Nutritional Requirements and Related Diseases of Small Animals
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 (120–550 g). The first 2 wk of a puppy’s life is spent eating, seeking warmth, and sleeping. External food sources beyond bitch’s milk are rarely needed, unless the bitch cannot produce enough milk or the puppy is orphaned. In these cases, the puppy must be handreared. 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 8–12 mo of age in small and medium breeds and by 10–16 mo in large and giant 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 usually 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 adult maintenance and reproduction (see Table: AAFCO Nutrient Requirements for Dogs a and see Table: AAFCO Nutrient Requirements for Cats a). The National Research Council (NRC) also publishes nutrient profiles for dogs and cats for various life stages, most recently in 2006 (see Table: 2006 NRC Nutrient Requirements for Adult Dogs (Maintenance) a and see Table: 2006 NRC Nutrient Requirements for Kittens After Weaning a). Both AAFCO and NRC list minimum nutrient requirements and maximum nutrient requirements for nutrients with potential toxicity.
AAFCO Nutrient Requirements for Dogs a
AAFCO Nutrient Requirements for Cats a
2006 NRC Nutrient Requirements for Adult Dogs (Maintenance) a
2006 NRC Nutrient Requirements for Puppies after Weaning a
2006 NRC Nutrient Requirements for Adult Cats (Maintenance) a
2006 NRC Nutrient Requirements for Kittens After Weaning a
In developed countries, nutritional diseases are rarely seen in dogs and cats, especially when they are fed good quality, commercial, complete and balanced diets. Nutritional problems occur most commonly when dogs and cats are fed imbalanced homemade diets, when cats are fed diets formulated for dogs, or when dogs or cats are fed certain human foods. Dog or cat foods or homemade diets derived from a single food item are inadequate. For example, feeding predominantly meat or even an exclusive hamburger and rice diet to dogs or cats can induce calcium deficiency and secondary hyperparathyroidism. Raw, freshwater fish contain thiamine antagonists and can induce thiamine deficiency when fed to cats. Feeding liver can induce vitamin A toxicity in both dogs and cats.
Cats have some dietary requirements that are different from those of dogs and can develop nutritional deficiencies when fed diets formulated to meet the nutritional needs of dogs. For example, unlike dogs, cats require dietary sources of vitamin A, arachidonic acid, and taurine. Cats also require higher quantities of fat and protein than dogs, as well as of the amino acid arginine and the vitamins niacin and pyridoxine (vitamin B6). Cats lack the enzyme glucokinase, which unfortunately has led some to believe that cats cannot digest dietary carbohydrates. Cats produce the enzyme hexakinase, which allows them to digest and use properly processed dietary carbohydrates.
Well-intentioned owners occasionally cause problems by feeding dogs and cats certain human foods. For example, raisins and grapes contain an unknown substance that is toxic to dogs and can cause kidney damage. Chocolate contains theobromine and much smaller amounts of caffeine, both of which are methylxanthines. Dogs and cats metabolize theobromine much more slowly than people. Initial signs of toxicity include GI signs, such as vomiting and diarrhea. This can progress to polyuria, muscle tremors, cardiac arrhythmias, seizures, and death. Macadamia nuts are also potentially toxic to dogs and cats and can cause weakness, depression, vomiting, ataxia, muscle tremors, hyperthermia, and tachycardia. As few as six macadamia nuts can be toxic to dogs. Onions and garlic contain thiosulfate, which can cause oxidative damage to RBCs and result in anemia. Onions are more toxic than garlic. Guatemalan avocados contain a substance called persin, which can cause dyspnea, pulmonary edema, and pleural and pericardial effusion in goats and possibly dogs. Food high in fat, such as chicken skin, can result in some dogs developing pancreatitis. Broccoli toxicity has been reported to occur in dairy cattle, but it is a poorly documented problem in dogs and cats. Sugar-free foods containing xylitol can cause liver damage in dogs. Raw food diets (see Raw Meat–based Diets) are also not recommended for dogs and cats. Raw meat products may contain pathogens. (Also see Food Hazards.)
Nutrient deficiencies have also been seen in dogs and cats fed “natural,” “organic,” or “vegetarian” diets produced by owners with good intentions. Many published recipes have been only crudely balanced by computer, if at all, using nutrient averages. In addition, most homemade diets do not undergo the scrutiny and rigorous testing applied to commercial complete and balanced diets. If pet owners wish to feed their pets homemade diets, the diets should be prepared and cooked using recipes formulated by a veterinary nutritionist.
Some nutritional diseases are seen secondary to other pathologic conditions or anorexia, or both. Owner neglect is also a frequent contributing factor in malnutrition.
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 or joules. The caloric content of pets foods is usually expressed in kilocalories (kcal), which is 1,000 calories. 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.
Energy requirements for dogs and cats are not a linear function of body weight. Recent evidence indicates that dogs maintained in households require fewer calories per day than dogs kept in kennels, but 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, percent lean body mass, age, and environment. Even when specific formulas are used, any given animal may require as much as 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.
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. Likewise, the precise ME values for many cats 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 and 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/kg0.75 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 was done under thermoneutral (68°–72°F [20°–22°C]) conditions. However, unacclimatized adult cats increased their daily caloric intakes nearly 2-fold when environmental temperatures of 23°C and 0°C were studied.
Energy requirements are quite variable among dogs and cats. Animals with the same body weight can have 3-fold variation in daily kcal requirements, which are affected by age, neutering status, physiologic status (growth, gestation, lactation, etc), physical activity, environmental temperature, and any underlying abnormalities. Any recommendations for kcal requirements are only starting points and may need to be modified based on the response of the individual dog or cat.
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 is an exponential and a linear formula for calculating RER. The exponential formula (RER = 70 [body wt in kg0.75]) can be used for animals of any body weight, whereas the linear formula (RER = 30 × [body wt in kg] + 70) is restricted for 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 Daily Maintenance Energy Requirements for Dogs and Cats.
Daily Maintenance Energy Requirements for Dogs and Cats
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 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 wt. Another approach considers 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 MER in kcal of 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 many different disorders, including those of the GI, respiratory, and urinary systems.
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. Most commercial dog foods contain a combination of plant- and animal-based proteins, with protein digestibilities of 75%–90%. Digestibility is less for plant protein ingredients, protein 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; however, these are not the types of temperatures typically used to produce commercial pet foods.
Healthy adult dogs need a minimum of 2.62 g of protein of high biologic value per kg metabolic body wt (ie, BWkg0.75)/day (NRC guidelines). Healthy puppies 4–14 wk old and >14 wk old need a minimum of 9.7 g and 12.5 g of protein of high biologic value per kg metabolic body wt/day, respectively. The cat has a higher protein requirement than most species, and healthy adult cats need a minimum of 3.97 g of protein of high biologic value per kg metabolic body wt/day. Healthy kittens need a minimum of 9.4 g of protein of high biologic value per kg metabolic 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 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.
General guidelines for dietary protein requirements in dogs and cats exist, but requirements vary depending on the digestibility of the protein in the diet. If an animal is consuming a diet containing predominantly plant protein sources, protein requirements may be higher than if the animal is consuming a diet containing predominantly animal protein sources. The dietary requirement for protein in healthy adult dogs is satisfied when the dog’s metabolic need for amino acids and nitrogen is satisfied. Optimal diets for growing puppies should contain a minimum of 22% protein as dry matter (AAFCO guidelines) or 45 g protein/1,000 kcal ME for puppies 4–14 wk old and 35 g protein/1,000 kcal ME for puppies >14 wk old (NRC guidelines). Adult dogs require a minimum of 18% protein as dry matter (AAFCO guidelines) or ~20 g protein/1,000 kcal of ME required (NRC guidelines).
Optimal diets for growing kittens should contain at least 24%–28% ME as protein or 30% protein as dry matter (AAFCO guidelines) or 45 g protein/1,000 kcal ME (NRC guidelines). Optimal diets for adult cats should contain ~20% ME as protein or 26% protein as dry matter (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. 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.
Signs produced by protein deficiency or an improper protein:calorie ratio may include any or all of the following: reduced 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, 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 dogs or cats.
Dietary fat consists mainly of triglyceride with varying amounts of free fatty acids and glycerol. Lipids can either be simple (triglycerides, wax) or complex (containing many other elements).
Triglycerides 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 fatty acids that cannot be synthesized in the body; most fatty acids consumed in the diet are long-chain fatty acids. Most nutrients consumed are digested and absorbed in the small intestines, where they then enter the blood supply via the portal vein and are delivered to the liver. When long-chain fatty acids are consumed, they are digested and absorbed into the small-intestinal epithelial cells; however, they are not transported directly into the blood supply but rather enter the lymphatics first. There are conflicting studies regarding the fate of dietary medium-chain fatty acids. Most studies suggest that medium-chain fatty acids do not require initial transport in the lymphatics and instead can be absorbed from the intestines directly into the blood supply via the portal vein.
Fatty acids are either saturated, indicating there are no double bonds, or unsaturated, indicating there are one or more double bonds. Fatty acids that contain more than one double bond are called polyunsaturated fatty acids (PUFA). PUFA are designated as either omega-3, omega-6, or omega-9 fatty acids, depending on the location of the first double bond. The more double bonds a fatty acid contains, the more prone it is to rancidity if not properly preserved. Saturated fatty acids are used primarily for energy in the body, whereas unsaturated fatty acids are found in cell membranes and blood lipoproteins.
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 calorie consumption and obesity. Fat is a concentrated source of energy, yielding ~2.25 times the ME (as an equal dry-weight portion) of soluble carbohydrate or protein. The addition of too much dietary fat relative to other nutrients may result in excessive energy intake and subsequent suboptimal intakes of protein, minerals, and vitamins.
Dietary fats also facilitate the absorption, storage, and transport of the fat-soluble 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.
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. Rancid fats in the diet can also result in fat-soluble vitamin deficiency.
Dietary fat requirements vary with age and species. Optimal diets for growing puppies should contain a minimum 8% fat as dry matter (AAFCO guidelines) or 5.9 g of fat per kg metabolic body wt/day (NRC guidelines) or 21.3 g fat/1,000 kcal ME (NRC guidelines). Optimal diets for adult dogs should contain a minimum 5% fat as dry matter (AAFCO guidelines) or 1.3 g of fat per kg metabolic body wt/day (NRC guidelines) or 10 g fat/1,000 kcal ME (NRC guidelines). Optimal diets for growing kittens and adult cats should contain a minimum 9% fat as dry matter (AAFCO guidelines), 4.7 g of fat per kg metabolic body wt/day for kittens, 4.7 g of fat per kg metabolic body wt/day for kittens, 2.2 g of fat per kg metabolic body wt/day for adult cats (NRC guidelines), or 22.5 g fat/1,000 kcal ME for growing kittens and adult cats (NRC guidelines).
Dogs and cats have a dietary requirement for specific EFA, including linoleic acid, an unsaturated EFA found in appreciable amounts in corn and soy oil. Cats also have a dietary requirement for another unsaturated EFA, arachidonic acid. Unlike dogs, cats cannot readily convert linoleic to arachidonic acid, which must be obtained from animal sources. Recommendations for dietary intake of both linoleic acid and arachidonic acid are ~5 g and 0.2 g/kg diet, respectively, for kittens and adult cats. Both linoleic acid and arachidonic acid are omega-6 fatty acids.
Recent studies suggest that α-linolenic acid (an omega-3 fatty acid) is also essential in dogs and possibly in cats. This omega-3 fatty acid is found primarily in flaxseed oils. The amount of dietary α-linolenic acid needed likely depends on the linoleic acid content. Although the required amounts of this omega-3 fatty acid are presently unknown, current minimal recommendations include 0.8 g/kg diet of α-linoleic acid when linoleic acid is 13 g/kg diet (dry-matter basis) for puppies and 0.44 g/kg diet of α-linoleic acid when linoleic acid is 11 g/kg diet (dry-matter basis) for adults. In addition, the longer chain omega-3 fatty acid, docosahexaenoic acid (DHA), may be conditionally essential for normal neurologic growth and development of puppies and kittens. Puppies fed diets containing DHA perform better in learning experiments and are easier to train than puppies fed diets without DHA. Eicosapentaenoic acid (EPA) is another longer chain omega-3 fatty acid that has been shown to be beneficial in the diet for treatment of certain skin, renal, and GI conditions, as well as cancer, arthritis, and hyperlipidemia. These longer chain omega-3 fatty acids are found primarily in marine sources of lipids. Very little α-linolenic acid gets converted to DHA/EPA in dogs and cats. Therefore, when choosing omega-3 fatty acids to treat certain medical conditions, it is best to choose marine sources. NRC recommends a level of 0.025 g/1,000 kcal ME of a combination of DHA and EPA for both kittens and adult cats. NRC recommends levels of DHA and EPA in the diet of 0.13 g/1,000 kcal ME for puppies and 0.11 g/1,000 kcal ME for adult dogs.
Most commercial adult dog foods typically contain 5%–15% fat (dry-matter basis). Puppy diets usually contain 8%–20% fat (dry-matter basis). One reason for this wide range of fat content is the purpose of the diet; work, stress, growth, and lactation require higher levels than 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 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 in pet foods include low- and high-molecular-weight sugars, starches, and various cell wall and storage nonstarch polysaccharides or dietary fibers. The four carbohydrate groups functionally are absorbable (eg, monosaccharides such as glucose, galactose, and fructose), digestible (eg, disaccharides, some oligosaccharides), fermentable (eg, lactose, some oligosaccharides), and poorly fermentable (eg, fibers such as cellulose, which is an insoluble fiber).
Although there is no minimum dietary requirement for simple carbohydrates or starches for dogs and cats, certain tissues, such as the brain and RBCs, require glucose for energy. If inadequate amounts of dietary carbohydrates are available, the body will synthesize glucose from glucogenic amino acids and glycerol. Cats normally use glucogenic amino acids and glycerol to synthesize glucose, which is one reason why cats are classified as true carnivores. However, dogs usually synthesize glucose from dietary carbohydrates. The use of dietary protein to synthesize energy in dogs diverts amino acids away from functions such as synthesis of nonessential amino acids and building muscle. Carbohydrates can become conditionally essential when energy needs are high, such as during growth, gestation, and lactation. 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. However, properly cooked nonfibrous carbohydrates are utilized well by both cats and 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 in both dogs and cats.
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. Although there is no dietary requirement for fiber in dogs and cats, there are health benefits of having certain fiber sources in the diet. Fiber is resistant to hydrolysis by mammalian digestive secretions but is not an inert traveler through the GI tract. Increased levels of fiber in diets 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.
The diverse nature of fiber has led to numerous classification methods. One way fiber is classified is based on its solubility. Soluble fibers have greater water-holding capacity than insoluble fibers. Fiber sources such as beet pulp, cellulose, and rice bran have low solubilities, while gum arabic, methylcellulose, and inulin have high solubility. Psyllium contains both low-soluble and high-soluble fiber. Although the classification of fiber based on its solubility is still used, fiber is better classified based on its rate of fermentability. Fermentation 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 short-chain fatty acids acetate, propionate, and butyrate. Short-chain fatty acids have numerous benefits, including supplying energy to the large-intestinal epithelial cells, stimulating intestinal sodium and water absorption, and lowering the pH in the large intestines—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 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 short-chain fatty acids and some nutrients (eg, some B vitamins and vitamin K). Beneficial bacteria also function as immunomodulators and reduce liver toxins (eg, blood amine and ammonia).
Dietary fructooligosaccharides (FOS) and mannanoligosaccharides (MOS) also promote the survival and growth of beneficial bacteria in the GI tract. FOS are nondigestible oligosaccharides consisting of chains of fructose molecules. Dietary sources of FOS include beet pulp, psyllium, and chicory. Beneficial bacteria are able to use FOS as a metabolic fuel, whereas pathogenic bacteria cannot. FOS also enhance the effectiveness of the GI immune system. MOS are similar to FOS, except the predominant sugar molecule in MOS is mannose instead of fructose. Dietary sources of MOS include natural fibers found in yeast cells. MOS use a different mechanism than FOS to inhibit the growth of harmful bacteria. Pathogenic bacteria attach to the intestinal wall using finger-like projections called fimbriae. Fimbriae bind to specific mannose residues on intestinal cells. Fimbriated mannose-specific pathogens can also bind to MOS instead of adhering to the intestinal epithelium, and harmful bacteria are then excreted in the feces.
Several chemical methods are used 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, which is what is listed on pet food labels, quantifies insoluble dietary fiber, which is primarily cellulose, some lignin, and a small amount of hemicellulose. However, it does not measure a large portion of insoluble dietary fiber, nor any of the soluble dietary fiber. Therefore, crude fiber is not an accurate measure of total dietary fiber. The physiologic effects of fiber are not uniform across all fiber types, and relying solely on fiber content listed on pet food labels does not accurately reflect either fiber content and fiber types in commercial pet foods, or the physiologic effects from a diet.
Most commercial dog and cat foods are fortified with vitamins to levels that exceed minimal requirements. There is no AAFCO dietary requirement for vitamin C for dogs and cats, because they are able to synthesize it in the liver. Although dogs and cats can synthesize vitamin C in levels sufficient to prevent signs of deficiency, supplementation may provide additional health benefits because vitamin C functions as a free radical scavenger and an antioxidant in the body.
There is also no AAFCO dietary requirement for vitamin K for dogs and cats, because intestinal bacteria are able to synthesize it. However, any condition that alters the intestinal microflora, such as antibiotic therapy, may result in vitamin K deficiency. As a result, NRC recommends vitamin K at 0.33 mg/1,000 kcal ME in puppies, at 0.45 mg/1,000 kcal ME in adult dogs, and at 0.25 mg/1,000 kcal ME in kittens and adult cats.
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 adverse 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) throughout a period of months, toxic reactions may be seen. Only clinically relevant vitamin-related imbalances are described below.
Excessive consumption of liver can lead to hypervitaminosis A and may produce skeletal lesions, including deforming cervical spondylosis, ankylosis of vertebrae and large joints, osseocartilagenous hyperplasia, osteoporosis, inhibited collagen synthesis, decreased chrondrogenesis in growth plates of growing dogs, and narrowed intervertebral foramina.
Unlike most other mammals, cats cannot convert β-carotene to vitamin A, because they lack the intestinal dioxygenase enzyme necessary for β-carotene cleavage. 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 to cause 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 growing kittens. See AAFCO Nutrient Requirements for Cats a, 2006 NRC Nutrient Requirements for Adult Cats (Maintenance) a, and 2006 NRC Nutrient Requirements for Kittens After Weaning a for dietary levels of vitamin A and other nutrients recommended by AAFCO and NRC.
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.
In cats, steatitis results from a diet high in PUFA, 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 in other species.
Thiamine deficiency generally does not develop in cats fed properly prepared, commercial, complete, and balanced 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 can be classified into three 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 mcg 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 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) for therapeutic effect is common. Limited evidence exists for the recommendations of dietary mineral requirements for cats in 2006 NRC Nutrient Requirements for Adult Cats (Maintenance) a and 2006 NRC Nutrient Requirements for Kittens After Weaning a; many are based on the mineral content of successfully fed diets.
Calcium and phosphorus deficiencies are uncommon in well-balanced growth diets. Exceptions may include high-meat diets high in phosphorus and low in calcium and diets high in phytates, which inhibit absorption of trace minerals. 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. 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.
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 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 cause of hyperthyroidism that develops in older cats with increased blood thyroxine and triiodothyronine is unknown.
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, 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.