The Importance of Antioxidants in Life
Antioxidants are believed to help alleviate many diseases, from cancer to heart problems. All antioxidants, however, are not created equal and the consumer should understand what these compounds are and what they are capable of doing. Research suggests that certain antioxidants can indeed be helpful.
It's hard to open a magazine about health and nutrition without seeing the word "antioxidants" peppered throughout the articles and ads. People read grandiose claims that these substances can prevent cancer, heart disease, Alzheimer's, Parkinson's, diabetes, and cataracts, not to mention increase longevity and give their skin a youthful glow. Millions of Americans take supplements containing one or more antioxidants every day, sometimes on their doctor's advice.
Yet many individuals are confused. They may buy products that contain antioxidants without fully understanding what these compounds are and what they can and cannot do. Vitamins C and E, beta-carotene and other carotenoids (chemicals that give plants their red, orange, or yellow color), and the mineral selenium are the most-studied antioxidants. Although the nutrients act on cells in similar ways, they are not interchangeable or uniformly beneficial. In fact, they differ from one another considerably, and researchers are just beginning to understand the particular role each one may play in warding off disease. Vitamin E, for example, appears to help prevent heart disease, while the carotenoids may protect the retina of the eye against degeneration.
Free Radical Damage
Their Common Enemy
All cells in the human body use oxygen to break down the carbohydrates, fats, and proteins that give them energy. Just as a car creates exhaust as it burns fuel, metabolically active cells produce byproducts called free radicals. These are oxygen molecules that have lost one electron (a negatively charged particle) and seek to stabilize themselves by stealing an electron from a nearby molecule.
Free radicals are not choosy about where they get their electrons, and they will readily attach to -- and damage -- proteins, fats, and other chemicals in the body. When free radicals attack DNA, a cell's genetic material, they may induce the cell to undergo changes that lead to uncontrolled cell proliferation -- which is cancer.
Fortunately, cells come equipped with a defense system for combating free radicals; they naturally produce a variety of antioxidants, chemicals that limit the activity of free radicals and repair their damage. However, some researchers believe that, as people age, their cells generate fewer of these protective compounds, or the ones they produce become less efficient at doing their job. Theoretically, an age-related increase in uncontrolled free radicals may explain why many diseases become more common as people grow older.
It's no wonder, then, that antioxidants have generated so much publicity. The following is a summary of the published research on antioxidant nutrients to help people separate fact from hype.
Researchers suspect that free radicals may promote some cases of heart disease. When these molecules damage low-density lipoprotein (LDL) cholesterol inside blood vessels, white blood cells flock to the site and consume the LDL in an attempt to repair the injury. White cells swollen with LDL particles become foam cells; these lipid-laden cells are thought to be the beginning of atherosclerotic plaques in the arteries.
A consistent body of research indicates that vitamin E may protect people against heart disease, possibly by preventing free radicals from damaging LDL particles. The bulk of the evidence is from observational investigations -- that is, studies in which the participants reported their intake of E on questionnaires. They were not asked to take the vitamin as part of a clinical trial.
Because the amount of E thought to provide cardiac benefit could not be obtained solely from dietary sources (which include vegetable oils, nuts, and whole grains), most studies have focused on individuals who get their E through supplements. The data generally indicate that taking doses ranging from 100 to 800 IU (International Units) per day may lower the risk of heart disease by 30%-40%.
Americans typically get about 17 IU of vitamin E a day through diet; the current Reference Daily Intake (RDI) is 30 IU. However, some doctors now recommend that their patients -- both healthy individuals and those with coronary artery disease -- take E supplements containing higher doses to prevent or slow heart disease.
Neither the American Heart Association nor professional medical societies endorse E supplements, though, mainly because most of the published research is observational. To date, there have only been two controlled clinical trials evaluating E.
In a Finnish trial of nearly 30,000 male smokers tracked for 5-8 years, the supplements did not appear to reduce the risk of heart disease compared to a placebo, but the dose of E was relatively small -- about 50 IU daily. By contrast, a British study that compared the effects of high doses, 400 or 800 IU, to a placebo among 2,002 men and women with clogged arteries showed a large benefit for E: those taking supplements had a 77% reduced risk of nonfatal heart attack over the next 1-3 years.
There is no consensus about the optimal amount of E people should take. Doses up to 800 IU are considered safe for most individuals. However, people taking warfarin and other anticoagulant medications should check with their doctor before downing E supplements because large doses can amplify the effects of these agents and increase the risk of internal bleeding.
Scientists suspected that vitamin C and beta-carotene would also prevent heart disease, but most of the research to date has not borne this out. Evidence from observational studies, however, suggests that getting carotenoids through food may lower coronary disease risk. Beta-carotene is just one of several hundred carotenoids contained in dark green, yellow, and orange fruits and vegetables.
The strongest evidence that antioxidants may prevent cancer comes from epidemiologic studies that compared people who ate a lot of fruits and vegetables, which are rich in the compounds, to those who didn't. Indeed, hundreds of investigations from around the world have found an association between a plant-rich diet and a lower risk of colon, stomach, lung, breast, and prostate malignancies. The American Cancer Society estimates that about one-third of US cancer deaths are linked to diets low in fruits and vegetables and high in fat.
The best way to get a variety of antioxidants, as well as other beneficial plant chemicals, is to eat an assortment of fruits and vegetables. A good rule of thumb is to consume at least three colors each day because plants with different pigments are thought to contain different antioxidants.
There is some evidence that lycopene, for example, a carotenoid found in tomatoes that makes them red, may lower the risk of prostate cancer. The body best absorbs Lycopene when tomatoes are cooked; tomato paste and sauce are good sources of the substance.
Clinical trials examining the protective effect of individual antioxidant supplements on cancer rates have generally come up short, with the following possible exceptions: Data from the Finnish smokers trial pointed to an association between vitamin E and prostate cancer risk. Among 30,000 male smokers, those who took 50 IU of E daily were 32% less likely to develop prostate cancer, and 41% less likely to die from it, than those who didn't take E supplements.
There is promising preliminary evidence from an observational study and one small clinical trial indicating that the mineral selenium may protect against prostate cancer and possibly lung, colon, and esophageal malignancies. However, experts believe it's still too early to recommend that people take selenium supplements.
Scientists theorize that free radicals play a role in causing cataracts by damaging the proteins in the lens of the eye. There is some evidence that the antioxidant vitamins C, E, and beta-carotene may reduce the risk of developing cataracts.
Meanwhile, the carotenoids appear to be the key antioxidants in preventing macular degeneration, the progressive deterioration of the macula, the central portion of the retina. (For more on macular degeneration, see Harvard Health Letter, August 1998.) In a 1994 study published in the Journal of the American Medical Association, researchers compared the dietary habits of 356 people with macular degeneration to 520 controls who did not have the eye disease. People with the highest reported consumption of carotenoids had a 43% lower risk of macular degeneration than those with the lowest dietary intake. The beneficial effect may have been due to lutein and zeaxanthin, antioxidants found in dark green, leafy vegetables that also exist naturally in the retina.
A study published in the April 27, 1997, issue of the New England Journal of Medicine suggested that vitamin E modestly slowed the progression of mid-stage Alzheimer's disease. However, the dose of E was larger than is commonly recommended for anyone -- 2,000 IU -- and the noted benefits were slight. There is no evidence that E supplements can protect healthy people from developing Alzheimer's disease.
Because, however, there is growing evidence that antioxidants may help thwart some chronic conditions, the Food and Nutrition Board of the National Academy of Sciences plans to issue a report in late fall listing the recommended intakes of these nutrients for healthy people. Until then, people should remember that, with the possible exception of E, the most beneficial mix of these substances isn't available at any pharmacy; for that, you'll have to visit your local produce stand.
Antioxidants at a Glance
||Reference Daily Intake (RDI)
||Citrus fruits, strawberries, tomatoes, cantaloupe, broccoli, asparagus, peppers, spinach, potatoes.
||Consuming more than the RDI in supplement form has not been found to prevent heart disease or cancer.|
||Vegetable oils (soy, corn, olive, cottonseed, safflower, and sunflower), nuts, sunflower seeds, wheat germ.
||Doses of 100-800 IU may lower heart disease risk by 30%-40%.|
||Dark green, yellow, and orange vegetables including spinach, collard greens, broccoli, carrots, peppers, and sweet potatoes; yellow fruits such as apricots and peaches.
||No association between beta-carotene supplementation and heart disease or cancer risk; some evidence that carotenoid-rich foods protect against heart disease and macular degeneration.|
||None available; RDAs are 70 mcg for men and 55 mcg for women.
||Egg yolks, tuna, seafood, chicken, liver, whole grains, and edible plants grown in selenium-rich soil. (Found west of the Mississippi).
||Preliminary evidence suggest that 150-200 mcg may lower prostate cancer risk.|
|Copyright © 1999 Information Access Company. Copyright © 1999 President and Fellows of Harvard College. Harvard Health Letter. Feb. 1999; 24(5)|