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The importance of vitamin A for dogs

Dog liver treats

Vitamin A for dogs: Can you have too much of a good thing? The short answer is yes, vitamin A toxicity is real... but if you're sensible your dog will be fine.

Just like Goldilocks and the three bears’ porridge, when it comes to vitamin A things need to be just right. On the one paw, this fat-soluble vitamin is essential for good health. On the other, give too much for too long and it makes dogs sick. 

Benefits of vitamin A for dogs

The body is an amazing thing. It converts food into the raw materials to make cells and energy to power them. But unlike a vehicle that can't repair itself when damaged, the body can‚ÄĒthanks in part to vitamin A.¬†

While all vitamins for dogs have their roles, this vitamin is the project manager when it comes to repair and regeneration of living tissue‚ÄĒespecially the cells that line the gut, lungs, and eye, along with skin itself. This is why vitamin A¬†is needed for everything from good eyesight to a strong immune system.

Let's look at an example: the gut. The acids and harsh enzymes that digest food are pretty rough on the digestive tract. To keep the intestine healthy means a never-ending cycle of repair and replace for its lining. Vitamin A deficiency causes the biological equivalent of a construction workers' strike, with the gut left in poor repair. This is one of the causes of diarrhea. 

But don't forget the whole body is deficient, not just the gut, so other organs also start to break down. For the eye, this means the light sensitive layer (the retina) can't work properly, leading to vision loss and blindness. In the lungs, this can lead to a poor ability to fight infections and serious complications, such as pneumonia. 

But what of the benefits of vitamin A for dogs? Flip this coin over and we find vitamin A helps strengthen the immune system to better fight off bugs; creates healthy tissue in the lungs, gut, and urinary tract; and ensures the eyes are fit for their purpose. In growing pups, vitamin A also helps make strong bones. Last but not least, vitamin A has antioxidant properties that fight the damage done by free radicals that contribute to aging and can cause cancer. 

How much vitamin A do dogs need?

Getting the right amount of fat-soluble vitamin A is a balancing act. On one side of the see-saw is deficiency and all its problems, but on the other side is a risk of toxicity. So how much vitamin A does a dog need to hit that sweet spot in the middle?

Data suggests adult dogs need around 22‚Äď47 IU (international units) for each kilogram of their body weight. This means an average-sized dog needs around 330‚Äď700 IU per day. Now, bear in mind that manufactured pet foods by law must supply enough vitamin A for good health; any vitamin A given in a supplement is extra on top (and not needed).

Many natural foods are good sources of vitamin A for dogs. For example:

  • A 3 oz serving of liver contains around 1,990 IU of Vitamin A
  • An average size baked sweet potato contains around 400 IU Vitamin A
  • Half a cup of raw carrot contains around 140 IU Vitamin A
  • A half cup of spinach contains around 170 IU Vitamin A

The other good news is the safe range of vitamin A for dogs is pretty wide. Clever old Mother Nature has safety mechanisms in place and it takes determined effort over a long time to achieve either deficiency or toxicity. 

For a start, the body can store vitamin A and most dogs have enough in reserve to last several weeks. On the overdose side, we're talking about the dog that's given a hefty daily supplement for years or a dog that eats only liver and nothing else. 

It may occur to you that a dog on a raw diet is eating several rich sources of vitamin A. Could this be a problem? Actually, yes, in theory it is possible to create nutritional problems, such as vitamin A overdose, on a raw diet. This is just one reason why getting advice from a qualified veterinary nutritionist is a good idea. Any diet, raw or otherwise, must be balanced properly to be healthy for your dog.

As a rule of thumb, there are sensible measures you can take to achieve a happy balance. The golden rule is not to feed any one meat or vegetable to excess. So in the case of liver (which stores vitamin A and is a naturally rich source), only offer this once or twice a week. And if you think this is too cautious, take a lesson from the Eskimos. They feed their sledging dogs raw meat, but always toss the liver into the sea to stop their dogs from eating it. This is because the Eskimo recognized the signs of vitamin A toxicity in their dogs and worked out the cause was too much liver! 

Likewise, avoid giving foods like sardines or egg yolks every day. Mix things up with more 'off' days than 'on.' If you're worried about deficiency, then supplementing the diet with carrots and green leafy veg will help as it's hard to overdose on carotenoids. But at the end of the day, rest easy knowing the body harvests and stores the vitamin A it needs from those once-weekly liver dinners. 

Natural sources of vitamin A for dogs

Plants contain the building blocks of vitamin A, but not the vitamin itself. These building blocks are known as the carotenoids. Once eaten, they are assembled by the body to create vitamin A. As the name 'carotenoids' suggests, the vegetables rich in pre-vitamin A are those with a yellow-orange color (like carrots), along with green leafy vegetables and sweetcorn. 

Because the body must work to make vitamin A from plants, it's more difficult to overdose. A fascinating fact is some animal species are better at making vitamin A than others. For example, rats and chickens are twice as efficient as dogs at manufacturing vitamin A. And if you are wondering, from 1 mg of beta-carotene a rat makes around 1700 iu of vitamin A. 

For meat eaters, the liver stores vitamin A and is a rich source. Milk fat and egg yolks are also good. But topping the chart as the major supplementary source of vitamin A for dogs are fish oils, which are best taken from whole fish versus supplements.

Vitamin A toxicity

So what's all this about vitamin A toxicity? For a start, it's a real thing. In one first-hand example, a cat was treated like a princess by a doting owner who only fed her liver‚ÄĒher favorite food. It took years for the problem to appear, but the end result was a crippled cat because her bones fused together due to long term vitamin A toxicity.

How much vitamin A is too much?

The sensible owner should not worry about vitamin A toxicity (or deficiency, for that matter). It takes effort over a long period of time to cause this condition. Those pets at most risk are those:

  • Given a high dose vitamin A supplementation every day for months to years
  • Fed liver or other vitamin A rich foods every day for months to years

For those people wanting to put a figure on things, vitamin A toxicity has¬† been experimentally created by feeding 100x the recommended daily amount over a long period of time. For an average sized dog, that means giving 33,000‚Äď700,000 IU a day for months. It's useful to know that a tablespoon of cod liver oil (a rich source of vitamin A for dogs) contains approximately 1,300 IU. A quick sum shows a dog would need around 25 tablespoons a day before they reach toxic levels.¬†

Also vitamin A toxicity doesn't happen overnight (neither does vitamin D toxicity). On average, it takes around three to four years for severe signs of toxicity to develop. But the bad news is that changes to the skeleton are permanent and can't be reversed… so best not risk it. 

Signs and symptoms of vitamin A toxicity

The early signs of vitamin A toxicity include: 

  • Pain
  • Lameness
  • Tooth loss
  • Poor appetite
  • Weight loss¬†
  • Poor skin and a dry coat
  • Stomach upsets
  • A tendency to bleed

These signs get worse over time to cause bone deformities, fused joints, spontaneous fractures, and internal bleeding. 

The final word: Do dogs need vitamin A supplementation?

Nope, skip the vitamin A supplement and spend the money on something tasty instead. The best thing you can do is to make sure your best buddy has a well-balanced, varied diet. Remember, just as treats should be given sparingly, so should vitamin A-rich foods such as liver or eggs. Follow these rules and your dog will hit the sweet spot for healthy levels of vitamin A. 

The article is written by Pippa Elliot, BVMS, MRCVS on November 9 2020. Medically Reviewed by Joanne Istille.

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