Do Dogs Need Vitamin D Supplements?


We’re living in strange times. Never in living memory have so many people been asked to ‘stay home’ for so long. There’s a lot of focus on the bigger negative effects of being indoors for so long: mental; physical; social; and so on. A less obvious downside of lockdown that recent news reports have brought to our attention is vitamin D deficiency. Health experts are concerned many people are becoming deficient in this crucial vitamin because of a lack of going outdoors in the sun. As Britain doesn’t get great sun at the best of times, they are recommending certain groups of people should be taking vitamin D supplements to top up. Naturally, as a nation of animal lovers, questions have arisen as to whether our four-legged friends, who may also be indoors more than usual, should be given vitamin D supplements as well. So today we will be answering the question ‘do dogs need vitamin D supplements?’

What Even Is Vitamin D?

It’s probably important to explain what vitamin D is. Why it’s actually important first, so prepare yourself for a little biology lesson! Vitamin D is a molecule that has a number of effects on mammalian bodies, both ours and our pets. We will refer to human vitamin D throughout this section, but the biology is similar in almost all mammals, including dogs. 

Vitamin D’s primary function is to maintain what we call ‘calcium homeostasis’, or normal calcium levels. Calcium is an important component of bones and muscles. It is also used for muscle contraction, signalling between cells and coagulation of blood if we are injured. Too much or too little calcium can be disastrous, so vitamin D is crucial to maintain calcium homeostasis.

The way vitamin D does this is actually quite complex, and involves a number of different organs. The inactive form of vitamin D gets into our body either via dietary intake or via the skin (more on this later). While inactive it has no effect. In our necks, we have an organ called the parathyroid glands that detects blood calcium levels. If it detects that calcium is too low, it releases a hormone called parathyroid hormone. This hormone acts on the liver and kidneys. It tells them to convert inactive vitamin D into active vitamin D3 (calcitriol). Active vitamin D3 causes our intestines to absorb more calcium and reduces kidney excretion of calcium. In short, when calcium is low, vitamin D acts to increase it back to safe levels again. 

Vitamin D also has a number of secondary roles including phosphorus homeostasis (another key component of bones), remodelling of bones after damage or exercise, dental health, immune system regulation, vision and muscle growth. It is safe to say that vitamin D is a very important molecule for healthy people and dogs! 

Dogs and Vitamin D

We’ve mentioned above that we can get vitamin D from our skin. The way this works is ultraviolet (UV) light from the sun. It is absorbed by a precursor to vitamin D found in the skin, converting the precursor to inactive vitamin D. It can then go to the liver and kidneys to be converted to active vitamin D3. This is why there has been a lot of focus lately on the effects of low sunlight during the lockdown. If not enough sunlight hits our skin, we may not be producing enough vitamin D to meet our bodies’ requirements. But is this true for dogs?

While dogs do indeed convert some sunlight to vitamin D, unlike people and many other mammals, they are not very efficient at this. Most of a dog’s (and cat’s) vitamin D should come from their diet. As dogs are primarily carnivores (though can consume vegetable matter), they have evolved to absorb most of their dietary vitamin D from meat (whereas cats must consume all of their vitamin D from meat). Good sources of the vitamin for dogs include liver, fish, eggs, beef and dairy products. Having sufficient amounts of these in your dog’s diet will prevent vitamin D deficiency and low calcium causing weakened bones, poor muscle activity, heart disease and other conditions. If there are specific risks of vitamin D or calcium deficiency, vets can prescribe vitamin D supplement tablets for your dog.

Do Our Dogs Need to be Outdoors More Often?

What we can learn from the biology is that if you have a healthy dog with no signs of vitamin D or calcium deficiency, having less sun will not impact their vitamin D production to a great degree. It is very unlikely a dog with a normal diet would become vitamin D deficient due to a lack of sunlight. Sunlight, of course, has a number of other health benefits, so being outside frequently is highly recommended. Be wary that the same UV light that helps create a lot of vitamin D in humans, and a tiny amount in dogs, can be dangerous. UV light damages dog skin in the same way as it does human skin. Particularly lighter skin and skin with less hair. Dogs can be sunburnt and long-term UV light exposure can increase the risk of skin cancers. These can be prevented in the same way we protect our skin. Reducing long-term sunlight exposure, staying out of direct sunlight, and using suntan lotion (human suntan lotion can be used. Though dog-friendly products are available) on pink or white skin (skin, not fur, so part the hair to check).

It is not impossible, of course, that dogs can be vitamin D deficient. Some commercial pet foods contain low amounts of vitamin D in them. For some dogs, this can result in a deficiency. However, there is no easy way to tell just from the product what is sufficient for your dog. Every dog is different, requiring different amounts of vitamin D and absorbing more from their food. If you are suspecting your dog has low vitamin D, it is best to visit your vet first. They can perform blood tests that will indicate if your dog is deficient. From here, they can recommend a changed diet or vitamin D supplements. It is very important that you do not introduce more vitamin D into your dog’s diet. Get proper veterinary advice, as we will discuss next.

Too Much of a Good Thing?

Clearly vitamin D is important for animal health, but just like carbohydrates or fat, too much vitamin D (vitamin D toxicity) can be quite harmful. Excess vitamin D is usually removed from the body. However if there is a sudden huge intake of vitamin D, or a high intake over a long time, our dog’s bodies may not be able to cope. Vitamin D will have the same effect regardless of its amount. So vitamin D toxicity results in more calcium being pulled from bones, more being absorbed from the diet and less being excreted by the kidneys. This results in too high calcium levels, or hypercalcaemia.

Hypercalcaemia can lead to muscle damage, twitching and seizures, kidney and intestine damage, vomiting and constipation, and excess urination. In severe cases, it can be fatal. On top of this, the weak calcium-deficient bones are at greater risk of damage or fracture. Although dogs are more resistant to vitamin D toxicity than herbivorous animals, they are still at risk. This is why we advise you to never give human vitamin D supplements to your dog (as they often contain far too much vitamin D), and only increase their dietary vitamin D after veterinary advice. 

Closing Thoughts 

Now that you are an expert on all things sunlight, calcium and vitamin D. Hopefully you will see that there is no reason to supplement your dog with vitamin D if they are not getting outside as much during the lockdown. Your healthy, happy dog probably has sufficient vitamin D in their diet. If you aren’t convinced, discuss any concerns you have with your vet, and they can safely guide you on how best to increase your dog’s vitamin D intake if necessary. Don’t forget that humans are different, and we definitely do need a little sun now and then to keep our vitamin D topped up. So while the sun is out, catch some rays. But remember that if you have a furry little friend with you, you’ll both need some suntan lotion behind the ears!

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