Here is a list of terms used in phytochemical research.

Allium compounds: Another name for the organosulfides, or allyl sulfides, found in allium vegetables, which include garlic, onions, leeks, chives and shallots. Allium compounds such as diallyl sulfide and allyl methyl trisulfide may boost enzyme cancer detox systems and prevent bacteria from converting nitrates into substances that help make carcinogens. Garlic lowers cholesterol in people with elevated readings; diallyl sulfide is the suspected operative. Garlic also reduces blood clotting and lowers blood pressure. In addition to these and other possible health benefits, organosulfides give the allium family its pungency. But they may be lost in cooking.

Allyl methyl trisulfide: See "Allium compounds."

Alpha carotene: A powerful antioxidant carotenoid that the body converts to vitamin A, as needed. In population studies, alpha carotene is related to reduced risk of lung cancer. It may slow the proliferation of cancer cells. Carrots are a rich source.

Alpha tocopherol: The most common form of vitamin E, found both in the human body and in supplements. But gamma tocopherol is the primary source of vitamin E in the American diet, chiefly because so many foods such as margarine, salad dressings and packaged baked products are made with gamma-rich soybean oil.

Anthocyanins: Probably the most abundant flavonoid. See "Flavonoids."

Antioxidants: Antioxidants are chemical magnets that disarm highly reactive and damaging forms of oxygen, which are called collectively "free radicals." In chemicalspeak, these molecules are reactive because they have an extra electron to give away - and want to do it quick. Free radicals are the natural byproducts of energy metabolism in the cell but also come from outside sources. Although many phytochemicals are antioxidants, the most widely recognized and researched are beta carotene and vitamins C and E.

Beta carotene: A carotenoid that is stored in the liver, where the body converts it to vitamin A, as needed; found in dark, leafy greens and red, orange and yellow fruits and vegetables. A powerful antioxidant, beta carotene may play a role in slowing the progression of cancer. In population studies, it's related to decreased risk of lung cancer and oral cancers. It also may enhance immunity, help prevent cataracts and slow plaque buildup in arteries. But it is not without controversy: In a study of Finnish smokers, lung cancer increased among those taking supplements. Similar problems occurred in a study of former smokers, smokers and workers exposed to asbestos. However, a 12-year U.S. trial of more than 20,000 physicians, most of whom did not smoke, showed no such increase nor any protective effect - for cancer or heart disease. These findings don't negate beta carotene's promise, but they do complicate the picture for now.

Biochanin A: See "Phytoesterogens."

Vitamin C: The most effective water-soluble antioxidant, especially abundant in citrus fruits. Dr. Balz Frei, director of the Linus Pauling Institute at Oregon State University, calls it the "first line of antioxidant defense in human plasma." It works in concert with vitamin E to help slow LDL oxidation, as well as protecting against some cancers. It also protects parts of the eye against oxidative damage from ultraviolet light and may prevent cataracts.

Caffeic and ferulic acids: Phenolic acids that in animal studies prevent the formation of carcinogens in the stomach. Found in virtually all fruits and vegetables.

Campesterol: See "Phytosterols."

Carcinogens: Cancer-causing substances.

Carnosol: An antioxidant phenolic compound in rosemary that may prevent cholesterol oxidation and prevent cancer. Rosemary extracts are used in processed foods as a preservative, but flavor limits their application.

Carotenoids: A family of antioxidants that are also pigments in plants, giving foods such as tomatoes, watermelon and sweet potatoes their bright colors. Although more than 600 have been identified, only a handful are found in measurable quantities in the human body: alpha carotene, beta carotene, lycopene, lutein, zeaxanthin and cryptoxanthin. But around a dozen may be important. Carotenoids appear to play an anticancer role and enhance immunity. Lycopene is increasingly gaining ground as the most powerful antioxidant in the carotenoid family, particularly in relation to prostate and breast cancer. Two carotenoids found in the eye, lutein and zeaxanthin, are believed to protect against the leading cause of blindness in people over 65. Carotenoids also may play a role in heart health: In LDL oxidation, antioxidants are consumed in a sequence that begins with vitamin E; lycopene is next, followed by beta carotene. Although carotenoids appear to be heat resistant, sunlight breaks them down in the presence of oxygen, so don't cut up vegetables and leave them out on the counter for a long time before using them. Cooking foods lightly makes their carotenoids more readily available.

Catechins: A subclass of flavonoids found in tea. Up to 30% of the dry weight of green tea leaves is catechins. Scientists believe catechins to be one of the important active substances that gives green tea extract its cancer-preventive and possibly curative properties in animal studies. But population studies show no such clear-cut protective effect.

Chalcones: See "Flavonoids."

Cholesterol: A important component of blood lipids (fats) manufactured by the liver that's also the precursor of the steroid hormones, such as the sex and "fight or flight" hormones. Too much of some kinds, specifically low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL), if oxidized, can collect inside artery walls as plaque, restricting blood flow, reducing vessel flexibility and leading to heart disease. High-density lipoprotein (HDL) helps move LDL cholesterol out of the system. Vitamin E, lycopene and beta carotene protect LDL from oxidation; their antioxidant activity is enhanced in the presence of vitamin C. People concerned with cholesterol should watch their intake of foods containing saturated fats, which stimulate the liver to make more cholesterol. Dietary cholesterol from animal-based foods has little effect on blood cholesterol in healthy people.

Coumarins: A class of widely occurring phenolic compounds, especially abundant in citrus fruits, that may help the enzymes that fend off cancer.

Cryptoxanthin: A carotenoid that's been associated with a decreased risk of cervical cancer. Abundant in many orange fruits, especially mango, tangerines, oranges and papaya.

Curcumin: A phenolic compound that gives turmeric and mustard spices their yellow color and exhibits anticancer, anti-inflammatory and antioxidant properties.

Diadzein: See "Genistein."

Diallyl sulfide: An allium compound that may have an anticancer role and is suspected of being the active ingredient in garlic that lowers cholesterol. See "Allium compounds."

Dithiolthiones: Organosulfur compounds that are abundant in cruciferous vegetables and may aid the enzymes that fend off carcinogens and other outside invaders. They also may inhibit the development of cancer.

Vitamin E: The most potent fat-soluble antioxidant, as well as one of the most widely recognized and researched. It occurs in eight chemical forms of varying potency; alpha tocopherol is the most common. But gamma tocopherol is the main type found in the American diet because so many products are made with soybean oil. Many kinds of research suggest that vitamin E works in concert with vitamin C, interfering with LDL oxidation and protecting against heart disease. But another part of this protection, its anticlotting function, may promote excessive bleeding in some people. Vitamin E also may play a role in immunity and in recovery from exercise-induced stress. In one study it delayed the onset of debilitating symptoms in Alzheimer's disease. It also shows anticancer promise. Getting enough E for such benefits from diet alone without overdoing fat is difficult because it's found primarily in oils, prompting many health experts to recommend taking supplements. Recommendations range from 100 to 800 IUs of vitamin E daily, with most in the 200 IUs to 400 IUs range, and some specify the natural form, d-alpha tocopherol, or mixed tocopherols. One study suggests that alpha and gamma tocopherol work in concert more effectively than individually against some particularly virulent free radicals. But the vitamin E research is far from definitive, and the supplement recommendations, controversial.

Ellagic acid: A phenolic acid with possible anticancer properties. Found in nuts, particularly walnuts, and fruits such as strawberries, cranberries and blackberries. But there is question as to its bioavailability (52).

Ferulic acid: See "Caffeic acid."

Fiber: Population studies suggest that a fiber-rich diet helps prevent both cancer and heart disease. Scientists suspect that one kind, insoluble fiber, prevents colon cancer in particular, possibly by increasing bulk and speeding waste through the colon, binding with carcinogens and producing anticancer substances along the way. Whole wheat and wheat bran are rich sources. The second type, soluble fiber, appears to lower cholesterol and is abundant in oats, barley, legumes and vegetables such as potatoes. Most fruits, vegetables and grains contain a combination of the two types. Americans currently consume about 13 grams of fiber a day; the Daily Values on food labels, based on a 2,000-calorie diet, suggest 25 grams.

Flavanones: See "Flavonoids."

Flavonoids: A broad subcategory of plant phenolics (or phenolic compounds) made up of more than 4,000 compounds that are found in fruits, vegetables, wine and tea, especially green tea. "Plants have evolved to produce flavonoids to protect against fungal parasites, herbivores, pathogens and oxidative cell injury," writes Natalie Cook in a 1996 overview. "Conversely, flavonoids produce stimuli to assist in pollination and guide insects to their food source. For example, anthocyanins produce the pink, red, mauve, violet and blue colors of flowers, fruits and vegetables." The many potential effects of flavonoids include defending cells against carcinogens, curbing the oxidation of LDL cholesterol and preventing blood clotting. Major flavonoid classes include flavonols, flavanones, catechins, anthocyanins, isoflavones, dihydroflavonols and chalcones.

Flavonols: See "Flavonoids" and "Quercetin."

Folic acid: In the realm of cancer study, a deficiency of this nutrient may lead to chromosome and/or DNA damage that can open the way for cancer. In heart research, low folate causes high levels of homocysteine in the blood, which increases the risk for stroke and heart attack. Found in dark leafy greens.

Free radicals: Highly reactive molecular byproducts of energy metabolism that can damage cells and DNA. Free radicals also come from environmental sources such as cigarette smoke, auto and industrial emissions and sunlight. A leading theory of aging holds that free radicals are largely responsible for the declines and diseases associated with aging.

Genistein: An isoflavone, like daidzein, uniquely abundant in soyfoods; some of it is converted in the intestines to a compound that acts as a weak estrogen (phytoestrogen); the subject of hundreds of studies. Scientists believe it may be a significant anticancer force, particularly with hormone-related cancers such as breast cancer. It also may offer protection against cardiovascular disease by reducing blood clotting and/or cholesterol levels. Further, it may play a role in bone health and in relieving menopausal symptoms. See "Phytoestrogens."

Glutathione: A water-soluble antioxidant found in onions and potatoes that may detoxify cancer-causing substances. It also supports the actions of other antioxidants, such as vitamins C and E and beta carotene.

HDL cholesterol: See "Cholesterol."

Indole-3-carbinol: See "Indoles."

Indoles: Found in cruciferous vegetables, indoles may prevent carcinogens from reaching their intended goal inside of cells. They're formed from glucosinolates, which are particularly abundant in brussels sprouts, rutabaga and mustard greens. One, indole-3-carbinol, may help protect against estrogen-related cancers, such as breast cancer.

Inositol hexaphosphate: See "Phytic acid."

Isoflavones: Genistein and daidzein are the most prominent; found almost exclusively in soybeans and soyfoods; some are converted in the intestine to compounds with estrogen-mimicking functions; may help prevent hormone-related cancers, such as breast cancer. Sometimes scientists will refer to foods as "containing isoflavones" as a kind of shorthand. See "Genistein."

Isothiocyanates: Among the most effective cancer-prevention agents known. These organosulfur compounds boost the cancer-fighting power of certain enzymes. One, sulphorophane, appears to be especially potent. They are partially responsible for the pungency of some cruciferous vegetables.

Kaempferol: A flavonoid, like quercetin, found broadly in fruits and vegetables.

LDL cholesterol: See "Cholesterol" and "Oxidation."

Lignans (also called phenolic lignans): Plant phenolics converted in the intestines to a type of phytoestrogen ("plant" estrogen) with antioxidant properties. As a weak estrogen, lignans may affect hormone-related cancers by tying up the estrogen receptors on cells. Lignans are abundant in flax seed and flour, whole grain products and some berries. Vegetables and other grains are also sources.

Limonene: This monoterpene, which shows so much promise for cancer treatment, is the same substance that gives lemon scent to furniture polish and grease-cutting power to detergent. (One scientist once described how it dissolved a researcher's plastic pipette.) It is found in citrus oils, as well as garlic and the oils of other plants; it is used in Japan to dissolve gallstones. Limonene and its chemical cousin, perillyl alcohol, show powerful anticancer effects in animals. In rats, limonene caused the complete regression of mammary tumors. Human studies are underway with cancer patients.

Lutein: A powerful antioxidant and one of two carotenoids found in the eye. These yellow pigments are believed to filter out harmful blue light and protect against age-related macular degeneration, the leading cause of blindness in people over 65. Studies show that eating lots of spinach and collard greens - rich in lutein and its carotenoid partner, zeaxanthin - may substantially lower the risk of this irreversible disease. More resistant to cooking than other carotenoids, it's also associated with decreased lung cancer risk.

Lycopene: Emerging as the most powerful antioxidant of the carotenoid family. The pigment gives tomatoes their red color and also makes grapefruit and watermelon pink. The most concentrated carotenoid in the prostate, lycopene is linked to reduced prostate cancer risk in population studies. In one study, it inhibited cancer cell proliferation more effectively than alpha carotene or beta carotene. It is also stirring interest as a possible breast cancer preventative.

Monoterpenes: A broad category of compounds that may prevent, slow and/or reverse the progression of some cancers as well as affect blood clotting and cholesterol. The two most notable are limonene and perillyl alcohol. Found in the essential oils of citrus fruits, cherries, spearmint and dill.

Monounsaturated fats: Especially abundant in olive oil and canola oil. Monounsaturated fats slightly lower total cholesterol; this action may be due to their phytosterols.

Nitrates, nitrites: Nitrosamines are known to cause cancer. Nitrites in smoked and fermented foods and nitrates, found naturally in some foods and changed to nitrites by bacteria in the mouth, combine with amines in the stomach as protein breaks down to form nitrosamines. Vitamins C and E and phenolic compounds, such as quercetin, block this reaction in the stomach and may thus prevent cancer. Nitrates and nitrites are also found in some cured meats.

Organosulfides: The mostly smelly compounds in the allium (onion-garlic) and cruciferous (broccoli-kale) families. Dithiolthiones, including sulforophane, and indoles are the dominant ones in the cruciferous vegetables; they work primarily against cancer. Allium compounds such as diallyl sulfide are operative in the allium vegetables, especially garlic; they have a variety of anticancer and heart health functions.

Oxidation: Occurs when something is chemically united with certain types of oxygen with the help of an oxidizing agent. Combustion - fire - is the result of oxidation. It also occurs when metals rust or cut apples or potatoes turn brown. (Squeezing lemon juice on apples to prevent discoloration is an example of an antioxidant in action.) In the body, highly reactive free radical forms of oxygen grab onto other compounds in cells, causing structural damage to cell protein or fats or to the DNA within the nucleus. Polyunsaturated fat molecules in cell membranes and LDL cholesterol are particularly susceptible to free radical damage. Oxidized LDL cholesterol changes readily into substances that contribute to lesions in blood vessel walls, building up as plaque that gradually shrinks the circumference of the vessels and makes them less flexible.

Perillyl Alcohol: The limonene cousin that has been shown in animal studies to shrink tumors in animals, including stubborn pancreatic tumors. Found in citrus oils, this monterpene is being tested on humans. The intervention trials are using amounts far greater than what is ordinarily consumed from fruits and vegetables.

Phenolic compounds (or plant phenols): A broad category of antioxidant compounds that includes flavonoids, phenolic acids (which includes ellagic acid, tannic acid and vanillin) and hydroxycinnamic acid derivatives (caffeic, chlorogenic and ferulic acids, curcumin, coumarins). Lignans are another class of phenolic compounds. Found in almost all fruits, vegetables and grains, phenolic compounds affect the quality, appeal and stability of foods with antioxidant action, flavor and color. They give wine its characteristic hues, flavors and astringency. Besides scavenging for free radicals, some phenolic compounds appear to interrupt cancer development in other ways. Some also hinder LDL oxidation. It's not yet known how well plant phenols are absorbed from foods.

Phytic acid (inositol hexaphosphate): A heat- and acid-stable phytate in cereal grains, nuts and seeds, especially abundant in sesame seeds and soybeans. Although a high-fiber diet is thought to protect against some cancers, the argument has been advanced that phytic acid, not fiber, may provide the protection. It appears to slow the formation of cancer in lab and animal studies. It also may help control blood sugar, cholesterol and triglycerides.

Phytoestrogens: So-called "plant" estrogens that are produced in the intestines from certain flavonoids, isoflavones (most notably genistein, biochanin A and daidzein) and lignans. Often scientists simply say foods "contain" isoflavones as a kind of shorthand. Phytoestrogens are 250 to 1,000 times weaker than human estrogen but still impact the body. They are suspected of blocking estrogens by tying up estrogen receptors on cells, thus affecting hormone-related cancers, including breast and prostate cancer. They also may decrease hot flashes and other symptoms of menopause, although most of the evidence is anecdotal so far. One scientist likens phytoestrogens to a key that can fit a lock but not open it, effectively blocking the real key. Some phytoestrogens are similar to tamoxifen, a drug used to treat some breast cancers. Soyfoods are rich sources. In one study, tofu was found to contain the most isoflavones of the foods tested, though amount varied by brand (53).

Phytosterols: Plant sterols that in modest amounts can lower cholesterol and that show anticancer activity in lab and animal studies. Nuts (almonds, cashews, peanuts), seeds (sesame, sunflower), whole wheat, corn, soybeans and many vegetable oils are good sources. Some scientists speculate that phytosterols are responsible for the cholesterol-lowering properties of mono- and polyunsaturated oils. Some key sterols that lower cholesterol are beta-sitosterol, stigmasterol and campesterol.

Polyunsaturated fats: Like monounsaturated fats, polyunsaturated fats lower cholesterol. Two groups of polyunsaturated fats, omega-3s and omega-6s, are essential fatty acids, which the body requires but cannot manufacture. Omega-6s are in seeds and in vegetable and seed oils. Omega-3s are in green leafy vegetables, canola oil and soybeans. In slightly different form, omega-3s are found in fish and especially concentrated in cold-water fish such as salmon, trout, sardines and mackerel. (The source of these plant substances in fish are plankton and algae.) Because omega-3s and omega-6s compete for the same enzymes in the body, excessive intake of omega-6 can lead to a relative omega-3 deficiency.

Pro-oxidant: Any chemical compound that enhances oxidation. Under some conditions, some phytochemicals have been shown to act as pro-oxidants, one reason scientists wave people off supplements. Americans often assume - wrongly - that if a little of something is good, a lot must be better. If you're talking spinach, you probably can't (or won't) eat too much; if you're talking milligrams of sulphorophane in a capsule, you might do harm. With the exception of vitamin E, too little is known about most phytochemicals to mega-dose.

Protease inhibitors: Proteins that are plentiful in plants. Lab and animal studies show that they may aid DNA repair, which can slow cancer cell division and help return a cell to its normal state. They also may prevent tumors from releasing proteases that destroy neighboring cells. Found especially in soyfoods, also seeds and legumes.

Quercetin: The most studied flavonoid because it is among the most abundant; a more potent antioxidant than vitamin E, according to some research. Onions are the richest source; it's also found in wine and tea. (Many sources say "onions, tea, wine and apples" because these were the main dietary sources in a major Netherlands study.) Among other functions, it may block carcinogens as well as slow the growth and spread of cancer cells. It also may prevent the conversion of nitrites in the stomach to compounds that become building blocks for carcinogens. Quercetin appears to survive the heat of cooking, and about 5% to 10% of the quercetin from onions is absorbed by the body.

Resveratrol: A naturally occuring phenolic fungicide in grapes (and wine) that may that protect the heart. Peanuts also contain resveratrol.

Retinol: Another name for vitamin A. See "Carotenoids."

Saponins: Naturally occurring compounds found in most vegetables and herbs, but especially abundant in soybeans and other beans and legumes. Lab and animal research with saponins suggests they may prevent cancer cells from multiplying. They may also help control blood sugar, cholesterol and triglycerides.

Selenium: A trace mineral that may alter the course of cancer by helping certain enzymes protect cells against damage. The amount found in produce is directly related to the amount in the soil where it is grown, and selenium is readily taken up by the body. Its antioxidant function may prevent premature aging. Garlic contains selenium, and one scientist has used enriched soil to increase the amount in garlic bulbs.

Silymarin: A flavonoid present in artichokes that has been used in Europe to treat alcohol-related liver diseases. This strong antioxidant protects against liver toxicity in animals and plays a cancer-protective role.

Sitosterol (beta-sitosterol): See "Phytosterols."

Stigmasterol: See "Phytosterols."

Sulphorophane: See "Isothiocyanates."

Terpenes: Monoterpenes and triterpenes comprise the terpenes under investigation. Most of the attention is focused on two monoterpenes: limonene and perillyl alcohol.

Vanillin: A phenolic compound in vanilla beans and cloves.

Zeaxanthin: A strong antioxidant and one of two yellow carotenoids found in the eye that are believed to filter out harmful blue light and protect against age-related macular degeneration, the leading cause of blindness in people over 65. Studies show that eating lots of spinach and collard greens - rich in zeaxanthin and its carotenoid partner, lutein - may substantially lower the risk for this irreversible condition. Also in the eye, the antioxidants may help scavenge free radicals caused by exposure to sunlight. Zeazanthin is also associated with decreased lung cancer risk. Corn and eggs are also good sources.