Cats are obligate carnivores. From a nutritional perspective, this means they evolved consuming prey and require certain nutrients found naturally or in adequate amounts only in animal tissues. In addition, cats require more protein and have uniquely higher requirements for arginine and sulfur-containing amino acids, and some B-vitamins, compared with dogs. Wild and feral cats rely on prey as the primary basis of their diets, as this is what they are able to obtain. The prey provides energy and nutrients, just as ingredients in prepared foods provide energy and nutrients. If the available prey provides all the nutrients and energy the animal needs, it will survive. If not, the individual, or the species, would fail to thrive. The opportunistic consumption of prey-based diets does not mean cats are physiologically limited to such a diet. The same nutrients can be provided from other ingredients, such as those used in either commercial or home-prepared diets. “Essential” nutrients are those the animal needs, which cannot be synthesized in adequate amounts endogenously, so must be provided by the diet. “Nonessential” nutrients are also needed but are produced within the body as long as appropriate precursors are available. Common examples include the many nonessential amino acids (e.g. alanine, glutamine, tyrosine, etc.) and glucose. Glucose is required and used as an energy source for most cells in the body. Because glucose can be produced endogenously, predominantly from amino acids, it is not considered an essential dietary nutrient, yet it is physiologically essential. Dietary carbohydrates Nutritional differences between prey-based diets and commercial pet foods have contributed to perceptions commercial dry foods may not be appropriate for cats, predominantly due to dietary carbohydrates (CHO). Some authors have suggested CHO contribute to developing diabetes mellitus and/or obesity in cats and should not be part of feline diets.1,2 It is true most cats do not require a dietary source of CHO, as noted above. This is also true for dogs, humans, and most other mammalian species. However, glucose is so critical to survival, numerous systems are in place to ensure a consistent supply on a cellular level. Dietary CHO can be used as a ready source of glucose, otherwise, protein is used as the primary source of glucose via gluconeogenesis. Cats, like humans, adjust their hepatic gluconeogenesis in response to physiological need,3 and also adjust protein oxidation in response to alterations in protein and CHO intake so long as the minimum requirement for protein is met.4 Thus, when protein is relatively low, dietary CHO provide a protein-sparing effect and protein oxidation is reduced. Cats do have unique features regarding CHO metabolism, including a lack of sweet taste receptors, lack of glucokinase, and relatively lower levels of digestive enzymes. Despite these features, cats are very capable of digesting and metabolizing CHO. Multiple studies have shown the apparent digestibility of CHO in commercial and experimental cat foods exceeds 80-90 percent, very similar to that shown in dogs.5 While cats lack glucokinase, they have abundant other hexokinases, so normal cats are not limited in their total ability to phosphorylate and metabolize CHO. Still, they may do so more slowly compared to dogs. The kinetics of the hexokinases in cats are consistent with their normal eating habits of many small meals per day, regardless of the CHO content. It has been suggested high-CHO diets will produce prolonged hyperglycemia, ultimately leading to “beta cell exhaustion” and diabetes mellitus. Although post-prandial blood glucose may be greater following high carbohydrate diets, research in healthy cats showed that only when diets contained more than 50 percent CHO and very low protein (~22 percent) did hyperglycemia occur.6 However, even with this extreme diet, the degree of hyperglycemia was well below that shown to cause “glucose toxicity.”6 Epidemiological data does not support a role for high CHO diets (i.e. dry cat food) as a risk factor for developing diabetes.6 Rather, obesity and inactivity appear to be major risk factors for feline diabetes. Therefore, while low-CHO diets can be beneficial for the management of feline diabetes, it does not appear dietary CHOs are a causative factor in diabetes. Dietary protein The protein requirement of cats is greater than for dogs and most other species. There is some controversy regarding the optimum amount of protein cats should receive and the extent of their abilities to adapt to lower protein intake. Early research showed cats appeared to lack the ability to down-regulate protein catabolism and gluconeogenesis wherein proteins are catabolized and used to produce glucose.7 This was subsequently disproven, as cats do increase or decrease gluconeogenesis and protein oxidation depending upon the intake of protein and available CHO,4 as well as based on blood glucose concentrations.3,8 As long as protein intake is sufficient, cats are well able to adapt to various intakes of protein and CHO. Cats can adapt to and maintain nitrogen balance on as little as 2.5 g protein/kg body weight daily. Yet, multiple studies challenge whether maintaining nitrogen balance is sufficient for optimum health, regardless of species.5,9 Animals fed low protein diets (but sufficient to maintain nitrogen balance) appear to be at greater risk from infectious diseases, environmental stresses and intestinal parasites due to impaired immune function. In both dogs and cats, it takes two to three times more protein to maintain protein turnover or lean body mass (LBM) than is needed to maintain nitrogen balance: adult cats need about 34 percent of calories (about 5 g/kg body weight) as protein to maintain LBM.5,9 For weight management, higher protein diets help cats lose fat and retain LBM and may help reduce the risk for weight regain.10-12 Cats do not actually require meat to provide their protein: they require essential and nonessential amino acids, which can come from a variety of sources. The perception cats cannot digest or utilize vegetable source proteins is false.13 Regardless of source, proteins are metabolized as amino acids. The body cannot differentiate whether a specific amino acid came from animal or plant protein. It is true vegetable proteins, such as soy and corn-, wheat- or rice-glutens, would not provide all of the essential amino acids cats require, so should not be used as a sole source of protein in a diet.13 However, vegetable proteins can be both highly digestible and a good source of many amino acids. Properly cooked and used with complementary amino acid sources, they can contribute available nutrients to a complete, balanced diet. Cats, as obligate carnivores, require nutrients mainly from animal tissues, with higher needs for protein, arginine, sulfur amino acids, and certain B-vitamins than dogs. Water: Do cats require wet foods? It has been suggested cats fed dry food diets may be subclinically dehydrated, and that all cats should be fed wet diets to increase water intake.14 In fact, healthy cats are well able to regulate and balance total body water via adjustments in drinking and urinary concentration and volume.5 Cats fed dry foods will voluntarily drink more water, while those fed wet foods will drink less. Unless wet foods, which contain about 75-82 percent water, constitute about 75 percent of cats’ intake or more, total water intake will remain about the same as those fed dry food. Cats fed a diet of 100 percent wet food will be forced to consume more water and will urinate more to balance this. This approach has been used therapeutically in conditions such as urolithiasis and constipation. Dry foods are more economical and convenient to use, provide improved oral health, and are preferred by some cats and some owners. Wet foods are generally more palatable, have the potential to increase water intake, and may aid in portion control to help manage body weight. Despite the general palatability of wet foods, some cats and some owners may dislike them. In cats that need encouragement to increase water intake, some methods include multiple water bowls placed in “safe” locations, water fountains or moving water, and flavored or supplemented waters. The success of each of these options varies with the individual cat. Dry foods offer convenience, cost-effectiveness, and dental benefits, while wet foods are often tastier, may improve hydration, and can help with portion control for weight management. Feeding aging cats Unlike dogs, cats appear to go through four distinct life stages: growth (< 1 year), adult (~1 to 7 years), mature/senior (~7 to 12 years of age), and geriatric (~13 years and older).14 While individuals “age” at different rates, the population means differ among the life stages in terms of typical body condition and nutrient requirements. Obesity is most prevalent in mature cats, but geriatric cats are more likely to be underweight and lose body weight and LBM over time. Concurrent with this is a decrease in the ability to digest fat and protein, as well as an increase in apparent maintenance energy requirements. While data are limited, it appears geriatric cats, like geriatric humans, also have increased protein needs.14 It is important to perform a nutritional assessment on all patients on a regular basis, but particularly important with aging cats. Core components of a nutritional assessment include medical history and physical examination with body weight, body condition score (BCS) and muscle condition score (MCS), a comprehensive dietary history including food, treats, and appetite, and assessment of any changes. Should any “red flags” be identified during a screening evaluation, a more in-depth evaluation should be completed. Examples of flags include altered gastrointestinal function, changes in appetite and/or body weight, abnormal BCS or MCS, poor skin or haircoat condition, or changes in health. The extended nutritional assessment considers animal, dietary, and environmental/human factors. Increases or decreases in body weight or condition should trigger further evaluation. Weight loss in geriatric cats can be evidence of subclinical disease. Limited data and anecdotal observations suggest preventing weight loss in nonobese cats can prolong life.16 If weight loss is evident (assessed as a percentage loss of body weight), further evaluation should determine if this is associated with increased or decreased calorie intake. Weight loss in the face of increased calorie intake suggests an underlying disease, such as hyperthyroidism or diabetes mellitus. Decreased intake may also be caused by illness but may also be attributed to hyposmia or ageusia or to feeding management. Many “senior cat diets” are lower in calories, which may be appropriate for the middle-aged cats that tend to gain weight. However, these diets are not suitable for geriatric cats with increased energy needs. Highly digestible diets with higher calorie and protein content would be more appropriate for healthy, lean geriatric cats. Loss of LBM is common in aging cats, as in other species.15 Pronounced loss of LBM is associated with increased morbidity and mortality. Inadequate protein intake can accelerate loss of LBM. Unless medically necessary, protein should not be restricted in aging cats. Feeding management Feeding management—the what, when, how, and where food is provided—must meet the needs of both cat and owner. Some veterinarians recommend wet or canned food for aging cats to encourage water intake. When wet food is accepted by both cat and owner and added hydration is needed, this is a sensible approach. Some cats and some owners, however, prefer dry foods, and this also is completely acceptable. Some cats are easily stressed, which can contribute to inappropriate intake. Having a set feeding schedule can be important. When an owner is not able to accommodate a set schedule, automated feeders may be helpful. Cats in multiple cat households may benefit from separate feeding bowls placed out of sight of each other. Bowls should be placed in quiet areas to keep cats from being disturbed while eating. If feeding bowls are placed in raised locations, it is necessary to assure that senior cats can continue to easily access the food. Food bowls placed several inches above the floor may enhance access for cats with arthritis. Wide, shallow bowls may be preferred for some cats. Food toys, such as puzzles or food balls, can encourage mental and physical activity and reduce stress. These may be especially helpful in obese-prone cats. In geriatric cats, introduction should be carefully monitored as an inability to access the food could contribute to more stress. Other options to encourage feeding-related activity include tossing kibbles for cats to chase or hiding food throughout the home. If a dietary change becomes necessary, it is best to do so slowly and only once the cat is medically stable. Provide both the old and new diets at the same time. One suggestion is to place the new food in the familiar feeding bowl and the familiar food in a new bowl, side by side. Allow the cat to become accustomed to the new food before slowly decreasing the amount of the prior diet. For sick or inappetent cats or picky eaters, several weeks may be needed to complete the transition. If medications need to be provided in food, it should be put into a small amount of a different but palatable food, separate from the main meal. Summary Cats have absolute requirements for certain nutrients. Yet, cats, like other species, have great flexibility regarding the source of those nutrients. This metabolic flexibility allows cats to thrive on a wide range of nutrient profiles from a wide range of ingredients. This is very good news for clinicians who may need to select a certain nutrient profile or a certain type of food to meet the needs of either the patient or the client. A cat’s life stage can impact its energy and nutritional needs, as well as optimum feeding management. Reduced digestive function is common in geriatric cats along with increased energy requirements. Weight loss in senior cats should be evaluated and, if possible, corrected. If no underlying disease can be identified, it is important to assure that there are no environmental restrictions to intake and that a suitable diet is being fed. Dottie Laflamme, DVM, PhD, DACVIM (Nutrition) is a retired veterinary nutritionist, having worked in the research department of Ralston Purina and Nestle Purina for nearly 30 years. Her research focused on therapeutic nutrition and nutrition for aging cats and dogs. Laura Eirmann, DVM, DACVIM (Nutrition), is currently the clinical nutritionist at the Oradell Animal Hospital in Paramus, N.J. References Zoran DL. The carnivore connection to nutrition in cats. J Am Vet Med Assoc 2002;221:1559–67. Rand JS, Fleeman LM, Farrow HE, et al. Canine and feline diabetes mellitus: nature or nurture? J Nutr 2004;134:2072S–80S. Kley S, Hoenig M, Glushka J, et al. The impact of obesity, sex, and diet on hepatic glucose production in cats. Am J Physiol Regul Integr Comp Physiol 2009; 296:R936–43. Green AS, Ramsey JJ, Villaverde C, et al. Cats are able to adapt protein oxidation to protein intake provided their requirement for dietary protein is met. J Nutr 2008;138:1053-1060. Laflamme DP. Understanding the nutritional needs of healthy cats and those with diet-sensitive conditions. Vet Clin N Am 2020; 50:905-924. Laflamme DP, Backus RC, Forrester D, Hoenig M. Evidence does not support the controversy regarding carbohydrates in feline diets. J Am Vet Med Assoc. 2022;260:506-513. Rogers QR, Morris JG, Freedland RA. Lack of hepatic enzymatic adaptation to low and high levels of dietary protein in the adult cat. Enzyme 1977;22:346-356. Hoenig M, Jordan ET, Glushka J, et al. Effect of macronutrients, age and obesity on 6- and 24-hour postprandial glucose metabolism in cats. Am J Physiol Regul Integr Comp Physiol 2011;301:R1798-R1807. Laflamme DP, Hannah SS. Discrepancy Between Use of Lean Body Mass or Nitrogen Balance to Determine Protein Requirements for Adult Cats. J Fel Med Surg. 2013;15:691-697. Laflamme, DP, Hannah SS. Increased dietary protein promotes fat loss and reduces loss of lean body mass during weight loss in cats. Int J Appl Res Vet Med. 2005;3:62–68. Vasconcellos RS, Borges NC, Goncalves KNV, et al. Protein intake during weight loss influences the energy required for weight loss and maintenance in cats. J. Nutr. 2009;139:855–860. Des Courtis X, Wei A, Kass PH, et al. Influence of dietary protein level on body composition and energy expenditure in calorically restricted overweight cats. J Anim Physiol Anim Nutr 2015;99:474-482. Laflamme DP. Understanding the nutritional needs of healthy cats and those with diet-sensitive conditions. Vet Clin N Am Sm Anim Pract 2020;50:905-924. https://vetmed.tamu.edu/news/pet-talk/choosing-a-cat-food/ Laflamme DP, Gunn-Moore D. Nutrition for aging cats. Vet Clin N Am Sm Anim Pract 2014;44:761-774. Cupp CJ, Kerr WW. Effect of Diet and Body Composition on Life Span in Aging Cats. Proc Companion Anim Nutr Summit. Focus on Gerontology. Nestlé Purina PetCare. Clearwater Beach, FL. 2010;36-42.
