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Vitamin D Metabolism in the Hooded Seal (Cystophora cristata)¹

Department of Zoology ^* Department of Nutritional Sciences, College of Biological Science, University of Guelph, Guelph, Ontario, Canada N1G 2W1

Fish-eating mammals, such as seals, appear to ingest levels of vitamin D that are toxic to most mammals. To determine how seals cope with high vitamin D intakes, the metabolism of tritiated cholecalciferol ([³H]D₃) was investigated in hooded seal (Cystophora cristata) pups during their postweaning fast and pups and adults consuming herring alone or supplemented with 400,000 KI D₃ daily. [³H]D₃ was metabolized to 25-[³H]OHD₃ and 24,25-[³H](OH)₂D₃. 1,25-[³H](OH)₂D₃ was not detected, but plasma levels of 1,25-(OH)₂D were similar to those in other mammals and were not affected by vitamin D intake. Plasma vitamin D, 25-OHD and 24,25-(OH)₂D increased with vitamin D intake, but 25-OHD did not increase to the extent seen in other mammals. The supplemented seals showed no evidence of toxicity. Levels of 24,25-(OH)₂D were higher in the unsupplemented seals (4 to 33 ng/mL) than reported in other mammals with similar 25-OHD levels and did not decrease with 25-OHD. High levels of 24,25-(OH)₂D relative to 25-OHD have also been found in hooded seals in the wild. The half-lives of vitamin D, 25-OHD and 24,25-(OH)₂D were shorter than those reported for most other mammals. Increased conversion of 25-OHD to 24,25-(OH)₂D and a high capacity for vitamin D storage in their large blubber mass appeared to be factors in the resistance of seals to vitamin D toxicity.

KEY WORDS: • vitamin D metabolism • vitamin D toxicity • pinnipeds

¹ Financial support for this study was provided by operating grants (to K. R. and H. H. D.) from the Natural Sciences and Engineering Research Council of Canada and scholarships (University of Guelph, Ontario Graduate and Norman James Aquatic Mammalogy) to K. M. K.

² To whom reprint requests should be sent.

Manuscript received 27 May 1987. Revision accepted 2 October 1987.