Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D

As in herbivores and omnivores, the biosynthesis of vitamin D3 in the skin exposed to ultraviolet (uv) light is generally expected to also occur in the dog and the cat. The purpose of this in vitro study was to measure the concentrations of vitamin D3 and its precursor 7-dehydrocholesterol (7DHC) in dog and cat skin before and after a quantitatively and qualitatively standardized exposure to uv light. The results are compared to those obtained by the same method in the skin of the rat. The efficiency of extracting 7DHC and vitamin D3 from skin was 72 +/- 8% and 67 +/- 3%, respectively. In dog and cat skin the concentrations of nonesterified 7DHC were below the detection limit of the HPLC system. Therefore, skin extracts were saponified and total 7DHC and vitamin D3 concentrations were measured by normal-phase HPLC. Before irradiation with uv-B light the total concentrations of 7DHC were 1858 +/- 183, 1958 +/- 204, and 17,620 +/- 2345 ng/cm2 skin (mean +/- SEM; n = 5) for the dog, the cat, and the rat, respectively. The corresponding concentrations of vitamin D3 were 211 +/- 44, 193 +/- 18, and 161 +/- 32 ng/cm2 skin for the dog, the cat, and the rat, respectively. Irradiation of standard solutions of 7DHC with 0.15 J uv-B light/min resulted in a time-dependent decrease in 7DHC and a concomitant increase in previtamin D3. After exposure of skin to a total of 2.25 J uv-B light no significant changes in concentrations in vitamin D3 were found in extracts of the skin of the dog and the cat, whereas a 40-fold increase in the vitamin D3 concentration occurred in the skin of the rat. It is concluded that in the skin of the dog and the cat only low concentrations of esterified 7DHC are present and that this 7DHC is also inadequately converted to vitamin D3. As shown previously there is no detectable increase in vitamin D3 in the dog exposed to uv irradiation in vivo. Therefore, these low 7DHC concentrations are not caused by high turnover of 7DHC but are due to restricted availability of this vitamin D3 precursor in the skin of the dog. Thus, the dog and the cat are, unlike herbivores and omnivores, not able to synthesize vitamin D3 adequately in the skin and are mainly dependent on its dietary intake, i.e., vitamin D3 is an essential vitamin for the dog and cat.