Vitamin C Supplementation

The popularity of vitamin-mineral supplementation in the human health field, and in particular the prevalence of vitamin C supplementation, is unquestionable. Despite the fact that dogs and cats manufacture vitamin C on their own in the liver, the need for dietary vitamin C in pets continues to be discussed by both pet owners and veterinarians.

Vitamin C, or ascorbic acid, is required for the formation of collagen. Collagen is a primary constituent of teeth, bone, and connective tissue, and is produced by bone-forming cells during skeletal growth and development. Ascorbic acid is produced in the liver from two sugars, either glucose or galactose. Unless inadequate amounts are synthesized by the liver or there is an unusually high metabolic requirement, supplementation of the diet with ascorbic acid is unnecessary.

Ascorbic acid may occasionally be included in a petfood because of its ability to act as a natural antioxidant. However, since it is water soluble and not easily combined with the fats in petfood, its effectiveness as an antioxidant is limited. However, when combined with other antioxidants such as vitamin E and butylated hydroxytoluene, ascorbic acid has been shown to work cooperatively with these antioxidants.

Whether or not supplementation of a canine or feline diet with vitamin C is necessary, particularly during times of increased demand, remains controversial. Vitamin C requirements do increase with stress and many pet owners, breeders, and veterinarians believe that stressed animals may be unable to manufacturer sufficient ascorbic acid themselves to deal with stressful situations. This belief was most likely based on a study involving human athletes which indicated that ascorbate levels in the blood were found to decline during stress. However, several subsequent controlled studies with human athletes showed vitamin C supplementation to have no beneficial effects on work capacity. No studies at present support the need for vitamin C supplementation for working and stressed dogs.

Further support for vitamin C supplementation arose from a published report that suggested hypertrophic osteodystrophy (HOD), a bone disease of young dogs, might be due to a deficiency of vitamin C within the body, based on a report that found similarities between bone abnormalities found in HOD and scurvy (vitamin C deficiency) in humans. Later evidence showed that the two conditions were not the same disorder and controlled studies on the efficacy of supplementing with vitamin C in the treatment of HOD failed to support the concept that low levels of ascorbic acid were a cause of HOD.

The association between vitamin C and skeletal disorders in general persisted, and eventually included several other developmental bone disorders, including osteochondritis dessicans (OCD) and canine hip dysplasia (CHD). Growing dogs were supplemented with ascorbic acid in the hope that these diseases might be prevented. However, there is no evidence to support the preventive efficacy of such supplementation.

In humans, the general belief is that vitamin C is at best beneficial and at worst, harmless. Ascorbic acid supplementation in dogs and cats, on the other hand, may be detrimental under certain circumstances since any excess ascorbic acid is excreted as oxalate in the urine. Elevated concentrations of urine oxalate may play a role in the formation of calcium oxalate crystals or stones in the urinary tract. For this reason, avoidance of ascorbic acid supplementation is currently recommended for the prevention of calcium oxalate urinary crystals or bladder stones.