Introduction
It has been suggested that cats be fed foods similar to their natural diet (ie, high protein and fat with relatively little carbohydrate)1 to provide optimum nutrition and help prevent and manage feline obesity and diabetes mellitus.2,3 The natural prey of feral cats is reported to contain 2% to 12% of calories from carbohydrate and 52% to 63% of calories from protein.1,4 In contrast, commercial dry cat foods, and many wet foods, contain greater amounts of carbohydrates.5,6 In 1 study, commercially available dry cat foods commonly purchased in the US contained 34% to 48% of calories as carbohydrates (determined as nitrogen free extract [NFE], mean = 41%) and 29% to 36% calories as protein (mean = 33%).5 Commercial dry and wet cat foods purchased in the UK averaged 34.6% and 2.29% NFE, respectively, on a dry matter basis.6
The divergence of nutritional profiles of commercial cat foods from the natural diet of feral cats contributed to debate regarding the impact of feeding increased amounts of carbohydrates on the health and well-being of domestic cats, which are obligate carnivores.3,7 The purpose of this article is to review the role of dietary carbohydrates within the context of feline nutrition.
Dietary Carbohydrates in Commercial Cat Foods
Dietary carbohydrates include simple sugars (mono- and disaccharides), complex carbohydrates (polysaccharides and starches), and indigestible carbohydrates (dietary fibers).7 As used in this article, carbohydrates refer to digestible carbohydrates that can be broken down by mammalian digestive enzymes and provide a dietary source of the monosaccharide, glucose. Although simple sugars are a large component of carbohydrates in “Western” human diets, their use in pet foods is very limited.8,9 Starches and complex carbohydrates are the focus of this review because they are the most common carbohydrates found in commercial dry and wet cat foods, which are consumed by well over 90% of cats in the US, Australia, and Europe.10–12
Carbohydrates are an important component of foods and are found predominantly in plant materials including grains, beans and legumes, vegetables, potatoes and fruits.7 In extruded dry commercial cat foods, carbohydrates provide an important structural component in addition to their role as an energy source. Although provision of all essential nutrients is important, the most important role of a diet is to provide metabolic energy, which can be derived from dietary protein, fat, or carbohydrate. Compared to the production of animal sources for dietary protein and fat, production of carbohydrate-rich ingredients has less adverse impacts on the environment.13 Also, when completely metabolized, digestible carbohydrates yield carbon dioxide and water, whereas nitrogenous waste and metabolic acid equivalents are produced when dietary protein is metabolized for energy.
Raw starches are not digestible and may decrease apparent digestibility of other nutrients (eg, dietary protein); however, they are not present in commercial cat foods.14,15 As noted, simple sugars are used rarely in commercial dry or canned cat foods and are metabolized differently from cooked starches.16,17 Studies evaluating effects of feeding raw starch or large quantities of simple sugars in experimental diets are therefore not relevant to the effects of commercial wet or dry cat foods.18 Semimoist foods may contain significant amounts of simple sugars; however, they comprise a very small percentage of total pet foods sold. All further reference to commercial cat foods in this article refers to cooked wet (canned or tinned or in pouches) or dry foods.
Diets used in studies evaluating the effects of dietary carbohydrates necessarily require changes in other macronutrients (ie, fat and protein) because a diet is the sum of its parts and it is not possible to change only a single macronutrient. Therefore, diets that are low in carbohydrates will, by default, contain more protein, fat or both. Likewise, high carbohydrate diets will have less protein, fat, or both. Thus, any effects associated with increasing or decreasing dietary carbohydrates in these studies cannot be clearly distinguished from corresponding changes in fat or protein. Diets may also differ in other dietary factors, such as micronutrients, dietary fiber or ingredients, which can limit the ability to definitively conclude an effect from dietary carbohydrate.
Do cats require dietary carbohydrates?
Essential nutrients are those needed by the body which either cannot be synthesized de novo or synthesized in sufficient amounts and, therefore, must be included in the diet. Like other mammals, cats have a metabolic need for glucose (a carbohydrate), which is required by the brain and nervous tissues, red blood cells, renal medulla and other tissues.19,20 The sugars provided by dietary carbohydrates also provide building blocks for other necessary structures such as lactose produced by the mammary gland, ribose for nucleic acid synthesis, carbon skeletons for amino acid synthesis, vitamin C synthesis, and the sugar residues found in glycoproteins, glycolipids and glycosaminoglycans in the body.7 However, because the physiological requirement for glucose can be met either by dietary carbohydrates or via gluconeogenesis from dietary protein, a dietary source of carbohydrates is not required for adult cats so long as protein intake is adequate to support gluconeogenesis. This is similar to the situation in most adult, nonlactating mammals.7
Although adult cats do not have a dietary requirement for carbohydrates, they do have a requirement for energy, which can be met from dietary proteins, fats, or carbohydrates.21 Cats have a requirement for essential fatty acids, which can be met with relatively low levels of total dietary fat.22 Cats also require a certain minimum intake of protein and essential amino acids that must be provided by the diet.23,24 Additional fat or protein in excess of requirements, as well as dietary carbohydrates, are used as energy sources. Therefore, there is nutritional flexibility regarding the utilization of energy sources and, once the protein and fatty acid requirements are met with a diet formulation, the balance of energy needs may be provided with a variable combination of carbohydrates, fat, and more protein.24,25 This allows cats to thrive on foods that range widely in macronutrient content.
There is some evidence that cats may voluntarily limit their carbohydrate intake when dietary amounts exceed 40% (dry matter basis). Researchers using geometric analytical techniques or simple choice methods to evaluate intakes of cats given options among various diets identified that cats appeared to limit their maximum carbohydrate intake.26 The authors concluded that cats would limit their intake on high carbohydrate foods, especially those with low protein. The greater intake of higher protein diets is consistent with other studies.27,28 However, the choices cats made were somewhat reliant on the diets available to them, and the results of the studies suggesting a “carbohydrate intake ceiling” were refuted in a subsequent study using different diets designed to have similar palatability.29 In this latter study, the diets contained more carbohydrates (33% to 52% ME) than the options from the prior study, and varied in protein (23% to 42% ME) and fat (23% to 44% ME), but were balanced to be similar in palatability. Cats in that study chose to consume 43%, 30% and 27% of calories from carbohydrate, protein and fat, respectively.29
Are cats unique relative to carbohydrate utilization?
To obtain energy from dietary carbohydrates, cats must be able to digest them. Amylase and intestinal disaccharidases are necessary for the digestion of starch and complex carbohydrates. While both cats and dogs lack salivary amylase, amylase is found in the pancreas and chyme, although at a lower concentration in cats compared with other species.30 Data from ex vivo studies are mixed regarding intestinal disaccharidase activity in cats versus dogs; 2 studies identified lower amounts in cats and 1 indicated greater disaccharidase activity in cats.31–33 In addition to differences in digestive enzymes, cats lack hepatic glucokinase, and sweet taste receptors.30,32,33 These differences are cited as evidence that cats lack the metabolic flexibility to adapt to higher carbohydrate diets.2 However, multiple studies14,15,34,35 have documented that cats are capable of digesting properly cooked and processed carbohydrates, often with apparent digestion in excess of 90%.This is comparable to findings in dogs fed very similar diets.8,34
Postabsorption, cats demonstrate considerable ability to adapt and to metabolize different macronutrients as energy sources, whether from carbohydrates, fat or protein.24,25,36–38 When dietary carbohydrates were increased from 6.1 to 9.5 g/100 Kcal, cats increased their oxidation of carbohydrates.37 Similarly, when fed diets with decreasing amounts of dietary protein (12.2 to 2.1 g/100 Kcal) in exchange for increasing amounts of dietary carbohydrates (3.2 to 15.3 g of carbohydrate/100 Kcal), cats metabolized (oxidized) less protein and more carbohydrates in the higher carbohydrate diets, sparing dietary protein.24 In the higher protein, lower carbohydrate diets, the opposite was true, and cats oxidized more protein. Cats were able to adapt their metabolism based on the sources of energy available so long as the diet provided at least 4 g protein/100 Kcal.24 In another study38 wherein dietary fat and carbohydrates were exchanged, cats switched their metabolism to increase oxidation of the increased nutrient and decrease oxidation of the restricted nutrient. This study showed that cats, like other mammalian species, adapt whole-body nutrient metabolism in response to changes in dietary macronutrient content, but may require more than 14 days to fully adapt to altered macronutrient concentrations.38
Are cats unique relative to glucose metabolism?
Blood glucose concentrations are determined by a balance between glucose entry into the bloodstream from both dietary and endogenous sources (eg, hepatic glucose production [HGP]), and glucose uptake (both insulin-dependent and insulin-independent) by cells. Factors other than diet can affect blood glucose concentrations including stress, gastric emptying time, meal size, and physical activity or muscular exertion.
It has been suggested that, in cats, gluconeogenesis is “more or less permanently switched on”19 and that the condition is associated with an insulin resistance that may lead to obesity, excessive insulin secretion, pancreatic amyloid deposition, and diabetes mellitus when “high” carbohydrate diets are fed.39,40 This perception was in part supported by 1 study41 showing no decrease in activities of hepatic gluconeogenic enzymes when cats were fed a high carbohydrate diet containing sugar and raw starch versus a high protein, low carbohydrate diet. However, newer studies42,43 that have quantified glucogenic metabolite fluxes show that while cats do have abundant capacity for gluconeogenesis, they also have the ability to appropriately moderate HGP. Gluconeogenesis is the main contributor to HGP, but glycogenolysis accounts for approximately 35% to 40% of HGP in fasted and fed cats, respectively.43 Even insulin-resistant cats compensate for peripheral insulin resistance by downregulating HGP to maintain normal blood glucose concentrations in both the fasted and fed state.43,44 Based on these studies, cats do regulate gluconeogenesis, and this regulation of glucose homeostasis appears to be very similar in cats and humans.45
What is the impact of dietary carbohydrates on blood glucose in cats?
It is well documented that altering dietary carbohydrates can influence fasting or postprandial blood glucose concentrations in cats and other species.42,46–55 Diets in feline studies vary widely in carbohydrate content (carbohydrates: 0.8 to 16.3 g/100 Kcal), but also in other macronutrients (protein: 6.1 to 13.5 g protein/100 Kcal; fat: 1.1 to 7.5 g fat/100 Kcal).46–55 Therefore, any effects of diets on postprandial blood glucose are at least partly confounded by changes in dietary protein, fat, or both, and it cannot be concluded that differences in postprandial blood glucose concentrations are due solely to dietary carbohydrate. For example, 1 study of 3 low carbohydrate diets, wherein protein calories were exchanged for fat calories, showed that increasing dietary protein resulted in increased glucose production.56 A complexity in interpreting the meaning of postprandial glucose concentrations is that potency of amino acids for stimulating insulin release in cats appears to be high.57 Also, the contribution of ingested glucose to circulating glucose is likely varied substantially by dietary amino acid and fat effects on gastric emptying though secreted cholecystokinin.58,59 A further challenge is that there are no published data establishing an upper reference limit for normal postprandial glycemia in cats, so it is not possible to designate a specific value that differentiates physiological (healthy) from pathological postprandial glycemia in cats. In addition, studies have varied in experimental methods including study duration, testing protocols, duration of fasting before sampling, and duration of postprandial period, which can impact the results. For example, 1 study showed that the mean peak postprandial glucose concentration was greater following a single, extraordinarily high carbohydrate meal in cats fasted approximately 24 hours (151 mg/dL or 8.4 mmol/L) compared with when the same diet was fed ad libitum (128 mg/dL or 7.1 mmol/L).53 Studies based on fasting cats 18 to 24 hours prior to assessing postprandial blood glucose values may be expected to represent a “worst case scenario” rather than “typical,” since cats naturally eat many meals per day and the majority of pet owners typically feed either ad libitum or several meals daily.10 Cats meal fed once or twice daily or fed ad libitum, without previous fasting, showed relatively little intra-day variability in blood glucose concentrations.54,55,60
Based on the published studies, postprandial blood glucose concentrations (peak and mean) are significantly greater in most cats fed diets containing 12.4 to 14.6 g carbohydrate/ 100 Kcal compared with cats fed diets containing 3.3 to 7.3 g carbohydrate/100 Kcal. The highest blood glucose was reported in overweight cats fed the highest (14.6 g/100 Kcal) carbohydrate and lowest protien diet following an 18 to 24 hour fast, with a mean peak glucose of 166 mg/dL (9.2 mmol/L), and individual cats reaching peak postprandial glucose concentrations of 241 mg/dL (13.4 mmol/L).50 As comparing postprandial blood glucose concentrations to reference ranges for fasted cats does not allow for meaningful conclusions, one might instead compare these values to those associated with glucose toxicity. Feline and rodent studies,61–65 wherein blood glucose was artificially increased for days to weeks, confirmed that prolonged hyperglycemia can severely compromise β cell function. The effects of glucose appear to depend on the severity and duration of hyperglycemia, and may also depend on the presence or absence of hyperlipidemia.64,66 In healthy cats made hyperglycemic (blood glucose of 450 to 540 mg/dL or 25 to 30 mmol/L), insulin concentrations fell to basal levels within 3 days despite continued high glucose concentrations, and β cell function was grossly abnormal after 10 days of hyperglycemia.63 Similar adverse results were observed in a different study67 when glucose was infused into healthy cats to achieve glucose concentrations of 522 mg/dL (29 mmol/L) over 6 weeks. However, when glucose concentrations were maintained at 310 mg/dLL(17 mmol/L) over the same period, the effects were much less pronounced and no morphologic changes in β-cells were identified, suggesting a dose- response effect.67 To the authors’ knowledge, there are no published studies describing effects of mild to moderate hyperglycemia (blood glucose < 250 mg/dL or 14 mmol/L) in cats.
Based on the sum of the evidence and the lack of published data to document adverse effects from postprandial blood glucose concentrations in the ranges reported in the dietary studies, it must be concluded that pathological hyperglycemia was not induced even in cats fed very high carbohydrate diets. However, glucose intolerant cats may benefit from lower dietary carbohydrates and feeding management that avoids single large meals daily.
What are the advantages and disadvantages of low carbohydrate diets for cats?
To address the advantages of low carbohydrate diets, one must define low, normal, and high carbohydrates. Since any effects of dietary carbohydrate will depend, in part, on the type and source of carbohydrate as well as the other constituents of the diet, it is difficult to quantify a specific amount of dietary carbohydrate that is “excessive.” When defined as an amount that induces adverse effects in healthy cats, the challenge is even greater due to limited data. Another way to quantify excessive dietary carbohydrates is an amount that would compromise the ability of a diet to provide all other nutrient requirements, especially protein. The Association of American Feed Control Officials cite the minimum protein requirement for nutritionally balanced diets for adult cats as 6.5 g/100 Kcal or 26% of dry matter.68 While this is more than adequate to maintain nitrogen balance, there is some evidence to suggest that cats may need more protein than this to support lean body mass.24,67,69,70 When dietary carbohydrate exceeds about 50% of the calories, it is difficult to assure that all other nutrient needs are met. Therefore, dietary carbohydrates in excess of approximately 50% of metabolizable energy (ME) may be defined as apparently “excessive” for healthy cats. This amount of carbohydrate is consistent with the highest levels tested that showed significantly increased blood glucose concentrations53 and, in some cats, loose stools.50
In diabetic cats, “low” carbohydrate diets providing 5% to 26% ME as carbohydrate (1.4 to 7.6 g/100 Kcal) have been associated with improved glycemic control and diabetic remission and are recommended for cats with diabetes mellitus.71–76 However, comparative data defining a specific amount that is advantageous in this regard are minimal. One study73 identified that a diet with carbohydrates at 3.5 g/100 Kcal provided greater benefits for diabetic cats than one containing 7.6 g/100 Kcal. In that study, 68% of cats fed the lower carbohydrate diet reverted to a non–insulin dependent state, compared with 41% of cats fed the moderate carbohydrate diet. Limitations to interpretation of that study were that the diets also differed in other components and remission was not clearly defined nor based on glycemic assessment other than fructosamine. In healthy cats, postprandial mean and peak glucose did not differ when cats were fed diets containing 4.4 and 6.7 g of starch/100 Kcal, but these values increased significantly when the cats were fed diets containing 9.4 g of starch/100 Kcal.55 Based on these data, we propose that “low carbohydrate” diets for cats with type-2 diabetes mellitus should contain no more than approximately 26% of calories (about 7.4 g/100 Kcal) as carbohydrate in a diet with at least 40% of calories (about 11.5 g/100 Kcal) from protein.
While low carbohydrate diets appear to provide a benefit for cats with diabetes mellitus, they tend to be high in calories. As noted earlier in this article, a diet is the sum of its parts, so when carbohydrate is decreased, the balance must be made up from protein and fats. While most low carbohydrate diets also are high in protein, they typically contain more fat. Fat contains more than twice the calories of carbohydrates and excess calories from fat are more easily stored in adipose tissue,75 so high fat diets increase the risk for weight gain and obesity. Multiple studies47,53,77,78 have confirmed that increasing dietary fat results in greater occurrence of undesired weight gain in cats.
Based on the available evidence, it appears that low-carbohydrate, high-fat diets increase the risk for obesity in cats, while higher-carbohydrate, lower calorie diets might reduce that risk. Epidemiological studies indicated that high carbohydrate dry diets were associated with lower risk of obesity in pet cats, compared with high fat dry diets.79,80 Likewise, other epidemiological surveys did not confirm high carbohydrate foods as a risk factor for obesity in cats.80–86 Research in colony cats also refutes the theory that increased dietary carbohydrate favors development of obesity.47,78 Even when protein and calories were controlled, the lower carbohydrate, higher fat diet resulted in increased body fat compared with higher carbohydrate diets.47 In another study, cats were fed diets high in either protein (46% ME with 7.8 g carbohydrate/100 Kcal), fat (47% ME with 7.7 g carbohydrate/100 Kcal), or carbohydrates (47% ME, 13.6 g carbohydrate/100 kcal).53 Energy intake during ad libitum-feeding was reduced with the high-carbohydrate diet compared with the high-protein and the high-fat diets. Finally, a study evaluating 2 commercial dry foods of very similar energy densities and fat concentrations, but differing greatly in protein and carbohydrate content, were fed in excess of energy needs while weight gain was evaluated in cats.50 Substantially greater weight gain occurred with the lower carbohydrate diet (6.7 g/100 Kcal) compared with the higher carbohydrate diet (14.5 g/100 Kcal).50
Some studies12,87 have documented a mild increase in risk for obesity in cats fed dry food. Per the information provided above, this appears to be independent of carbohydrate content. Rather, obesity risks associated with dry food may be due to the greater calorie density of dry versus wet food on a volume basis or may be related to feeding management. Dry foods are more likely to be fed free choice or ad libitum, which may contribute to excessive consumption and weight gain in some cats.12,83
Do high carbohydrate dry foods cause diabetes mellitus in cats?
Despite the lack of evidence suggesting that dietary carbohydrate increases the risk for diabetes, there remains some controversy about the role of commercial dry foods in these conditions. Dry foods have been implicated as they tend to be high in carbohydrates.3,88
Three retrospective case-controlled epidemiological studies evaluated risk factors for development of feline diabetes, but only 2 specifically solicited input on the diets being fed at the time diabetes was diagnosed.11,89,90 One of the studies suggested that feeding either dry or canned food alone was associated with greater risk for diabetes mellitus (OR, 2.2 or 3.0, respectively) compared with feeding a mix of dry and canned foods; however, this study did not document that the foods were those fed before the diagnosis was made and may, therefore, reflect changes made following diagnosis of diabetes.89 The other 2 studies focused on the diet prior to diagnosis and did not find an increased risk with dry foods.11,90 On the contrary, one of these studies documented that although dry food was consumed by 85% of both diabetic and control cats, dry foods constituted an estimated 79% of the dry matter intake in control cats compared with only 44% in diabetic cats at the time of diagnosis (P < 0.05), suggesting a potential protective effect from dry foods.11
Clinical Summary and Conclusions
Although adult cats, as with most other mammals, do not require a dietary source of carbohydrates, they do have physiological requirements for the carbohydrate, glucose. Cats have considerable metabolic flexibility and carbohydrates can provide a protein-sparing effect, meaning that less protein is required for gluconeogenesis when dietary carbohydrates are provided. So long as dietary protein needs are met, cats are able to adapt well to either high-fat or high-carbohydrate diets.
Dietary carbohydrates, within nutritionally balanced diets, do not appear to have adverse effects in healthy cats. However, low-carbohydrate, high-fat diets have been identified as increasing the risk for development of obesity in cats. Depending on the specific formulation, we suggest that diets for healthy cats should not exceed 40% to 50% of calories from carbohydrates to assure all other nutrient needs are met. While higher compared to lower carbohydrate diets may lead to greater postprandial blood glucose concentrations, there is no evidence that the concentrations reported are detrimental rather than physiological. There is no evidence to conclude that high carbohydrate diets lead to diabetes mellitus in cats. However, although the evidence is very limited, it appears that low-carbohydrate diets (≤ 26% of ME) may help diabetic cats improve glucose control and achieve remission.
Acknowledgments
At the time of writing, Dr. Laflamme was employed by Nestlé Purina Petfood and continues to consult for the pet food industry; Dr. Forrester was employed by Hill’s Pet Nutrition, Inc; and Drs. Backus and Hoenig have received research funding from various pet food companies. The authors declare that employers and sponsors had no input into this manuscript.
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