Vitamin D

Vitamin D is a fat-soluble vitamin that regulates the intestinal absorption of [calcium](/glossary/calcium) and [phosphorus](/glossary/phosphorus) and supports bone mineralisation. A defining peculiarity sets dogs and cats apart from people: they synthesise very little vitamin D in sun-exposed skin and depend almost entirely on dietary intake, which makes accurate dosing in a complete food critical, because the gap between deficiency and toxicity is narrow. FEDIAF considers a nutritional maximum of about 3,000 IU per 100 g of dry matter safe for cats of all life stages, while for growing giant-breed puppies the accepted maximum is much lower, around 320 IU per 100 g of dry matter (FEDIAF, 2021). Deficiency causes rickets and skeletal abnormalities in young animals. Excess, which usually traces back to over-supplementation or a formulation error rather than diet alone, causes hypercalcaemia with calcification of the kidneys and blood vessels and can be fatal; product recalls have indeed occurred over excessive vitamin D levels in finished foods. The practical takeaway follows directly: vitamin D supplements should never be added to an already complete diet without veterinary guidance, because the margin for error is small. On a label, vitamin D3 appears among the added vitamins in international units. See the [Petipedia glossary](/glossary) for related fat-soluble vitamins.

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