Frequently Asked Questions About Phytonutrients
1. What are phytonutrients and where are they found?
The term "phyto" originated from a Greek word meaning plant. Phytonutrients are certain organic components of plants, and these components are thought to promote human health. Fruits, vegetables, grains, legumes, nuts, and teas are rich sources of phytonutrients. Unlike the traditional nutrients (protein, fat, vitamins, minerals), phytonutrients are not "essential" for life, so some people prefer the term "phytochemical."
2. What are the major classes of phytonutrients?
Some of the common classes of phytonutrients include:
| Carotenoids |
| Flavonoids (Polyphenols), including Isoflavones (Phytoestrogens) |
| Inositol Phosphates (Phytates) |
| Lignans (Phytoestrogens) |
| Isothiocyanates and Indoles |
| Phenols and Cyclic Compounds |
| Saponins |
| Sulfides and Thiols |
| Terpenes |
About Carotenoids
Of all the phytonutrients, we probably know the most about carotenoids, the red, orange, and yellow pigments in fruits and vegetables. The carotenoids most commonly found in vegetables (and in plasma) are listed below along with common sources of these compounds. Fruits and vegetables that are high in carotenoids appear to protect humans against certain cancers, heart disease, and age-related macular degeneration.
| Carotenoid | Common Food Source |
| alpha-carotene | carrots |
| beta-carotene | leafy green and yellow vegetables (eg. broccoli, sweet potato, pumpkin, carrots) |
| beta-cryptoxanthin | citrus, peaches, apricots |
| lutein | leafy greens such as kale, spinach, turnip greens |
| lycopene | tomato products, pink grapefruit, watermelon, guava |
| zeaxanthin | green vegetables, eggs, citrus |
For a more detailed discussion of carotenoid content of fruits and vegetables see Chug-Ahuja et al, Journal of the American Dietetic Association, 1993; vol 93: pp 318 and Mangels et al. Journal of the American Dietetic Association, 1993; vol 93: pp 284-296. For carotenoid values of specific foods see the USDA-NCC Carotenoid Database for U.S. Foods on the web site of the Nutrient Database Laboratory.
About Polyphenols
Polyphenolic compounds are natural components of a wide variety of plants; they are also known as secondary plant metabolites. Food sources rich in polyphenols include onion, apple, tea, red wine, red grapes, grape juice, strawberries, raspberries, blueberries, cranberries, and certain nuts. The average polyphenol/ flavonoid intake in the U.S. has not been determined with precision, in large part, because there is presently no U.S. national food database for these compounds (USDA scientists and their colleagues are in the process of developing a database for foods rich in polyphenols). It has been estimated that in the Dutch diet a subset of flavonoids (flavonols and flavones) provide 23 milligrams per day. Earlier estimates of dietary intake that approximated 650 milligrams per day (Kuhnau, World Review of Nutrition and Dietetics, 1976; vol 24: pp 117) are generally thought to be too high as the estimate was based on data that were generated by "old" (less specific) methodology. Scientists at the Food Composition Laboratory, Beltsville Human Nutrition Research Center are currently developing new methodology for the accurate measurement of polyphenols in foods.
Polyphenols can be classified as nonflavonoids and flavonoids. The flavonoids quercetin and catechins are the most extensively studied polyphenols relative to absorption and metabolism.
| Nonflavonoids | Sources |
| ellagic acid | strawberries, blueberries, raspberries |
| coumarins | |
| Flavonoids | Sources |
| anthocyanins | Fruits |
| catechins | tea, wine |
| flavanones | citrus |
| flavones | Fruits and vegetables |
| flavonols | Fruits, vegetables, tea, wine |
| isoflavones | soybeans |