Vitamin D is essential for the health of all living creatures. It aids in the development of bones and teeth, protects against osteoporosis, regulates the heart and ensures proper thyroid function, assists with blood clotting and assures proper absorption of phosphorous and calcium in the digestive tract. Animals, including dogs, can get their daily dose through sunlight synthesis, or from a dietary supplement.
In humans and in many animal species, exposure to ultraviolet light in the form of sunshine is enough to produce a sufficient amount of vitamin D to maintain good health. The way it works is, a precursor for vitamin D called 7-dehydrocholesterol (7-DHC) exists in the skin and is converted to vitamin D when exposed to sunlight. That vitamin D is then metabolized in the liver and kidneys for use throughout the body. Dogs receive some vitamin D from sunlight exposure, but they have some major differences in their biology that makes dietary vitamin D much more useful to them.
Dogs have much lower vitamin D requirements than other animals, and numerous studies have shown that carnivores do not produce nearly as much vitamin D through sunlight synthesis as other animals. This lack of ultraviolet vitamin D production isn't because they're covered in fur, either. Fur-bearing herbivores and omnivores produce significant amounts of vitamin D through skin synthesis. Rather, dogs have very low levels of the vitamin D precursor 7-DHC in their skin. For example, the levels of 7-DHC in the skin of rats is 950 percent higher than the levels found in the skin of dogs.
Vitamin D in Carnivores
Since 7-DHC is a precursor to both cholesterol and vitamin D, and it takes far more time for the skin to synthesize vitamin D than cholesterol, there is one theory that the majority of 7-DHC in a dog's skin is largely converted to cholesterol before it can be converted into vitamin D. The lack of significant skin synthesis of vitamin D in dogs further leads researchers to believe dogs satisfy the majority of their vitamin D requirements through diet. The organs of prey dogs would normally feed on are very high in vitamin D content.
Vitamin D Deficiency
Most commercially prepared dog foods contain sufficient vitamin D to meet your dog's nutritional requirements. If you make your own food for him, you may need to provide a vitamin D supplement. Lack of enough vitamin D can cause rickets in dogs, a disease that affects bone formation. In tests on animals with rickets, dogs were the only ones who couldn't be cured through exposure to sunlight and instead were healed through dietary vitamin D. Too much dietary vitamin D can be toxic to your pet, so be sure to consult with your veterinarian before supplementing his diet.