nutrients Review Plant Proteins: Assessing Their Nutritional Quality and Effects on Health and Physical Function Steven R. Hertzler *, Jacqueline C. Lieblein-Boff, Mary Weiler and Courtney Allgeier Scientific and Medical Affairs, Abbott Nutrition, 2900 Easton Square Place, Columbus, OH 43219, USA; jacqueline.boff@abbott.com (J.C.L.-B.); [email protected] (M.W.); [email protected] (C.A.) * Correspondence: [email protected] Received: 2 November 2020; Accepted: 27 November 2020; Published: 30 November 2020 Abstract: Consumer demand for plant protein-based products is high and expected to grow considerably in the next decade. Factors contributing to the rise in popularity of plant proteins include: (1) potential health benefits associated with increased intake of plant-based diets; (2) consumer concerns regarding adverse health effects of consuming diets high in animal protein (e.g., increased saturated fat); (3) increased consumer recognition of the need to improve the environmental sustainability of food production; (4) ethical issues regarding the treatment of animals; and (5) general consumer view of protein as a “positive” nutrient (more is better). While there are health and physical function benefits of diets higher in plant-based protein, the nutritional quality of plant proteins may be inferior in some respects relative to animal proteins. This review highlights the nutritional quality of plant proteins and strategies for wisely using them to meet amino acid requirements. In addition, a summary of studies evaluating the potential benefits of plant proteins for both health and physical function is provided. Finally, potential safety issues associated with increased intake of plant proteins are addressed. Keywords: plant protein; protein quality; PDCAAS; DIAAS; vegetable protein; protein requirements; amino acids 1. Introduction Protein is a nutrient that has been trending increasingly positive in the minds of consumers, with demand rising for both plant and animal sources of protein [1]. In addition, there is a growing body of clinical evidence, especially in older adults, supporting health benefits associated with protein at or above current dietary protein intake recommendations. Among these health benefits are increases in lean body mass [2–6], functional benefits such as increased leg power [4] or gait speed [6], and improved bone density [7–9]. Thus, on the one hand, there is likely to be a continued push for protein-rich options in the food marketplace. On the other hand, the global production of an increased volume of food protein, especially high-quality animal protein, could present environmental sustainability challenges. The production of 1 kg of high-quality animal protein requires feeding 6 kg plant protein to livestock, which introduces the subsequent strain on land and water resources, as well as potential increases in greenhouse gas emissions, associated with livestock agriculture [1,10]. Wider and prudent use of plant proteins in the diet can help to supply adequate high-quality protein for the population and may reduce the potential for adverse environmental consequences. This review presents information on: (1) the nutritional quality of plant proteins; (2) strategies for wisely using plant proteins to meet indispensable amino acid requirements; (3) effects of plant proteins on health and physical function; and (4) potential health and safety concerns associated with plant proteins. Nutrients 2020, 12, 3704; doi:10.3390/nu12123704 www.mdpi.com/journal/nutrients Nutrients 2020, 12, x FOR PEER REVIEW 2 of 28 Nutrients 2020, 12, 3704 2 of 27 2. Determination of Protein Quality 2. DeterminationTwo requirements of Protein for Qualitya protein to be considered high quality, or complete, for humans are havingTwo requirementsadequate levels for of a proteinindispensable to be considered amino acids high (see quality, Table or 1) complete, to support for humanshuman aregrowth having and adequatedevelopment levels and of indispensable being readily amino digested acids and (see absorbed. Table1) to support human growth and development and being readily digested and absorbed. Table 1. Indispensable, dispensable, and conditionally indispensable amino acids in the human diet. TableAdapted 1. Indispensable, from [11]. dispensable, and conditionally indispensable amino acids in the human diet. Adapted from [11]. Indispensable Dispensable Conditionally Indispensable IndispensableHistidine Alanine Dispensable Arginine Conditionally Indispensable HistidineIsoleucine Aspartic Alanine acid Cysteine Arginine IsoleucineLeucine Asparagine Aspartic acid Glutamine Cysteine LeucineLysine Glutamic Asparagine acid Glycine Glutamine LysineMethionine Seri Glutamicne acidProline Glycine Methionine Serine Proline Phenylalanine Tyrosine Phenylalanine Tyrosine ThreonineThreonine TryptophanTryptophan Valine Valine VariousVarious methods methods for for evaluating evaluating protein protein quality quality have have been been developed developed over theover years, the years, but amino but amino acid scoringacid scoring is currently is currently the recommended the recommended method method by the Food by the and Food Agricultural and Agricultural Organization Organizatio of the Unitedn of the NationsUnited (FAO) Nations and (FAO) the U.S. and National the U.S. Academy National ofAcademy Sciences of [11 Sciences,12]. 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