Phytonutrients are plant-based compounds that have health benefits but are not essential to life. Also called phytochemicals or dietary phytochemicals, phytonutrients are sometimes defined as bioactive substances associated with reduced risk for various chronic diseases. At least 900 different phytonutrients have been identified, and there are probably thousands more awaiting characterization.
Unlike macronutrients (proteins, carbohydrates, fats) and micronutrients (vitamins, minerals), which are needed for growth, metabolism, and other body functions, phytonutrients are not as essential. Despite this, phytonutrients appear to have a wide range of positive activities in the human body. Herbs and spices used for flavoring foods are associated with a long list of potential beneficial effects on human health. Over 40% of medicines prescribed in the United States contain chemicals derived from plants. Botanists and chemists actively search the worldwide plant kingdom for new phytochemicals.
The majority of the health benefits from phytonutrients are attributable to their antioxidative and antiinflammatory activities. Oxidative stress and chronic inflammation are considered primary causes of agerelated diseases and conditions. Oxidative stress results from the accumulation of free radicals. These are unstable molecules that are normal products of metabolism, as well as by-products of environmental toxins. Oxidative stress and damage caused by free radicals are associated with the development of cancer, cardiovascular disease, osteoporosis, gastrointestinal disorders, diabetes, and neurodegenerative disorders. The majority of known phytonutrients have antioxidant activities that counteract free radicals and help protect the body from the effects of oxidation. Antioxidants may help prevent, delay, and/or fight cancer, heart disease, and eye disorders such as cataracts. Cauliflower and other cruciferous vegetables contain phytonutrients with anti-inflammatory, antibacterial, antifungal, and anti-cancer properties. Some medicinal plants derive their healing properties from the antioxidant and anti-inflammatory activities of their phytonutrients.
In addition to anti-inflammatory activities that help prevent coronary artery disease, phytonutrients in apples, grapes, berries, plums, and other fruits and vegetables may improve blood vessel function, reduce the negative effects of LDL (“bad”) cholesterol, and help prevent coronary artery disease. Other phytonutrients appear to enhance the immune response, facilitate communications between cells, alter cellsignaling pathways that regulate gene expression, or have antibacterial properties. Antibacterial phytonutrients may interfere with the adhesion of pathogens (disease-causing organisms) to human cells.
Some phytonutrients, called phytoestrogens, can function as human estrogens and affect estrogen metabolism. They may help reduce menopausal symptoms and help prevent osteoporosis.
Most phytonutrients are plant pigments or secondary metabolites (by-products of plant metabolism). They often play defensive roles in plants, protecting them from harmful ultraviolet radiation, disease, pests, predators, and other plants. Medicinal plants are often rich in phytonutrients, and many herbs and spices contain phytonutrients with therapeutic properties. Many phytochemicals are used as medicines and digestive aids.
There are three main classes of phytonutrients:
More often, phytonutrients are classified according to their chemical structures. Major classes include:
CAROTENOIDS. Carotenoids are the red, yellow, and orange pigments in fruits and vegetables. Carotenes are antioxidant carotenoids that may decrease the risk of heart disease and stroke and protect against some cancers and age-related macular degeneration. Alpha-carotene in carrots; beta-carotene in leafy green and yellow vegetables such as carrots, broccoli, sweet potato, and pumpkin; and beta-cryptoxanthin in citrus fruits, peaches, and apricots are converted to vitamin A in the body. These may improve immunity and help alleviate arthritis. Other carotenoids include lutein and zeaxanthin, which can help maintain a healthy heart and normal vision, help protect the eyes from age-related macular degeneration and cataracts, and possibly help fight cancer. Lycopene—the red pigment in tomatoes, pink grapefruit, watermelon, and guava—is another important carotenoid.
POLYPHENOLS. Polyphenols and phenolic acids are secondary metabolites with antioxidant activities that are found in a wide variety of plants. Nonflavonoid polyphenols include coumarins such as p-coumaric acid, curcuminoids in turmeric, resveratrol in grapes and red wine, and ellagic acid. Polyphenolic flavonoids include:
OTHER PHYTONUTRIENTS. Other common phytonutrients include:
Lignans in whole grains have strong antioxidant and phytoestrogenic activities. Intestinal microflora help convert plant lignans into the mammalian lignans, enterolactone and enterodiol, which may help protect against chronic diseases including diabetes, heart disease, and hormone-related cancers. Lignans may decrease the risk of breast cancer by helping fat cells remove excess estrogen.
Carbohydrate-rich foods such as fruits, vegetables, beans, and whole and enriched grains are the major sources of phytonutrients. Among the best sources are blueberries and other berries, oranges, tomatoes, green beans, soybeans, dark leafy greens, carrots, sweet potatoes, and green teas. Phytonutrients in berries are heat stable, and brief cooking may even increase their antioxidative activities. Other beans, pomegranates, grapefruit, and nuts are also high in phytonutrients. Black beans are similar to plums in antioxidant content and have ten times more antioxidants than tomatoes. Dark greens such as broccoli, kale, watercress, arugula, and collard greens may have anticancer activities. Sun-exposed loose-leaf greens have far more phytonutrients than tightly closed greens, and external leaves are richer in polyphenols than leaves in the center. Among cooking oils, extravirgin olive oil has the most phytonutrients.
For specific phytonutrients:
Although phytonutrients are most often associated with fruits and vegetables, whole grains are full of phytonutrients such as phenolic acids. The total phenolic acid concentration in whole grains corresponds to the total antioxidant activity in the grain. Phytonutrients in whole grains appear to be important for digestive health. Whole grains have significantly more phytonutrients than refined grains. Refining whole grains into white flour removes the bran and inner germ layers that are rich in phytonutrients, but processing, including heating and milling, helps release some phytonutrients from their bound forms so that they are more available to the body. Phytonutrients are also released during digestion in the colon, so that their effects can be exerted both locally and throughout the body after absorption into the bloodstream. For example, an enzyme in the human intestine and in intestinal microflora releases ferulic acid from cereal bran.
Phytonutrients in different foods appear to complement each other. Their effects may be increased when combined, such as when mixing fruits such as strawberries, raspberries, blueberries, and cranberries in juices or smoothies. The anti-inflammatory effects of whole-grain phytonutrients may work with dietary fiber and minerals in whole grains and with phytonutrients in fruits and vegetables.
Little information is available about the phytonutrient content of typical diets and specific foods, nor are the amounts required for health benefits known. The U.S. Department of Agriculture (USDA) maintains databases of nutrient compositions of foods, including phytonutrients. It is known that most Americans do not consume the recommended daily servings of fruits and vegetables containing phytonutrients.
A food's antioxidant capacity, or total antioxidant potential, is measured in Trolox equivalents (TE), based on the antioxidant capacity of a vitamin E derivative:
Phenolic acid content is measured in mg of gallic acid equivalents per 100 g. Corn is the highest at 265, followed by wheat (136), oats (111), and rice (95). Compared with whole-wheat flour, refined wheat flour has lost:
Freshness and preparation are important for obtaining phytonutrients. Antioxidants and other phytonutrients break down from oxidation when exposed in light and air. Fresh produce may lose half of its phytonutrients during storage and cooking. Thus, frozen produce can have more phytonutrients than older fresh produce.
Although ample evidence supports the health benefits of diets rich in fruits, vegetables, legumes, whole grains, and nuts, evidence that these effects are due to specific phytonutrients is very limited. Because plantbased foods are complex mixtures of bioactive compounds, information on the potential health effects of individual phytochemicals is linked to information on the health effects of foods that contain them. Furthermore, very little is known about the ways in which phytonutrients interact. The health effects of most phytonutrients have not been studied in isolation, and the isolated compounds may not provide all of the health benefits of whole plant foods. For example, garlic pills, extracts, and other supplements may lack the potential cholesterol-lowering phytonutrients found in whole garlic. Carotenoids and flavonoids are believed to have more health-promoting properties when consumed together in food rather than as separate supplements. For people who need supplemental nutrition, whole-food supplements are preferable.
Manufacturers regularly introduce new combinations of so-called “functional beverages”—juices, teas, soft drinks, and flavored waters with phytonutrients and other ingredients—that promise to improve mood and memory, increase energy, relieve stress, or dissolve fat. Claims for phytonutrient supplements and functional beverages are unproven and have not been found to have any particular benefits in healthy people. In addition, ingredients and amounts may be neither identified nor standardized, and their safety is unknown. Such products are no substitute for healthy diets and lifestyles.
Phytonutrients in supplements are in a concentrated, more potent form than in whole foods and may cause allergic reactions in some people. People should seek medical advice before taking phytonutrient supplements, especially pregnant and breastfeeding women and anyone with a medical condition. For example, cauliflower contains goitrogens that can interfere with the functioning of the thyroid gland.
Phytonutrients interact in plants and presumably in the human body as well; however, very little is known about these interactions. For example, naringin increases the oral bioavailability of calcium-channel blockers and may enhance the effects of these drugs, resulting in a serious drop in blood pressure. Naringin also inhibits the breakdown of various substances such as caffeine, coumarin, and estrogens.
See also Anti-inflammatory diets ; Antioxidants ; Cancer ; Carotenoids ; Macronutrients ; Nutrigenomics .
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Margaret Alic, PhD