Recent dog food recalls have sounded the alarm about the dangers of diets that contain too much vitamin D. In dogs, high levels of this vitamin can cause vomiting, joint issues, kidney failure, and even death. So how much vitamin D should your dog be getting? And why are large amounts so dangerous?

In both humans and dogs, vitamin D supports multiple aspects of good health. But for dogs, high levels of this vitamin can be toxic. Depending on how much a dog is exposed to and for how long, vitamin D poisoning can cause a variety of symptoms:

• increased thirst
• loss of appetite
• excessive drooling
• diarrhea
• vomiting
• weight loss
• joint issues

Vitamin D’s best-known role in the body is to regulate the absorption of calcium and phosphorus. That’s why it’s so important for healthy bones. But too much vitamin D can lead to too much calcium, which is dangerous because it causes body tissues to harden (calcify).

The heart, arteries, gastrointestinal tract, and kidneys are especially likely to suffer damage as a result of this hardening of tissues. In dogs, large enough amounts of vitamin D can cause kidney failure within a matter of days.

So how much vitamin D is too much? The answer is, we don’t know exactly.

The organizations that establish the dietary requirements for pet foods—the American Association of Feed Control Officials (AAFCO), the National Research Council (NRC), and the European Pet Food Industry Federation (FEDIAF)—all agree that vitamin D is an essential nutrient for dogs. But their definitions of acceptable intake fall into a broad range. And no safe upper limits have been defined.

Part of the problem is that, although the role of vitamin D in multiple aspects of human health continues to receive growing attention, much less research has been done on how vitamin D works in dogs.

Vitamin D is produced by plants and by the skin of most mammals in response to ultraviolet light. The form produced by plants is called D2; the form produced by skin is D3. Unlike humans, dogs get almost no vitamin D from the effect of sunlight on their skin, so nearly all of their vitamin D must come from their diet.

The vitamin D in your dog’s food is absorbed by the intestines, then converted by enzymes in the liver into a slightly different molecule. That molecule is finally metabolized in the kidneys into an active form of vitamin D that can be used by the body. This usable form of vitamin D, called calcitriol, is actually a hormone and is stored mostly in fat tissue.

We know dogs need vitamin D to grow and maintain strong bones. In addition, this vitamin is partly responsible for controlling inflammation. Newer research has found that it’s also involved in regulating genes and maintaining cell health.

That broad range of acceptable vitamin D amounts in dog food reflects the fact that there hasn’t been enough research done yet on how much dogs actually need.

Different commercial dog foods may contain radically different amounts of vitamin D, depending on each manufacturer’s choice within that acceptable range outlined by the AAFCO and similar organizations.

In the dog foods that were recently recalled for elevated levels of vitamin D—in some cases more than 70 times the amount that was intended—a mistake in the “vitamin premix” was reportedly the source of the problem.

But even when the premade mix of synthetic vitamins that’s added to a pet food contains the intended amount of vitamin D, the level in the final product may still be too high if the manufacturer hasn’t correctly accounted for the natural vitamin D content of the other ingredients. In addition, synthetic vitamins don’t always behave the same way food-based vitamins do. (If a pet food label lists “vitamin D” as an ingredient, that’s a synthetic vitamin.)

Most research on this vitamin in dogs has focused on the effects of deficiency. But even evaluating a dog’s vitamin D status by blood test is problematic, according to a recent summary of the canine research on this subject.

For one thing, researchers can’t measure how much of that final, usable form of vitamin D (calcitriol) a dog has; they can only measure the intermediate form that’s produced by the liver. Recent studies estimate that 75% of dogs don’t have enough of that preliminary form, but the meaning of that statistic is somewhat unclear.

Puppies whose diet doesn’t contain enough vitamin D develop weak, soft bones (a condition known as rickets). Low measures of vitamin D in adult dogs have been associated with chronic kidney disease, inflammatory bowel disease (IBD), hyperparathyroidism, congestive heart failure, and some cancers.

But researchers haven’t been able to show that larger amounts of vitamin D reduce the risk of most of these diseases either. In fact, it may be that some diseases—especially cancer—actually cause vitamin D deficiency by interfering with the body’s ability to process the vitamin into its usable form.

Nine brands of dry dog food were recalled in late 2018 when they were found to contain overly high levels of vitamin D. In January 2019, Hill’s Pet Nutrition recalled multiple varieties of its canned dog food for the same problem. The Hill’s recall was expanded in March. You can find more information about the specific brands and products involved in these recalls here.

The manufacturers of the recalled dog food were first alerted to the problem by concerned pet parents whose dogs had become sick. It’s still unclear how many cases of illness or death have been caused by the recalled foods. If you suspect your dog may have been sickened by excess vitamin D, you can find additional help, including links for reporting a complaint, on the FDA website.‍