Also called · cholecalciferol · sunshine vitamin · D3
What it is
Vitamin D is a fat-soluble vitamin that regulates the absorption of calcium and phosphorus. It is directly involved in forming and maintaining bone.
Humans make it in the skin from ultraviolet light. Hence the name sunshine vitamin, and the received wisdom about getting some sun.
That pathway does not work in dogs and cats. Their skin holds less of the precursor 7-dehydrocholesterol than human skin, and what is there is depleted by high activity of the enzyme that converts it to cholesterol, leaving no raw material. Experiments confirm that ultraviolet exposure does not raise production.
When dog, cat and rat skin were exposed to identical ultraviolet light, production in the rat rose fortyfold — while in the dog and cat it did not rise at all. (How et al., Gen Comp Endocrinol, 1994)
What it does
All of it comes from the diet
Since they cannot make it, eating is the only source. Complete diets always have it added, and both AAFCO and NRC set a minimum and a maximum.
More walks will not fill the gap
“Plenty of sun makes strong bones” is received wisdom built on humans. Walks are good for muscle, joints and mental health, but they have nothing to do with vitamin D supply.
It is the nutrient most often missing from home cooking
A diet of meat and vegetables alone runs short of vitamin D easily. In a growing animal that means rickets; in an adult, osteomalacia.
How to feed it
If you are feeding a complete diet, no supplementation is needed. The added amount is formulated to cover it.
Meet it daily through the main diet. It is not a nutrient to catch up on in batches.
What to watch for
Being fat-soluble it accumulates. In excess, blood calcium rises and calcium deposits in the kidneys, heart and blood vessels. Sharing a human vitamin D supplement is dangerous, and there have been several recalls caused by over-addition during manufacture.
Some rodenticides use vitamin D analogues. Vomiting, increased thirst and urination, and lethargy appear, and it can progress to acute kidney failure. If you suspect it, go to a vet immediately.
It is already in a complete diet. Joint and skin supplements sometimes contain it too, so using several products together can push past the upper limit.
Common questions
No. They cannot make it from sunlight in the first place, so indoors and outdoors make no difference. The food covers it.
We would not suggest it. The dosage units are different and it accumulates. Excess is the greater danger, not deficiency.
Concerns this ingredient relates to
General nutrition information. It does not replace a diagnosis or a prescription. If your animal has a medical condition or is on medication, talk to your veterinarian first.