Vitamin C and Calcium Oxalate Stones

By Ken Tudor, DVM on Sep. 13, 2012

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.