Dogs and cats require specific dietary nutrient concentrations based on their life stage. The Association of American Feed Control Officials (AAFCO) publishes nutrient profiles for dogs and cats in the two main life stage categories of nutritional requirements: Adult Maintenance and Growth and Reproduction. AAFCO lists the nutrient requirements on a dry matter basis and on a unit per 1,000 kcal metabolizable energy (ME) basis. The National Research Council (NRC) also published nutrient profiles for dogs, cats, puppies, and kittens (last published in 2006). NRC uses the following life stages when listing nutrient requirements:
growth after weaning
adult maintenance
late gestation
peak lactation
Both AAFCO and NRC list minimum nutrient requirements and maximum nutrient requirements for nutrients with potential toxicity. However, neither AAFCO nor NRC recognizes that nutritional requirements may change in healthy, older adult dogs and cats, Research shows that the adult maintenance life stage for nutrient requirements is not a monolithic life stage applicable to any age of adult dogs or cats. Instead, the adult life stage must be divided into life stages such as young adult, mature adult, and geriatric or super senior life stages to adequately address the differences in metabolic and physiologic changes occurring as adult animals age(1, 2).
In higher-income countries, nutritional diseases are rarely seen in dogs and cats when owners feed a good-quality, commercial, complete, and balanced diet or a homemade diet formulated by a Board Certified Veterinary Nutritionist. Nutritional problems occur most commonly when dogs and cats are fed unbalanced homemade diets and when cats are fed diets formulated for dogs, vegetarian diets, or diets (such as grain-free diets) that use ingredients not commonly used in pet food or in higher amounts than have previously been shown to be safe.
Cats have different dietary requirements than dogs and can develop nutritional deficiencies when fed diets formulated for dogs. For example, unlike dogs, cats require dietary sources of vitamin A, arachidonic acid, and taurine. Cats also have higher requirement levels for some amino acids, such as arginine, and the vitamins niacin and pyridoxine (vitamin B6).
Dog or cat foods and homemade diets derived from a limited number of food items are often inadequate. For example, feeding predominantly meat or even an exclusive hamburger and rice diet to dogs or cats can induce calcium deficiency and secondary nutritional hyperparathyroidism. Feeding only liver can induce vitamin A toxicity in both dogs and cats. The form in which the food is fed (raw versus cooked) can impact nutrient availability. For example, some raw forms foods contain antinutritional factors that are destroyed by cooking, such as avidin found in raw eggs, which destroys biotin; thiaminases found in raw fish, which destroy thiamine; and trypsin inhibitors found in raw soybeans, which interfere with protein digestion. Alternatively, cooking methods and temperatures can impact nutrient levels and availability. Cooking can improve the availability of certain nutrients, or it may reduce the amount of other nutrients. These factors must be considered when formulating diets.
Energy in Nutritional Requirements of Small Animals
The most useful measure of energy for nutritional purposes is metabolizable energy (ME), which is defined as that portion of the total energy of a diet that is retained within the body. It is typically measured in calories in the US or joules in many other countries. A calorie is a very small unit of energy, so the unit of measure most commonly used in dog and cat nutrition is kilocalorie, equivalent to 1,000 calories. A kilocalorie is often abbreviated as kcal or Calorie (with the "c" capitalized). Dogs and cats require sufficient energy to allow for optimal body weight, body condition score (BCS), and muscle condition score (MCS) throughout the various life stages for that individual animal.
Energy requirements for dogs and cats are not a linear function of body weight. Dogs maintained in households often require fewer calories per day than dogs kept in kennels; however, considerable variability exists. Breed differences also affect caloric needs independent of body size; eg, Newfoundlands appear to require fewer calories/day than Great Danes. Other factors that determine daily energy needs include activity level, life stage, percentage of lean body mass, age, and environment.
Caloric Requirements:
Many formulas are available to calculate caloric requirements for dogs and cats. A simple method for healthy dogs and cats starts with calculating the resting energy requirement (RER). The RER is the energy requirement for a healthy but fed animal, at rest in a thermoneutral environment. It includes energy expended for recovery from physical activity and feeding.
There are two formulas for calculating RER. The exponential formula (RER = 70 [body weight in kg0.75]) can be used for animals of any body weight, whereas the linear formula (RER = 30 × [body weight in kg] + 70) is restricted to use in animals that weigh > 2 kg and < 45 kg.
The maintenance energy requirement (MER) is the energy requirement of a moderately active animal in a thermoneutral environment. It includes energy needed to obtain, digest, and absorb food in amounts to maintain body weight, as well as energy for spontaneous activity. The formulas to calculate MER take into account age and neuter status.
Formulas for daily maintenance energy requirements (kcal/day) are listed in the table.Any specific formulas used to calculate energy requirements for dogs or cats should be considered a starting point. As with people, animals of the same weight can vary in their energy requirement needed to maintain an ideal body weight. Any given animal may require as much as 30% more or less of the calories calculated for its body weight than another animal with the same body weight.
If dogs or cats consuming an adult maintenance diet require significantly less (10%–30% less) weight or volume of food than recommended on the pet food label to maintain an ideal weight and BCS, the diet could be modified to a less energy-dense diet. Standard maintenance diets are formulated to meet the nutrient requirements of a moderately active adult animal consuming a reasonable quantity or volume of the food. If maintenance diets are fed in a calorically restricted fashion, there is a reduction in the intake of all nutrients in the diet, and maintenance diets are not formulated to be fed in that way. Over-the-counter, commercial weight-management diets are formulated to be less energy dense, with the nutrient levels in the diet adjusted to ensure the animal receives all the required nutrients while consuming fewer calories. Nonetheless, severely restricting caloric intake using a weight-management diet may result in nutrient deficiencies as well. It is good practice to check for underlying medical conditions, such as hypothyroidism in dogs, whenever the amount of calories needed to maintain an animal at an ideal body weight and BCS seems unusually low. In some cases, a therapeutic weight-loss diet may need to be fed to ensure the animal is receiving the necessary amounts of nutrients while consuming a limited amount of calories. Unlike adult maintenance diets, these diets are formulated to be low in calories while still providing all the nutrients the animal requires.
Nutrient Classifications
The six classes of nutrients are water, protein, fat, carbohydrates, vitamins, and minerals. Only protein, fat, and carbohydrate provide energy; vitamins, minerals, and water do not.
Water in Nutritional Requirements of Small Animals
Water is the most important nutrient; a lack of water can lead to death in a matter of days. Clean, fresh water should be available at all times. Multiple water sources encourage consumption. This is particularly important in cats, which often do not drink a lot of water.
Several approaches have been used to estimate daily water needs. There are general guidelines for daily fluid requirements in dogs and cats, but individual variations exist. The quantity of water required depends on a number of different factors, including the animal’s diet, environment, activity level, and health status.
The moisture content of canned pet foods varies from 60% to > 87%. Dry pet foods contain 3%–11% water, and semimoist foods contain 25%–35% water. As a result, dogs and cats consuming predominantly canned food generally drink less water than those consuming predominantly dry diets.
In a thermoneutral environment, most mammalian species need ~44–66 mL/kg body weight.
When provided ample amounts of water, healthy animals can effectively self-regulate their intake, and in general, water intake should not be restricted. Water deficiency can be seen as a result of poor husbandry or disease. Dehydration is a serious problem in many different disorders, including those of the GI, respiratory, and urinary systems.
Protein in Nutritional Requirements of Small Animals
Protein is required to increase and renew the nitrogenous components of the body. A primary function of dietary protein is as a source of essential amino acids and nitrogen for the synthesis of nonessential amino acids. Amino acids supply both nitrogen for the synthesis of all other nitrogenous compounds and energy when catabolized.
Ten amino acids are essential in the diet of dogs: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Cats have a dietary requirement for an additional amino acid, taurine. Other nonessential amino acids may become conditionally essential when an animal has an underlying disorder that either interferes with synthesis of the amino acid or results in its excessive consumption or loss.
Protein requirements of dogs and cats vary with age, activity level, temperament, life stage, health status, and protein quality of the diet. Many commercial dog foods contain a combination of plant- and animal-based proteins, with varying levels of protein digestibility. Digestibility below 80% is classified as low; digestibility between 80% and 85% is classified as average; digestibility between 86% and 93% is classified as high; and digestibility greater than 93% is considered very high.
Heat can have variable effects on dietary protein. For example, heat can inactivate antinutritional factors present in some raw forms of protein. If excessive heat is used when manufacturing food, it can impact protein digestibility. Most cooked commercial dry diets are manufactured using the extrusion method of cooking, and most cooked canned foods are manufactured using a retort method of cooking. Extrusion uses a combination of moisture, pressure,temperature, flow rate, residence time, and mechanical shear from rotating screws. Cooking time in the extruder can vary from 10 to 270 seconds, and cooking temperature can vary from 80°C to 200°C (175°F to 392°F). A minimum temperature of 74°C (165°F) is needed to kill bacteria, parasitic worms, and protozoa in food, and a minimum of 100°C (212°F) is needed to kill bacterial spores in food. Although a small amount of protein may be lost during the cooking process, most manufacturers that meet World Small Animal Veterinary Association (WSAVA) guidelines are aware of this and add additional amounts of protein to the food prior to cooking it.
The biologic value of a protein is related to the number and types of essential amino acids it contains and to its digestibility and metabolizability. The higher the biologic value of a protein, the less protein is needed in the diet to supply the essential amino acid requirements. Egg has been given the highest biologic value, and organ and skeletal meats have a higher biologic value than plant-based proteins.
Diets for growing puppies and reproduction should contain a minimum of 22.5% protein as dry matter or 56.3 grams per 1,000 kcal ME (AAFCO guidelines) or 45 g protein/1,000 kcal ME for puppies 4–14 weeks old and 35 g protein/1,000 kcal ME for puppies > 14 weeks old (NRC guidelines). Adult dogs require a minimum of 18% protein as dry matter or 45 grams per 1,000 kcal ME (AAFCO guidelines) or ~20 g protein/1,000 kcal of ME required (NRC guidelines).
Diets for growing kittens and reproduction should contain a minimum of 30% protein as dry matter or 75 grams of protein per 1,000 kcal ME (AAFCO guidelines) or 45 g protein/1,000 kcal ME (NRC guidelines). Diets for adult cats should contain a minimum of 26% protein as dry matter or 65 grams of protein per 1,000 kcal ME (AAFCO guidelines) or 40 g protein/1,000 kcal ME (NRC guidelines). Growing kittens are more sensitive to the quality of dietary protein and amino acid balance than are adult cats.
Taurine is an essential amino acid in diets for cats, and taurine deficiency can cause dilated cardiomyopathy and central retinal degeneration in cats. As a result, cats must have some animal-based protein sources in their diet because plant-based protein sources are devoid of taurine. Alternatively. the diet can be supplemented with synthetic taurine.
Without sufficient energy from dietary fat or carbohydrate, dietary protein ordinarily used for growth or maintenance of body functions is converted to energy instead. To prevent protein deficiency, energy-dense diets must ensure that protein requirements are met in a smaller volume of food.
Signs produced by protein deficiency or an improper protein:calorie ratio may include any or all of the following:
decreased growth rates in puppies and kittens
anemia
weight loss
skeletal muscle atrophy
dull, unkempt hair coat
anorexia
reproductive problems
persistent unresponsive parasitism or low-grade microbial infection
impaired protection via vaccination
rapid weight loss after injury or during disease
failure to respond properly to treatment of injury or disease
Fats in Nutritional Requirements of Small Animals
Triglycerides, also called fatty acids, are divided into short, medium, and long chain, based on the number of carbon atoms in the fatty acid chain. Essential fatty acids are long-chain triglycerides (LCTs) that cannot be synthesized in the body; most fatty acids consumed in the diet are long-chain fatty acids.
The majority of nutrients consumed in the diet are digested and absorbed in the small intestine, where they then enter the blood supply via the portal vein and are delivered to the liver. When LCTs are consumed, they are digested and absorbed into the small-intestinal epithelial cells. However, they are not transported directly into the portal vein but rather enter the lymphatics before they eventually reach the blood supply. In contrast, medium-chain triglycerides (MCTs) do not appear to require initial transport in the lymphatics and instead can be absorbed from the intestines directly into the blood supply via the portal vein. This offers an advantage in dogs for the management of some GI disorders, such as lymphangiectasia, that involve the lymphatic system.
More recently, MCTs are being used in dogs with canine cognitive dysfunction. As dogs age, glucose metabolism in the brain becomes less efficient, which can have adverse affects on memory, learning, and awareness, leading to a decline in cognitive function. Certain MCTs from plant-based oils can serve as an alternative energy source for the brain. In addition, MCTs are also being used in dogs with refractory seizures to decrease seizure frequency.
Dietary fats also facilitate the absorption, storage, and transport of the fat-soluble vitamins (A, D, E, and K) and are a source of essential fatty acids (EFAs).
All mammals, including dogs and cats, have a dietary requirement for linoleic acid, an omega-6 fatty acid, which is found in appreciable amounts in vegetable oils, such as corn and soy oil. Cats have an additional requirement for arachidonic acid, another omega-6 fatty acid, which is absent in vegetable oils and fats but found in fat from meat, poultry, and eggs. Unlike dogs, cats cannot readily convert linoleic to arachidonic acid.
More recently, omega-3 fatty acids (alpha-linolenic acid [ALA], eicosapentaenoic acid [EPA], and docosahexaenoic acid [DHA]) have been added to the list of EFAs required during growth and reproduction in both dogs and cats. The best dietary source of ALA is flaxseed oil, and the best dietary sources of EPA and DHA are oily fish, krill oil, and algae oil.
Most commercial adult dog foods typically contain 5%–15% fat (dry-matter basis). Puppy diets usually contain 8%–20% fat (dry-matter basis). The wide range of fat content in different diets provides for the different energy requirements of a given animal, to accommodate the varying demands of work, stress, growth, or lactation as opposed to maintenance. As much as 60% of the calories in a cat’s diet may come from fat, although diets that contain 8%–40% fat (dry-matter basis) have also been fed successfully. Because fat can add considerably more calories to a finished diet, the amount of protein relative to energy must be balanced appropriately to the life stage and typical intakes expected for an animal’s size and needs.
EFA deficiencies are extremely rare in dogs and cats fed properly preserved complete and balanced diets formulated according to AAFCO profiles. Deficiencies of EFAs induce one or several clinical signs, such as a dry, scaly, lusterless coat; inactivity; or reproductive disorders such as anestrus, testicular underdevelopment, or lack of libido. Fatty acid supplements are often recommended for dogs with dry, flaky skin and dull coats; however, underlying metabolic conditions should always be evaluated first.
Carbohydrates in Nutritional Requirements of Small Animals
Resistant starches are classified as type 1, starches physically inaccessible to digestive enzymes, and type 2, those resistant to digestion due to the nature of the starch granules within the plant, such as the type of starch found in raw potatoes. Fiber is also resistant to digestion by digestive enzymes produced in the GI tract of dogs and cats; however, bacteria in the GI tract can produce enzymes capable of breaking fiber down.
Carbohydrates are the dietary source of glucose, which is required by certain tissues in the body. For example, the brain, nervous tissue, RBCs, renal medulla, testes, pregnant uterus, and the mammary gland during lactation all require glucose for energy. However, because glucose can be produced by the body via gluconeogenesis from amino acids or triglycerides, dietary carbohydrates are not considered essential nutrients in the diet for adult, nonreproducing dogs and cats.
Dogs usually use glucose from dietary carbohydrates, but if adequate amounts of dietary carbohydrates are not provided in their diet, gluconeogenesis will occur, diverting amino acids from functions such as synthesis of nonessential amino acids and building muscle. In contrast, cats are normally in a state of continuous gluconeogenesis, using glucogenic amino acids and glycerol to synthesize glucose, and they cannot downregulate gluconeogenesis during periods of fasting and starvation. Therefore, if adequate levels of protein and fat are not provided in the diet, cats will use endogenous glucogenic amino acids and glycerol to produce glucose.
Both dogs and cats can digest properly cooked starches, such as those from grains, with > 90% efficiency. Postabsorption, both dogs and cats will use the glucose from dietary carbohydrates to help meet their physiologic demand for glucose. Cats are adapted to a moderate intake of dietary complex carbohydrates from which glucose is absorbed into the blood at a slow and steady rate. Cats graze on food, eating as many as 18 small meals per day, resulting in a slow and steady release of glucose into the blood. In many species, including dogs and cats, either poorly digestible carbohydrates or an overload of simple sugars in the GI tract may cause adverse changes in intestinal metabolism, including osmotic diarrhea and flatulence.
Carbohydrates can become conditionally essential in dogs when energy needs are high, such as during growth, gestation, and lactation. Pup survival and metabolic status of the bitch at whelping appear to be improved by a diet in which 20%–30% of the energy is derived from carbohydrates.
Carbohydrates can also become conditionally essential in queens during lactation, protecting against weight loss and improving milk production. For lactating queens, the diet should contain at least 10% dry matter digestible carbohydrates.
Fiber
Fiber is defined as the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the small intestine and have complete or partial fermentation in the large intestine. Fiber is resistant to hydrolysis by mammalian digestive enzymes in the small intestines; however, beneficial bacteria in the large intestines are capable of fermenting fiber and producing other compounds, such as short-chain fatty acids (SCFAs), also known as volatile fatty acids (VFAs). Fiber also serves as an energy substrate for beneficial bacteria.
Although there is no dietary requirement for fiber in dogs and cats, there are health benefits from having certain fiber sources in the diet. Fiber adsorbs microbial toxins and xenobiotics, both of which can have deleterious effects in the GI tract. Certain types of dietary fiber can reduce bile acid solubility in the water in feces, thereby reducing some of the direct effects these acids can have on the intestinal epithelium. Other types of fiber can also adsorb nutrients, which can be beneficial in situations where slowing down the absorption rate of glucose in the intestines is desired, (eg, diabetic patients). However, fiber can also bind with nutrients, and a diet too high in fiber can interfere with the absorption of nutrients in the GI tract, potentially causing nutrient deficiencies.
The three primary classification systems of fiber are solubility, fermentability, and viscosity. The diverse nature of fiber makes it difficult to make generalized statements applicable to all fiber types, such as that soluble fibers are fermentable fibers and insoluble fibers are nonfermentable fibers. It is also important to realize that two fibers with the same properties in one classification system may have entirely opposite properties in another classification system.
Solubility and Viscosity
Fiber is categorized as insoluble and soluble. Insoluble fiber has no significant interaction with water, and therefore it has no appreciable water-holding capacity as the ingesta passes through the large intestines. It is analogous to ingesting plastic beads: plastic beads increase fecal volume as they pass through the GI tract, but they have no ability to absorb water and increase the moisture content of the feces. Insoluble fiber can have a laxative effect in the GI tract, but it is not due to the fiber interacting with water to increase the moisture content in the feces. One way it produces a laxative effect is promoting peristalsis. Insoluble fiber increases fecal volume or bulk, and in turn this causes stretch in the colonic smooth muscle, thereby stimulating peristalsis and decreasing intestinal transit time. A second way coarse, insoluble fiber can promote a laxative effect is through a mechanically irritating effect on the large intestinal mucosa that stimulates secretion of water and mucus from the intestinal mucosa into the lumen of the large intestine. In contrast, if finely ground, insoluble fiber is fed, it does not cause irritation of the intestinal mucosa, and therefore will not be followed by a laxative effect. Examples of insoluble fiber include cellulose, beet pulp, and rice bran.
Soluble fiber dissolves in water, and the fiber's water-holding capacity throughout the entire GI tract depends upon its viscosity. Viscosity refers to the gel-forming properties of some soluble fibers, and this gel determines the fiber's water-holding capacity. Soluble fibers can be categorized as viscous or nonviscous, depending upon the way the polymer sugar chains in the fiber interact with one another. Fibers with highly branched, bushlike polymers do not pack in a regular array when mixed with water, and therefore have no effect on viscosity. These are known as soluble, nonviscous fibers and include inulin, fructooligosaccharides (FOS), and wheat dextrin. Fibers with linear or strain-chain polymers pack into a regular array and form crosslinks with adjacent polymer chains when mixed with water to form a gel. These are known as soluble, viscous fibers. The gel slows down gastric emptying time and increases transit time through the small intestines. Examples of soluble, viscous fibers include psyllium, beta-glucan, and raw guar gum. Soluble, viscous fiber is often used in pet food to form a gravy in canned diets. The water content of the feces is inversely proportional to stool viscosity.
Fermentability
Fermentation is the process that occurs when intestinal bacteria convert fiber into other compounds, such as short-chain fatty acids (SCFAs), also known as volatile fatty acids (VFAs), as well as some vitamins, such as vitamin K. Short-chain fatty acid production in the large intestine is important in dogs and cats because SCFAs provide approximately 70% of the energy needs for colonocytes in dogs. SCFAs also lower the pH in the colon, which may enhance colonic peristalsis and decrease transit time, but a more acidic pH may also act as a defense barrier in the GI tract and protect against the colonization of pathogenic bacteria.
All fiber is fermentable to some degree and ranges from low, or poorly fermentable, to high, or rapidly fermentable. Fermentation of fiber produces desirable compounds, such as SCFAs, but it also produces undesirable compounds, such as methane and hydrogen sulfite gases. As a result, moderately fermentable fiber is often used in pet food to balance the formation of desirable and undesirable compounds. Examples of moderately fermentable fiber include beet pulp, rice bran, and gum arabic.
Manufacturers of fiber-containing diets should be aware of the impact fiber amount and type can have on digestibility of certain nutrients and make sure that the fiber type and amount in the diet do not result in nutritional deficiencies when consumed.
Increased levels of fiber in diets
increase fecal output
normalize transit time
alter colonic microbiota and fermentation patterns
alter glucose absorption and insulin kinetics
at high levels, can depress diet digestibility of certain nutrients
Fiber sources such as beet pulp, cellulose, and rice bran have low solubility, while gum arabic, methylcellulose, and inulin have high solubility. Psyllium, which is found in common over-the-counter products and supplements, contains both low-soluble and high-soluble fiber.
Although the classification of fiber based on its solubility is still used, fiber is also often classified based on its rate of fermentability. Fermentability is defined as the capacity of fiber breakdown by intestinal bacteria, and this definition more accurately assesses the potential benefits of fiber in the GI tract.
Fermentation of fiber produces the SCFAs acetate, propionate, and butyrate. Short-chain fatty acids have numerous benefits, including supplying ~70% of the energy needed by large-intestinal epithelial cells, stimulating intestinal sodium and water absorption, and lowering the pH in the large intestine—an environment that favors survival of beneficial bacteria in the GI tract.
Conversely, fermentation also produces less desirable substances such as gases, ammonia, and phenols. Highly fermentable fibers are rapidly metabolized by intestinal bacteria and produce large amounts of gas that can result in cramping and diarrhea. Production of less desirable fermentation products can be minimized by using a moderately fermentable fiber source; examples include beet pulp, inulin, and psyllium. Beet pulp also provides good fecal quality in dogs without affecting other nutrient digestibility when included at ≤ 7.5% (dry-matter basis).
Dietary fermentable fiber also functions as a prebiotic in dogs and cats. Prebiotics are defined as nondigestible food ingredients that selectively stimulate the growth or activity of beneficial bacteria in the intestines, such as Bifidobacterium and Lactobacillus. They also inhibit the survival and colonization of pathogenic bacteria. The beneficial bacteria produce SCFAs and some nutrients (eg, some B vitamins and vitamin K). Beneficial bacteria can also function as immunomodulators, reduce liver toxins (eg, blood amine and ammonia), and help with anxious behavior in dogs.
Crude fiber, which is listed in the guaranteed analysis on pet food labels, quantifies only some insoluble dietary fiber and none of the soluble dietary fiber. Therefore, crude fiber is unfortunately not an accurate measure of total dietary fiber.
Fiber Designation on Pet Food Labels Changing
The physiologic effects of fiber are not uniform across all fiber types, and relying solely on the percentage of crude fiber listed on pet food labels does not accurately reflect fiber content or fiber type in the product, nor the anticipated physiologic effects from a given diet. A more useful method of reporting fiber would be to report total dietary fiber, which includes both insoluble and soluble fibers. Specific types of fiber are often used in the management of certain conditions, and therefore knowing not only the amount of total dietary fiber in the diet but also how much of the fiber is insoluble versus soluble is important.
Lipid-soluble Vitamins in Nutritional Requirements of Small Animals
Vitamins are defined as organic substances present in food that are essential for normal metabolism. They are classified by physical and chemical properties into lipid-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B and C).
Vitamin A
Vitamin A is an essential nutrient required for numerous functions in the body, such as normal vision, growth, reproduction, immune function, maintenance of healthy epithelial tissues, and expression and regulation of many genes. Beta-carotene is a precursor of vitamin A found in plants, and dogs can readily use plant and animal sources of vitamin A. In contrast, cats cannot convert beta-carotene to vitamin A because they lack the intestinal dioxygenase enzyme necessary for beta-carotene cleavage. As a result, cats require a preformed source of vitamin A in their diet, such as that supplied by liver, fish oil, or synthetic vitamin A.
Vitamin A deficiency in dogs and cats can result in ocular changes, such as the following:
night blindness (nyctalopia) and severe conjunctival dryness (xerophthalmia)
conjunctivitis
corneal opacity and ulceration
keratitis
retinal degeneration
photophobia
In addition, dogs may experience anorexia, body weight loss, skin lesions, ataxia, metaplasia of bronchiolar epithelium, pneumonitis, and increased susceptibility to infections. In puppies, vitamin A deficiency can result in defective remodeling of the cranial foramina, resulting in stenosis and degeneration of the cochlear nerve and eventually deafness.
Hypovitaminosis A in cats may also cause stillbirths and congenital anomalies (hydrocephaly, blindness, hairlessness, deafness, ataxia, cerebellar dysplasia, and intestinal hernia), as well as squamous metaplasia of the respiratory tract, conjunctiva, endometrium, and salivary glands.
Hypervitaminosis A (toxicity) is rare in dogs, who appear to be less sensitive than cats to excess intake of dietary vitamin A. The most common clinical signs of vitamin A toxicity in dogs are skeletal malformation, spontaneous fractures, and internal hemorrhage; however, ankylosis of vertebrae and large joints, osseocartilaginous hyperplasia, osteoporosis, inhibited collagen synthesis, decreased chondrogenesis in growth plates of growing dogs, and narrowed intervertebral foramina can also occur.
In cats, vitamin A toxicity is more commonly seen and can occur when growing kittens are fed a diet high in liver. This results in extensive osseocartilaginous hyperplasia of the first three cervical vertebrae, which restricts neck movement and limits the kittens' ability to effectively groom themselves, resulting in unkempt fur.
See AAFCO nutrient requirements for cats and dogs and NRC nutrient profiles for cats, dogs, kittens, and puppies for dietary levels of vitamin A and other nutrients.
Vitamin D
Vitamin D is an essential nutrient in the diet of both dogs and cats. Unlike humans or many other mammals, both dogs and cats have very limited quantities of 7-dehydrocholesterol, a precursor that is converted into cholecalciferol (D3) when skin is exposed to UV light.
The main function of vitamin D is to enhance intestinal absorption and mobilization of calcium and phosphorus, as well as promote retention and bone deposition of both minerals. Excesses or deficiencies of vitamin D can result in changes in serum calcium and phosphorus levels. The biologically active form of vitamin D in the body is 1,25-dihydroxycholecalciferol, also known as calcitriol.
Clinical signs of vitamin D deficiency include rickets in young animals, enlarged costochondral junctions, and osteomalacia and osteoporosis in adult animals. Classic signs of rickets are rare in puppies and kittens fed complete and balanced commercial diets; they are most often present when homemade diets are fed without supplementation.
Hypervitaminosis D has been associated with excessive amounts of vitamin D inadvertently added to commercial pet foods or with ingestion of rodenticides containing cholecalciferol (D3).Vitamin D toxicity causes hypercalcemia and hyperphosphatemia with irreversible soft tissue calcification of the kidney tubules, heart valves, and large-vessel walls and can eventually be fatal.
Vitamin E
Vitamin E functions as an antioxidant in the body.
Clinical signs of vitamin E deficiency are most often attributed to membrane dysfunction as a result of oxidative degradation of polyunsaturated membrane phospholipids and disruption of other critical cellular processes. In dogs, clinical signs of vitamin E deficiency include the following:
degenerative skeletal muscle disease associated with muscle weakness
degeneration of testicular germinal epithelium and impaired spermatogenesis in males and birth of weak and dead puppies in females
retinol degeneration
lipofuscinosis (brown pigmentation) of intestinal smooth muscle
anorexia
listlessness
coma
In cats, clinical signs of vitamin E deficiency include the following:
focal interstitial myocarditis
focal myositis of skeletal muscle
periportal mononuclear infiltration in the liver and steatitis
Naturally occurring deficiencies of vitamin E have occurred in cats given large quantities of canned tuna, resulting in the development of pansteatitis (yellow fat disease). Kittens and adult cats may also develop anorexia, listlessness, hyperesthesia on palpation of the ventral abdomen, nodular adipose tissue, and fat depots discolored by brown or orange ceroid pigments.
Vitamin E is generally considered one of the least toxic fat-soluble vitamins; however, very high doses of vitamin E can antagonize absorption of the other fat-soluble vitamins, and clinical signs associated with deficiencies of vitamin A, D, and K can occur.
Vitamin K
Although Vitamin K is an essential vitamin in both dogs and cats, gut biota typically can provide adequate amounts. As a result, although vitamin K1 and K2 are ingested in the diet, the vitamin K in tissues is most commonly of bacterial origin.
Any condition that alters the intestinal microbiota, such as antimicrobial treatment, may result in vitamin K deficiency.
Vitamin K deficiency in both dogs and cats results in prolonged clotting times and excessive bleeding. Vitamin K toxicity is extremely rare.
Water-soluble Vitamins in Nutritional Requirements of Small Animals
Most water-soluble vitamins serve as enzymatic cofactors. Water-soluble vitamins have much less potential for toxicity than lipid-soluble vitamins because any oral intake of excessive levels is not readily stored in the body.
Water-soluble vitamins are usually readily excreted if excess amounts are consumed. They are thought to be far less likely to cause toxicity or adverse effects when ingested in megadoses. Vitamin B12 is the only water-soluble vitamin stored in the liver.
Vitamin C
Vitamin C (ascorbic acid) has many functions in the body, including as an antioxidant and free radical scavenger playing a role in collagen synthesis. There is no AAFCO dietary requirement for vitamin C for dogs and cats because they are able to synthesize it from glucose in the liver. Nonetheless, supplementation may provide additional health benefits. Vitamin C may also be used as a natural preservative in pet food to recharge mixed tocopherols used to prevent oxidation and rancidity of fat in the diet.
Excessive supplementation of vitamin C has raised concerns in dogs and cats with a history of calcium oxalate urolithiasis because the breakdown of ascorbic acid occurs nonenzymatically and results in oxalate formation and increased amounts of oxalate excreted in the urine.
Thiamine (Vitamin B)
Thiamine deficiency generally does not develop in dogs and cats fed properly prepared complete and balanced commercial diets. Thiamine deficiency can be associated with high intake of thiamine antagonists, such as thiaminases, the enzymes that degrade thiamine. Thiaminases are found in raw freshwater fish and clams. They can produce a deficiency by rapid destruction of dietary thiamine.
Thiaminases are destroyed by cooking fish; therefore, although canned commercial cat foods may contain fish, the heat associated with canning is sufficient to destroy thiaminase. Destruction of thiamine has also resulted from treatment of food with sulfur dioxide or overheating during drying or canning, but deficiencies are rare.
Acute thiamine deficiency involves the nervous system and produces severe neurologic signs, while chronic thiamine deficiency produces changes in the myocardium and peripheral nerves. Clinical signs of thiamine deficiency in dogs include ataxia and cardiac hypertrophy, while those in cats include anorexia, failure to grow, neurologic dysfunction, seizures, and muscle weakness. Muscle weakness most commonly appears as ventroflexion of the head, the same clinical sign often seen in cats with potassium deficiency.
Niacin
Niacin is a generic term used to describe compounds that exhibit vitamin B3 activity (eg, nicotinic acid and nicotinamide).
Deficiency of niacin in dogs results in a condition known as black tongue disease, which is characterized by bloody discharge from the GI tract and inflammation of the mucous membranes in dogs fed niacin-deficient diets. Both pellagra in humans and black tongue disease in dogs can be cured by supplementing the diet with niacin.
Deficiency of niacin is rare in dogs fed complete and balanced diets because dogs can synthesize niacin from the essential amino acid tryptophan. Clinical signs that have been reported in dogs with niacin deficiency include the following:
anorexia
weight loss
bloody diarrhea
impaired intestinal absorption of water, sodium, potassium, and glucose
reddening of the inside of the upper lip that progresses to inflammation and ulceration of the buccal and pharyngeal mucosa
profuse salivation with blood-tinged saliva
horrible odor
Cats cannot synthesize substantial amounts of niacin from tryptophan and therefore must have their total niacin requirement provided in the diet.
Niacin deficiency is uncommon in cats fed complete and balanced diets and has primarily been reported in kittens fed a purified diet without niacin. These kittens ceased growing after 10–15 days, developed diarrhea, lost weight, and died by day 50. No skin or oral lesions were observed.
Biotin
Biotin functions as an essential cofactor for four different carboxylase reactions in mammals. Carboxylases are needed for metabolism of lipids, glucose, and some amino acids. Biotin can be provided in the diet or synthesized by intestinal microbes.
Naturally occurring biotin deficiency is very rare in dogs and cats; however, it can be caused by administering oral antimicrobials that kill intestinal bacteria that synthesize biotin. The other way that biotin deficiency can occur is by feeding raw egg whites. While egg yolks are a good source of biotin, raw egg whites contain a glycoprotein called avidin that binds biotin and prevents its absorption in the GI tract. Clinical signs of biotin deficiency include poor growth, dermatitis, lethargy, and neurologic abnormalities.
Folate
Folate functions as a cofactor in nucleotide biosynthesis, phospholipid synthesis, amino acid metabolism, neurotransmitter production, and creatinine formation. Folate is also involved with rapidly dividing cells. Folate, paired with cobalamin (vitamin B12), is involved in methyl group transfer in the production of methionine from homocysteine.
Folate can be provided in the diet or synthesized from intestinal microbes. One way to diagnose antibiotic-responsive diarrhea (ARD), also known as SIBO (small intestinal bacterial overgrowth), and dysbiosis is to measure folate levels in the blood, which will be increased with ARD.
Clinical signs of folate deficiency in dogs and cats include weight loss, anorexia, anemia, diarrhea, and poor hair coat quality. In cats, longterm folate deficiency may also result in increased plasma iron concentrations and megaloblastic anemia.
Cobalamin (Vitamin B)
Cobalamin is also involved with rapidly dividing cells. Cobalamin can be provided in the diet, primarily via animal products because nonenriched plant products are essentially devoid of cobalamin, or synthesized by some intestinal microbes. The ileum is the major site of cobalamin absorption in mammals.
Cobalamin deficiency can result in anemia and irreversible neurologic damage due to demyelination. Evaluating patients for both folate and cobalamin deficiency is important for appropriate treatment.
Cobalamin deficiency in dogs has been induced by intestinal bacterial overgrowth and due to genetic abnormalities of cobalamin metabolism (intestinal malabsorption of cobalamin) in Giant Schnauzers, Border Collies, Beagles, and Komondors. Puppies with this condition can show clinical signs of anorexia, failure to thrive, neutropenia with hypersegmentation, anemia, giant platelets, and megaloblastic changes in the bone marrow.
Inborn errors of metabolism can be assessed by measuring serum levels of cobalamin or homocysteine or by analysis of urine to detect methylmalonic aciduria. Parenteral administration of cobalamin is required in dogs with this genetic abnormality.
Cobalamin deficiency in cats is primarily a result of inborn errors of metabolism, bacterial overgrowth in the intestines, inflammatory bowel disease, intestinal lymphoma, cholangiohepatitis or cholangitis, and pancreatitis. Clinical signs include weight loss, diarrhea, vomiting, anorexia, unkempt or even greasy-appearing hair coat, and thickened intestines. If cats have subnormal absorption of cobalamin, they may not respond well to oral supplementation and instead may require parenteral administration of cobalamin every two weeks to maintain normal concentrations in the blood.
Minerals in Nutritional Requirements of Small Animals
Minerals can be classified as the following:
macrominerals (sodium, potassium, calcium, phosphorus, magnesium) required in gram amounts/day
trace minerals of known importance (iron, zinc, copper, iodine, fluorine, selenium, chromium) required in milligram or microgram amounts/day
A balanced amount of the necessary dietary minerals in relation to the energy density of the diet is important. As intake of a mineral exceeds the requirement, an excessive amount may be absorbed, or a large amount of the unabsorbed mineral may prevent adequate intestinal absorption of other minerals. Indiscriminate mineral supplementation should be avoided because of the likelihood of causing a mineral imbalance.
Mineral deficiency is rare in well-balanced diets. Manipulation of dietary intake of calcium, phosphorus, sodium, magnesium (dogs and cats), and copper (dogs) is common in therapeutic diets intended for the management of specific diseases.
Macrominerals
Calcium and phosphorus are in close balance with one another in the body, and dietary phosphorus levels are often governed by calcium levels in the diet with the goal of achieving the right Ca:P ratio for each species, size, and life stage. Calcium absorption in the intestines occurs by both active transport and passive transport. Active transport is a saturable process regulated by the active form of vitamin D, while passive transport is a nonsaturable process.
After birth, calcium absorption mainly occurs by passive transport, and young animals from weaning to about 6 months old may not be able to regulate intestinal absorption of calcium when excess calcium is in the diet. After about 6 months, the contribution of passive transport to total calcium absorption decreases, and active transport becomes more prominent, allowing better regulation of calcium absorption.
Homeostasis of calcium in the body involves several organs and parathyroid hormone and calcitonin, while homeostasis of phosphorus in the body is a coordinated effort between intestinal absorption and renal excretion. Calcium and phosphorus deficiencies are uncommon in dogs and cats consuming complete and balanced diets. However, calcium deficiencies have occurred when feeding high- or all-meat diets high in phosphorus and low in calcium.
In dogs and cats, the requirements for dietary calcium and phosphorus are increased over maintenance during growth, pregnancy, and lactation. In dogs, the optimal calcium:phosphorus ratio should be ~1.2–1.4:1; however, minimum and maximum ratios by AAFCO are 1:1 to 2.1:1. Less phosphorus is absorbed at the higher ratios, so an appropriate balance of these two minerals is necessary.
Insufficient supplies of calcium or excess amounts of phosphorus decrease calcium absorption and result in irritability, hyperesthesia, and loss of muscle tone, with temporary or permanent paralysis associated with nutritional secondary hyperparathyroidism. Skeletal demineralization, particularly of the pelvis and vertebral bodies, develops with calcium deficiency. By the time there is a pathologic fracture and the condition can be confirmed radiographically, bone demineralization is severe. Often, there is a history of feeding a diet composed almost entirely of meat, liver, fish, or poultry.
Excess intake of calcium is more problematic for growing (weaning to 1 year) large- and giant-breed dogs. Excessive supplementation (> 3% calcium [dry-matter basis]) causes more severe clinical signs of osteochondrosis and decreased skeletal remodeling in young, rapidly growing large-breed dogs than in dogs fed diets with lower dietary calcium (1%–3% [dry-matter basis]). The clinical signs of lameness, pain, and decreased mobility have not been reported in small-breed dogs or more slowly growing breeds that were fed the higher calcium amounts.
Magnesium is an essential cofactor of many intercellular metabolic enzyme pathways and is rarely deficient in complete and balanced diets. However, when calcium or phosphorus supplementation is excessive, insoluble and indigestible mineral complexes form within the intestine and may decrease magnesium absorption. Clinical signs of magnesium deficiency in puppies are listlessness, lethargy, and muscle weakness. Excessive magnesium is excreted in the urine.
Trace Minerals
Iodine deficiency is rare when complete and balanced diets are fed but may be seen when high-meat diets are used (dogs and cats) or when diets contain saltwater fish (cats). Kittens with iodine deficiency show clinical signs of hyperthyroidism in the early stages, with increased excitability, followed later by hypothyroidism and lethargy. Abnormal calcium metabolism, alopecia, and fetal resorption have been reported. The condition can be confirmed by thyroid size (12 mg/100 g body weight) and histopathology at necropsy.
Iron and copper found in most meats are used efficiently, and nutritional deficiencies are rare except in animals fed a diet composed almost entirely of milk or vegetables. Deficiency of iron or copper is marked by a microcytic, hypochromic anemia and, often, by a reddish tinge to the hair in a white-haired animal.
Deficiency of zinc results in emesis, keratitis, achromotrichia, delayed growth, and emaciation. Zinc availability can sometimes be reduced by other components in commercial diets. Siberian Huskies and Alaskan Malamutes may have a genetic defect that impairs intestinal absorption of zinc and present with dermatosis that responds to zinc supplementation (canine zinc-responsive dermatosis).
References
Purina Institute. Accessed October 12, 2023. https://www.purinainstitute.com
Cat Care For Life. Accessed October 12, 2023. https://www.catcare4life.org/cat-owners/lifestages/
