The truth about antioxidants
OPINION:I’ve taught many students that one of the most dangerous things they do is breathe. An article by Melinda Moyer (The Myth of Antioxidants) in Scientific American (February 2013), however, points out that the “antioxidants” some people take to try to counter any ill-effects of the process may do no good at all.
We inhale air in order to get oxygen to our cells where it is used to oxidise (“burn”) food, thereby releasing energy. Oxidation means removing electrons from food molecules and passing them on to oxygen. Some of the electrons interact with oxygen in a manner that generates free radicals – reactive oxygen species (ROS). ROS are extremely reactive and must be controlled by the cell lest they cause harm by randomly reacting with cellular components. The cell has other chemicals called antioxidants that “mop up” ROS, preventing their damaging effects. Despite this, ROS may cause some low-level damage, including genetic damage by reacting with DNA. ROS levels and cellular damage increase with age.
In the 1950s a theory was proposed by Denham Harman that the main reason we age is because ROS damage accumulates gradually over the years. It was known that exposure to ionising radiation causes ROS production in cells and that diets rich in antioxidants eased the effects of radiation. Harman suggested that, if free radicals speed up ageing then antioxidants should slow it down.
Harman fed antioxidants to mice and reported that they lived longer. In 1969 the first antioxidant enzyme (superoxide dismutase) in the cell was discovered and it seemed that such enzymes evolved to counter free-radical damage. The free-radical theory of ageing took root strongly.
However, other scientists found it very difficult to replicate Harman’s experimental findings. More sophisticated experiments became possible in the 1990s when mice were genetically manipulated to produce more or less of their natural antioxidant enzymes, and consequently live with more or less ROS and correspondingly suffer more or less ROS damage. These experiments repeatedly showed that ROS levels in animals have no effect on how long they lived.
Things got worse for Harman’s theory. Work published in 2010 described experiments on worms genetically modified to produce high levels of free radicals. The worms lived longer than normal ones but, if fed antioxidants, their lifespan fell back to normal.
Other work showed that mice, genetically altered to produce higher levels of another antioxidant enzyme called catalase, live longer. So now we have evidence that free radicals can lengthen lifespan or shorten it, and that antioxidants can lengthen (if we accept Harman’s reports) or shorten lifespan.
The situation is clearly more complex than described by Harman’s original theory. My biochemistry department colleague Prof Jim Heffron, who studies antioxidants, says: “There is a fine balance in cells; low levels of antioxidants lead to oxidative stress and high levels of ROS resulting in chronic inflammation, cancer and cardiovascular diseases. High levels of antioxidants produce low levels of ROS and may lead to low blood pressure and reduced defence against microorganisms and some auto-immune disorders.
“ROS is essential for regulating cell function and antioxidants are necessary to maintain the ‘normal’ level of ROS. So too little ROS and too much is bad for you and the same is true for antioxidants. It is the balance that is important. It is comforting to know that the best way to get the right dose of antioxidants is to consume plenty of fruits and vegetables.”
The principal antioxidant vitamins taken as supplements are vitamin E, Beta-carotene (converted to Vitamin A in body) and vitamin C. In 2007, a review of 68 clinical trials was published in the Journal of the American Medical Association that concluded that antioxidant supplements do not reduce risk of death. When the review concentrated on the more rigorously controlled trials, the data indicated that in some cases antioxidant supplements increased the risk of death by up to 16 percent. I used to take a vitamin supplement but I now propose to stop.
William Reville is an emeritus professor of biochemistry and public awareness of science officer at UCC.