Fruits and Vegetables: A Phytochemical Pharmacy

Do the nutritional wonders of fruits and vegetables ever cease? For the past 20 years we've heard that fruits and vegetables are the cornerstone of health, supplying us with a wealth of vitamins, minerals, fiber, and complex carbohydrates. More recently, scientists have found another group of compounds within fruits and vegetables that may promote health--phytochemicals, which occur naturally in plants.

Phytochemicals are the biologically active substances in plants that give them their color, flavor, odor, and protection against plant diseases. During the past few years, scientists have discovered that many of these plant chemicals may also protect the body against disease. Studies have consistently found that eating greater amounts of fruits and vegetables may reduce the risk of heart attack, macular degeneration (the chief cause of blindness among the elderly), and most cancers. Consequently, hundreds of these plant substances are being investigated now for their role in preventing cancer and other degenerative diseases.

How do they work?

Many phytochemicals work as powerful antioxidants, protecting cells and organs from damage caused by free radicals. Free radicals are toxic oxygen molecules produced by cigarette smoke, X-rays, air pollutants, sunlight, and as a by-product of our metabolism. They are capable of oxidizing other molecules in our body, causing destruction and aging of cells. Along with aging, free radicals are thought to be involved in many ailments from cataracts to cancer. Antioxidants neutralize free radicals, decreasing their damaging effects.

Phytochemicals appear to interfere with virtually every stage of cancer growth. Certain phytochemicals seem to halt cancer at its inception by blocking the enzyme that activates cancer genes or by preventing various substances from forming cancer-causing agents called carcinogens. Others stop carcinogens from damaging cells, tissues, and organs, or help the body produce enzymes that destroy carcinogens. Still others suppress the spread of cancer by interfering with the reproduction of cells that already have been exposed to carcinogens.

Phytochemicals also may reduce the risk of coronary artery disease. Various phytochemicals have been found to lower blood pressure and cholesterol levels as well as block the oxidation of "bad" LDL-cholesterol, preventing it from harming arteries.

How they were discovered

During the 1970s, Lee Wattenberg, PhD, of the University of Minnesota, found that animals fed broccoli, Brussels sprouts, and other members of the cabbage family (called cruciferous vegetables) had lower rates of cancer than the control animals. Likewise, scientists at John Hopkins University found that lab animals fed cruciferous vegetables had a 90 percent reduction in their cancer rate after being exposed to aflatoxin, a deadly cancer-causing agent. Since that time, several phytochemicals have been isolated from cruciferous vegetables and found to have potent anti-cancer properties.

Also in the 1970s, German scientists discovered that the Japanese, who eat large amounts of soy foods, had 30 times more genistein, a component of soybeans, in their urine and considerably lower cancer rates than individuals on a Western diet (Plant Medica. 43: 101-120, 1981).

What studies have found

Allium compounds (from Garlic and Onions): A study of more than 41,000 women, known as the Iowa Women's Health Study (American Journal of Epidemiology, January 1, 1994, Vol. 139, No. 1) found that a diet consisting of garlic, fruits, and vegetables reduced the risk of colon cancer by 35 percent. Another study found that people in China's Shandong Province who eat garlic and onions regularly experience almost 40 percent less stomach cancer.

Lycopenes (from tomatoes and other red fruits): An Italian study of 5,500 people, found that eating tomatoes, which are rich in lycopene, was more effective in preventing digestive tract cancers than eating green vegetables. Individuals who ate tomatoes at least seven times a week had half the risk of developing these cancers as those who ate tomatoes only once a week.

Other studies have seen similar results. A six-year study from Harvard Medical School involving 48,000 men, aged 40 to 75, discovered that those who ate tomato-based foods four to seven times a week cut their risk of prostate cancer by 22 percent and those who ate tomatoes more than ten times a week cut their risk by 35 percent. Prostate cancer is the most commonly diagnosed cancer in men and the second highest cause of cancer mortality in men in the United States.

Beta carotene (from yellow, orange, and dark green vegetables and fruits): Although study after study has shown that a diet rich in beta carotene could reduce the risk of cancer, heart disease, and eye disorders, some recent studies using synthetic beta carotene or synthetic beta carotene supplements failed to show similar results. Both the 12-year Physicians' Health Study and four-year Beta Carotene and Retinol Efficacy Trial (CARET) failed to find any evidence of benefit in those taking synthetic beta carotene supplements. Other research findings suggest that a combination of carotenes offers more benefits than synthetic beta carotene alone. One such study found a 33 percent reduced risk of heart disease in men with high blood levels of carotenoids. Lutein and zeaxanthin (carotenes from dark leafy green vegetables): A recent study on macular degeneration (the chief cause of blindness among the elderly) found that people who ate greens, such as spinach and collards, at least five times a week had almost half the risk of macular degeneration, compared to those who rarely ate greens. Greens are a rich source of carotenoids. Two of these carotenoids, lutein and zeaxanthin, which are in the retina of the eye as well as in leafy greens, appear to form a pigment that filters out destructive forms of light and protects the eye. The National Health and Nutrition Examination Survey found that those eating a diet high in vegetables and fruits exhibited a similar reduction in the risk of eye-disease. Genistein (from soy foods, mung beans, and alfalfa sprouts): Studies show that when animals are feed soybeans or extracted genistein, breast cancer rates drop 40 to 65 percent. Genistein is an estrogen look-alike that can plug into estrogen receptor sites in breast tissue, thereby competing with estrogen and possibly deterring breast cancer.

Which is better--raw or cooked?

Some phytochemicals are partially destroyed by heat while others are heat-stable. Carotenoids lose some of their effect when cooked because they absorb more oxygen. Yet cooking enhances the availability of sulforaphane, a phytochemical in cruciferous vegetables, and produces more indoles (see page 40) in broccoli. Cooking also appears to help release lycopene from cells in tomatoes. Thus tomato sauce and other canned or cooked tomato products offer more protection than raw tomatoes. Lightly microwave or steam vegetables instead of boiling to prevent phytochemicals from escaping into the cooking water.

What about supplements containing phytochemicals?

Individual chemicals that are isolated and extracted and then either made into a pill or combined with other plant chemicals to produce a synthetic product should, in theory, provide the same effects as those in whole plants. Numerous studies have found that animals given compounds extracted from fruits or vegetables and exposed to carcinogens exhibit a reduced incidence of cancers similar to that seen in animals that are fed the same fruits or vegetables (Cancer Causes and Control 2: 427, 1991).

Supplements are not intended to replace whole foods, but they may offer some added protection to those who need extra nutrients due to illness--especially cancer, and those who do not or are not able to eat the USDA recommended five to nine servings of fruits and vegetables each day. However, considering that the knowledge of phytochemicals and their properties is still evolving, it may be wishful thinking that a few of these compounds will provide all the health benefits of whole foods which contain hundreds. According to one analysis, citrus fruits alone contain 58 known anti-cancer compounds.

Consumer guide

Below are some of the more widely studied phytochemicals grouped by the foods they are most concentrated in.

Fruits

Limonene (a monoterpene compound): Best sources: Citrus fruits. How it works: Increases production of enzymes that may break down carcinogens, and stimulates cancer-killing immune cells. Benefits: May help protect against breast cancer. Ellagic acid: Best sources: Grapes, apples, strawberries, and raspberries. How it works: Slows tumor growth by blocking production of enzymes used by cancer cells. Benefits: May prevent carcinogens from damaging a cell's DNA, thus helping prevent the formation of new cancer cells.

Anthocyanin/Proanthocyanin Bioflavonoids (polyphenol compounds): Best sources: Blueberries, bilberries, cranberries, red grapes, strawberries, and citrus fruits. It's also available in supplements as bilberry extract and grape-seed extract. How they work: Prevent eye tissue from free radical damage, inhibit the production of prostaglandins that can cause blood clotting, and help the body dispose of potential cancer-causing chemicals. Benefits: May reduce the risk of macular degeneration, heart disease, and some cancers.

Orange, Red, & Dark Green Vegetables & Fruits

Alpha-carotene (a carotenoid): Best sources: Pumpkin, carrots, cantaloupe, kale, yellow corn, and seaweed. Also available in natural carotene supplements. How it works: Slows the growth of cancer cells. Benefits: May reduce risk of lung cancer and boost the immune system.

Beta carotene (a carotenoid): Best sources: Sweet potatoes, carrots, pumpkin, cantaloupe, apricots, peaches, spinach, and other leafy greens. Natural beta carotene supplements are also available. How it works: Functions as an antioxidant, protecting LDL lipids from free radical damage. Benefits: May reduce the risk of heart disease, bladder, colon, and skin cancer, as well as stimulate the immune system.

Lycopene (a carotenoid): Best sources: Watermelons, tomatoes, guava, and pink grapefruit. Lycopene is also available in supplement form. How it works: Functions as an antioxidant, protecting the protein, fat, and DNA in cells from free radical damage. Benefits: May reduce risk of cardiovascular disease, colon, pancreatic, and prostate cancer.

Lutein and Zeaxanthin (carotenoids):Best sources: Kale; spinach; beet, collard, and mustard greens; and sweet red peppers. How they work: Form a pigment in the eye that filters out damaging forms of light, and function as antioxidants, protecting against cell damage. Benefits: May reduce the risk of macular degeneration (the most common cause of blindness in the elderly). They also may reduce the risk of lung, colon, and prostate cancer, and improve immune response.

Capsaicin: Best sources: Chili peppers--the hottest peppers contain the most capsaicin (also available in supplements). How it works: Impedes carcinogens such as nitrates and cigarette smoke from attaching to cellular material, preventing formation of cancer cells. May also kill bacteria that can cause ulcers. Benefits: Increases circulation and may reduce the risk of lung and other cancers.

Catechin flavonoid (a polyphenol compound): Best sources: Green Tea (also available in supplements). How it works: Functions as an antioxidant, protecting cells from free radical damage, and hinders blood platelets from sticking together. Benefits: May help protect against stomach, liver, and lung cancer, and lower cholesterol levels.

Cruciferous Vegetables

Cruciferous vegetables include: Bok choy, broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, kohlrabi, mustard greens, radishes, rutabaga, turnip greens, and turnips.

Indoles: Best sources: Cruciferous vegetables. How they work: Stimulate enzymes that diminish the effectiveness of the hormone estrogen, and improve immune response. Benefits: May reduce the risk of breast and ovarian cancer.

Isothiocyanates: Best sources: Cruciferous vegetables. How they work: Increase production of enzymes that block carcinogens from damaging cells. Benefits: May slow tumor growth and reduce the risk of lung cancer.

Sulforaphane: Best sources: Cruciferous vegetables. How it works: Activates the liver to make enzymes that bind to carcinogens and transport them out of cells. Benefits: Suppresses the development of tumors in animals.

Root Vegetables

Gingerol: Best source: Ginger root (also available in supplements). How it works and the benefits: Increases the production of substances that protect the stomach lining, preventing the formation of ulcers; stimulates gastric activity, causing the stomach to empty more quickly; stimulates the gall bladder, promoting healthy digestion; and helps alleviate nausea caused by pregnancy and motion sickness. To use: Add 1/2 teaspoon dried ginger powder to one cup hot water or tea, or simmer a few slices of fresh ginger root in three cups of water for 10 minutes.

Glycyrrhizin: Best source: Dried licorice root (also available in supplements). How it works: Prevents the conversion of testosterone into a more potent form that may promote the growth of prostate cancer, and stimulates the production of liver enzymes that reduce the level of estrogen. Benefits: Helps prevent breast cancer in animals and may reduce the risk of prostate cancer. To use: Use as a stir-stick to flavor your tea, or use one teaspoon of powder in a cup of hot water.

Allyl sulfide (an organo-sulfur compound): Best sources: Garlic or garlic supplements, onions, scallions, leeks, and chives. How it works: Increases the production enzymes that break down potential carcinogens and make them easier to excrete. Benefits: May decrease the risk of stomach and colon cancer, and lower LDL-cholesterol ("bad") levels. Garlic powder or tablets have been found to im-prove immune response.

Beans & Grains

Genistein (an Iso-flavonoid):Best sources: Soybeans and soybean products (miso, tofu, soy milk, tempeh, soy flour), peanuts, mung beans, and alfalfa sprouts. Also available as supplements such as soy protein powders. How it works: Blocks enzymes that turn on cancer genes, and inhibits the growth of new blood vessels around cancer cells that are needed to feed growing tumors. Genistein also blocks the entry of estrogen into cells, possibly deterring breast cancer, and may inhibit the uptake of testosterone in the prostate. Benefits: Its anti-hormonal effects may protect against both breast and prostate cancer. In test tubes, it inhibits the growth of all types of cancer cells, including those in the breast, colon, lung, prostate, skin, and blood (leukemia). Other compounds in soybeans may reduce blood cholesterol levels.

Phytosterols and Saponins: Best sources: Soybeans and dried beans. How they work: Suppress the growth of cancer cells in the large intestine and enhance immunity. Benefits: May slow the development of colon cancer.

Protease inhibitors: Best sources: Soybeans and dried beans. How they work: Prevent the conversion of normal cells into cancerous cells. Benefits: Slow tumor growth.

Phytic acid: Best sources: Grains (oats, rice, rye, wheat) soybeans, peanuts, and sesame seeds. How it works: Binds to iron and carries it out of the body, preventing iron from producing cancer-causing free radicals. Benefits: May reduce the risk of colon cancer.