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Pet vet: Understanding the importance of vitamin D for pets

Vitamin D has been emerging as a very interesting substance. In some respects it acts as a hormone, while in other aspects as a vitamin. Its wide range of actions in the body underscore both the importance of vitamin D for health and the complexity of its role in the body. Vitamin D in this article refers generically to vitamin D and its metabolites in the body.

Vitamin D from cod liver oil was first shown to prevent rickets in dogs in 1914. Rickets is a disease of the growing bone in young animals. Following this discovery, the role of vitamin D in absorption of calcium and phosphorous from the small intestine was determined. Additional work showed that vitamin D also played an important role in maintaining blood calcium levels by regulating mobilization of calcium from bone.

Over time it has become increasingly clear that vitamin D has an essential role in maintaining health beyond its function for healthy bones and calcium metabolism. It is estimated that vitamin D controls up to 5 percent of the human genome. One study of white blood cells showed that nearly 300 genes were influenced by vitamin D. This effect on the genes results in a wide-ranging impact on cell and organ function. For example, vitamin D has an effect on insulin production, inflammation reduction, and heart and immune function.

In humans, sunlight exposure is a primary source of vitamin D. This is not the case for dogs and cats. They are not able to produce significant vitamin D through exposure to sunlight. Vitamin D must be included in their diet. Since vitamin D is a fat-soluble (dissolves in fat instead of water) vitamin, it is stored in the fat tissue of the body.

Before commercial dog foods, dogs obtained their vitamin D from eating the fat of their prey. Commercial pet foods have added vitamin D in amounts that are sufficient to prevent rickets. However, these amounts do not appear to be sufficient to meet all the vitamin D needs. This is complicated by variation in the ability to absorb vitamin D from the diet. Once in the body, vitamin D must undergo processing in the liver and then the kidney to become the active form.

A study published in 2015 found a wide variation in the vitamin D levels in the blood of dogs. Interestingly, although the study groups were small, there was a significant difference in vitamin D levels between breeds. For example, the German Shepherds had significantly higher levels than Golden Retrievers. The authors note implies breed differences in ability to absorb vitamin D from the intestines. Another interesting observation was a difference in vitamin D levels between male and female dogs and if they were spayed or neutered, implying some sex hormone influence on vitamin D absorption.

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These findings are important because there has been an association between low vitamin D levels and cancers including lymphoma and hemangiosarcoma, and health problems including infections, inflammatory bowel disease, and heart and kidney disease in dogs. Another study from 2014 determined that the vitamin D blood level needed to prevent abnormal bone development (20-25 ng/ml) was not sufficient to prevent or reduce risk of other diseases. As a result, experts now recommend levels of 100-120 ng/ml. Based on this new recommended value, 85 percent of dogs in the 2015 study were in the insufficient range.

Our own observations are consistent with these studies. The majority (more than 95 percent) of our tested patients fall into the insufficient range. These are patients with a variety of problems ranging from osteoarthritis to cancer. One observation has been the overall improvement in joint comfort once the vitamin D blood levels reach the sufficient range. Many of these patients become more active and initiate more play behaviors.

Since vitamin D is fat soluble and stored in the body, there is concern for toxicity when over-supplemented. Therefore, it is always best to test blood levels before starting vitamin D supplements and to periodically retest because of the variability of absorption.

Contact your veterinarian if you have questions about vitamin D or would like to have vitamin D testing performed.

Ron Carsten was one of the first veterinarians in Colorado to use the integrative approach, has lectured widely to veterinarians and has been a pioneer in the therapeutic use of food concentrates to manage clinical problems. He practices integrative veterinary medicine in Glenwood Springs.