Phytochemicals also are known to repair damaged cells so they don't cause cancer and can kill damaged cells and remove them from the body, says Thomson. They also can enhance the immune system, help lower cholesterol and blood pressure and possibly reduce menopausal symptoms, she says.
The fiber found in whole grains, fruits and vegetables can speed carcinogens through the digestive tract and can dilute carcinogenic substances, according to Integrative Medicine Inc.
And there's a simpler benefit, too. A diet high in plant-based foods will make you feel better, Thomson says.
Phytochemicals alone have no nutritional value, says Tammy Thornton, a registered and licensed dietitian at Washington County Health Department. However, not ingesting them reduces the body's ability to fight disease, she says.
"They protect you from being among these environmental hazards," says Thornton.
Where to get them
With names like carotenoids, flavonoids, monoterpenes and indoles, phytochemicals may sound too exotic to be in everyday foods. But if you fill your grocery cart with items such as tomato and soy products, citrus fruits, cabbage, broccoli, garlic, onions, grains, caraway seeds and thyme, you're taking some healthful steps.
"I eat by the rainbow," says Thomson, a registered dietitian who is studying at University of Arizona in Tucson whether a diet high in vegetables and fruits could prevent the recurrence of breast cancer.
When she goes through the produce section, Thomson buys vegetables of varying colors and strives to try a new type every two or three weeks.
For those who don't care for certain types of vegetables, Thomson suggests trying different methods of preparation. Someone who doesn't like fresh beets may like the canned variety, for instance, or if baked sweet potatoes don't appeal, try mashing them instead.
While fruits contain some phytochemicals, too, "they're a far second to vegetables," says Thomson.
She recommends eating five servings of vegetables and fruits a day. Americans are averaging about three a day, Thomson says, with the white potato being a favorite.
A serving would be a half-cup of cooked vegetables, one cup of raw vegetables, a whole piece of fresh fruit or a half-cup of canned fruit, Thornton says.
The best approach is to incorporate a wide variety of fruits, vegetables and grains in the diet, says Thornton.
When choosing grains, steer away from plain white bread because "there's just nothing in it" nutritionally except starch, Thornton says. Rice and pasta are other good sources of fiber, as are couscous, bulgur, barley and kasha, according to American Institute for Cancer Research.
There's no need to omit meat from the diet, but if more fruits and vegetables are consumed, people will find they will need less meat to feel full, Thornton says.
"That's a good way to start a lower-fat way of eating," Thornton says.
-- where to find phytochemicals