General Description
A yellow-orange member of the family of plant pigments called carotenoids,
beta-carotene is an antioxidant nutrient that the body converts
into vitamin A. Of the more than 600 carotenoids that have been
identified, beta-carotene has the greatest pro-vitamin A activity
and has been the most extensively researched. Other notable members
of the carotenoid family include lycopene, alpha carotene, lutein,
zeaxanthin, capsanthin, and cryptoxanthin. While most of these carotenoids
are not significant sources of vitamin A, they may still play a
useful role as dietary antioxidants. Although most beta-carotene
research has focused on pro-vitamin A activity, the nutrient appears
to provide significant health benefits beyond its role as a vitamin
A precursor.
Food sources
Beta-carotene is abundant in orange, yellow, and dark green vegetables
and fruits. Some of the best sources include spinach, carrots,
sweet potatoes, bell peppers, squash, broccoli, and apricots.
Beta-carotene is also available as a dietary supplement in both
natural and synthetic forms, with the synthetic variety being
the most common. Natural forms are usually identified on labels
as "natural source," "D. salina," "algal source" or "palm source."
Health applications
Primary functions
Beta-carotene is an antioxidant that is particularly effective in
quenching singlet oxygen radicals. It also serves as a source of
vitamin A, which is essential for immune function, the growth and
maintenance of the skin, and the proper function of the eyes.
Immune function
Beta-carotene appears to play an important role in immune system
function, both as a vitamin A precursor and as an antioxidant.
Studies conducted on people with human immunodeficiency virus
(HIV) have shown that beta-carotene supplementation can increase
the activity of natural killer cells and other white blood cells.1,2
Other studies suggest that beta-carotene may play a role in cancer
prevention.3-8 Diets high in beta-carotene-rich foods
are associated with a reduced risk for cancers of the stomach,
lungs, and skin.9-12 However, studies using synthetic
beta-carotene supplements have failed to show any reduction in
cancer risk.13-16 Some studies indicate that beta-carotene
supplements may actually increase the risk of lung cancer in people
who smoke.14-16 It is possible that natural beta-carotene
may have anticancer properties not present in synthetic forms,
or that foods rich in beta-carotene contain other antioxidant
vitamins and flavonoids that work synergistically to prevent tumor
formation.
Photosensitivity
Research indicates that beta-carotene may help protect the skin
from the sun's ultraviolet (UV) rays. Studies conducted on subjects
with sunlight-sensitive conditions such as polymorphic light eruption,
light urticaria, and protoporphyria showed significant improvement
in sunlight tolerance.17-20 Beta-carotene's light-protective
properties appear to stem from its role as a free-radical scavenger.21
Dosage/toxicity
The most common supplemental beta-carotene intake is 25,000 IU
(15 mg) per day, although many studies have used twice that amount
with no adverse effects. Many nutritional researchers consider
beta-carotene to be the safest source of supplemental vitamin
A because high doses of vitamin A can lead to toxic accumulations
in the liver, whereas beta-carotene is only converted into vitamin
A as needed and does not cause toxic build-up. Excessive beta-carotene
intake can cause yellowing of the skin, a harmless effect that
passes when intake is reduced. Nutritionists often recommend combining
beta-carotene with other antioxidants such as vitamins C and E,
selenium, and zinc. |
