Amino Acids in Dog Nutrition and Health

Evaluation of Effective Energy Values of Six Protein Ingredients Fed to Beagles and Predictive Energy Equations for Protein Feedstuff

This study evaluated the nutrition composition, the nutrient digestibility, and the energy value of six protein ingredients used in pet food by the difference method in six beagles within a 7 × 6 incomplete Latin square design. The results showed that the apparent total tract digestibility of gross energy (GE) and organic matter (OM) in beagles fed the fish meal (FM) and corn gluten meal (CGM) diets was higher than for those fed the meat and bone meal (MBM), soybean meal (SBM), mealworm meal (MM), and yeast extract (YE) diets (p < 0.05). The digestible energy (DE), metabolizable energy (ME), and net energy (NE) of the MM diet were greater than the other diets, and MBM was the lowest (p < 0.05). The ME of protein ingredients was positively correlated with organic matter and negatively correlated with the ash content. The NE of protein ingredients was positively correlated with the crude protein content and negatively correlated with the ash content. The study resulted in predictive energy equations for protein ingredients that were more accurate than the NRC's predictive equation of ME when the ash content of the ingredient was more than 30% DM. In conclusion, the nutrient digestibility and energy value of corn gluten meal were similar to those of fish meal and those of soybean meal were similar to yeast extract. All predictive energy equations for six protein feedstuffs had slight differences with measured energy values.

Beyond the Bowl: Understanding Amino Acid Requirements and Digestibility to Improve Protein Quality Metrics for Dog and Cat Foods

The determination of amino acid (AA) requirements for mammals has traditionally been done through nitrogen (N) balance studies, but this technique underestimates AA requirements in adult animals. There has been a shift toward researchers using the indicator amino acid oxidation (IAAO) technique for the determination of AA requirements in humans, and recently in dogs. However, the determination of AA requirements specific to adult dogs and cats at maintenance is lacking and the current requirements outlined by the National Research Council are based on a dearth of data and are likely underreporting the requirements of indispensable AA (IAA) for the population. To ensure the physiological requirements of our cats and dogs are met, we need methods to accurately and precisely measure digestibility. In vivo methods, such as ileal cannulation, are most commonly used, however, due to ethical considerations, we are moving away from animal models and toward in vitro methods. Harmonized static digestion models have the potential to replace in vivo methods but work needs to be done to have these methods more accurately represent the gastrointestinal tract (GIT) of cats and dogs. The Digestible IAA Score (DIAAS) is one metric that can help define protein quality for individual ingredients or mixed diets that uses AA SID estimates and ideally those can be replaced with in vitro AA digestibility estimates. Finally, we need accurate and reliable laboratory AA analyses to measure the AA present in complete diets, especially those used to quantify methionine (Met) and cysteine (Cys), both often limiting AAs in cat and dog diets. Together, this will guide accurate feed formulation for our companion animals to satisfy requirements while avoiding over-supplying protein, which inevitably contributes to excess N excretion, affecting both the environment and feed sustainability.

Characteristics of the Digestive Tract of Dogs and Cats

As for other mammals, the digestive system of dogs (facultative carnivores) and cats (obligate carnivores) includes the mouth, teeth, tongue, pharynx, esophagus, stomach, small intestine, large intestine, and accessory digestive organs (salivary glands, pancreas, liver, and gallbladder). These carnivores have a relatively shorter digestive tract but longer canine teeth, a tighter digitation of molars, and a greater stomach volume than omnivorous mammals such as humans and pigs. Both dogs and cats have no detectable or a very low activity of salivary α-amylase but dogs, unlike cats, possess a relatively high activity of pancreatic α-amylase. Thus, cats select low-starch foods but dogs can consume high-starch diets. In contrast to many mammals, the vitamin B12 (cobalamin)-binding intrinsic factor for the digestion and absorption of vitamin B12 is produced in: (a) dogs primarily by pancreatic ductal cells and to a lesser extent the gastric mucosa; and (b) cats exclusively by the pancreatic tissue. Amino acids (glutamate, glutamine, and aspartate) are the main metabolic fuels in enterocytes of the foregut. The primary function of the small intestine is to digest and absorb dietary nutrients, and its secondary function is to regulate the entry of dietary nutrients into the blood circulation, separate the external from the internal milieu, and perform immune surveillance. The major function of the large intestine is to ferment undigested food (particularly fiber and protein) and to absorb water, short-chain fatty acids (serving as major metabolic fuels for epithelial cells of the large intestine), as well as vitamins. The fermentation products, water, sloughed cells, digestive secretions, and microbes form feces and then pass into the rectum for excretion via the anal canal. The microflora influences colonic absorption and cell metabolism, as well as feces quality. The digestive tract is essential for the health, survival, growth, and development of dogs and cats.

Recent Advances in the Nutrition and Metabolism of Dogs and Cats

Domestic dogs (facultative carnivores) and cats (obligate carnivores) have been human companions for at least 12,000 and 9000 years, respectively. These animal species have a relatively short digestive tract but a large stomach volume and share many common features of physiological processes, intestinal microbes, and nutrient metabolism. The taste buds of the canine and feline tongues can distinguish sour, umami, bitter, and salty substances. Dogs, but not cats, possess sweet receptors. α-Amylase activity is either absent or very low in canine and feline saliva, and is present at low or substantial levels in the pancreatic secretions of cats or dogs, respectively. Thus, unlike cats, dogs have adapted to high-starch rations while also consuming animal-sourced foods. At metabolic levels, both dogs and cats synthesize de novo vitamin C and many amino acids (AAs, such as Ala, Asn, Asp, Glu, Gln, Gly, Pro, and Ser) but have a very limited ability to form vitamin D3. Compared with dogs, cats have higher requirements for AAs, some B-complex vitamins, and choline; greater rates of gluconeogenesis; a higher capacity to tolerate AA imbalances and antagonism; a more limited ability to synthesize arginine and taurine from glutamine/proline and cysteine, respectively; and a very limited ability to generate polyunsaturated fatty acids (PUFAs) from respective substrates. Unlike dogs, cats cannot convert either β-carotene into vitamin A or tryptophan into niacin. Dogs can thrive on one large meal daily and select high-fat over low-fat diets, whereas cats eat more frequently during light and dark periods and select high-protein over low-protein diets. There are increasing concerns over the health of skin, hair, bone, and joints (specialized connective tissues containing large amounts of collagen and/or keratin); sarcopenia (age-related losses of skeletal-muscle mass and function); and cognitive function in dogs and cats. Sufficient intakes of proteinogenic AAs and taurine along with vitamins, minerals, and PUFAs are crucial for the normal structures of the skin, hair, bone, and joints, while mitigating sarcopenia and cognitive dysfunction. Although pet owners may have different perceptions about the feeding and management practice of their dogs and cats, the health and well-being of the companion animals critically depend on safe, balanced, and nutritive foods. The new knowledge covered in this volume of Adv Exp Med Biol is essential to guide the formulation of pet foods to improve the growth, development, brain function, reproduction, lactation, and health of the companion animals.

Functions and Metabolism of Amino Acids in the Hair and Skin of Dogs and Cats

The hair and skin of domestic cats or dogs account for 2% and 12-24% of their body weight, respectively, depending on breed and age. These connective tissues contain protein as the major constituent and provide the first line of defense against external pathogens and toxins. Maintenance of the skin and hair in smooth and elastic states requires special nutritional support, particularly an adequate provision of amino acids (AAs). Keratin (rich in cysteine, serine and glycine) is the major protein both in the epidermis of the skin and in the hair. Filaggrin [rich in some AAs (e.g., serine, glutamate, glutamine, glycine, arginine, and histidine)] is another physiologically important protein in the epidermis of the skin. Collagen and elastin (rich in glycine and proline plus 4-hydroxyproline) are the predominant proteins in the dermis and hypodermis of the skin. Taurine and 4-hydroxyproline are abundant free AAs in the skin of dogs and cats, and 4-hydroxyproline is also an abundant free AA in their hair. The epidermis of the skin synthesizes melanin (the pigment in the skin and hair) from tyrosine and produces trans-urocanate from histidine. Qualitative requirements for proteinogenic AAs are similar between cats and dogs but not identical. Both animal species require the same AAs to nourish the hair and skin but the amounts differ. Other factors (e.g., breeds, coat color, and age) may affect the requirements of cats or dogs for nutrients. The development of a healthy coat, especially a black coat, as well as healthy skin critically depends on AAs [particularly arginine, glycine, histidine, proline, 4-hydroxyproline, and serine, sulfur AAs (methionine, cysteine, and taurine), phenylalanine, and tyrosine] and creatine. Although there are a myriad of studies on AA nutrition in cats and dogs, there is still much to learn about how each AA affects the growth, development and maintenance of the hair and skin. Animal-sourced foodstuffs (e.g., feather meal and poultry by-product meal) are excellent sources of the AAs that are crucial to maintain the normal structure and health of the skin and hair in dogs and cats.

Characteristics of Nutrition and Metabolism in Dogs and Cats

Domestic dogs and cats have evolved differentially in some aspects of nutrition, metabolism, chemical sensing, and feeding behavior. The dogs have adapted to omnivorous diets containing taurine-abundant meat and starch-rich plant ingredients. By contrast, domestic cats must consume animal-sourced foods for survival, growth, and development. Both dogs and cats synthesize vitamin C and many amino acids (AAs, such as alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and serine), but have a limited ability to form de novo arginine and vitamin D3. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for AAs (particularly arginine, taurine, and tyrosine), B-complex vitamins (niacin, thiamin, folate, and biotin), and choline; exhibit greater rates of gluconeogenesis; are less sensitive to AA imbalances and antagonism; are more capable of concentrating urine through renal reabsorption of water; and cannot tolerate high levels of dietary starch due to limited pancreatic α-amylase activity. In addition, dogs can form sufficient taurine from cysteine (for most breeds); arachidonic acid from linoleic acid; eicosapentaenoic acid and docosahexaenoic acid from α-linolenic acid; all-trans-retinol from β-carotene; and niacin from tryptophan. These synthetic pathways, however, are either absent or limited in all cats due to (a) no or low activities of key enzymes (including pyrroline-5-carboxylate synthase, cysteine dioxygenase, ∆6-desaturase, β-carotene dioxygenase, and quinolinate phosphoribosyltransferase) and (b) diversion of intermediates to other metabolic pathways. Dogs can thrive on one large meal daily, select high-fat over low-fat diets, and consume sweet substances. By contrast, cats eat more frequently during light and dark periods, select high-protein over low-protein diets, refuse dry food, enjoy a consistent diet, and cannot taste sweetness. This knowledge guides the feeding and care of dogs and cats, as well as the manufacturing of their foods. As abundant sources of essential nutrients, animal-derived foodstuffs play important roles in optimizing the growth, development, and health of the companion animals.

Nutrition and Aging in Dogs and Cats

Aging is often associated with chronic inflammation and declining health. Both veterinarians and owners of aging dogs and cats are interested in nutritional solutions and strategies to prevent signs of age-related disease, increase longevity, and improve quality of life. Physiological decreases in muscle mass, decreased immunity, and a decrease in sense acuity are some of the changes often seen in otherwise healthy senior pets; however, there may also be an increase in risk for pathologies such as renal, cardiovascular, musculoskeletal, and neoplastic diseases. Aging may also lead to cognitive decline and even cognitive dysfunction. Some nutritional strategies that may be helpful with the prevention and treatment of age-related diseases include supplementation with ω3 polyunsaturated fatty acids and antioxidant nutrients that can help modulate inflammation and benefit osteoarthritis, renal disease, cancer, and more. Supplementation with medium-chain triglycerides shows promise in the treatment of canine cognitive dysfunction as these may be metabolized to ketone bodies that are utilized as an alternative energy source for the central nervous system. Additionally, a high intake of dietary phosphorus in soluble and bioavailable forms can lead to renal disease, which is of greater concern in senior pets. There are no published guidelines for nutritional requirements specific to senior pets and as a result, products marketed for senior dogs and cats are highly variable.

Birth weight, puppies' growth and health with limited-ingredient novel protein diet vs standard diet in late pregnancy

Great attention has been given in the last years to the protein source of dog food, and commercial limited-ingredient diets with a single protein are available also for late pregnancy. This work compared the effect of a fish-based limited ingredient diet (LID), and of a standard mixed-protein diet (Mixed), fed to the bitches from the last three weeks of pregnancy and to the puppies at weaning, on birth weight, growth and health of the puppies. From a breeder's records, the weight of 22 Lagotto Romagnolo (LR) and 10 Appenzeller Cattle Dog (ACD) bitches on the day of mating, and of their 199 puppies, were extracted. The effect of diet on puppies' weight on day 0, 6, 30 and 60 was analyzed, considering litter size and sex. The analyses were repeated on puppies' weights normalized on the relative dam's non-pregnant bodyweight. Birth weight was available for 146 puppies, 82 LR and 64 ACD. Median birth weight of LR puppies was 287.5 g (170-400 g); sex ratio was 1.11 (males/females, N = 80). Median birth weight of ACD puppies was 390 g (240-525 g); sex ratio 1.15 (males/females, N = 58). Diet did not significantly affect birth weight in both breeds; however, it showed a significant effect on normalized birth weights (LR, P = 0.016; ACD, P = 0.034), with higher values for LID. At day 30, ACD puppies showed significantly higher weights with the Mixed diet (P = 0.002), and, at day 60, diet significantly affected the normalized weight in both breeds (LR, P = 0.019; ACD, P = 0.001), with higher values for the Mixed type. LID may help the dam to invest the energy in the growth of her litter, however, the same diet resulted in lower puppies' weights around weaning, compared to the Mixed diet. Although our results should be confirmed with larger numbers of animals and more breeds, they set some points worth to be further investigated. The choice of a limited-ingredient single-protein diet can affect litter weight and weight at weaning. Whether, administered to dams and puppies, it can prevent later pathologies, like chronic gastrointestinal diseases or food allergies, is a field of research deserving full attention.

Amino acid nutrition and metabolism in domestic cats and dogs

Domestic cats and dogs are carnivores that have evolved differentially in the nutrition and metabolism of amino acids. This article highlights both proteinogenic and nonproteinogenic amino acids. Dogs inadequately synthesize citrulline (the precursor of arginine) from glutamine, glutamate, and proline in the small intestine. Although most breeds of dogs have potential for adequately converting cysteine into taurine in the liver, a small proportion (1.3%-2.5%) of the Newfoundland dogs fed commercially available balanced diets exhibit a deficiency of taurine possibly due to gene mutations. Certain breeds of dogs (e.g., golden retrievers) are more prone to taurine deficiency possibly due to lower hepatic activities of cysteine dioxygenase and cysteine sulfinate decarboxylase. De novo synthesis of arginine and taurine is very limited in cats. Thus, concentrations of both taurine and arginine in feline milk are the greatest among domestic mammals. Compared with dogs, cats have greater endogenous nitrogen losses and higher dietary requirements for many amino acids (e.g., arginine, taurine, cysteine, and tyrosine), and are less sensitive to amino acid imbalances and antagonisms. Throughout adulthood, cats and dogs may lose 34% and 21% of their lean body mass, respectively. Adequate intakes of high-quality protein (i.e., 32% and 40% animal protein in diets of aging dogs and cats, respectively; dry matter basis) are recommended to alleviate aging-associated reductions in the mass and function of skeletal muscles and bones. Pet-food grade animal-sourced foodstuffs are excellent sources of both proteinogenic amino acids and taurine for cats and dogs, and can help to optimize their growth, development, and health.

Evaluation of high-protein diets differing in protein source in healthy adult dogs

Given the dynamic market for protein-based ingredients in the pet food industry, demand continues to increase for both plant- and animal-based options. Protein sources contain different amino acid (AA) profiles and vary in digestibility, affecting protein quality. The objective of this study was to evaluate the apparent total tract digestibility (ATTD) of canine diets differing in protein source and test their effects on serum metabolites and fecal characteristics, metabolites, and microbiota of healthy adult dogs consuming them. Four extruded diets were formulated to be isonitrogenous and meet the nutrient needs for adult dogs at maintenance, with the primary difference being protein source: 1) fresh deboned, dried, and spray-dried chicken (DC), 2) chicken by-product meal (CBPM), 3) wheat gluten meal (WGM), and 4) corn gluten meal (CGM). Twelve adult spayed female beagles (body weight [BW] = 9.9 ± 1.0 kg; age = 6.3 ± 1.1 yr) were used in a replicated 4 × 4 Latin square design (n = 12/treatment). Each period consisted of a 22-d adaptation phase, 5 d for fecal collection, and 1 d for blood collection. Fecal microbiota data were analyzed using QIIME 2.2020.8. All other data were analyzed using the Mixed Models procedure of SAS version 9.4. Fecal scores were higher (P < 0.05; looser stools) in dogs fed DC or CBPM than those fed WGM or CGM, but all remained within an appropriate range. Dry matter ATTD was lower (P < 0.05) in dogs fed CBPM or CGM than those fed DC or WGM. Crude protein ATTD was lower (P < 0.05) in dogs fed DC or CGM than those fed WGM. Dogs fed CBPM had lower (P < 0.05) organic matter, crude protein, and energy ATTD than those fed the other diets. Fecal indole was higher (P < 0.05) in dogs fed CBPM than those fed WGM. Fecal short-chain fatty acids were higher (P < 0.05) in dogs fed DC than those fed CGM. Fecal branched-chain fatty acids were higher (P < 0.05) in dogs fed DC or CBPM than those fed WGM. Fecal ammonia was higher (P < 0.05) in dogs fed DC or CBPM than those fed WGM or CGM. The relative abundances of three bacterial phyla and nine bacterial genera were shifted among treatment groups (P < 0.05). Considering AA profiles and digestibility data, the DC diet protein sources provided the highest quality protein without additional AA supplementation, but the animal-based protein diets resulted in higher fecal proteolytic metabolites. Further studies evaluating moderate dietary protein concentrations are needed to better compare plant- and animal-based protein sources.

Dietary management of chronic enteropathy in dogs

Chronic idiopathic enteropathy is a clinical condition defined by the exclusion of infectious, metabolic or neoplastic causes of gastrointestinal signs and is categorised by a response to treatment including management with diet change, immunosuppressant medication or interventions that directly target the microbiome (e.g. antibiotics, faecal transplantation or probiotics). Animals that fail these therapies are categorised as non-responsive or refractory chronic idiopathic enteropathy. This specific categorisation implies that nutritional intervention is only needed for a subset of patients with enteropathy. However, often dogs with chronic idiopathic enteropathy are malnourished, have nutrient malabsorption or have gastrointestinal inflammation that occurs as a result of a breakdown in tolerance to luminal antigens including microorganism or dietary components. Thus, all dogs with chronic idiopathic enteropathy benefit from a nutritional assessment and targeted nutritional intervention. Among dogs presenting for chronic idiopathic enteropathy, the response rate to diet alone is roughly 50% in the referral population giving the impression that the overall response could be even higher especially when more than one nutritional intervention is attempted and strict adherence is maintained. The objectives of this review article are to outline the nutritional approach to a dog with chronic idiopathic enteropathy, including the nutritional assessment, and to highlight areas for nutritional intervention.

Nutrition and Metabolism: Foundations for Animal Growth, Development, Reproduction, and Health

Consumption of high-quality animal protein plays an important role in improving human nutrition, growth, development, and health. With an exponential growth of the global population, demands for animal-sourced protein are expected to increase by 60% between 2021 and 2050. In addition to the production of food protein and fiber (wool), animals are useful models for biomedical research to prevent and treat human diseases and serve as bioreactors to produce therapeutic proteins. For a high efficiency to transform low-quality feedstuffs and forages into high-quality protein and highly bioavailable essential minerals in diets of humans, farm animals have dietary requirements for energy, amino acids, lipids, carbohydrates, minerals, vitamins, and water in their life cycles. All nutrients interact with each other to influence the growth, development, and health of mammals, birds, fish, and crustaceans, and adequate nutrition is crucial for preventing and treating their metabolic disorders (including metabolic diseases) and infectious diseases. At the organ level, the small intestine is not only the terminal site for nutrient digestion and absorption, but also intimately interacts with a diverse community of intestinal antigens and bacteria to influence gut and whole-body health. Understanding the species and metabolism of intestinal microbes, as well as their interactions with the intestinal immune systems and the host intestinal epithelium can help to mitigate antimicrobial resistance and develop prebiotic and probiotic alternatives to in-feed antibiotics in animal production. As abundant sources of amino acids, bioactive peptides, energy, and highly bioavailable minerals and vitamins, animal by-product feedstuffs are effective for improving the growth, development, health, feed efficiency, and survival of livestock and poultry, as well as companion and aquatic animals. The new knowledge covered in this and related volumes of Adv Exp Med Biol is essential to ensure sufficient provision of animal protein for humans, while helping reduce greenhouse gas emissions, minimize the urinary and fecal excretion of nitrogenous and other wastes to the environment, and sustain animal agriculture (including aquaculture).

Functional Molecules of Intestinal Mucosal Products and Peptones in Animal Nutrition and Health

There is growing interest in the use of intestinal mucosal products and peptones (partial protein hydrolysates) to enhance the food intake, growth, development, and health of animals. The mucosa of the small intestine consists of the epithelium, the lamina propria, and the muscularis mucosa. The diverse population of cells (epithelial, immune, endocrine, neuronal, vascular, and elastic cells) in the intestinal mucosa contains not only high-quality food protein (e.g., collagen) but also a wide array of low-, medium-, and high-molecular-weight functional molecules with enormous nutritional, physiological, and immunological importance. Available evidence shows that intestinal mucosal products and peptones provide functional substances, including growth factors, enzymes, hormones, large peptides, small peptides, antimicrobials, cytokines, bioamines, regulators of nutrient metabolism, unique amino acids (e.g., taurine and 4-hydroxyproline), and other bioactive substances (e.g., creatine and glutathione). Therefore, dietary supplementation with intestinal mucosal products and peptones can cost-effectively improve feed intake, immunity, health (the intestine and the whole body), well-being, wound healing, growth performance, and feed efficiency in livestock, poultry, fish, and crustaceans. In feeding practices, an inclusion level of an intestinal mucosal product or a mucosal peptone product at up to 5% (as-fed basis) is appropriate in the diets of these animals, as well as companion and zoo animals.

Hepatic Glucose Metabolism and Its Disorders in Fish

Carbohydrate, which is the most abundant nutrient in plant-sourced feedstuffs, is an economically indispensable component in commercial compound feeds for fish. This nutrient can enhance the physical quality of diets and allow for pellet expansion during extrusion. There is compelling evidence that an excess dietary intake of starch causes hepatic disorders, thereby further reducing the overall food consumption and growth performance of fish species. Among the severe metabolic disturbances are glycogenic hepatopathy (hepatomegaly caused by the excessive accumulation of glycogen in hepatocytes) and hepatic steatosis (the accumulation of large vacuoles of triacylglycerols in hepatocytes). The development of those disorders is mainly due to the limited ability of fish to oxidize glucose and control blood glucose concentration. The prolonged elevations of blood glucose increase glucose intake by the liver, and excess glucose is stored either as glycogen through glycogenesis in hepatocytes or as triglycerides via lipogenesis in tissues, depending on the species. In some fish species (e.g., largemouth bass), the liver has a low ability to regulate glycolysis, gluconeogenesis, and glycogen breakdown in response to high starch intake. For most species of fish, the liver size increases with lipid or glycogen accumulation when they have a high starch intake. It is a challenge to develop the same set of diagnostic criteria for all fish species as their physiology or metabolic patterns differ. Although glycogenic hepatopathy appears to be a common disease in carnivorous fish, it has been under-recognized in many studies. As a result, understanding these diseases and their pathogeneses in different fish species is crucial for manufacturing cost-effective pellet diets to promote the health, growth, survival, and feed efficiency of fish in future.

Pigs (Sus Scrofa) in Biomedical Research

Much of biomedical oriented research is conducted with animal models. Over the years, rodents (primarily rats and mice) have emerged as the preferred species for basic biochemistry, cell biology, physiology and nutrition studies. In the past, dogs have been used for the evaluation of dietary protein quality and other aspects of animal nitrogen metabolism and physiology, cardiovascular and endocrine research. At an increasing rate, pigs have also been used as a model species in biomedical research. Pigs are readily available in various mature sizes and genotypic/phenotypic traits, and there are many anatomic, nutritional and physiologic similarities between human beings and pigs. Many notable reviews summarizing the role of pigs in biomedical studies have already been published and these are cited below. The present review focuses on characteristics that make pigs an excellent biomedical animal model in particular in obesity, diabetes and cardiovascular research. To procure an animal model for obesity, irrespective of species used, these animals must be fed a dense caloric diet (high fat) to achieve an experimental working model within a reasonable period. This review also focuses on a putative role of gastrointestinal microbiota in obesity as obese animals exhibit a shift in the distribution of gastrointestinal microbial phyla from lean animals. But to date such results have not pinpointed a treatable cause for obesity. Sometimes, the choice of sampling sites for microbial assessment in many reports can be questioned as the microbial content and phyla distribution in easily collected fecal samples may differ from those obtained directly from the small intestine and upper colon. While pigs are still utilized in many countries for medical surgery practice, this has been discontinued in US medical schools.

Composition of Amino Acids in Foodstuffs for Humans and Animals

Amino acids (AAs) are the building blocks of proteins that have both structural and metabolic functions in humans and other animals. In mammals, birds, fish, and crustaceans, proteinogenic AAs are alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. All animals can synthesize de novo alanine, asparagine, aspartate, glutamate, glutamine, glycine, proline, and serine, whereas most mammals (including humans and pigs) can synthesize de novo arginine. Results of extensive research over the past three decades have shown that humans and other animals have dietary requirements for AAs that are synthesizable de novo in animal cells. Recent advances in analytical methods have allowed us to determine all proteinogenic AAs in foods consumed by humans, livestock, poultry, fish, and crustaceans. Both plant- and animal-sourced foods contain high amounts of glutamate, glutamine, aspartate, asparagine, and branched-chain AAs. Cysteine, glycine, lysine, methionine, proline, threonine, and tryptophan generally occur in low amounts in plant products but are enriched in animal products. In addition, taurine and creatine (essential for the integrity and function of tissues) are absent from plants but are abundant in meat and present in all animal-sourced foods. A combination of plant- and animal products is desirable for the healthy diets of humans and omnivorous animals. Furthermore, animal-sourced feedstuffs can be included in the diets of farm and companion animals to cost-effectively improve their growth performance, feed efficiency, and productivity, while helping to sustain the global animal agriculture (including aquaculture).