This conversion takes place in the outer skin layers. Small amounts of vitamin D are also obtained directly through the diet, usually from meat such as liver or fish oils. For this reason, some researchers view it as a hormone rather than a vitamin. For our purposes we will consider it a vitamin. Vitamin D plays a major role in regulating the calcium and phosphorous levels within the bloodstream.
Vitamin D stimulates the kidney conservation of calcium and therefore helps the body to retain it.
Because of its interplay with calcium, vitamin D is extremely important in bone formation and nerve and muscle control. Deficiencies of vitamin D were very prevalent in the past, but only occasionally surface today. Low levels of vitamin D will cause a bone demineralization referred to as rickets. Again, supplementation is highly advised in both puppies and kittens and to a lesser extent in adults. Vitamin D toxicities, as with vitamin A, are extremely rare.