Vitamin D is a complex vitamin with a sting in the tail: if a dog doesn't get enough vitamin D they may develop rickets, but give too much and the result is poisoning and organ failure. The best approach to offering the right amount of vitamin D is to feed a good quality, balanced diet. Our best buddies rarely need a vitamin D supplement and giving one may do more harm than good.
The importance of vitamin D for dogs
All vitamins for dogs play important roles. For Vitamin D, that role is making bones strong. Think of vitamin D as the foot on the gas pedal that controls the amount of calcium going to the bones. But it is wrong to think of vitamin D as acting on its own. To grow those healthy bones takes a three-way see-saw between vitamin D, calcium, and phosphorus. What affects one of these three triggers a knock-on effect with the others; just like pushing hard on one end of the see-saw. The vital role played by vitamin D in this, is to help the body feed the right amount of calcium to growing bones and to keep adult bones healthy.
You might have heard vitamin D being called the ‘sunshine vitamin.’ This is because people make their own vitamin D (without needing it in food) when the skin is exposed to UV light on a sunny day. But dogs differ from people in that our best buddies can't do this and their vitamin D is supplied by dog food. That protective layer of fur has something to do with this, but also their skin just doesn't react with sunshine in the same way as ours. It's thought that dogs evolved this way because the need for vitamin D in dogs was met by their natural meat-based diet. When a wild dog makes a kill, their food (especially the blood, fat, and liver) is rich in this vital nutrient, so making their own from sunshine was a waste of energy.
What happens if a dog doesn't get enough vitamin D?
Vitamin D deficiencies result in weak bones and cartilage, or a condition called rickets. This affects growing pups and results in bowed legs and a rubbery skeleton due to lack of calcium. But adults can also suffer vitamin D deficiency, which causes swollen, painful joints and brittle bones that can fracture because of poor calcium content. An example of this is the dog that breaks a leg after a normal activity, such as jumping down off the sofa.
How much vitamin D do dogs need?
There is no easy answer as to how much vitamin D a dog needs. The right vitamin D level depends on things such as if the dog is growing, their age, their breed, and even their sex. Another big factor is how much calcium and phosphorus the dog gets in their food (remember that three-way see-saw!). To make things even trickier, while too little vitamin D is troublesome, we know that giving too much is dangerous and may poison the dog.
The recommended amount of vitamin D that is safe for a dog varies widely, and suggesting a one-size-fits-all dose is not safe. For owners who feel strongly about boosting their dog's intake, we recommend looking for foods rich in vitamin D, such as egg yolks, liver, and fish—but even then, it should only be part of a balanced diet. Commercial foods go through rigorous tests and checks to make sure they are perfectly balanced and avoid over- or under-supplementing vitamin D.
Can you give your dog too much vitamin D?
Too much vitamin D acts like a toxin and is dangerous (which is also true for vitamin A). To illustrate this, know that many rat poisons work by giving vermin an ultra-high dose of vitamin D. But it isn't just one large dose of vitamin D that's dangerous, because smaller quantities given over many weeks or months also do harm.
There have been sad cases of pets given vitamin D supplements by their caring but misinformed, owners. Over the months, the extra vitamin D causes an imbalance with calcium release from their pet’s bones, and this calcium has to be stored somewhere else. In these unfortunate pets, the calcium was laid down in soft tissue, including the central nervous system and heart, which lead to their untimely deaths.
Symptoms of vitamin D toxicity
A dog that has a massive overdose of vitamin D (say if they scavenge some rat bait) is an emergency situation. Always contact a vet immediately if you suspect a dog has got at something they shouldn't have. One massive dose of Vitamin D can cause extreme thirst, sickness, diarrhea, and collapse.
A dog given a vitamin D supplement they don't need over the course of weeks or months is also worrying, but in a different way. Long-term raised levels of vitamin D means the body seeks out somewhere to off-load high blood calcium levels. The answer is storing calcium in soft tissues such as the kidney, liver, gut, heart, skin, and eyes. When enough calcium builds up, it interferes with organ function and can lead to organ failure. The signs of vitamin D toxicity are different depending on what tissue is involved. Symptoms range from joint pain leading to lameness, tummy upsets because of calcium in the gut wall, mental dullness due to calcium in nerves, or excessive thirst and weight loss due to organ failure.
Do dogs need vitamin D supplements?
No, most normal healthy dogs do not need a vitamin D supplement. It is safer to feed a good quality diet based on animal protein, which contains the right balance of calcium and phosphorus without a supplement. By doing this, Mother Nature takes care of the math and provides the correct amount of vitamin D.
There are very few medical conditions that might require vitamin D to be given as a supplement. These exceptions include some rare genetic disorders where the dog is unable to absorb vitamin D and certain serious bowel conditions. But be aware: these dogs will already be ill because of their underlying health problem and the vet would advise you if a supplement is required. Therefore, the safe option is never to give a vitamin D for dogs supplement unless advised to do so by a vet.