Unlike man, rats and our common poultry and livestock, dogs and cats have a nutritional requirement for vitamin D even when sufficient sunlight is available, since vitamin D3 is not produced in skin through action of UV irradiation on 7-dehydrocholesterol in sufficient quantities to prevent rickets (How et al., 1994a, b; 1995). Hazewinkel et al. (1987) indicated that rickets in dogs could not be prevented or treated by ultraviolet radiation; these dogs developed clinical, biochemical and histological signs of rickets. In the skin of the dog and cat the concentrations of the precursor 7-dehydrocholesterol are low and the precursor is inadequately converted to vitamin D. It is suggested that carnivores do not need to provide their own vitamin D, since fat, liver and blood of their prey will fulfill this need (How et al., 1995).
Vitamin D requirements of cats and dogs are suggested to be sufficiently high to produce normal growth, calcification, production and reproduction, provided that diets contain recommended levels of calcium and available phosphorus. Species differences can be illustrated by the fact that adequate intakes of calcium and phosphorus in a diet that contains only enough vitamin D to produce normal bone in the rat or pig will quickly cause the development of rickets in chicks. Cats and dogs have low vitamin D requirements when calcium, phosphorus and the ratio of the minerals are correct. The need for vitamin D depends to a large extent on the ratio of calcium to phosphorus. As this ratio becomes either wider or narrower than the optimum, the requirement for vitamin D increases, but no amount will compensate for severe deficiencies of either calcium or phosphorus. A calcium to phosphorus ratio of 1.2:1 is suggested for dogs (NRC, 1985) with no optimum ratio yet established for cats (NRC, 1986).
Amounts of dietary calcium and phosphorus and the physical and chemical forms in which they are presented must be considered when determining requirements for vitamin D. High dietary calcium concentrations can precipitate phosphates as insoluble calcium phosphate. Soluble calcium salts are more readily absorbed, and oxalates tend to interfere with absorption, but some of this interference can be overcome by dietary vitamin D. Correspondingly, while the phosphorus of inorganic orthophosphate tends to be well absorbed, other factors being favorable, that of phytic acid, which is the predominant phosphorus compound of unprocessed cereal grains and oilseeds, seems to be poorly available to monogastric species. Phosphorus absorption is mostly independent of vitamin D intake, with the inefficient absorption in rickets being secondary to failure of calcium absorption, and the improvement upon vitamin administration being a result of improving calcium absorption.
