101/Antioxidants The Practical Issues

By Robert DiSilvestro, PhD

In the early 1970s, I asked my introductory biochemistry professor about vitamin E. He answered by asking, “How important can an antioxidant be?” The implication was that antioxidants do something, but there isn’t a practical reason to think much about them. As it turns out, there may be no aspect of nutrition with a wider range of influence on health than antioxidants. Although the degree to which antioxidants can remedy existing symptoms of various health problems is still being investigated, eating antioxidants is likely to have a positive effect on your health.

To understand antioxidants, you first must understand free radicals. Free radicals are reactive molecules with an unpaired electron, and they show up in our bodies as a result of various biological and environmental processes. The body makes free radicals to kill bacteria, and they are a byproduct of some body reactions. They also enter the body due to outside toxins such as exposure to cigarette smoke and air pollutants. Free radicals look for a match for their unpaired electrons, and they complete this partnership by taking electrons from other molecules. A molecule that loses an electron can break apart and turn into a new free radical. This starts a chain reaction of electron thefts and broken molecules until something stops the process, such as the meeting of two free radicals. These damaging interactions are called oxidant reactions. Antioxidants can prevent free-radical–induced oxidant damage. This can work in various ways, so we need a variety of antioxidants.

Vitamin E was the first nutrient to receive attention as an antioxidant. Vitamin E is a chain-breaking anti-oxidant in that it stops the chain reactions described above. Vitamin E does this for chain reactions involving lipids (fats and fat-like molecules) such as those in cell membranes and LDL (the blood transporter of the so- called bad cholesterol). This can be relevant to many health issues, but vitamin E’s impact on cardiovascular disease is especially interesting. One reason is that oxidant damage to LDL initiates atherosclerosis (hardening of the arteries). In one study, the response was greater with increased vitamin E doses, but some effect was seen with as little as 25 IU. In contrast, another study suggested at least 400 IU was necessary to cause an effect. The latter intake typically requires supplements. Natural vitamin E supplements (d-alpha-tocopherol) are more potent in the body than synthetic (dl-alpha-tocopherol). In addition, some evidence suggests supplements containing a family of vitamin E compounds are more potent than d-alpha-tocopherol on its own. The best food sources of vitamin E are vegetable oils and foods high in those oils, such as almonds and wheat germ.

Vitamin E supplementation has been criticized by some recent studies, which suggest daily intake of 400 IU of vitamin E can actually be harmful. This conclusion is still speculative, though, since the effects seem small and the results cannot be fully interpreted until more statistical analysis is conducted. Also, the negative findings of one of the studies were attributable to more variables than just vitamin E intake.

Another disappointing outcome of these recent studies is that vitamin E supplements did not seem to prevent cardiovascular disease. In considering these findings, though, we need to take into account three important points: The study participants were in poor health before the studies began and may have been beyond the point at which vitamin E would have helped; the subjects were very diverse, especially in terms of drugs and other therapy being used, clouding the differences between treatment groups; and vitamin E may work best in cooperation with other antioxidants.

One potential partner is vitamin C, which in test tubes has the same chain-breaking antioxidant actions as vitamin E. However, unlike vitamin E, which works with lipids, vitamin C works with molecules that dissolve in water. In other test tube experiments, vitamin C can recycle vitamin E to make its antioxidant functions more potent, and studies are starting to support the efficacy of vitamins C and E in combination. The vitamin C intake needed for optimal antioxidant action may exceed the recommended dietary allowance (RDA). You can easily get two to four times the RDA of vitamin C by consuming citrus fruits, their juices, and certain other fruits and vegetables, such as berries, melons, asparagus, leafy green vegetables, tomatoes, or red peppers.

Selenium is part of antioxidant enzymes (protein catalysts that accelerate specific reactions). A selenium enzyme family eliminates hydrogen peroxide, which is easily converted into free radicals in the body. The selenium provided by the typical American diet is plenty to ensure high blood levels of this enzyme family, but optimal tissue levels might need additional intake. Some research suggests that increasing daily selenium intake to 200 mcg, either through supplementation or by eating Brazil nuts, which are high in selenium, may lower cancer risk, but more research is needed to establish whether this effect is significant and whether it is caused by selenium’s antioxidant functions.

Three other minerals—zinc, magnesium, and copper—also have antioxidant functions. Zinc and magnesium can stabilize cell membranes, which makes them less apt to make free radicals. In addition, these minerals suppress production of molecules that signal cells to make radicals. Also, copper and zinc inhibit stored body iron from making free radicals. Many people consume less than the RDA of zinc, magnesium, and copper. Moreover, many conditions, such as rheumatoid arthritis and diabetes, raise needs for these minerals. Although some people can eat adequate levels of these minerals, others are likely to be helped by supplements. This issue can get complicated and should be discussed with a dietitian.

Besides essential nutrients such as vitamins and minerals, plant foods and herbs contain compounds called phytochemicals. They are not required for basic body function but can have health-promoting properties. In other words, you can live without them, but you may live longer and better with them. One example is flavonoids, which are found in many plants, although the types and amounts vary with different sources. Green tea, soy protein, and certain grape products, including red wine, have especially high amounts of flavonoids. Although it’s well known that green tea flavonoids are good antioxidants and soy products promote health, many people may not realize that soy’s value is derived in part from its antioxidant flavonoids (a particular kind called isoflavones). Current research makes it very likely that green tea and soy intake provides benefits relevant to a variety of health issues. However, how much you should consume and how much benefit you can expect are still hazy issues. Green tea and soy protein flavonoids can be found in supplemental form, although studies suggest that these flavonoids might work best when consumed in whole-food products.

Food oils do not usually contain phytochemicals, but an exception is olive oil, which contains chemical relatives of flavonoids. High intake of supplement versions of these flavonoids may help prevent or treat rheumatoid arthritis, though the research is still in the early stages. Even so, using olive oil instead of other oils can provide an antioxidant boost.

Another class of antioxidant phytochemicals is the carotenoids, which are especially good at getting rid of one particular oxidant. One of the carotenoids, beta-carotene, is found in carrots and other orange-colored fruits and vegetables. Beta-carotene is a unique molecule in nutrition. About 15 percent of it is converted to vitamin A, and the rest acts as a phytochemical. One study of smokers suggested that high doses of supplemental beta-carotene could be dangerous. Eating carrots and other food sources of beta-carotene is usually safe, though in fair-skinned people, high dietary intake of beta-carotene can cause the skin to temporarily appear yellowish.

Numerous other carotenoids are not converted to vitamin A; they are strictly phytochemicals. Two of these are lycopene (found in tomato products) and lutein (found primarily in leafy green vegetables and, to some extent, in fruits such as oranges). Lycopene seems to be especially active in the prostate and liver, and lutein has high activity in the eyes. Once again, it’s hard to relate an exact intake of these compounds to an exact reduction in disease risk. Even so, it would seem to be a good idea to eat the richest sources of these carotenoids regularly. Along these lines, Dr. Edward Giovannucci, a nutrition professor at Harvard, has written: “Overall, when you look at the limited data that exist on diet and prostate cancer, the consistency of the indication of a potential benefit of lycopene is remarkable.”

Interestingly, in a rat study from our laboratory, tomato extract produced a better liver antioxidant effect than did synthetic isolated lycopene. Still, this difference may not hold for all future studies.

Be careful when considering versions of well-known vitamin supplements that advertise added lycopene or lutein. Typically, the amount added is too small to have any effect. For example, one product has a few hundred micrograms of lycopene, whereas a can of tomato juice can have 28 milligrams (28,000 micrograms).

Many other antioxidants exist, but the ones discussed here have the most scientific evidence in their favor. Of course, more research is needed to further clarify the effects and usefulness of all these antioxidants, but it is clear that they have actions that should help prevent or treat many health problems.