There has been a lot written about antioxidants, in the popular press as well as in the scientific literature. Foods that contain antioxidants are often promoted for possibly advantageous health effects. Most high potassium foods are high in antioxidant values. But how the antioxidant compounds in foods contribute to health remains poorly understood. It may not be at all related to their antioxidant capacity.
Essential to life, free radicals are involved in many chemical cellular processes. Most of these chemical reactions are well controlled, but have a small leakage of free radicals. These leaked free radicals can damage cell structures, such as cell membranes, proteins and DNA. Antioxidants are believed to reduce this damage by intercepting the free radicals before they cause harm.
Because many epidemiological studies have shown that diets high in fruits and vegetables reduce disease and prolong life, researchers speculated that the high content of antioxidants in these foods might be responsible for these favorable effects. So a great many research studies were done to find out if these foods did their magic by fighting free radicals with their antioxidants.
Initial reports seemed to confirm the researchers' suspicions. Bigger clinical trials were then done. The results were mixed. When the clinical trials were grouped together for metanalysis, no advantage to antioxidant supplements could be found even though the foods containing them did help.
A recent metanalysis from March 2012 (1) confirmed these prior studies. The duration of the clinical trials in this analysis averaged only 3 years with the longest being just 12 years. Also the people in the studies were older and most already had disease.
There are several problems with this and other clinical trials. First, to see an effect on a cellular system so critical to life would require a much longer period. Because the handling of oxidation (it provides energy for all our body's functions) is so critical, there is an intricate network of metabolic pathways involved with multiple alternative pathways when one is altered. Allowing oxidation to be quickly altered would not result in a stable species.
Also, because the people in the studies were older and most already had disease, it would be difficult to show an effect from a strategy meant to prevent disease. The design of the studies means the supplements would have to reverse disease, not just slow down (prevent) damage. Most of the damage already present would be very resistant to improvement.
This is too much to expect from an antioxidant, since it works by catching the free radicals in the act – not by undoing damage already done. It is like expecting an anti-missile missile to repair a building destroyed by a missile instead of intercepting the missile before it does the damage.
But how much antioxidant is the right amount and which antioxidants are the most important cannot be known at present. There are several tests to measure the antioxidant value of a food, the most commonly known being the ORAC. The problem is that the tests all measure the antioxidant ability of the food in a test tube. These values are difficult to correlate with antioxidant effectiveness inside the cell.
Different antioxidants act at different locations in the cell. The water soluble antioxidants do not work at the same cellular locations as the fat soluble antioxidants. And there are different locations that each antioxidant works at, since the size and configuration of the antioxidant molecules differ.
So it makes no sense to reduce this complex activity to a single number and claim that the higher the antioxidant number, the better the compound is at fighting free radicals in the body. The USDA has recognized the misleading aspect of antioxidant numbers and has taken down its pages giving the antioxidant values of foods.
This doesn't mean you shouldn't get antioxidants. They help fight free radicals. The best source is the fruits and vegetables themselves. But choosing them just on the basis of antioxidant capacity may not be a good way to choose them.
And as far as supplements, how much and which ones you should use for the best results are not fully known. It probably makes sense to get a water soluble one, such as vitamin C, and a fat soluble one, such as vitamin E.
None of the large clinical trials show any benefit though, and for some supplements such as vitamin A and beta-carotene there even may be some harm. But there are limitations to the trials, so that long term supplementation may turn out to be helpful. But at present, long term supplementation is based on faith rather than evidence. Not enough is known yet about supplement use.
Foods with antioxidants have been shown to improve health though. Whether the antioxidants help by fighting free radicals directly or in some other fashion is yet to be determined. Vitamins such as A, C, and E are important, but other antioxidants, such as the polyphenols and sulforaphanes, may be equally or more important. They are found in abundance in the high potassium foods. By choosing a variety of fruits and vegetables, you should get a good balance of the compounds needed for good health.
1. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Cochrane Database Syst Rev. 2012 Mar 14;(3):CD007176. doi: 10.1002/14651858.CD007176.pub2. (abstract)