Indoor cats get vitamin D from their food. They have to as they cannot create it (synthesise it) from sunlight in their skin as humans do. It does not make any difference if a cat is an indoor cat or an indoor/outdoor cat. Vitamin D is calcitriol. It is an essential dietary component. Domestic cats do not need vitamin D supplements unless your veterinarian says so. Overdosing on vitamin D can be toxic for domestic cats. Cats also must get vitamin A in their diet (retinol is the active form of vitamin A). An overdose of vitamin D causes a range of clinical signs. Within 8-48 hours the cat may suffer a loss of appetite and weakness. Following these symptoms the cat may vomit, suffer from constipation, increased drinking and peeing due to kidney failure (after a few days) and may become dehydrated. See your vet about poisoning due to an overdose vitamin D. Many household products e.g. human vitamin pills contain vitamin D. These might poison a dog.