfilled.” Our focus, therefore, is on the requirements for amino acids, and so we attempt to meet our remit as follows: the most basic demand for the production of food proteins, assuming an adequately nourished, healthy population, is determined by the physiological requirements for a utilizable source of nitrogen and for the nine indispensable amino acids (with two conditionally indispensable amino acids, tyrosine and cystine) needed for support of maintenance, growth, and tissue repair. Adequate knowledge about the quantitative characteristics of these requirements is fundamental. Therefore, we begin by reviewing the status of current international estimates of amino acid requirements and argue that for the indispensable amino acids they are seriously inadequate for planning purposes, especially for adults, who will represent a sizable proportion of the world's future 8 billion people, and their food needs. The requirement for dietary protein consists of two components: (1) total nitrogen to serve the needs for synthesis of the nutritionally dispensable (nonessential) and conditionally indispensable amino acids, as well as for other physiologically important nitrogen-containing compounds, and (2) the nutritionally indispensable (essential) amino acids (IAA) that cannot be made by human tissues at rates commensurate with metabolic needs and so must be supplied from an exogenous source (diet, parenteral amino acid mixture). With respect to the human requirements for total nitrogen (protein), the current international recommendations are now broadly accepted (FAO/WHO/UNU, 1985).