Vitamins may be defined as organic substances that play a required catalytic role within the cell (usually as components of coenzymes or other groups associated with enzymes) and must be obtained in small amounts through the diet. Vitamin requirements are specific for each organism, and their deficiency may cause disease. Vitamin deficiencies in young animals usually result in growth failure, various symptoms whose nature depends on the vitamin, and eventual death.

Although a vitamin is usually defined as an organic chemical which an animal or human must obtain from the diet in very small amounts, this is not entirely true. Vitamin A does not occur in the plant kingdom, but the pigment carotene is universally present in green plants, and most animals can split a molecule of carotene into two molecules of vitamin A. The exceptions are cats and probably other carnivores, which under natural conditions have to obtain the preformed vitamin by consuming the tissues of other animals. Niacin, too, is not an absolute requirement, since most animals (cats again being an exception) can synthesize it from the amino acid tryptophan if the latter is present in excess of its use for protein synthesis.

Vitamin D is not a true vitamin: most species do not need it in their diet, because they obtain an adequate supply through the exposure of skin to sunlight, which converts a sterol present in dermal tissue to vitamin D. The vitamin is subsequently metabolized to form a hormone that acts to control the absorption and utilization of calcium and phosphate. Animals such as rodents, which normally have little exposure to sunlight and search for food mostly at night, appear to have evolved so as to be independent of vitamin D so long as their intakes of calcium and phosphate are well-balanced.

Vitamin C (ascorbic acid) is an essential chemical in the tissues of all species, but most can make it for themselves, so that for them it is not a vitamin. Presumably, species that cannot synthesize vitamin C—they include humans, guinea pigs, and fruit-eating bats—had ancestors that lost the ability at a time when their diet was rich in ascorbic acid.

Bacteria vary greatly in their need for vitamins. Many are entirely independent of outside sources, but at the other extreme some of the strains of bacteria found in milk (i.e., Lactobacillus) have lost the ability to synthesize the B vitamins that they need. This property has made them useful for assaying extracts of foods for their vitamin B content. Indeed, many vitamins of this group were first discovered as growth factors for bacteria before being tested with animals and humans. The mixed bacterial flora in the guts of animals are, on balance, synthesizers of the B vitamins. Consequently, ruminant animals do not have to obtain them from an external source. On the other hand, the ability of hindgut fermenters to absorb vitamins from their large intestine is uncertain. Rats and rabbits, whose nutritional needs have been studied intensively, have both been found to engage in coprophagy, the eating of fecal pellets that are vitamin-rich as a result of bacterial fermentation in the hindgut.

For one B vitamin—cobalamin, or vitamin B₁₂—bacterial fermentation is the only source, though it can be obtained indirectly from the tissues or milk of animals that have obtained it themselves from bacteria. The generalization that “the animal kingdom lives on the plant kingdom” is therefore not the whole truth, because animals rely partly on bacteria for this one micronutrient.