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Types of Phytochemicals and their Effects on the Human Body

Phytochemicals are, as the name suggests, all chemicals found in plants. There are many different ways to categorize thousands of phytochemicals we know today. Nutritionists tend to categorize them by their possible health effects in the human body. For instance, certain phytochemicals act as antioxidants and are simply called “antioxidants” rather than by their specific chemical name. Similarly, some phytochemicals act as phytoestrogens, affecting metabolism of the female sex hormone estrogen in the human body and may thus be identified by this action (for more information also see the section “Potential Health Effects of Phytochemicals”).

Another way of organizing phytochemicals is into groups that share a similar chemical structure. It is common that phytochemicals in the same class also share similar effects on the human body. Such classes of phytochemicals include:

Phenols. Sometimes also referred to as phenolic compounds, phenols are a large class of phytochemicals, some of which are associated with antioxidant properties and are believed to lower the risk of heart disease and cancer. Phenols have a chemical structure that includes one or more hydroxyl groups attached to an aromatic ring. When more than one hydroxyl group is involved, we speak of polyphenols. Examples of phenolic phytochemicals include:

  • Flavonoids e.g., resveratrol found in red grapes, peanuts and blueberries may help counteract damage to the arteries caused by a high-fat diet. Resveratrol may also prevent cellular damage and resulting cancer cell growth as well as heart disease by limiting blood clot formation and inflammation. Another type of flavonoid, catechins found in cocoa, tea and blackberries may help to remove and inhibit the development of carcinogens. Other examples in this category include quercetin or cyanidin.
  • Isoflavones such as daidzein or genistein may act as phytoestrogens, potentially reducing the risk of certain types of cancer. Good sources of isoflavones are soybeans and soy products, red clover, legumes, apples, carrots, cherries, garlic and potatoes.
  • Phenolic acids. Various phenolic acids are linked to specific effects in the body. For instance, capsaicin present in hot peppers is believed to help regulate blood clotting whereas ellagic acid found in berries, pomegranate and walnuts may trigger enzyme activity that helps to remove carcinogens from the body. Other well-known phytochemicals in this category include curcumin, tannins or vanillin.
  • Lignans which can be found in flaxseed, sesame seeds and whole grains may inhibit the action of estrogen in the body, thereby possibly decreasing the risk of cancer.

Terpenes. This large class includes phytochemicals believed to possess antioxidant properties and associated with a reduced risk of heart disease and cancer. Terpenes may be identified as hydrocarbons that come from isoprene which is the major type of hydrocarbon emitted by plants and trees. This class of phytochemicals includes:

  • Carotenoids. Examples include beta-carotene, lycopene, lutein and zeaxanthin. The best known representative of this category is beta-carotene (provitamin A) which can be found in fruits and vegetables that are yellow or orange in color (e.g., cantaloupes, apricots, papayas, sweet potatoes, carrots). It is also present in dark green vegetables such as broccoli or spinach and many other cruciferous vegetables. Lycopene is abundant in guavas, watermelon, tomatoes, papaya, sweet red peppers as well as pink and red grapefruit. Lutein is found in egg yolks, kiwi and green leafy vegetables (e.g., spinach, kale, broccoli, romaine lettuce) and is linked to a reduced risk for macular degeneration.
  • Saponins present in legumes, asparagus, tomatoes, alfalfa sprouts, garlic and potatoes have been associated with improving immune system response and inhibiting the growth of cancer cells.
  • Monoterpenes found in the peels of citrus fruits are thought to trigger enzyme activity that may neutralize carcinogens in the body and inhibit cancer cell growth.

Thiols (organosulfur compounds). Phytochemicals in this class contain sulfur and are believed to stimulate enzymatic activity in the body, which may help to prevent carcinogens from damaging DNA. Thiols may emit a pungent smell. Some types, such as indoles, may also act as phytoestrogens. The class of thiols includes indoles, isothiocyanates and allyl sulphides such as allicin. Indoles and isothiocyanates are found in cruciferous vegetables (kale, broccoli, cabbage, Brussels sprouts, cauliflower, horseradish, collared greens, mustard and turnips). Allicin, often associated with garlic, is not present in garlic in its natural state but it becomes available after garlic has been chopped or crushed.

Phytic acids. These phytochemicals, also known as inositol hexaphosphate or phytate, bind to minerals and are believed to inhibit the development of free radicals, which may lead to reduced risk of cancer. Sources of phytic acids include nuts, oats, whole grains and legumes.

Phytosterols. These molecules are structurally similar to animal cholesterol. They are thought to compete with cholesterol from animal-based foods for absorption into the body, thus helping to lower cholesterol levels. Rich sources of phytosterols include lettuce, nuts, capers, flaxseed and cucumbers.

Protease inhibitors. They may prevent enzyme production in cancer cells, which may slow tumor growth and/or inhibit malignant changes. Good sources of protease inhibitors include Brussels sprouts, green tea and potatoes.

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