You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Vitamin D is extremely important for normal skeletal development and maintenance as it affects the absorption and utilisation of calcium and phosphorus in the body.

Vitamin D is an essential nutrient which can be derived from three sources :

• It  can be manufactured (as cholecalciferol) by the body in the skin from about 10 substances (called pro-vitamins) - providing the animal is exposed to sunlight. 
• It is present in foods derived from  plants (mainly as ergocalciferol - vitamin D₂)
• It is present in foods derived from animals (mainly as cholecalciferol - Vitamin D₃)

Vitamin D is absorbed from the gastrointestinal tract with fats, and it is one of the so-called fat soluble vitamins. If the diet is deficient in fats, or if the animal has a disease which prevents fat absorption (such as liver disease with an absence of bile salts) vitamin D absorption will be decreased. Since 1968 it has been known that these two forms of vitamin D (vitamin D₂ and D₃) have to be converted by the body into active forms - the most important being called 1,25-dehydroxycholecalciferol, and this conversion involves the liver, the intestine and the kidneys. The final stage takes place in the kidney.

In the body most vitamin D is found circulating in the blood where it acts as a hormone, and it:

• Increases the absorption of the minerals calcium and phosphorus from the gastrointestinal tract
• Affects the movement of calcium and phosphorus into, and out of, bone.
• Increases the absorption of calcium, and also affects the absorption of phosphorus, from urine in the kidneys.

Basically, vitamin D acts to increase the amount of calcium and phosphorus in the blood mainly by increasing absorption from the intestines, and decreasing losses in urine.  The production and activity of vitamin D is very closely linked to the activity of another hormone involved in regulating calcium and phosphorus in the body - parathyroid hormone.

Vitamin D is found in many naturally occurring foods , but it is particularly high in fish liver oils, for example blue fin tuna liver oil contains 4 million IU vitamin D/100g, and ordinary cod liver oil contains 10,000 IU vitamin D/100g. Ironically, too much vitamin D can be as harmful as too little vitamin D.....and both poisoning and deficiency disease are seen from time to time in veterinary patients. The accurate diagnosis of vitamin D deficiency, or vitamin D poisoning, can be difficult if there is also an inadequate amount, excessive amounts or an imbalance of calcium and phosphorus in the animals diet. 

Vitamin D Deficiency

In young animals Vitamin D deficiency causes true "rickets" which is a disorder of bone and cartilage due to poor mineralisation with calcium and phosphorus. As a result the bones are weak and bend easily, the chest may not form properly and joints are very swollen. Teeth may not erupt properly. In adults there is demineralisation (called osteomalacia) of bone with pain and fractures occurring. Unfortunately the term "rickets" is still widely used today by some veterinary clinicians to describe various types of developmental skeletal disease in young dogs ...but true rickets is extremely rare in animals fed manufactured "complete" rations, and can only be diagnosed on radiographs and with laboratory tests. Rickets is only likely to be seen in animals fed a poor ration and kept indoors out of sunlight.

Vitamin D Poisoning

Too much vitamin D causes high blood calcium concentrations due to demineralisation of bone and increased absorption from the intestine. This leads to calcium being deposited in soft tissues where it can cause damage particularly in vital organs such as the kidneys, lungs and heart, and at the same time the bones become thin. Other signs include:

• Inappetance
• Weight loss
• Diarhoea
• Dehydration
• Death

Toxic side-effects have been reported to occur in puppies receiving as little as 10,000  IU of vitamin D/kg body weight/day, and in another study in puppies on 60-120 ng vitamin D/kg/day for 4 weeks.

Vitamin D Supplements

In general Vitamin D supplements are not needed by animals unless they are on an inadequate ration and get no exposure to sunlight.

There may be clinical reasons to provide supplementation, for example,  when there is disease of the intestine, liver or  kidneys. Interestingly, vitamin D can be absorbed into the bloodstream from a high vitamin D oil that is rubbed into the skin. 

Updated October 2013