I have written often about the popular notion that vitamins, dietary supplements, herbs, and other things which can be identified as “antioxidants” based on in vitro laboratory studies must automatically be good for our pets. This sort of simplistic reasoning is rarely justified in biology, and there is ample reason to doubt the hype that free radicals and oxidation are always bad and that antioxidants are always good.
In terms of specific supplements, there is large-scale clinical research evidence showing that even essential antioxidant vitamins, such as Vitamin E and Vitamin C, are not only not beneficial in excess but can increase the risk of cancer, heart disease, and other serious ailments and interfere with some medical treatments, such as chemotherapy. The common alternative medicine mantra that such supplements prevent cancer and other diseases, or help “boosting the immune system” in patients with serious illness, is inaccurate and potentially dangerous.
A recent review in Scientific American magazine does a nice job of summarizing the growing research evidence of the last decade showing that the theory of oxidative damage as a primary driver of aging and disease is deeply flawed, and that antioxidant supplements (as opposed to fruits and vegetables in their whole form) can often do more harm than good.
Moyer, M.W. The Myth of antioxidants: The hallowed notion that oxidative damage causes aging and that vitamins might preserve our youth is now in doubt. Scientific American. February, 2013. 64-67.
Though intended for the general public, and thus not a systematic review, this article does a good job of identifying several areas in which theories about the role of free radicals and antioxidant appears to have failed to hold up. The first is the notion that oxidative damage is a key component of aging and that reducing oxidation and supplementing antioxidant should lengthen life. The article cites a number of laboratory studies in invertebrates and mammals showing that animals genetically engineered to have reduced activity of natural antioxidant systems do, in fact, have increases in free radicals. However, these animals live longer than animals with better antioxidant function! And supplementing these subjects with antioxidants reduces their life expectancy to that of animals with normal anti-oxidative functions.
The author also reviews some of the early studies produced by the first proponents of the oxidative damage theory, and points out that attempts to replicate these have usually failed. There are even studies showing that application of an herbicide which increases free radical production can increase lifespan in some laboratory species, and again antioxidant supplementation eliminates this increase in life expectancy.
More nuanced theories suggest that, as is usually the case, the truth is complex and depends on context and many factors. There are undoubtedly situations in which free radicals and oxidative damage are harmful. But there may also be situations in which increases in free radicals may be beneficial. Such oxidative compounds increase as a consequence of exercise, for example, and antioxidant vitamins appear in some research to attenuate the benefits of exercise. And oxidative damage is one mechanism by which the immune system and some medicines destroy infectious organisms and cancer cells. Antioxidant that interfere with these beneficial activities can actually cause harm.
There is much work yet to be done to refine our understanding of the positive and negative effects of oxidation and antioxidants, but one thing is clear. Simplistic notions of antioxidant supplements as automatically beneficial are nonsense and not a sound basis on which to recommend supplements, herbal remedies, or other preventative or therapeutic products.