veterinary sciences Review Cats and Carbohydrates: The Carnivore Fantasy? Adronie Verbrugghe 1,* and Myriam Hesta 2 1 Department of Clinical Studies, Ontario Veterinary College, University of Guelph, 50 Stone Road E, Guelph N1G 2W1 ON, Canada 2 Laboratory of Animal Nutrition, Faculty of Veterinary Medicine, Ghent University, Heidestraat 19, Merelbeke B-9820, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +1-519-824-4120 Academic Editor: Jacquie Rand Received: 28 June 2017; Accepted: 8 November 2017; Published: 15 November 2017 Abstract: The domestic cat’s wild ancestors are obligate carnivores that consume prey containing only minimal amounts of carbohydrates. Evolutionary events adapted the cat’s metabolism and physiology to this diet strictly composed of animal tissues and led to unique digestive and metabolic peculiarities of carbohydrate metabolism. The domestic cat still closely resembles its wild ancestor. Although the carnivore connection of domestic cats is well recognised, little is known about the precise nutrient profile to which the digestive physiology and metabolism of the cat have adapted throughout evolution. Moreover, studies show that domestic cats balance macronutrient intake by selecting low-carbohydrate foods. The fact that cats evolved consuming low-carbohydrate prey has led to speculations that high-carbohydrate diets could be detrimental for a cat’s health. More specifically, it has been suggested that excess carbohydrates could lead to feline obesity and diabetes mellitus. Additionally, the chances for remission of diabetes mellitus are higher in cats that consume a low-carbohydrate diet. This literature review will summarise current carbohydrate knowledge pertaining to digestion, absorption and metabolism of carbohydrates, food selection and macronutrient balancing in healthy, obese and diabetic cats, as well as the role of carbohydrates in prevention and treatment of obesity and diabetes mellitus. Keywords: carbohydrates; carnivore; diabetes mellitus; feline; glucose metabolism; hyperglycaemia; macronutrient selection; obesity 1. Introduction The domestic cat’s wild ancestors (Felis silvestris) are known to be obligate carnivores. From a nutritional perspective, this means that in their natural habitat cats consume small prey, including rodents and birds, which are high in protein, moderate in fat and include only minimal carbohydrates. Data from dietary habits of feral cats combined with compositional data of the consumed prey species revealed a typical diet containing a crude protein, crude fat and nitrogen-free extract (NFE) content of 52%, 46% and 2% of metabolisable energy (%ME), respectively [1]. These dietary habits have led to specific and unique nutritional requirements. In cats, the dietary requirements for protein, arginine, taurine, methionine and cystine, arachidonic acid, niacin, pyridoxine, vitamin A and vitamin D are greater than for omnivores due to metabolic differences [2–6]. This evolutionary background has served as a basis for several myths about cat nutrition. For example, sometimes the fact that cats are strict carnivores is interpreted as meaning that cats can only obtain their nutritional requirements through consuming animal tissue. This is incorrect from a nutritional perspective as animals, including cats, need nutrients and not specific ingredients [7]. The fact that cats evolved consuming low-carbohydrate prey and the increased understanding of the unique feline carbohydrate metabolism has led to speculations that high-carbohydrate diets could be detrimental to feline health. The most commonly Vet. Sci. 2017, 4, 55; doi:10.3390/vetsci4040055 www.mdpi.com/journal/vetsci Vet. Sci. 2017, 4, 55 2 of 22 expressed concern is that cats poorly digest and metabolise carbohydrates, while others warn that excess carbohydrates could lead to feline obesity and/or diabetes mellitus [4,5,8–10]. Therefore, researchers evaluated high-carbohydrate diets as a risk factor for the development of feline obesity and diabetes mellitus; others examined the use of low-carbohydrate diets for the prevention and treatment of these metabolic diseases. Vet. Sci. 2017, 4, 55 2 of 22 2. Definition, Classification and Function of Dietary Carbohydrates metabolise carbohydrates, while others warn that excess carbohydrates could lead to feline obesity Whenand/or discussing diabetes mellitus the benefits [4,5,8–10]. and Therefore, adverse researchers effects of evaluated dietary carbohydrates,high-carbohydrate onediets hasas a to keep in mindrisk the factor definition, for the development classification of andfeline function obesity an ofd diabetes carbohydrates. mellitus; others Carbohydrates examined the or use saccharides of low-carbohydrate diets for the prevention and treatment of these metabolic diseases. (CH2On) are biological molecules that are composed of carbon, hydrogen and oxygen [11]. 2. Definition, Classification and Function of Dietary Carbohydrates 2.1. Classification of Carbohydrates When discussing the benefits and adverse effects of dietary carbohydrates, one has to keep in Classificationmind the definition, is based classification on chemical and function structure of carbohydrates. and degree ofCarboh polymerisationydrates or saccharides and includes: monosaccharides,(CH2On) are disaccharides, biological molecules oligosaccharides that are compos anded polysaccharides of carbon, hydrogen [11 ,and12] (Figureoxygen 1[11].). Monosaccharides (glucose, fructose and galactose) are also called absorbable carbohydrates as these compounds do not 2.1. Classification of Carbohydrates require enzymatic digestion and can be absorbed directly. Disaccharides (lactose, sucrose, maltose and trehalose) areClassification readily digested is based by on intestinal chemical enzymesstructure and in most degree mammals of polymerisation [11–13]. Theand oligosaccharideincludes: monosaccharides, disaccharides, oligosaccharides and polysaccharides [11,12] (Figure 1). group with three to nine sugar molecules is resistant to enzymatic digestion and is fermented by Monosaccharides (glucose, fructose and galactose) are also called absorbable carbohydrates as these microbialcompounds enzymes do innot the require large enzymatic intestine digestion [12,13 and]. The can be polysaccharides absorbed directly. orDisaccharides complex carbohydrates,(lactose, encompassingsucrose, maltose 10 or moreand trehalose) sugar units,are readily can digested be further by intestinal defined enzymes based in on most the mammals digestibility [11–13]. and the nature ofThe the oligosaccharide glycosidic bonds group betweenwith three theto nine sugar sugar units molecules [12]. Starchesis resistant are to enzymatic complex digestion carbohydrates and that have α-glycosidicis fermented bondsby microbial and are enzymes therefore in the enzymatically large intestine [12,13]. digested The by polysaccharidesα-amylase in or the complex mammalian small intestinecarbohydrates, [11– 13encompassing]. Starch is 10 the or mostmore abundantsugar units, digestible can be further carbohydrate defined based produced on the during digestibility and the nature of the glycosidic bonds between the sugar units [12]. Starches are photosynthesis and serves as energy storage in plants [11,12]. Similarly, glycogen, a glucose polymer complex carbohydrates that have α-glycosidic bonds and are therefore enzymatically digested by linked withα-amylaseα-glycosidic in the mammalian bonds, functionssmall intestine as long-term [11–13]. Starch energy is the storage most inabundant animals. digestible Glycogen is presentcarbohydrate in most animal produced tissues, during with ph theotosynthesis highest contentand serves in as liver energy and storage skeletal in muscleplants [11,12]. [12]. Dietary fibre, anotherSimilarly, class glycogen, of complex a glucose carbohydrates, polymer linked is with composed α-glycosidic of cell bonds, wall functions polysaccharides as long-term (cellulose, hemicellulosesenergy storage and some in animals. pectins), Glycogen non-cellulose is present in polysaccharides most animal tissues, (pectins, with the gums highest and content mucilage) in and liver and skeletal muscle [12]. Dietary fibre, another class of complex carbohydrates, is composed of structural non-polysaccharides (lignin). The sugar units of dietary fibre are linked by β-glycosidic cell wall polysaccharides (cellulose, hemicelluloses and some pectins), non-cellulose polysaccharides bonds,(pectins, so dietary gums fibre and ismucilage) not digestible and structural in the non-polysaccharides small intestine. 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