Amino acids, feedstuffs quality and formulation: Interview with Guoyao Wu (Texas A&M)

A: Results of studies by my group have established the concept that animals have dietary requirements for not only so-called “nutritionally essential” amino acids (EAAs) but also so-called “nutritionally nonessential” amino acids (NEAAs). The proteinogenic amino acids that are not formed de novo by animals must be provided in diets. The current National Research Council (NRC; e.g., the 2012 version of Swine Requirements) may not provide the best estimates of amino acid requirements for animals such as growing, lactating, and gestating swine. Tryptophan and branched-chain amino acids are prototypes for growing pigs and lactating sows, respectively.

When fed diets containing no or insufficient NEAAs, animals can grow but their growth or production performance is suboptimum. Examples in support of this notion are many, including arginine for gestating and lactating swine; glutamine for suckling piglets; arginine, glutamate, glutamine, glycine, and proline for post-weanling nursery pigs; and glycine for growing-finishing pigs. Thus, we must move beyond the long-standing “ideal protein” concept, which ignored all the NEAAs, in animal nutrition. “Optimum ratios and amounts of all proteinogenic AAs” should be considered in formulating diets for farm animals, including swine, poultry, and fishes. This new nutritional concept provides the much-needed scientific basis for improving amino acid nutrition in animals. As explained in our article (Wu and Li. Experimental Biology and Medicine 2022;247:1191-1201), we have proposed optimal ratios and amounts of true digestible amino acids based on experimental results. The quantifiable criteria include the following: tissue- and cell-specific requirements for amino acids, increases in the number and litter weight of live-born piglets, the growth and survival of animals, the milk yield of lactating dams and their subsequent pregnancy rates, egg production and quality (in the case of poultry), immunity and resistance to infectious pathogens, as well as feed efficiency.

A: Solutions to the mentioned problems are to develop low-protein diets containing adequate NEAAs. Glycine, glutamine, and proline are particularly important for poultry due to the following reasons. First, glycine and glutamine serve as intermediate sinks for dietary nitrogen in the body and are required for the production of uric acid as the major end product of amino acid-derived ammonia. When fed conventional diets, poultry do not synthesize sufficient glycine, glutamate, or glutamine. Second, unlike mammals, the skeletal muscle (e.g., breast muscle) of poultry has a limited ability to transport glucose due to the absence of glucose transporter-4 and instead uses glutamine as a major energy source for protein synthesis, growth, and tissue integrity. Third, poultry has a limited arginase activity for proline synthesis and thus their diets must contain sufficient proline to meet metabolic needs. We recently discovered that, in chickens, proline is a major substrate for the renal synthesis of polyamines (putrescine, spermidine, and spermine) for other tissues. These unique substances are essential for DNA and protein synthesis and, therefore, the growth, development, and reproduction of poultry.

A: Feedstuffs and forages are ultimately the only sources of amino acids for animals when their diets are not supplemented with crystalline amino acids or their sources. All plant proteins contain relatively low amounts of glycine and proline, and non-legume feedstuffs and forages are generally deficient in tryptophan, lysine, methionine, and threonine, relative to animal needs. In addition, all plants lack taurine (a nutritionally essential amino acid in carnivores) and creatine (essential for the development, integrity and function of tissues, particularly the brain, skeletal muscle, and reproductive tract), and contain only a negligible amount of 4-hydroxyproline (a potent anti-inflammatory amino acid in the intestine). Animal-sourced by-products are excellent sources of all proteinogenic amino acids, 4-hydroxyproline, taurine, and also provide creatine.

Feather meal is an abundant source of arginine, glycine, proline, and 4-hydroxyproline for animals, whereas mucosal peptones contain bioactive small peptides to protect the small intestine from oxidative stress and inflammation. Small amounts of animal-sourced feedstuffs can help balance amino acids in diets. Finally, feed quality is influenced by a plethora of factors, such as processing methods, variation in nutrient content and availability, and the presence of molds, toxins (e.g., mycotoxins and gizzerosine), or anti-nutritional factors (e.g., trypsin inhibitors and phytates).

A: Diets must provide all nutrients in proper amounts and balances. Because protein is the single most expensive component of animal diets, nutritionists must pay attention to NEAAs. The use of EAAs to form NEAAs in the body is biologically inefficient and economically wasteful, and therefore must be avoided. Some animals are very sensitive to dietary protein intake due to ammonia toxicity. For example, increasing the content of crude protein in the diet of gestating gilts from 12% to 14% reduces the number of total piglets born alive per litter by 0.21. In modern animal production, diets are often supplemented with one or more nutrients, probiotics, or prebiotics. However, economic benefits, including reductions in the costs of labor and health management, should be considered when a dietary supplement is included. Examples of beneficial, cost-effective supplements include arginine for gestating swine during early pregnancy, citrulline for gestating ruminants during early pregnancy; arginine for preweaning piglets with intrauterine growth restriction (IUGR); arginine, glutamate, glutamine, glycine, and proline for weanling pigs; and glycine for post-weaning IUGR pigs during the growing-finishing periods. Clearly, in contrast to recommendations from some nutritionists, synthetic amino acids can be beneficially supplemented to the diets of gestating swine to improve their reproductive performance. Finally, it must be borne in mind that the requirements of animals for nutrients are dynamic, and, therefore, environmental factors (e.g., ambient temperatures) and the diseased state of animals must be considered in the proper diet formulation. More does not necessarily mean better. For example, overfeeding energy to gestating swine increases embryonic mortality during early pregnancy and reduces feed intake during the subsequent lactation period.

A: There are several common misconceptions about poultry and swine nutrition. First, poultry and swine do not have dietary requirements for NEAAs. As noted previously, NEAAs must be sufficient in diets to optimize the health, growth and production performance, and feed efficiency of animals. Second, poultry and swine diets must be formulated according to the “ideal protein” concept. This concept might be ideal in the 1960s–1990s when our knowledge of cell- and tissue-specific metabolism of amino acids in animals was limited but has now been recognized to be flawed in animal nutrition. Third, energy is a nutrient, and glucose and fatty acids provide most energy for all tissues and cell types. Energy is not a nutrient but is contained in nutrients, primarily protein, amino acids, lipids, and starch. Amino acids, fatty acids, and glucose have different metabolic fates and different physiological roles in a tissue- and cell-specific manner. The proportions of energy from these three types of macronutrients can affect the metabolic profiles and health (e.g., obesity development) of animals, such as poultry and swine. The sources of energy substrates vary with cell types. For example, sufficient glutamine, glutamate, and aspartate (primary sources of energy in mammalian enterocytes) must be provided in diets to ensure the health of the small intestine. In the case of poultry, adequate glutamate and aspartate are also the primary sources of energy in their enterocytes, whereas dietary glutamine undergoes only limited degradation in the small intestine and is effectively available to skeletal muscle for ATP production. Dietary fiber, which is fermented in the hindgut to form large amounts of short-chain fatty acids (major metabolic fuels for epithelial cells of the large intestine), is crucial for the health of the gut. Fourth, NRC recommendations must be strictly followed in diet formulation. This is not true, as these values only serve as guidelines.

Animals have dynamic requirements for nutrients, particularly amino acids, under various physiological, pathological, and environmental conditions. Animals (e.g., poultry and swine) of different breeds or genetic backgrounds may have different capacities for growth (e.g., rate of muscle gain) and production (litter size, as well as egg size and quality) and, therefore, different requirements for dietary nutrients. Fifth, poultry and swine only need grains to thrive. These animals are omnivores rather than herbivores and can consume both plant-sourced feedstuffs and animal by-products. Without any supplements, grains alone cannot support the maximum growth or production performance of poultry and swine. Their diets must provide balanced and sufficient amounts of all nutrients, such as amino acids (including lysine, methionine, glutamine, glycine, and proline), essential fatty acids (e.g., n-3 and n-6 polyunsaturated fatty acids), vitamins (e.g., vitamins A, D, E, and K, as well as biotin, pyridoxine, and folic acid), and minerals (e.g., calcium, phosphorus, iron, and zinc). Sixth, poultry and swine diets must include antibiotics for rapid growth. Although in-feed antibiotics can reduce the incidence of disease such as diarrhea, they can be replaced effectively by alternatives such as essential oils, functional amino acids, and organic acids. Overuse of in-feed antibiotics in poultry and swine diets can result in antibiotic resistance in both humans (through the consumption of products such as meat, milk, and eggs) and animals, thereby posing a threat to public health. Adequate knowledge of principles of animal nutrition is necessary to dispel these common false myths in poultry and swine nutrition.