Frequently Asked Questions About Phytonutrients
- What are phytonutrients and where are they found?
- What are the major classes of
phytonutrients?
- How do phytonutrients protect against diseases?
- What is the evidence that fruit and vegetable consumption protects human
health?
- Are Americans eating enough fruits and vegetables?
- What is the present state of phytonutrient research?
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".
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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;93:318 and Mangels et al. Journal of the American
Dietetic Association, 1993;93:284-296. For carotenoid values of specific foods see the
USDA-NCC Carotenoid Database for U.S. Foods
on the website 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 mg per day. Earlier estimates of
dietary intake that approximated 650 mg per day (Kuhnau, World Review of Nutrition and
Dietetics, 1976;24:117) 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 non-flavonoids 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 |
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3. How do phytonutrients protect against disease?
The following are commonly proposed
mechanisms by which phytonutrients may protect human health. More research is needed to
firmly establish the mechanisms of action of the various phytochemicals.
Phytonutrients may:
|
serve as antioxidants |
|
enhance immune response |
|
enhance cell-to-cell communication |
|
alter estrogen metabolism |
|
convert to vitamin A (beta-carotene is
metabolized to vitamin A) |
|
cause cancer cells to die (apoptosis) |
|
repair DNA damage caused by smoking and
other toxic exposures |
|
detoxify carcinogens through the activation
of the cytocrome P450 and Phase II enzyme systems |
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4. What is the evidence that fruit and vegetable
consumption protects human health?
Evidence that fruit and vegetable
consumption protects human health is accumulating from large population (epidemiological)
studies, human feeding studies, and cell culture studies. Listed below are a few selected
population studies from the literature linking fruit and vegetable consumption to health.
For an excellent review concerning vegetables, fruit and cancer prevention, see Steinmetz
and Potter, Journal of the American Dietetic Association 1996;96:1027.
Evidence that Carotenoids are Protective
Fruit and vegetable consumption has been
linked to decreased risk of stroke -- both hemorrhagic and ischemic stroke. Each increment
of three daily servings of fruits and vegetables equated to a 22% decrease in risk of
stroke, including transient ischemic attack (Gillman et al. Journal of the American
Medical Association. 1995;273;1113).
Elderly men whose intake of dark green
and deep yellow vegetable put them in the highest quartile for consumption of these
vegetables had about a 46% decrease in risk of heart disease relative to men who ranked in
the lowest quartile. Men in the highest quintile had about a 70% lower risk of cancer than
did their counterparts in the lowest quintile. The differences in vegetable consumption
between high and low intake rankings was not striking. Men in the highest quartile or
quintile consumed more than two (>2.05 and >2.2) servings of dark green or deep
yellow vegetable a day; those in the lowest quartile or quintile consumed less than one
serving daily (<0.8 and <0.7). This suggests that small, consistent changes in
vegetable consumption can make important changes in health outcomes (Gaziano et al. Annals
of Epidemiology 1995;5:255 and Colditz et al. American Journal of Clinical Nutrition
1985;41:32).
Consumption of tomato products has been
linked to decreased risk of prostate cancer. Men in the highest quintile for consumption
of tomato products (10 or more servings a week) had about a 35% decrease in risk of
prostate cancer compared to counterparts whose consumption put them in the lowest quintile
(1.5 or fewer servings of tomato products a week) (Giovannucci et al. Journal of the
National Cancer Institute 1995;87:1767).
People in the highest quintile for
consumption of spinach or collard greens, plants high in the carotenoid lutein, had a 46%
decrease in risk of age-related macular degeneration compared to those in the lowest
quintile who consumed these vegetables less than once per month (Seddon et al. Journal of
the American Medical Association. 1994;272:1413).
Evidence that Polyphenols are
Protective
Flavonoid consumption has been linked to
lower risk of heart disease in some, but not all, studies. Elderly Dutch men in the
highest tertile of flavonoid intake had a risk of heart disease that was about 58% lower
than that of counterparts in the lowest tertile of intake. Those in the lowest tertile
consumed 19 mg or less of flavonoids per day, whereas those in the highest tertile
consumed approximately 30 mg per day or more (Hertog et al. Lancet. 1993;342:1007).
Similarly, Finnish subjects with the highest quartile of flavonoid intake had a risk of
mortality from heart disease that was about 27% (for women) and 33% (form men) lower than
that of those in the lowest quartile (Knekt et al. British Medical Journal. 1996;312:478).
However, in other studies the protective
effect of flavonoids could not be confirmed. For Welch men, flavonol intake did not
predict a lower rate of ischemic heart disease and was weakly positively associated with
ischemic heart disease mortality (Hertog et al. American Journal of Clinical Nutrition.
1997;65:1489). For U.S. male health professionals, data did not support a strong link
between intake of flavonoids and coronary heart disease (Rimm et al. Annals of Internal
Medicine. 1996;125:384).
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5. Are Americans Eating
Enough Fruits and Vegetables?
An excellent source of information on
fruit, vegetable and grain intake of Americans is USDA's 1994/96 Continuing Survey of Food
Intakes by Individuals conducted by the Food Surveys Research
Group, Beltsville Human Nutrition
Research Center. The following information comes from that survey.
About Vegetable Intake
On average, Americans consume 3.3
servings of vegetables a day. However, dark green vegetables and deep yellow vegetables
each represent only 0.2 daily servings.
On any given day, about 49% of the
population consumes at least the minimum number of servings of vegetables recommended (3
servings per day); 41% consume the number of servings recommended based on caloric intake
(3 servings per day for those consuming less than 2200 calories, 4 servings per day for
those consuming 2200-2800 calories, 5 servings per day for those consuming 2800 calories
or more). About 10% of the population consumes less than one serving of vegetable per day.
About Fruit Intake
On any given day about 29% of the
population consumes at least the minimum number of servings of fruit recommended (2
servings per day); 24% consume the number of servings recommended based on caloric intake
(2 servings per day for those consuming less than 2200 calories, 3 for those consuming
2800 calories, 4 for those consuming 2800 calories or more). About 48% consume less than
one serving of fruit a day.
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6.
What is the Present Status of the Art of Phytonutrients Research?
Population studies have linked fruit and
vegetable consumption with lowering the risk for chronic diseases including specific
cancers and heart disease. However, media and consumer interest in phytonutrients and
functional foods is far ahead of established proof that documents the health benefits of
these foods or food components for humans. Phytonutrients research is experiencing
remarkable growth. Hopefully, more specific information on phytonutrient consumption and
human health will be forthcoming in the near future. For now, it appears that an effective
strategy for reducing risk of cancer and heart disease is to increase consumption of
phytonutrient-rich foods including fruits, vegetables, grains and teas.