Posted on April 1, 2019 by maggielberger

By Dr. Guoyao Wu, [email protected] or 979.845.1817

The English word protein originated from the Greek ‘‘proteios,’’ meaning prime or primary. This term is very appropriate in human nutrition, because protein is the most fundamental component of tissues in the body. Dietary protein (the source of amino acids) provides the body with nitrogen, hydrocarbon skeletons, and sulfur and cannot be replaced by any other nutrients. Amino acids are physiologically essential precursors for the synthesis of proteins, peptides, and low-molecular weight substances (e.g., glutathione, creatine, nitric oxide, dopamine, serotonin, melanin, melatonin, and nucleotides) with enormous physiological importance. In addition, some amino acids serve as metabolic fuels in certain tissues and as regulatory molecules in multiple cell types. For example, dietary glutamate, glutamine and aspartate are major metabolic fuels for the mammalian small intestine in the fed state, whereas glutamine in the arterial blood is almost the exclusive source of energy for its epithelial absorptive cells in the post-absorptive state.  In addition, glutamine is a major source of energy for lymphocytes and macrophages to sustain immune responses. Thus, amino acids are essential for the optimal health, growth, development, reproduction, lactation, and survival of humans. This is indicated by metabolic disorders, kwashiorkor (caused by the severe deficiency of protein) and marasmus (caused by the severe deficiency of both protein and energy) in humans, particularly in many children of developing nations. Less severe forms of dietary protein deficiency occur in elderly subjects, including those in developed countries (e.g., the United States), therefore increasing their susceptibility to metabolic and infectious diseases.

Dietary requirements of humans for amino acids are affected by dietary, environmental, physiological and pathological factors. For example, exercise enhances the oxidation of amino acids to carbon dioxide and, therefore, dietary requirements for amino acids (and proteins). The Recommended Dietary Allowance of protein for the healthy adult with minimal physical activity is currently 0.8 g protein/kg body weight/day, which represents only minimum daily average dietary intake that meets the nutrient requirements of nearly all (97.5%) healthy adults.  Recently, American College of Sports Medicine, the American Dietetic Association, and Dieticians of Canada recommended that individuals with moderate and intense exercise consume 1.3 (ranging from 1.2 to 1.4) and 1.6 (ranging from 1.2 to 1.7) g protein/kg body weight/day, respectively.  A combination of exercise and adequate protein intake plays an important role in maintaining human health. This is particularly important for overweight or obese subjects.

Animal-source foods (e.g., meat, dairy products, egg, poultry, seafood, and other products) contain higher quantities and more balanced proportions of amino acids relative to human tissues, compared with plant-source foods (e.g., rice, wheat, corn, potato, vegetables, cereals, beans, peas, processed soy products, nuts, and seeds). For example, beef contains 63-68% protein on the dry matter basis, but most staple foods of plant origin (except for legumes) have a protein content < 12% (dry matter basis) and are deficient in most amino acids, including lysine, methionine, cysteine, tryptophan, threonine, and glycine. To meet the Institute of Medicine-recommended dietary allowance of methionine plus cysteine by the 70-kg adult human, daily intake of meat, wheat flour, or rice would be 45, 285, or 493 g dry matter, respectively.  Thus, consumption of animal products can meet adequate requirements of humans for amino acids, particularly children, while substantially reducing the need for plant-based foods or the ingestion of a large amount of starch. This is important for individuals whose metabolic profiles may be compromised by high intake of digestible carbohydrates.  The excessive amount of carbohydrates that would be consumed in wheat flour or white rice can be converted into fat in the body, thereby possibly contributing to the development of obesity, dyslipidemia, and other metabolic disorders. Conversely, adequate intake of lean meat promotes protein synthesis and sustains skeletal-muscle mass and function (including physical strength), while improving insulin sensitivity, ameliorating aging-associated sarcopenia, and reducing white fat accretion.

To date, there are myths about protein nutrition in humans due to inadequate understanding of the science. An ideal human diet would consist of both animal- and plant-source foods in appropriate amounts and proportions, while consuming adequate dietary fiber. Globally, plant- and animal-based foods contribute about 65% and 35% of protein, respectively, in human diets, and the opposite is true in North America. There is evidence that consumption of animal protein at < 65% of total dietary protein cannot meet the optimal requirement of elderly subjects for at least one nutritionally essential amino acid, leading to suboptimal protein nutrition. Proper combinations of large amounts of legumes with cereals could meet the protein requirements of adults with minimal physical activity, but likely not for the optimal growth or development in children. The global availability of legumes as a staple food is increasingly limited relative to the growing population, and in many parts of the world, these foods are not produced. Animal agriculture produces high-quality protein for human consumption.

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