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There are thousands of different identified phytochemicals and many different ways to categorize them.

In common usage, phytochemicals are often characterized by their probable protective function in the human body. For example, many different phytochemicals act as antioxidants (preventing cellular damage in the body) and may be referred to simply as “antioxidants” rather than by their specific phytochemical name. Likewise, certain phytochemicals act as phytoestrogens (potentially affecting metabolism of the hormone estrogen in the body) and may be identified by this action.    

Scientists often organize phytochemicals into groups that share a similar chemical structure. Phytochemicals in the same class may often have similar effects on the body. Phytochemical classifications include:

• Phenols. A large class of phytochemicals, some of which are believed to have antioxidant properties and are associated with a lowered risk of heart disease and cancer. Phenols may be identified by a chemical structure that includes one or more hydroxyl groups (an oxygen atom bonded to a hydrogen atom) attached to an aromatic ring (six carbon atoms that form a hexagonal pattern). Certain phenols may impart a fragrance. When more than one hydroxyl group is involved, the phytochemical is known as a polyphenol. Phenols are sometimes also referred to as phenolic compounds. Examples of phytochemicals in this class include:
  
  

  Subclass:	Types of:
  Flavonoids	Flavonols (e.g., quercetin, resveratrol) Flavon-3-ols (e.g., catechins) Anthocyanins (e.g., cyanidin) Isoflavones (e.g., daidzein, genistein)
  Phenolic acids	Capsaicin Curcumin Ellagic acid Tannic acid (tannins) Vanillin
  Hydroxycinnamic acids	Caffeic acid Ferulic acid
  Lignans

Flavonoids such as resveratrol (found in red wine and grapes, peanuts) may help neutralize damage to the arteries caused by a high-fat diet. This type of phytochemical may inhibit cancer cell growth and heart disease by limiting blood clot formation and inflammation. Catechins (found in tea, wine) may help remove and prevent the development of carcinogens.

Isoflavones (e.g., daidzein, genistein) may act as a phytoestrogens, possibly reducing the risk of certain types of cancer. Sources of isoflavones include soybeans and soy products (e.g., soy flour, soy milk, tofu), apples, cherries, carrots, potatoes and garlic.

Different phenolic acids are associated with specific effects in the body. For example, capsaicin (found in hot peppers) may help regulate blood clotting. Ellagic acid (found in berries, red grapes, kiwi) may trigger enzyme activity that helps to rid the body of carcinogens.

Lignans can be found in flaxseed and whole grains, and are believed to inhibit the action of estrogen in the body, possibly reducing the risk of cancer.

• Terpenes. Another large class of phytochemicals believed to have antioxidant properties and associated with a lowered risk of heart disease and cancer. They may be identified as hydrocarbons (compounds consisting of carbon and hydrogen only) that come from isoprene (the major type of hydrocarbon emitted by plants). This class of phytochemicals includes:

Subclass:	Types of:
Carotenoids	Alpha-carotene Beta-carotene Lycopene Zeaxanthin Lutein
Monoterpenes	Limonene
Saponins

Beta-carotene (a provitamin A) can be found in fruits and vegetables that are yellow or orange in color (e.g., apricots, cantaloupes, papayas, carrots, sweet potatoes). It may also be found in dark green vegetables such as spinach and broccoli. Lycopene is found in tomatoes, watermelon, guava and papaya, as well as pink and red grapefruit.

Lutein is found in kiwi, egg yolks and green vegetables (e.g., spinach, romaine lettuce, broccoli) and is associated with a potential reduced risk for macular degeneration. Monoterpenes in the peels of citrus fruits are believed to trigger enzyme activity that may neutralize carcinogens in the body and inhibit cancer cell growth. Saponins are found in legumes (including soybeans and soy products), tomatoes, alfalfa sprouts and potatoes, and have been associated with enhancing immune system response and inhibiting cancer cell growth.

• Thiols. Also known as organosulfur compounds, phytochemicals in this class contain sulfur and may stimulate enzyme activity in the body, which may help prevent carcinogens from damaging DNA. They may emit a pungent smell. Some types (e.g., indoles) may also act as phytoestrogens. This class of phytochemicals includes:

Subclass:	Types of:
Indoles	Indole-3-carbinol
Isothiocyanates	Sulforaphane
Allyl sulfides	Allicin

Indoles and isothiocyanates are found in cruciferous vegetables (broccoli, Brussels sprouts, cabbage, cauliflower, horseradish, mustard and collared greens, turnips and kale). Allicin is not present in garlic in its natural state, although this phytochemical becomes available after garlic has been crushed or chopped. 

Additional classes of phytochemicals include:

• Phytosterols. Structurally similar to animal cholesterol, phytosterols may compete with cholesterol from animal-based foods for absorption into the body, helping to decrease cholesterol levels. This may help protect against heart disease. Sources of phytosterols include soybeans and certain fat substitutes.
  
  
• Phytic acids.  Also known as inositol hexaphosphate or phytate, these phytochemicals bind to minerals and may prevent the development of free radicals, which may reduce the risk of cancer. Sources of phytic acids include whole grains, nuts, oats, whole soybeans, and seeds.
  
  
• Protease inhibitors. These phytochemicals may inhibit enzyme production in cancer cells, which may slow tumor growth and/or prevent malignant changes. Sources of protease inhibitors include Brussels sprouts, potatoes, soybeans and soy products.