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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

Vitamin C and Calcium Oxalate Stones

Cats and dogs are capable of meeting their nutritional needs for Vitamin C through the metabolism of the glucose in their diet or that produced by the liver. However, there is research that suggests the antioxidant properties of Vitamin C supplementation can benefit the management of medical conditions associated with “free radical” formation from oxygen metabolism that can damage normal cells.

Cancer and cancer therapy, dementia, heart disease, and asthma are some examples of conditions that involve oxidative damage. Typically this supplementation poses no major adverse side effects. This may not be true for pets with a predisposition for urinary calcium oxalate crystals and stones, however.

Vitamin C Metabolism in Pets

The normal metabolic breakdown of the amino acid glycine and ascorbic acid (the active ingredient in vitamin C) results in urinary oxalate. Dietary oxalic acid from diets high in vegetables and legumes (beans, soy, etc.) also contributes to urinary oxalate. For most pets this is not a problem. However, in breeds like the Miniature Schnauzer (which accounts for 25 percent of oxalate stones in dogs) and in many cats, this urinary oxalate in a mildly acidic urine results in the formation of calcium oxalate crystals or stones. In fact, calcium oxalate crystals and stones have surpassed the other major stone type, struvite, as the most common urinary stone problem.

Many attribute the change to the popularity of readily available commercial pet diets that became available to manage struvite disease and that contain ingredients that acidify urine. This has not been proven and bladder stone formation is so multifactorial that focus on crystal type and urine pH only fails to address the complexity of the problem. This is why so many pet owners and veterinarians are often disappointed by the failure of specific diets to eliminate the problem. I have lost count of the number of pets from which I have surgically removed stones that were on diets, supplements, or other therapies designed to prevent the stone formation of the type I removed. But I digress.

Vitamin C Supplementation for Pets

Because Vitamin C is not required in the cat or dog diet, not all pet vitamin supplements contain Vitamin C. Owners of pets with medical conditions that benefit from vitamin C often use human supplements. Although the RDA for Vitamin C in humans is 60mg, typical human Vitamin C-only supplements contain 500-1000mg. Part of this is owing to the work of Linus Pauling in the 60s and other subsequent work that suggested that mega-doses of Vitamin C in humans provided many preventative and positive health benefits. Because there is no established recommended dose of vitamin C for pets there are few established therapeutic doses.

Doses of 30mg, 60mg, and 100mg have been proposed by some researchers. Children’s Vitamin C only supplements range in dosages from 25-100mg per serving and are ideal for pets. Ask your veterinarian for the appropriate dose for your pet. For the normal pet the children’s doses and even the mega-doses may not be harmful. But any dose might be a problem for the “oxalate stone formers.”

Unfortunately many pet owners may not know whether their pet is at risk. Certainly Vitamin C should be avoided if a pet has a history of urinary oxalate stone formation. Supplementation should be avoided in breeds with high risk like the Schnauzers, Lhasa Apso, Yorkshire Terrier, Miniature Poodle, Shih Tzu, and Bichon Frise. Urinalysis testing for crystals can help identify other pets that may not be considered high risk. Multiple testing is recommended, especially during supplementation, because the urine concentration of crystals can vary with water consumption and kidney excretion patterns.

Vitamin C is a great therapeutic addition to veterinary treatment. It just may not be right for all pets.

Dr. Ken Tudor

Image: Dog’s life by marie-II / via Flickr

Comments  4

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  • Supplementation
    09/13/2012 11:28am

    Do you recommend Vitamin C supplementation for critters with cancer, dementia, heart disease, and asthma (other than the breeds mentioned)?

    Do you recommend Vitamin C supplementation for healthy critters as long as they're getting a balanced diet?

  • TheOldBroad
    09/13/2012 02:32pm

    Yes I do advise my clients to use vitamin C in those cases. Although the scientific evidence is not as compelling for its use with inflammatory conditions like arthritis, allergies and inflammatory bowel disease, I advise my owners to consider vitamin C in those conditions as well.
    Because cats and dogs make all of the vitamin C they need there is little research that has been done on vitamin C supplementation and potential prevention properties. Much research was done regarding the prevention of hip dysplasia and vitamin C, which suggested little effect and is not not recommended for large dogs at risk of hip dysplasia.
    Having said all that, I do supplement my homemade diets for normal, healthy dogs with vitamin C.
    Dr. T

  • 01/06/2014 05:05pm

    Hi, what kind of diet would you recommend for a Lhasa that has had crystals removed from his bladder already?

  • 09/14/2012 04:34am

    The latest scientific trend in nutrition is individuality. Every dog is different and might have different nutritional needs. For some dogs vitamin C might become "conditionally essential." For some dogs, supplementation might help manage their health issues. For some dogs, supplementing vitamin C is not a good idea. I believe, that individual assessment is a step that should not be skipped when making any nutritional decisions for dogs.

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