Definition and Classification

A vitamin is now generally accepted to be an organic compound that is (1) a component of a natural food, but is distinct from other nutrients such as carbohydrate, fat, protein, minerals and water; (2) present in most foods in minute amounts; (3) essential for normal metabolism in physiological functions such as growth, development, maintenance and reproduction; (4) a cause of a specific deficiency disease or syndrome if absent from the diet or improperly absorbed or utilized; and (5) not always synthesized by the host in sufficient amounts to meet physiological demands and therefore must be obtained from the diet. Vitamins are differentiated from the trace elements, also present in the diet in small quantities, by their organic nature.

Some vitamins deviate from the preceding definition in that they do not always need to be constituents of food (McDowell, 2000). Vitamin C (ascorbic acid), for example, can be synthesized by companion animals and farm livestock with the exception of fish. Nevertheless, a deficiency has been reported in some species that synthesize vitamin C, and supplementation with this vitamin has been shown to have therapeutic value for certain disease conditions or for maximizing performance. For example, swine, poultry and ruminants can synthesize vitamin C, but there is a favorable response to supplemental C when these animals are under stress. Also, some swine have been shown to have a genetic defect that limits synthesis of the vitamin. Likewise, for most species, niacin can be synthesized from the amino acid tryptophan (but not by the cat or by the fish species studied to date) and choline from the amino acid methionine. Vitamin D can be synthesized from action of ultraviolet (UV) light on precursor compounds in the skin. This is not true, however, for dogs and cats (and possibly other carnivores), which derive little or no vitamin D from UV light activity on skin (How et al., 1994a, b; 1995). Although there is often synthesis of niacin and vitamin D, these vitamins normally will be supplemented in modern animal diets. Except for fish, vitamin C supplementation is considered only under special circumstances such as environmental stress.

Classically, vitamins have been divided into two groups based on their solubilities in fat solvents or in water (Table 1). Thus, fat-soluble vitamins include A, D, E and K, while vitamins of the B complex and vitamin C are classified as water soluble. Fat-soluble vitamins are found in feedstuffs in association with lipids. The fat-soluble vitamins are absorbed along with dietary fats, apparently by mechanisms similar to those involved in fat absorption. Conditions favorable to fat absorption, such as adequate bile flow and good micelle formation, also favor absorption of the fat-soluble vitamins (Scott et al., 1982). Water-soluble vitamins are not associated with fats and alterations in fat absorption do not affect their absorption. The fat-soluble vitamins A and D and, to a lesser extent, E are generally stored in appreciable amounts in the animal body. Water-soluble vitamins are not stored and excesses are rapidly excreted, except for vitamin B12 and perhaps biotin. Table 2 lists 16 vitamins classified as either fat or water soluble. Although metabolically essential, not all of these vitamins would be dietary essentials for all species.

Definite vitamin requirements are given in the National Research Council publications for swine (1998), poultry (1994) and fish (1993). Thirteen vitamins are listed as required for swine and poultry: the four fat-soluble vitamins of A, D, E and K, and the water-soluble vitamins of thiamin, riboflavin, niacin, pantothenic acid, vitamin B6, biotin, folic acid, vitamin B12 and choline.

Poultry raised in intensive production systems are particularly susceptible to vitamin deficiencies (Scott et al., 1982). Reasons for this susceptibility are that (1) poultry derive little or no benefit from microbial synthesis of vitamins in the gastrointestinal tract; (2) poultry have high requirements for vitamins and (3) the high-density concentration of modern poultry operations places many stresses on the birds that may increase their vitamin requirements. Typical grain-oilseed meal poultry diets (e.g., corn-soybean meal) are generally supplemented with vitamins A, D3, E, K, riboflavin, niacin, pantothenic acid, B12 and choline (Scott et al., 1982). Thiamin and vitamin B6 are usually present in adequate quantities in the major ingredients such as corn-soybean meal-based diets. More attention is now being given to circumstances where supplemental biotin and folic acid are justified. Carnitine may be found to be of value in future studies.

The vitamins D and B12 are almost completely absent from diets based on corn and soybean meal. Vitamin K is generally added to poultry diets more than to other species' diets because birds have less intestinal synthesis due to a shorter intestinal tract and faster rate of food passage. Birds in cages have less access to feces (coprophagy) and therefore need higher levels of supplemental vitamins.

Vitamin supplementation of swine diets is obviously necessary, with vitamin needs having become more critical in recent years as complete confinement feeding has increased. Swine in confinement, without access to vitamin-rich pasture, and housed on slatted floors, which limit vitamins available from coprophagy, have greater need for supplemental vitamins. For swine the vitamins most likely to be marginal or deficient in corn-soybean diets are vitamins A, D, E, riboflavin, niacin, pantothenic acid and B12.

Almost all swine diets in the United States are now fortified with vitamins A, D, E, B12, riboflavin, niacin, pantothenic acid and choline. An increasing number of feed manufacturers are adding vitamin K and biotin along with others, including folic acid and B6 for specific management and feeding regimens. Diets are fortified with these vitamins even though not all experiments indicate a need for each of them. Most feed manufacturers add them as a precaution to take care of stress factors, subclinical disease level and other conditions on the average farm that may increase vitamin needs (Cunha, 1977). It appears that carnitine supplementation of weanling pigs has potential (Newton and Burtle, 1992) to improve performance.

Feeding fish in their aqueous environment involves considerations beyond those for feeding land animals. These aspects include the nutrient contribution of natural aquatic organisms in pond culture and the loss of nutrients if feed is not consumed immediately. Fish feeds require processing methods that provide special physical properties to facilitate feeding in water, and variation in feeding behavior requires special feeding regimens for various species (NRC, 1993). One problem with feeding fish relates to finding stable forms of vitamin C when feed is administered on the water.

Fish require 15 vitamins, the same 13 as poultry and swine require, as well as vitamin C and myo-inositol. The quantitative requirements for most of the vitamins have been established for chinook salmon, rainbow trout, common carp, channel catfish and yellowtail, while only some of the requirements are known for other fish species. For warm-water fish, intestinal microorganisms are a source of certain B vitamins and presumably vitamin K. In cold-water carnivorous fish, however, microorganisms are not a significant source of vitamins (Hepher, 1988; NRC, 1993).

Vitamins for which definite requirements are given in the Cattle (1989, 1996), Sheep (1985b), and Goats (1981) National Research Council (NRC) publications are A, D and E. It is well known that microorganisms in the rumen synthesize most B vitamins and vitamin K (Lardinois et al., 1944; Hungate, 1966). Rumen synthesis of the B vitamins and vitamin K develops rapidly in the young ruminant once solid feed is introduced into the diet. The rumen contents subsequently pass through parts of the digestive tract that are ideally suited for digestion and absorption of microbial products. Consequently, B vitamins and vitamin K synthesized in the rumen are readily available to the animal.

Although rumen microorganisms normally synthesize B vitamins and vitamin K in sufficient quantities to meet requirements, under special circumstances deficiencies have occurred and hence supplementation has proved beneficial for thiamin, niacin, biotin, vitamin B12, choline and vitamin K. Vitamin C can be synthesized in tissues by ruminants and most other animals under normal conditions; however, clinical cases of scurvy in ruminants have been described and supplemental vitamin C may be beneficial under certain conditions (e.g., stress). Since the conditions in which supplemental vitamin C may be beneficial are not well defined, recommended requirements are not included in the ruminant NRC publications.

Digestive systems of young ruminants, before full development of the rumen and its microflora, resemble those of monogastric animals. A reasonable assumption is that ruminants, at the tissue level, require the same vitamins as monogastric animals. Similarity of requirements has been shown for the young ruminant before development of the rumen (usually 6 to 8 weeks of age). Deficiencies of thiamin, riboflavin, vitamin B6, pantothenic acid, choline, biotin, niacin and vitamin B12 have been produced experimentally in young ruminants prior to the development of the rumen (Miller, 1979).

Vitamin B deficiency signs can be easily produced in young ruminants prior to development of a functioning rumen. However, natural milk given to young ruminants provides adequate B vitamins in addition to vitamin A and E for good health and adequate performance. Levels of various vitamins in colostrum and whole milk of cows are shown in Table 3. When young ruminants are fed milk replacers, however, it is advisable to verify the adequacy of vitamin intakes because supplementation may be needed until their rumens are functional. Such a determination is particularly important when a replacer contains appreciable quantities of non-milk protein. In preruminants receiving milk replacers, the reticulo-rumen develops slowly, and rumen synthesis of various nutrients, including B complex vitamins, may be limited.

There is a lack of experimental information on the level of vitamins required for well-balanced horse diets, as well as on which vitamins need to be added (Cunha, 1991). The vitamins most likely deficient for all classes of horses are vitamins A and E, with vitamin D also deficient for horses in confinement. Racehorses that are exercised only briefly in the early morning, with little exposure to sunlight, may be receiving inadequate vitamin D. Requirements for vitamins A, D, and E can be met with high quality (e.g., green color) sun-cured hay. Deficiencies of vitamin K and the B vitamins appear to be less likely in the mature horse than in other monogastric species, as many vitamins are synthesized in the cecum of the horse. It is not known, however, what quantities of the vitamins synthesized in the cecum are absorbed in the large intestine. Since it is unreliable to depend on intestinal synthesis, many horse owners use B vitamin supplementation of diets for the young horse and those being developed for racing or performance purposes (Cunha, 1991).

Horses grazing high-quality pastures are likely to need little or no supplementation because forages are a rich source of most fat- and water-soluble vitamins. In recent years however, vitamin supplementation has become more critical to the horse as the trend toward total confinement has increased. Currently, few horses receive a high level of vitamin intake from a lush green pasture or from a high-quality, leafy, green hay. Cunha (1991) suggested that a vitamin premix for horses contain vitamins A, D, E, K, thiamin, riboflavin, niacin, B6, biotin, pantothenic acid, folic acid, B12 and choline. Biotin supplementation is recommended as Comben et al. (1984) showed a benefit of the vitamin for hoof integrity.

Vitamin requirements have been recommended for dogs (NRC, 1985a; AAFCO, 1992) and cats (NRC, 1986; AAFCO, 1992). Despite the lack of precise information on the requirements of many vitamins for dogs and cats and the almost complete lack of information on vitamin bioavailability in important pet foods, there is a baseline of information on vitamin nutrition. Apparently dogs have some general similarities in vitamin requirements with other monogastric species (e.g., swine). Cats, however, show a specialization consistent with the evolutionary influence of a strict carnivorous diet. Cats lack the ability to synthesize taurine, cannot convert linoleic acid to arachidonic acid and are unable to cope with high levels of dietary carbohydrate. With regard to vitamins, cats cannot synthesize niacin from tryptophan or convert carotene to vitamin A. Unlike humans and most animals studied, both dogs and cats, as previously mentioned, have a nutritional requirement for vitamin D because insufficient quantities are synthesized in the skin from UV irradiation (How et al., 1994a, b; 1995). It is concluded that the cat, unlike the dog, is an obligate carnivore and is dependent on at least some animal-derived materials in its diet. Diets that do provide more animal protein (particularly organ meats) will likewise often provide more bioavailable vitamins for companion animals.

Approximately 75% of nonaccidental deaths in dogs are due to cancer, kidney failure and heart disease. Data are accumulating that suggest that many "age related" diseases such as cancer and heart disease are caused in part by free-radical damage. Free radicals can be generated by stress factors including weaning, housebreaking, rapid gains and disease conditions. Supplementing the pet diet with antioxidants such as vitamin E, vitamin C and beta-carotene can prevent or reduce the negative impact of free radical damage and thereby increase length and quality of life for companion animals.