Supplementation with antioxidants has been a well-loved and popular strategy for people and pets for decades, on the assumption that aging is associated with oxidative stress, and that antioxidants can slow some of these aging changes. As a practitioner who has seen clear turn-arounds in the quality of life of geriatric pets when I first began using antioxidants 18 or 19 years ago, I believe there is a shortage in some patients. I believe that antioxidants are clinically useful in old dogs and cats with inflammatory and degenerative conditions.
However, some naturopaths and veterinarians have recommended more antioxidants in healthy middle aged people and pets, presumably to slow aging changes before they become clinically obvious. I was one of them for a few years. But recent large clinical trials in people have suggested that taking extra antioxidants (mostly vitamins A,E,C and selenium) actually *increase* all cause mortality in people taking them for wellness and disease prevention (Bjelakovic 2007). It would appear that Vitamin C and selenium appear safest but that vitamins A and E are associated with problems. Of course, this paper does not prove a temporal relationship, explore whether different doses pose different risks, discuss the various forms of vitamins that might be involved, or really explore cause-specific mortality .... but still. It's a worry.
Now a study (Moyer 2009) shows that canine red blood cell concentrations of a major intracellular antioxidant (glutathione) are not different between young and old dogs. Similarly, plasma cysteine, an extracellular antioxidant, did not differ between young and old dogs. In well dogs. Granted, these were dogsowned by students and employees at the University of Wisconsin's veterinary school, and the dogs were fed a variety of diets. It might be possible to find a subgroup of dogs, eating particular types of diets, who did show evidence of antioxidant depletion. But the point is that this is a real world situation - most healthy old dogs are not antioxidant depleted. Oh, and many food companies are increasing antioxidant levels in their senior diets, which might not be a good thing.
As it turns out, 19 of the 35 older dogs were being fed diets that are supplemented in vitamin E, and 10 were being fed diets supplemented with Vitamin C. Calculations showed that although there was no real difference in vitamin C concentrations being fed to young and old dogs, the young dogs were recieving significantly more Vitamin E. These levels really don't approach those that we would use therapeutically in a nutraceutical supplement, however.
Now on the other hand, the same group from University of Wisconsin measured red blood cell glutathione, and plasma cysteine and Vitamin C in ill dogs (Viviano, 2008). Clinically ill, client-owned dogs and cats were compared to well dogs and cats. In this study, significantly lower erythrocyte glutathione concentrations were found in ill dogs compared with controls, and glutathione depletion correlated with illness severity as well as mortality. Interestingly, cats had higher ascorbate concentrations when ill compared with controls. So this looks like ill dogs are antioxidant depleted and that low glutathione levels lend a poor prognosis. In cats, the elevation in vitamin C is a strange finding and yet another example of why cats really cannot be treated like dogs or people.
So here's the take home. Don't supplement vitamins A,C,E, or selenium to healthy pets who are eating balanced diets. If you are feeding homemade diets, on the other hand, make sure that the diet has been computer balanced to provide at least the daily requirements of Vitamins A,E and selenium. And if a pet has a chronic inflammatory problem (like arthritis, immune-mediated diseases, cancer, kidney disease...) or undergoes a number of days of hospitalization, supplementation makes sense.
Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Mortality in randomized trials of antioxidant supplements for primary and secondary prevention: systematic review and meta-analysis.JAMA. 2007 Feb 28;297(8):842-57.
Moyer KL, Trepanier LA. Erythrocyte glutathione and plasma cysteine concentrations in young vs old dogs. J AM Vet Med Assoc 2009;234:95-99
Viviano KR, Lavergne SN, Goodman L, Vanderwielen B, Grundahl L, Padilla M, Trepanier LA. Glutathione, Cysteine, and Ascorbate Concentrations in Clinically Ill Dogs and Cats. J Vet Intern Med. 2008 Dec 16.