Nutritional Requirements and Related Diseases

Water

Energy

Protein

Fats

Carbohydrates and Crude Fiber

Dogs are a biologically diverse species, with normal body weight of 4-80 kg (2-175 lb). Normal birth weight of pups depends on breed type (120 to 550 g). The first 2 wk of a puppy’s life is spent eating, seeking warmth, and sleeping. External food sources beyond bitch’s milk is rarely needed unless the bitch cannot produce enough milk or the puppy is orphaned. In these cases, the puppy must be hand-reared. Growth rates of puppies are rapid for the first 5 mo; in this period, pups gain an average of 2-4 g/day/kg of their anticipated adult weight. The growth rate begins to plateau after 6 mo, and growth may be completed by 9-12 mo of age in small breeds and by 12-18 mo in large breeds. By comparison, the average mature body weight of domestic cats is 3.2 kg (7 lb) for toms, and 2.8 kg (6 lb) for queens. Normal birth weight of kittens is 90-100 g. The growth rate is exceptionally rapid for the first 3-4 mo, and kittens gain 50-100 g/wk. The growth rate begins to plateau at 150-160 days of age, and growth is completed within 200-220 days.

Dogs and cats require specific dietary nutrient concentrations based on their life stage. The Association of American Feed Control Officials (AAFCO) publishes dog and cat nutrient profiles for growth, maintenance, and reproduction. These are based in part on the 1974 and 1985 National Research Council (NRC) nutrient requirements for these species (Table: AAFCO Nutrient Requirements for Dogs and Table: AAFCO Nutrient Requirements for Cats). Updated NRC requirements have recently been established and will be published soon. These provide a comprehensive review of the nutrient requirements for various life stages and may be used by AAFCO to modify their profiles.

In developed countries, nutritional diseases are rarely seen in dogs and cats especially when they are fed good quality commercial rations or nutritionally balanced homemade diets. Dog or cat foods or homemade diets derived from a single food item are inadequate. For example, feeding predominately meat or even an exclusive hamburger and rice diet to dogs can induce calcium deficiency and secondary hypoparathyroidism. Feeding raw, freshwater fish to cats can induce a thiamine deficiency. Feeding liver can induce a vitamin A toxicity in both dogs and cats. Malnutrition has been seen in dogs and cats fed “natural,” “organic,” or “vegetarian” diets produced by owners with good intentions, and most published recipes have been only crudely balanced (by computer) using nutrient averages. Because the palatability, digestibility, and safety of these recipes have not been adequately or scientifically tested, it is difficult to characterize all of these homemade diets. Generally, most formulations contain excessive protein and phosphorus and are deficient in calcium, vitamin E, and microminerals such as copper, zinc, and potassium. Also, the energy density of these diets may be unbalanced relative to the other nutrients. Commonly used meat and carbohydrate ingredients contain more phosphorus than calcium. Homemade feline diets that are not actually deficient in fat or energy usually contain a vegetable oil that cats do not find palatable; therefore, less food is eaten causing a calorie deficiency. Rarely are homemade diets balanced for microminerals or vitamins.

Some nutritional diseases are seen secondary to other pathologic conditions or anorexia, or both. Owner neglect is also a frequent contributing factor in malnutrition.

Water:

Clean fresh water should be available at all times. Multiple water sources encourage consumption. Several approaches have been used to estimate daily water needs. In a thermoneutral environment, most mammalian species need ~44-66 mL/kg body wt. Another approach takes into account the fact that water needs appear to be highly associated with the amount of food consumed. In this case, daily maintenance fluid requirements in mL should equal the animal’s maintenance energy requirement in kcal of metabolizable energy (ME). A third technique sets daily water intake as 2-3 times the dietary dry matter intake. When provided ample amounts of water, healthy animals can effectively self-regulate their intake. Water deficiency can be seen as a result of poor husbandry or disease. Dehydration is a serious problem in disorders of the GI, respiratory, and urinary systems. During anorexia or increased fluid losses, a 2-5% (dogs) or 1-2% (cats) glucose and electrolyte solution should be administered PO, SC, or IV to adult dogs or cats at 60-80 mL/kg body wt/day (80-100 mL/kg in puppies and kittens) to maintain normal fluid balance.

Energy:

The most useful measure of energy for nutritional purposes is 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 or joules. Dogs and cats require sufficient energy to allow for optimal use of proteins and to maintain optimal body weight and condition through growth, maintenance, activity, pregnancy, and lactation. Of the 6 nutrient groups, only protein, fat, and carbohydrate provide energy, whereas vitamins, minerals, and water do not. Dogs and cats not consuming sufficient calories lose body weight and condition. Energy requirements for dogs and cats are not a linear function of body weight. Recent evidence indicates that pets maintained in households require fewer calories per day compared with dogs held in kennels, but considerable variability exists. Breed differences also affect caloric needs independent of body size. For example, Newfoundlands appear to require fewer calories/day than Great Danes. Other factors that determine daily energy needs include activity level, life stage, percent lean body mass, age, and environment. Even when specific formulae are used, any given animal may require up to 30% more or less of the calculated amount. Consequently, general recommendations may need to be modified within this 30% range, and body condition scoring should be regularly performed. In view of this variability, energy requirements for dogs are ~65 kcal/kg body wt for kennel or active adult dogs, ~50 kcal/kg body wt for inactive adults, ~120 kcal/kg for growing puppies, ~200 kcal/kg for lactating bitches (depending on litter size), and ~450 kcal/kg for heavily worked dogs. For cats, energy requirements are ~70 kcal/kg for lean adults, ~200 kcal/kg for growing kittens, and ~150 kcal/kg for lactating queens.

The precise ME values for many dog food ingredients have not been experimentally determined and are often estimated using those for other monogastric species (such as pigs) or calculated using Atwater physiologic fuel values modified for use with typical dog food ingredients. The precise ME values for many cat foods are not known, although it is believed that the factors used for dogs may apply. The modified Atwater ME values for dogs are 3.5 kcal/g for carbohydrate or protein and 8.5 kcal/g of fat. The impact of various environmental temperatures is described in the recent NRC publication on nutrient requirements of dogs and cats and has been documented under certain conditions. For example, energy requirements increased from 120 to 205 kcal/kg^0.75 in Huskies as ambient temperatures decreased from 14°C in summer to -20°C in winter. Effects of environmental temperature are not well characterized in cats because most of the research has been done under thermoneutral (68-72°F [20-22°C]) conditions. However, unacclimatized adult cats increased their daily caloric intakes by nearly 2-fold when environmental temperatures of 23°C and 0°C were studied.

Protein:

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 a variable amount of energy when catabolized. The amount of protein required depends on the age of the animal and protein quality and is different for dogs and cats.

Healthy adult dogs need ~2 g of protein of high biologic value per kg body wt/day. The cat has a higher protein requirement than most species, and healthy adult cats need ~4 g of protein of high biologic value per kg body wt/day. 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 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 do vegetable proteins.

The dietary requirement for protein is satisfied when the dog’s metabolic need for amino acids and nitrogen is satisfied. Optimal diets should contain 22-25% protein as dry matter for growing puppies, and 10-14% for adult dogs. Optimal diets should contain at least 24-28% ME as protein for growing kittens, and ~20% for adult cats. Growing kittens are more sensitive to the quality of dietary protein and amino acid balance than are adults. Protein suitable for cats must supply >500 mg of taurine/kg diet dry matter. Unless synthetic essential amino acids are added, some animal protein is necessary in the diet to prevent taurine depletion and development of feline central retinal degeneration or dilated cardiomyopathy.

Without sufficient energy from dietary fat or carbohydrate, dietary protein ordinarily used for growth or maintenance of body functions is less efficiently converted to energy. Too little high biologic protein in the diet, relative to the energy density, can cause an apparent protein deficiency.

Protein requirements of animals vary with age, activity level, temperament, life stage, and health status. Most commercial dog foods contain a combination of cereal and meat proteins, with protein digestibilities of 75-90%. Digestibility is less for protein ingredients of poor biologic value and for poor-quality diets. If excessive heat is used in processing, proteins can become chemically unavailable for digestion and absorption. The signs produced by protein deficiency or an improper protein to calorie ratio may include any or all of the following: weight loss, skeletal muscle atrophy (dogs), dull unkempt coat, anorexia, reproductive problems, persistent unresponsive parasitism or low-grade microbial infection, impaired protection via vaccination, rapid weight loss after injury or during disease, and failure to respond properly to treatment of injury or disease. High protein intakes per se do not cause skeletal abnormalities in dogs (including osteochondrosis in large breeds) or renal insufficiency later in life in cats.

Fats:

Dietary fat consists mainly of triglyceride with varying amounts of sterols and phospholipids. Fat is a concentrated source of energy, yielding ~2.25 times the ME (as an equal dry-weight portion) of soluble carbohydrate or protein. As much as 60% of the calories in a cat’s diet may come from fat, and diets that contain 8-40% fat (dry-matter basis) have also been fed successfully. Triglycerides are divided into short, medium, and long chain based on the number of carbon atoms in the fatty acid chain. Fatty acids are either saturated, indicating there are no double bonds, or unsaturated, indicating there are one or more double bonds. Dietary fatty acid profiles are reflected in the fatty acid composition of tissues and cell membranes. In general, as the fat content of a diet increases, so does the caloric density and palatability, which promotes excess consumption that results in obesity. Dietary fat also facilitates the absorption, storage, and transport of fat-soluble vitamins such as vitamins A, D, E, and K. They are also a source of essential fatty acids (EFA), which maintain functional integrity of cell membranes and are precursors of prostaglandins and leukotrienes. Animal fats are the most digestible component of the diet, and dogs can tolerate quite high dietary concentrations. However, the addition of too much dietary fat may result in excessive energy intake and subsequent suboptimal intakes of protein, minerals, and vitamins.

Dietary fats, especially the unsaturated variety, require a protective (natural or synthetic preservatives) antioxidation system. If antioxidant protection from a natural preservative system (eg, vitamin C or mixed tocopherols) or from synthetic preservatives (eg, BHA, BHT, ethoxyquin) in the diet is insufficient, dietary and body polyunsaturated fats become oxidized and lead to steatitis. Canine diets typically contain 5-15% fat (dry-matter basis) for adults. Puppy diets usually contain 8-20% fat (dry-matter basis). One reason for the wide range of fat content seen in commercial dog foods is the purpose of the diet—work, stress, growth, and lactation require higher levels than maintenance. However, because fat can add considerably more calories to a finished diet, it is important to remember that 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.

Cats cannot readily convert linoleic acid to arachidonic acid, which must be obtained from animal sources. Recommendations include both linoleic acid and arachidonic acid at ~5 g and 0.2 g/kg diet, respectively.

Dogs have a dietary requirement for linoleic acid, an unsaturated EFA that is found in appreciable amounts in corn and soy oil. Recent studies suggest that α-linolenic acid (ALA, an omega-3 fatty acid) is also essential in dogs and possibly cats. In addition, the longer chain omega-3 fatty acid, docosahexaenoic acid, may be conditionally essential for normal neurologic growth and development of puppies and kittens. The amount of dietary ALA needed likely depends on the linoleic acid content. Although required amounts of these omega-3 fatty acids are presently unknown, current minimal recommendations include 0.8 g/kg diet of ALA when linoleic acid is 13 g/kg diet (dry-matter basis) for puppies and 0.44 g/kg diet ALA when linoleic acid is 11 g/kg diet (dry-matter basis) for adults. Amounts for cats are currently unspecified. EFA deficiencies are extremely rare in dogs and cats fed complete and balanced diets formulated according to AAFCO profiles. Deficiencies of EFA induce one or several 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, but underlying metabolic conditions should always be evaluated first.

Carbohydrates and Crude Fiber:

Carbohydrates in pet foods include low- and high-molecular-weight sugars, starches, and various cell wall and storage nonstarch polysaccharides or dietary fibers. The 4 carbohydrate groups functionally are absorbable (eg, monosaccharides such as glucose and fructose), digestible (eg, disaccharides, some oligosaccharides), fermentable (eg, lactose, some oligosaccharides), and nonfermentable (eg, fibers such as cellulose, which is an insoluble fiber). Different carbohydrate sources have varying physiologic effects. In cats, carbohydrates apparently are not essential in the diet when ample protein and fats supply glucogenic amino acids and glycerol. Properly cooked nonfibrous carbohydrates are utilized well by dogs. In both dogs and cats, if starches are not cooked, they are poorly digested and may result in flatulence or diarrhea. Except for the occasional case of lactose or sucrose intolerance, most cooked carbohydrates are well tolerated. There is evidence that fermentable sources of carbohydrates (ie, digestible or soluble fibers) are useful in dogs; digestible versus nondigestible carbohydrate sources must be evaluated for their unique characteristics and intended purposes. Beet pulp, for example, contains both soluble and insoluble fiber and provides good stool quality in dogs without affecting other nutrient digestibility when included at ≤7.5% (dry-matter basis).

There are several chemical methods to determine the fiber level of a food; all extract the components of fiber to different degrees, which results in different estimates of fiber level for the same feedstuff. Crude fiber consists mainly of cellulose and lignin. It is resistant to hydrolysis by mammalian digestive secretions but is not an inert traveler through the GI tract. Increased levels of crude fiber in feline rations increase fecal output, normalize transit time, alter colonic microflora and fermentation patterns, alter glucose absorption and insulin kinetics, and at high levels, can depress diet digestibility.

Vitamins:

Most commercial dog and cat foods are fortified with vitamins to levels that exceed minimal requirements. There is no AAFCO dietary requirement for vitamins C or K for dogs. Cats have no documented dietary requirement for vitamin C. Deficiencies of fat-soluble vitamins (A, D, and E in dogs; A, D, E, and K in cats) and some of the 11 water-soluble B-complex vitamins have been produced experimentally. Water-soluble vitamins are usually readily excreted if excess amounts are consumed and are thought to be far less likely to cause toxicity or side effects when ingested in megadoses. Vitamin B₁₂ is the only water-soluble vitamin stored in the liver, and dogs may have a 2- to 5-yr depot. Fat-soluble vitamins (except for vitamin K in cats) are stored to an appreciable extent in the body, and when vitamins A and D are ingested in large amounts (10-100 times daily requirement) over a period of months, toxic reactions may be seen. Only clinically relevant vitamin-related imbalances are described below.

Vitamin A:

Excessive consumption of liver can lead to hypervitaminosis A and may produce skeletal lesions, including deforming cervical spondylosis, osseocartilaginous hyperplasia, osteoporosis, inhibited collagen synthesis, and decreased chrondrogenesis in growth plates of growing dogs.

Unlike most other mammals, cats cannot convert β-carotene to vitamin A because they lack intestinal carotenase. Therefore, cats require a preformed source in their diet, such as that supplied by liver, fish liver oils, or synthetic vitamin A. Signs of a vitamin A deficiency in cats are similar to those in other species, except that classic xerophthalmia, follicular hyperkeratosis, and retinal degeneration are rarely seen and usually are associated with concomitant protein deficiency. Nonetheless, cats fed diets deficient in vitamin A exhibited conjunctivitis, xerosis with keratitis and corneal vascularization, retinal degeneration, photophobia, and slowed pupillary response to light. Certain of these alterations also result from the retinal degeneration that is seen in taurine deprivation. Hypovitaminosis A in cats may exhaust vitamin A reserves of the kidneys and liver; affect reproduction causing stillbirths, congenital anomalies (hydrocephaly, blindness, hairlessness, deafness, ataxia, cerebellar dysplasia, intestinal hernia), and resorption of fetuses; and cause the same changes in epithelial cells noted in other animals. Squamous metaplasia of the respiratory tract, conjunctiva, endometrium, and salivary glands has been noted. Changes such as subpleural cysts lined by keratinizing squamous epithelium and extensive infectious sequelae are frequent in the lungs and are occasionally noted in the conjunctiva and salivary glands. Focal dysplasia of pancreatic acinar tissue and marked hypoplasia of seminiferous tubules, depletion of adrenal lipid, and focal atrophy of the skin have been reported. Borderline deficiency is more common, especially in chronic ill health. Retinol at 9,000 IU/kg of diet should meet dietary needs for vitamin A during gestation and lactation and exceed the needs of the growing kitten. Excessive consumption of liver can lead to hypervitaminosis A, which is characterized by new bone formation without osteolysis. Vitamin A toxicosis produces skeletal lesions of deforming cervical spondylosis, ankylosis of vertebrae and large joints, osseocartilagenous hyperplasia, osteoporosis, epiphyseal plate damage, and a narrowing of the intervertebral foramina.

Vitamin D:

Vitamin D deficiency results in rickets in young animals and osteomalacia in adult animals. Classic signs of rickets are rare in puppies and kittens and most often are seen when homemade diets are fed without supplementation. Rickets has been reported in kittens fed diets deficient in vitamin D, even though dietary amounts of calcium and phosphorus were normal. In rickets, serum calcium and phosphorus are decreased or low normal with a corresponding high parathyroid hormone level; bone mineralization is decreased, and the metaphyseal areas are enlarged. Osteomalacia rarely causes clinical signs in dogs or cats. Hypervitaminosis D causes hypercalcemia and hyperphosphatemia with irreversible soft-tissue calcification of the kidney tubules, heart valves, and large-vessel walls. Death in dogs is either related to chronic renal failure or acutely due to a massive aortic rupture. Death in cats is related to chronic renal failure.

Vitamin E:

In cats, steatitis results from a diet high in polyunsaturated fatty acids, particularly from marine fish oils when these are not protected with added antioxidants. Kittens or adult cats develop anorexia and muscular degeneration; depot fat becomes discolored by brown or orange ceroid pigments. Lesions are seen in cardiac and skeletal muscles and are similar to those described for other species.

Thiamine:

Deficiency generally does not develop in cats fed properly prepared commercial diets. Thiaminase, which tends to be high in uncooked freshwater fish, can produce a deficiency by rapid destruction of dietary thiamine. 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 now rare. Thiamine-deficient cats develop anorexia, an unkempt coat, a hunched position, and with time, convulsions that become more severe, leading later to prostration and death. At necropsy, small petechiae may be found in the cerebrum and midbrain. Diagnosis can be confirmed in the early stages by giving 100-250 mg thiamine, PO or IM, bid for several days. Recovery occurs in minutes to hours but, if the diet is not supplemented after this treatment, relapse can be expected. Thiamine deficiency may cause a number of other neurologic disorders, including impairment of labyrinthine righting reactions, seen as head ventroflexion and loss of the ability to maintain equilibrium when moving or jumping; impairment of the pupillary light reflex; and dysfunction of the cerebellum, suggested by asynergia, ataxia, and dysmetria.

Minerals:

Minerals can be classified into 3 major categories: 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 mg or µg amounts/day, and other trace minerals important in laboratory animals but that have an unclear role in companion animal nutrition (cobalt, molybdenum, cadmium, arsenic, silicon, vanadium, nickel, lead, tin). 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 intestinal absorption of other minerals in adequate amounts. Indiscriminate mineral supplementation should be avoided due to 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) for therapeutic effect is common. Limited evidence exists for the recommendations of dietary mineral requirements for cats made in Table: AAFCO Nutrient Requirements for Cats; many are based on the mineral content of successfully fed diets.

Macrominerals:

Calcium and phosphorus deficiency is uncommon in well-balanced growth diets. Exceptions may include high-meat diets that are high in phosphorus and low in calcium and diets high in phytates, which inhibit absorption of trace minerals. In both dogs and cats, the requirements for dietary calcium and phosphorus are increased over maintenance during growth, pregnancy, and lactation. In dogs, the calcium:phosphorus ratio should be ~1.2-1.5:1; a range of 1:1 to 2.5:1 is sufficient. Less phosphorus is absorbed at the higher ratios, so an appropriate balance of these 2 minerals is necessary. Also, insufficient supplies of calcium or excess 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 intakes of calcium are more problematic for growing (weaning to 1 yr) large- and giant-breed dogs. Excessive supplementation (>3% calcium [dry-matter basis]) causes more severe 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 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 depression, lethargy, and muscle weakness. Excessive magnesium is excreted in the urine. In cats, there is evidence that magnesium concentrations >0.3% (dry-matter basis) may be detrimental if the diet is too alkaline.

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). Deficient kittens show 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 wt) and histopathology at necropsy. The etiology of hyperthyroidism that develops in older cats with increased blood thyroxine and triiodothyronine is unknown.

Iron and copper found in most meats are utilized 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, retarded growth, and emaciation. Decreased zinc availability has been noted in canine diets containing excessive levels of phytate, which emphasizes the value of feeding trial tests over laboratory nutrient analyses of pet foods. Manganese toxicity has been reported to produce albinism in some Siamese cats; a deficiency of manganese in other species results in bone dyscrasia.