Mountains of research suggest dietary antioxidants have health benefits for pets and people. Conversely, some large clinical trials with antioxidant supplements did not detect benefits with the formulations tested.
The following reports focus on dietary antioxidant health benefits for pets. However, when considering antioxidants, it should be remembered that more is not necessarily better. Oxidative stress is involved in the pathogenesis of many diseases, but its complete annihilation may lead to negative clinical effects in our pets. In other words, excess supplementation may be harmful (see "Antioxidants: can they be harmful?").
Khoo showed that higher levels of dietary antioxidants or nutrients such as fish oil may be indicated for decreasing inflammation in cats (C. Khoo, et al ., 2007). He studied the effect of antioxidants on immune and inflammatory parameters in cats with inflammatory bowel disease (IBD).
Ten healthy and 10 IBD cats were fed wet food with low antioxidants (Ctrl) and test food (Aox) with added vitamin E, vitamin C and beta carotene for four weeks each in a randomized cross-over design. Both foods were completely balanced for adult cats. Serum vitamins E and C, DNA damage (comet assay), lymphocyte subsets and proliferation were measured at weeks four and eight.
Results showed that serum vitamins E and C were significantly increased in healthy and IBD cats on the Aox vs. Ctrl food. The Aox food tended to reduce the lymphocyte proliferation activity in both groups of cats. Results showed that IBD cats have a dysregulated and hyper-inflammatory immune response compared to healthy cats.
Milgram's work suggests that long-term maintenance on alpha-lipoic acid (LA) and acetyl-L-carnitine (ALC) help improve memory in older dogs (N.W. Milgram, 2007). These antioxidants apparently attenuate age-associated cognitive decline by slowing the rate of mitochondrial decay and cellular aging.
Beagle dogs between 7.6 and 8.8 years of age administered a twice daily supplement of LA and ALC over two months made significantly fewer errors in reaching the learning criterion on two landmark discrimination tasks compared to controls administered a methylcellulose placebo.
The improved performance on the landmark task of dogs supplemented with LA and ALC provides evidence of the effectiveness of this supplement in improving discrimination and allocentric spatial learning.
Park demonstrated that the dietary antioxidant bixin heightened cell-mediated and humoral immune response in cats (J.S. Park, et al ., 2007). Specifically, it inhibited DNA oxidative damage and inflammation in cats.
Bixin is an antioxidant compound extracted from the annatto seed. Female domestic cats were fed bixin daily for 16 weeks. Blood was sampled in weeks zero, six, 12 and 16. All cats were then vaccinated with an attenuated polyvalent vaccine in weeks 12 and 14. Skin hypersensitivity response (DTH) to saline, concanavalin A, the vaccine and histamine was assessed in weeks 12 and 16.
Cats fed 5 mg bixin generally showed the highest immune stimulatory and antioxidative action. In this treatment, bixin enhanced lymphoblastogenic response, populations of T helper and T cytotoxic cells, NK cytotoxicity and IgG production.
Bixin also inhibited DNA damage. At 10 mg, bixin stimulated DTH response to con A, percent of total T and T cytotoxic cells, and IgG production; however, it inhibited mitogen-induced lymphocyte proliferation. All doses of bixin reduced skin response to histamine and CD18 subpopulations.
The evidence for supplementing petfoods for health benefits is good but mixed. Research focused on dogs and cats is really just in its infancy. It's a highly complex field that will continue to impact pet food formulation.
Pets (and people) have evolved and adapted to symbiotically live with persistent, low-grade, oxidative stress. To some extent, this oxidative stress may actually benefit the physiological functioning of cells. Thus, using antioxidants to reduce oxidative stress to levels below some physiological threshold may trigger intracellular signaling pathways that damage cellular machinery.
Laviano contends this should be a critical area for future investigations (Laviano, 2007). He complains about the "media-driven assumption that oxidative stress is always harmful and antioxidant supplements are nearly always beneficial." He points out that when considering antioxidant therapy, more is not necessarily better.
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