Phytonutrients Explained
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".
| 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 |
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 (e.g. 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.
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 |
Common Food Source |
|
ellagic acid |
strawberries, blueberries, raspberries |
|
coumarins |
none |
| |
Flavonoids |
Common Food Source |
|
anthocyanins |
fruits |
|
catechins |
tea, wine |
|
flavanones |
citrus |
|
flavonols |
fruits, vegetables, tea, wine |
|
isoflavones |
soybeans |
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 |
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).
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.
Return to top
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.
http://www.barc.usda.gov/bhnrc/pl/pl_faq.html |
