When it comes to keeping our dogs healthy, we’re often tempted to focus on the basics – exercise, vaccinations, flea and worming preventatives and regular check-ups. However, one vital component which is sometimes overlooked is nutrition. Perhaps this is because we have become over reliant on perfectly designed and formulated diets, which we can buy straight off the shelves. Needless to say, nutrition is also incredibly important in our furry friends.  

There are many components of a balanced diet, falling broadly into the categories of macro- and micro- nutrients. And amongst the micronutrients are the vitamins, and amongst them is Vitamin D.  Often referred to as the “sunshine vitamin”, its production in the body is either stimulation by exposure to UVB rays, or it is ingested via certain foods. So how important is this vitamin in canine health? This article aims to answer this question.  

What is vitamin D and where does it come from? 

Potentially a good place to start is to answer this question, “what is vitamin D?”. Vitamin D, like all other vitamins, is a compound essential for life. So, “where does it come from?”. In answering this, you need to consider that there are two different types of vitamin D: vitamin D3, which is produced within the body, and vitamin D2, which is found in plants. Generally speaking, the term vitamin D refers to both of these.  

Vitamin D3, which is also known as cholecalciferol, is produced in the skin in response to sun exposure, or more specifically UVB rays. Providing that sun exposure is adequate, most mammals are theoretically capable of producing enough vitamin D in this manner to surpass daily requirements. However, research suggests that in dogs this rarely occurs in nature, due to differences in the levels of the precursor chemicals within the skin – and of course the thick UV-blocking hair coat on most breeds!  

As a result, the majority of their vitamin D will be coming from their diet. Most complete and balanced commercial dog foods are supplemented with vitamin D due to the risk of deficiency in domesticated pets. Therefore, prescribed supplementation via e.g. powders and capsules, are often not necessary. Foods rich in vitamin include liver, oily fish like salmon, egg yolks, and dairy products. Once ingested, vitamin D undergoes two hydroxylation steps – first in the liver and then in the kidneys – to become its active form, calcitriol. 

So, if the majority of commercial diets are adequately supplemented, which dogs are at risk of deficiency? 

In the majority of cases, the only dogs at risk of vitamin D deficiency are those on unbalanced raw and/or homemade diets. Without professional nutritional guidance, it can be difficult to accurately balance vitamin D in home-prepared meals. 

Apart from inadequate dietary intake, there are certain diseases which can interfere with vitamin D absorption/production which include: 

  • Malabsorption disorders including inflammatory bowel disease and exocrine pancreatic insufficiency. These problems reduce the amount of vitamin D available for absorption.  
  • Chronic kidney disease. As the kidney is responsible for activating vitamin D, diseases which affect the kidney can impaired their ability to convert vitamin D into its active form. Monitoring and managing vitamin D status in dogs with chronic kidney disease can be a key part of treatment – although it is complicated by over-activation of the parathyroid glands. 
  • Liver disease. Similarly to kidney disease, liver disease can affect the metabolism of vitamin D. 

Function of vitamin D 

Vitamin D is required for numerous processes in the body. Most notably, it is required for: 

  1. Bone health – vitamin D facilitates the absorption of calcium and phosphorus into the body – two minerals which are essential for strong bones and teeth. Without adequate vitamin D, puppies, like human babies, can develop a condition called rickets. In adult dogs, like humans, the equivalent condition is called osteomalacia. Although, due to modern day fortification and supplementation habits these conditions are rare, they can still occur in those with poor diets e.g. if pets are just fed table scraps. 
  1. Immune system regulation – vitamin D is required for a healthy immune system. It helps regulate immune responses, enhancing the body’s ability to fight off infections while reducing inappropriate inflammation. 
  1. Cardiovascular health – vitamin D is required for both heart and vascular health. 
  1. Neurological function – although less well established, vitamin D may play a role in supporting cognitive function and maintaining neurological health, especially in older dogs. 

Vitamin D deficiency 

Vitamin D deficiency in dogs can have significant health consequences, particularly in growing puppies and senior pets. Common signs include: 

  • Skeletal deformities in puppies (e.g., bowing of the limbs, joint pain). 
  • Muscle weakness or lethargy. 
  • Poor growth or weight gain. 
  • Bone pain or fractures. 
  • Delayed tooth eruption. 
  • Seizures or tremors in severe cases due to low calcium (hypocalcaemia). 

Vitamin D excess (toxicity) 

While deficiency is a concern, excessive intake of vitamin D can be equally dangerous. Excess vitamin D can be caused by: 

  • Over-supplementation. 
  • Ingestion of vitamin D rodenticides (e.g. some types of rat poison). 

Unlike water-soluble vitamins, excess vitamin D is stored in the body’s fat tissues and liver, making toxicity more likely if consumed in large quantities. Vitamin D toxicity (hypervitaminosis D) can lead to: 

  • Hypercalcaemia (elevated calcium levels). This can cause serious damage to the kidneys, heart, and other organs. 
  • Vomiting and diarrhoea. 
  • Loss of appetite. 
  • Increased thirst and urination. 
  • Muscle tremors or seizures. 
  • Kidney failure in severe cases. 

The role of the vet 

In cases where vitamin D absorption or production may be affected, such as those discussed above including inflammatory bowel disease, your vet will provide individualised care to minimise risks of deficiency. Additionally, in other high risk patients, including those receiving home made diets, your vet can identify and manage vitamin D imbalances. They are trained to interpret clinical signs, assess dietary risks, and recommend appropriate diagnostic tests, such as specific blood tests. In cases of deficiency, your vet may prescribe a carefully calculated supplement, often alongside adjustments to your dog’s diet. For dogs with toxicity or underlying conditions like kidney or liver disease, treatment becomes more complex and may involve hospitalisation and/or long-term monitoring. Working closely with your vet not only helps catch problems early but also ensures that any interventions are tailored to your dog’s individual needs. 

How to ensure correct Vitamin D levels in your pet 

  1. Feed a balanced diet – choose commercial dog foods labelled “complete and balanced”. These are formulated to provide adequate vitamin D for most healthy dogs. 
  1. Work with a veterinary nutritionist if feeding home prepared or raw diets – consult a veterinary nutritionist will ensure proper vitamin D levels are being provided to your pet. Balancing a homemade diet requires precise formulation. 
  1. Avoid unsupervised supplementation – never give your dog vitamin D supplements, especially those intended for humans, as dosages and requirements can be vastly different and may cause toxicity. Always consult your vet before introducing any supplement. 
  1. Be aware of rodenticide / rat poison risks – farm dogs are amongst those who are at higher risk of rat poison exposure. If exposure or ingestion does occur, you need to contact your vet as soon as possible because this is a medical emergency. 
  1. Routine health checks – regular veterinary check-ups, which can include blood work, can help detect vitamin D imbalances before clinical signs develop. This is especially important for dogs with chronic health conditions. 

Conclusion 

Vitamin D is a critical nutrient for dogs, with essential roles in bone health and immune function. It is incredibly likely that domesticated dogs do not receive adequate amounts of vitamin D from the sun due to being overly furry or spending a lot of time indoors. Therefore, dietary sources are essential in reducing risks of deficiency.  

Too little vitamin D can lead to bone deformities, poor growth, and immune dysfunction, while too much can also cause problems such as toxicity and kidney damage. Given the fine line between adequate and harmful levels, the safest approach is to feed a high-quality, balanced/formulated diet. 

If you suspect your dog may have a vitamin D imbalance – whether due to health concerns or dietary practices – veterinary advice should be sought. Through proper nutrition and individualised care, adequate vitamin D levels is easily achievable.  

Further reading: 

Vitamin D for Dogs – Purina

Sjaastad OV, Sand O & Hove K. 2010. Physiology of Domestic Animals. 2nd edition. Oslo: Scandinavian Veterinary Press. 804 pp. 

Are home-prepared diets for dogs safe?