The subject of anti-oxidants comes up fairly often on this blog. This is largely because the oxidation and free-radicals are often viewed as fundamentally destructive forces responsible for aging and many types of illness, and preventing the malign effects of these compounds by introducing anti-oxidants in the diet or as supplements is often lauded as a near miraculous way to prevent and treat disease. As is so often the case, however, the truth is far more complexed, nuanced, and riddled with uncertainties. The basic chemistry of what free-radicals and antioxidants are is well-described in this Wikipedia article.
The concept that free-radicals cause damage to cells and tissues and that antioxidants, produced by the body and ingested in foods, can help to mitigate this damage is a simple and relatively sound idea. This can be easily demonstrated in test-tube experiments and laboratory animals. However, extensive research has not generally supported the extension of this idea to the extremes it is often taken to by proponents of alternative diets and supplements who rely on the idea to promote their beliefs. In fact, such research has often found that antioxidants can actually cause harm, such as increasing cancer risk, and interfere with medical treatment, such as chemotherapy for cancer. It turns out, the body sometimes uses oxidation and free-radicals as a tool of the immune system, and suppressing these is not always a good thing even when it can actually be done. Life is always more complicated than our desire for simple answers leads us to imagine.
In terms of pet foods, a variety of antioxidants are used as preservatives and for presumed nutritional benefits. There is certainly evidence to support the use of antioxidants as food preservatives, though not to support the claims alternative medicine advocates often make that “natural” antioxidants, such as Vitamin E, are safer than “artificial preservatives.” The evidence is a lot less clear, however, about the purported health benefits of antioxidants added to food or given as supplements. A living organism eating food with thousands of chemical compounds in it over years is vastly more complex than the simple, in vitro models often used to support claims of health benefits from antioxidants.
Because health effects over long periods of time are difficult to measure, and to associate with particular substances in food, proxy markers of the effects of antioxidants are often used in studies of dietary supplementation. However, it is not clear how reliable such markers are in predicting the benefits to health and longevity that really matter, and studies using proxy markers cannot be viewed as proof that a supplement has a real benefit. Similarly, studies that evaluate supplements by measuring markers of immune function or other proxy values do not necessarily prove these supplements have clinically meaningful benefits.
There is little direct research on antioxidant supplementation, in food or as nutritional supplements, for producing health benefits in dogs and cats. A handful of studies show some potentially promising effects on proxy markers, and a couple seem to show some clinical benefits, but the evidence is scant and weak. The harmful effects of antioxidant supplementation in humans has only emerged with studies of large numbers of individuals over periods of time far longer than typical veterinary trials, so while few specific safety risks are known for common antioxidants, the assumption of safety made for most is not justified.
The bottom line is that most of the claims concerning the benefits of antioxidants are based on theory or indirect and limited evidence. The specific antioxidant given, the form in which it is given, the other components of the diet, the species, health status, and individual makeup of each animal, and many other factors all influence the effects of antioxidants. Whether such effects are strong enough to be clinically significant, and whether they are beneficial or harmful if they do have a real effect, is a complicated question, and simplistic, strong claims are not justified.
Here are previous posts related to this subject:
USDA Confronts Misleading Hype about Antioxidants in Foods
More Evidence Antioxidants May Increase Cancer Risk
Canine Nutrigenomics by Dr. Jean Dodds; Science as Windodressing
Evidence Concerning Vitamin and Mineral Supplements-Safety and Efficacy
SELECT- An Example of Why Clinical Trials Aren’t Just the Icing, They Are the Cake
And here are a few publications discussing the limited evidence regarding antioxidants and pets:
Antioxidants in Veterinary Nutrition. Steven C. Zicker, Karen J. Wedekind, Dennis E. Jewell. Vet Clin Small Anim 36 (2006) 1183–1198.
Acetyl-L-carnitine and alpha-lipoic acid supplementation of aged beagle dogs improves learning in two landmark discrimination tests. Milgram NW, Araujo JA, Hagen TM, Treadwell BV, Ames BN. FASEB J. 2007 Nov;21(13):3756-62.
Chronic antioxidant and mitochondrial cofactor administration improves discrimination learning in aged but not young dogs. Siwak CT, Tapp PD, Head E, Zicker SC, Murphey HL, Muggenburg BA, Ikeda-Douglas CJ, Cotman CW, Milgram NW. Prog Neuropsychopharmacol Biol Psychiatry. 2005 Mar;29(3):461-9.
Learning ability in aged beagle dogs is preserved by behavioral enrichment and dietary fortification: a two-year longitudinal study. Milgram NW, Head E, Zicker SC, Ikeda-Douglas CJ, Murphey H, Muggenburg B, Siwak C, Tapp D, Cotman CW. Neurobiol Aging. 2005 Jan;26(1):77-90.
Influence of dietary antioxidants and fatty acids on neutrophil mediated bacterial killing and gene expression in healthy Beagles. Hall JA, Chinn RM, Vorachek WR, Gorman ME, Greitl JL, Joshi DK, Jewell DE. Vet Immunol Immunopathol. 2011 Feb 15;139(2-4):217-28.
The effect of vitamin C supplementation in healthy dogs on antioxidative capacity and immune parameters. Hesta M, Ottermans C, Krammer-Lukas S, Zentek J, Hellweg P, Buyse J, Janssens GP. J Anim Physiol Anim Nutr (Berl). 2009 Feb;93(1):26-34.
A combination cocktail improves spatial attention in a canine model of human aging and Alzheimer’s disease. Head E, Murphey HL, Dowling AL, McCarty KL, Bethel SR, Nitz JA, Pleiss M, Vanrooyen J, Grossheim M, Smiley JR, Murphy MP, Beckett TL, Pagani D, Bresch F, Hendrix C. J Alzheimers Dis. 2012;32(4):1029-42.
Supplemental vitamin C appears to slow racing greyhounds. Marshall RJ, Scott KC, Hill RC, Lewis DD, Sundstrom D, Jones GL, Harper J. J Nutr. 2002 Jun;132(6 Suppl 2):1616S-21S.
Effect of feeding a weight loss food beyond a caloric restriction period on body composition and resistance to weight gain in dogs. Floerchinger AM, Jackson MI, Jewell DE, MacLeay JM, Paetau-Robinson I, Hahn KA. J Am Vet Med Assoc. 2015 Aug 15;247(4):375-84.
Effect of feeding a weight loss food beyond a caloric restriction period on body composition and resistance to weight gain in cats. Floerchinger AM, Jackson MI, Jewell DE, MacLeay JM, Hahn KA, Paetau-Robinson I. J Am Vet Med Assoc. 2015 Aug 15;247(4):365-74
Thank you. I really do appeciate the work you put into gathering and summarising this sort of information. It is so useful to have something this concise but with further information people can go to for challenging the ‘magical’ supplement proponents.