Pet Nutrition: Requirements and related diseases Part 2 Protein, Fats, Carbohydrates & Fiber

Purpose of this series is to explore the required specific dietary nutrient concentrations of cats and dogs based on their life stage. While the American Feed Control Officials (AAFCO) and the National Research Council (NRC) cooperate to publish dog and cat nutrient profiles for growth, maintenance, and reproduction, within these publications is little background to be found about the actual nutrients. And even less about the problems caused by malnutrition or imbalanced diets. To be more specific, in developed countries, nutritional diseases are rarely seen in dogs and cats, but only if and 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 using calculated 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 micro minerals 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 micro minerals or vitamins.
In part 1 of the series we talked about water and energy, today we are going to take a closer look at protein, fats, carbohydrates and fiber.
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 around 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 about 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 to 25% protein as dry matter for growing puppies, and 10 to 14% for adult dogs. Optimal diets should contain at least 24 to 28% ME as protein for growing kittens, and around 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 to 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 micro flora and fermentation patterns, alter glucose absorption and insulin kinetics, and at high levels, can depress diet digestibility.
Stay tuned for part 3 with a discussion about vitamins and minerals.
Notes: Contribution Merck Veterinarian Manual