How Too Much (or Too Little) Vitamin D Affects Cats

According to one study, high levels of Vitamin D increase survival rates, so why don’t experts recommend giving your cat a Vitamin D supplement?

The sun can’t help cats create Vitamin D in their bodies because of their fur. By: marchnwe

In May 2015, the University of Edinburgh vet school announced the results of a study showing that seriously ill cats with high levels of Vitamin D in their blood were more likely to be alive 1 month later than cats with low Vitamin D levels.

Indeed, this chimes with work done in the human field where low levels of Vitamin D are associated with health problems such as repeated infections, multiple sclerosis and cancer.

On the face of it, this sounds like an exciting discovery. Could it provide a key to improving our pets’ health?

Exciting News — Or Is It?

It’s easy to leap to the conclusion that if Vitamin D aids in recovery, then giving your cat a Vitamin D supplement could improve his health. However, it is dangerous to make this assumption because Vitamin D is toxic in high doses.

At this time, in the (in)famous words of former U.S. Secretary of Defense Donald Rumsfeld, “There are more unknowns than knowns.”

How or why Vitamin D benefited the sick cats is not known, which is just one reason the experts do not advise giving your cat a supplement.

Not Enough Is Known

The cats who survived in the study had high Vitamin D levels, but that’s just an observation and nothing more. The data is at the stage where it’s impossible to say what is significant (and helpful) and what isn’t.

The veterinary researchers at the University of Edinburgh are open about the fact that more work is needed to answer such questions as:

  • Could sick cats benefit from a Vitamin D supplement?
  • How does Vitamin D help sick cats get well?
  • Could Vitamin D decrease the risk of illness?
  • Why do Vitamin D levels fall in sick cats?

At the moment, the best we can say is that this looks like an exciting area for research to see if a simple supplement could benefit the health of our feline friends.

Researchers say what we learn about cats could help people, too.

A drooling cat is one possible signs of Vitamin D toxicity. By: firepile

A Useful Predictor of Survival

However, the researchers are positive about one thing: If your cat is sick, then knowing if he has high or low levels of Vitamin D could predict how likely he is to survive.

A simple blood test looking at Vitamin D levels could help the vet know if a seriously ill cat stands a fair chance of pulling through. This has many implications for decision making and balancing whether or not it’s fair to put a cat through a lengthy or invasive treatment. Any information that can help you reach a balanced decision about your pet’s health must be good.

All of this makes it tempting to consider giving your cat a Vitamin D supplement as a health insurance policy of sorts.

Well, the vets at Edinburgh advise against this. They argue 2 points:

  • A balanced diet provides plenty of Vitamin D.
  • Too much Vitamin D is toxic to cats.

A Balanced Diet

Cats’ fur prevents them from using sunshine to make Vitamin D like people do, but good-quality cat foods already contain great levels of Vitamin D.

Also, many of our cats’ favorite foods, such as oily fish, eggs and cheese, are already high in Vitamin D. Giving a supplement just isn’t necessary — and it’s dangerous to give too much of a good thing.

Vitamin D Toxicity

Your cat is unlikely to eat enough food to become poisoned with Vitamin D.

Indeed, the most common causes of toxicity are over-supplementation or eating rodents poisoned with Vitamin D–rich baits. For the latter, signs usually develop within 12–36 hours of eating the rat or mouse.

Signs of Vitamin D poisoning in cats include:

If you notice these signs, seek urgent veterinary attention. Bear in mind, though, that these symptoms are general, so not every drooling cat has Vitamin D toxicity.

The Best Option

The job of Vitamin D is to regulate blood calcium levels to build and maintain strong teeth and bones.

It is a fat-soluble vitamin, which means fat soaks it up. Too little means weak and rubbery bones, and too much means problems with nerve function and blood clotting.

All in all, a balanced diet is the best and safest option.


  • “Vitamin D Status Predicts 30 Day Survival Rate in Hospitalized Cats.” Titmarsh, Kilpatrick, Sinclair, Boag, Bode, Lalor et al. (2015). PLoS ONE, 10(5): e0125997. doi: 10.1371/journal.pone.0125997.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Nov. 2, 2018.

Dr. Pippa Elliott, BVMS, MRCVS

View posts by Dr. Pippa Elliott, BVMS, MRCVS
Dr. Pippa Elliott, BVMS, MRCVS, is a veterinarian with nearly 30 years of experience in companion animal practice. Dr. Elliott earned her Bachelor of Veterinary Medicine and Surgery from the University of Glasgow. She was also designated a Member of the Royal College of Veterinary Surgeons. Married with 2 grown-up kids, Dr. Elliott has a naughty puggle called Poggle, 3 cats and a bearded dragon.

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