I used to have a rather narrow view of fruits and vegetables: they were just a way to take in fiber, vitamins, and minerals while avoiding fat and calories. But I now know that these foods are more than just filler laced with the relatively narrow list of natural chemicals we know as nutrients.
My awakening came when researchers began publishing studies about antioxidants; these nutrients protect human cells from damage caused by highly reactive and destructive free radicals. Our bodies manufacture free radicals in the process of converting food into energy. But unchecked, they can destroy cell membranes, convert fats in the blood into artery-clogging plaque, and cause genes to mutate, perhaps initiating cancer.
I learned that scientists were most familiar with three antioxidants - vitamins C and E, and beta carotene, but that these were only the tip of the iceberg. There are hundreds of other disease-fighting phytochemicals (plant-based chemicals) such as beta-carotene found in plants.
One group, the carotenoids, contains more than 500 different orange-yellow plant pigments. The best known is beta-carotene, which is converted to vitamin A, a well-known and long-studied essential vitamin. But carotenoids do more than make plants look pretty. Residing in tiny droplets of fat in plant cells, they assist in photosynthesis by absorbing certain wavelengths of sunlight not absorbed by green chlorophyll; in effect, they help capture the sun's energy while preventing cells from being destroyed by excess light. Carotenoids also act as antioxidants.
The primary function of many phytochemicals is to protect plant cells from wind, rain, excess sunlight, invading organisms, and - perhaps most important-from cellular replication errors. When we eat fruits and vegetables containing these phytochemicals, our cells are protected in ways that we are only now beginning to understand.
Not surprisingly, the phytochemical research that has received the most attention is in the area of cancer prevention. Cancer often develops through many stages over many decades, and researchers have begun to pinpoint how phytochemicals can step in along the way and block the disease's progress. For example, they can prevent cells' exposure to carcinogens (cancer-causing substances). They can also come to the rescue later, when cells with abnormal genes begin to grow and multiply. The best-known example is how vitamin C blocks nitros-amine, the carcinogen believed to initiate stomach cancer.
Other phytochemicals also offer protection. For instance, sulforaphane, a substance in broccoli identified by Johns Hopkins researchers about three years ago, was found to block tumor growth by stimulating the body's production of cancer-fighting enzymes. Other sulfur-containing compounds, such as those found in garlic and onions, can also suppress the division of mutant cells that eventually form tumors. Genistein, found in soybeans, has the ability to literally starve tumors to death by blocking their blood supply. Genistein and daidzein, another soy phytochemical, act as phytoestrogens, which are believed to lower breast cancer risk by interfering with circulating levels of estrogen.
The list of disease-fighting phytochemicals continues to grow: Indoles in broccoli and cabbage, elegiac acid in nuts and berries, limonoids and flavonoids in citrus fruit rinds (my grandmother told me that the white of the orange was good for me), capsaicin in chili peppers, catechins in tea, lycopene in tomatoes and strawberries. Even herbs and spices, including rosemary, cumin, and saffron, are believed to have protective properties.
Yet researchers agree that true disease protection lies not in any single compound, but within the intricate chemistry of entire foods. For example, Cornell University researchers found that orange juice extracts, minus their vitamin C, reduced precancerous lesi by 40 percent. This suggests that vitamin C is not working alone when it interferes with cancer growth. (I suspect whole oranges may have worked even better.) And last year, the results of the 12-year Physicians Health Study by Harvard University researchers showed no difference in risk of heart disease or cancer between 11,000 physicians who took beta-carotene pills and 11,000 of them who took a placebo. Nor was a protective effect for beta-carotene observed in two other large health trials. This suggests that beta-carotene alone has no disease-fighting effect.
Once the hundreds of beneficial compounds are properly identified and their interactions understood, you could take 100 pills a day and still not enjoy the same benefits. There is a much simpler and better-tasting option; eat lots of fresh fruits and vegetables.
Elizabeth Hiser is a nutritionist and writer about health issues.
Photography by Suzanne DeJohn/National Gardening Association