A post going around says dogs with low vitamin D had nearly four times the risk of cancer, and that you should have your dog's level tested. The study is real. This page is what it found, what it cannot show, and the part the post hurries past: vitamin D is one of the few vitamins that can kill a dog when there is too much of it.
The study is by Kim Selting and colleagues, published in Veterinary and Comparative Oncology. They measured 25-hydroxyvitamin D, which is the form of vitamin D a blood test reads, in 282 healthy dogs and in 63 dogs brought in bleeding inside the abdomen, 43 of them with a malignant tumour and 20 with a benign one. That is 345 dogs, which is the "nearly 350" in the post.
Set against the healthy dogs, the dogs with less vitamin D were more likely to be among the cancer dogs, and the less they had, the more likely it was. The paper puts it as relative risk:
| Vitamin D in the blood | Relative risk of cancer |
|---|---|
| Under 40 ng/mL | 3.9 times |
| Under 60 ng/mL | 2.0 times |
| Under 80 ng/mL | 1.4 times |
| Over 100 ng/mL | 0.18 times |
Two other blood values, parathyroid hormone and C-reactive protein, which rises with inflammation, stopped changing once vitamin D reached about 100 ng/mL, and the authors proposed 100 to 120 ng/mL as the range that means "enough" for a dog.
This is the sentence the post leaves out. The cancer dogs were tested on the day they came in bleeding from a tumour. So the study shows that sick dogs had less vitamin D than healthy dogs. It does not show which came first. A dog that is ill, eating less and losing blood can have low vitamin D because it is ill.
A second study, in the Journal of Veterinary Internal Medicine in 2017, measured vitamin D in dogs newly diagnosed with three cancers and in healthy dogs, found the same kind of difference, and wrote the caution in plain words:
No study has given dogs vitamin D and then counted fewer cancers. So the honest version of the post is this: dogs with cancer tend to have lower vitamin D, and nobody yet knows whether raising it does anything.
The 40 ng/mL in the post is one row of the study's table. The authors' own line for enough is 100 ng/mL, and most dogs do not reach it. In 2015 the same group tested 320 dogs and found a median of 67.9 ng/mL in dogs fed commercial foods. By the 100 line, most dogs on a normal complete food count as insufficient, and the authors say so.
That range was also set with one laboratory's method. The 2017 paper notes it:
Laboratories use different methods and print different reference ranges on the report. So whether the word "low" appears on your dog's result depends partly on whose test it was.
The post is right about the sun. People make vitamin D in the skin; dogs and cats barely do. That was shown in 1994, in a paper whose title says it all: dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. The 2015 diet study puts it in one line:
So it comes from the food. A food sold as complete and balanced has to contain vitamin D: the AAFCO dog food nutrient profiles set a minimum of 500 IU per kilogram of dry matter and a maximum of 3,000. That is the number the makers formulate to, and it is why a dog on a complete food gets vitamin D every day without anyone thinking about it. Fatty fish such as salmon and sardines contain vitamin D too, and a spoonful is a fine topper. It is a topper, not a treatment.
Vitamin D is stored in fat, not flushed out, so extra builds up. In December 2018 the FDA warned about dry dog foods from nine makers, and the foods were pulled from sale.
The FDA's own summary is that vitamin D is an essential nutrient for dogs, but very high amounts can cause serious health problems like kidney failure or death. The signs it lists are vomiting, loss of appetite, increased thirst, increased urination, excessive drooling and weight loss.
The ASPCA Animal Poison Control Center's brief on vitamin D poisoning lists the other ways it happens: rat and mouse baits made of cholecalciferol, human vitamin D pills, calcium chews and multivitamins, and prescription psoriasis creams. Signs start 12 to 36 hours after the dog eats it, blood calcium rises, and the kidneys take the damage.
A test for 25-hydroxyvitamin D exists for dogs. Your vet draws blood and sends it to a laboratory; VDI Laboratory and several university diagnostic laboratories run it. There are good reasons to ask for it: a dog with an illness where vitamin D is part of the picture, a dog on a home-made diet with no vitamin D source, or a dog whose vet wants a baseline before a treatment. For a healthy dog on a complete food there is nothing yet to do with the number, because no study has shown that pushing it higher prevents anything.
If the number does come back low, the change goes through the vet: the diet first, and any supplement at a dose the vet sets and rechecks. The vitamin that was low in the cancer dogs is the same one that put nine brands of dog food on a recall list.