Antioxidants and Cancer Prevention (cont.)
Given the unexpected results of ATBC and CARET, and the finding of no effect
of beta-carotene in the PHS and WHS, NCI will follow the people who participated
in these studies and will examine the long-term health effects of beta-carotene
supplements. Post-trial follow-up has already been funded by NCI for CARET, ATBC,
the Chinese Cancer Prevention Study, and the two smaller trials of skin cancer
and colon polyps. Post-trial follow-up results have been published for ATBC, and
as of July 2004 are in press for CARET and are in progress for the Chinese
Cancer Prevention Study.
How might antioxidants prevent cancer?
Antioxidants neutralize free radicals as the natural by-product of normal
cell processes. Free radicals are molecules with incomplete electron shells
which make them more chemically reactive than those with complete electron
shells. Exposure to various environmental factors, including tobacco smoke and
radiation, can also lead to free radical formation. In humans, the most common
form of free radicals is oxygen. When an oxygen molecule (O2) becomes
electrically charged or "radicalized" it tries to steal electrons from other
molecules, causing damage to the DNA and other molecules. Over time, such damage
may become irreversible and lead to disease including cancer. Antioxidants are
often described as "mopping up" free radicals, meaning they neutralize the
electrical charge and prevent the free radical from taking electrons from other
Which foods are rich in antioxidants?
Antioxidants are abundant in fruits and vegetables, as well as in other foods
including nuts, grains and some meats, poultry and fish. The list below
describes food sources of common antioxidants.
- Beta-carotene is found in
many foods that are orange in color, including sweet potatoes, carrots,
cantaloupe, squash, apricots, pumpkin, and mangos. Some green leafy vegetables
including collard greens, spinach, and kale are also rich in beta-carotene.
- Lutein, best known for
its association with healthy eyes, is abundant in green, leafy vegetables such
as collard greens, spinach, and kale.
- Lycopene is a potent antioxidant found in tomatoes, watermelon,
guava, papaya, apricots,
pink grapefruit, blood oranges, and other foods. Estimates suggest 85 percent
of American dietary intake of lycopene comes from tomatoes and tomato
- Selenium is a mineral,
not an antioxidant nutrient. However, it is a component of antioxidant
enzymes. Plant foods like rice and wheat are the major dietary sources of
selenium in most countries. The amount of selenium in soil, which varies by
region, determines the amount of selenium in the foods grown in that soil.
Animals that eat grains or plants grown in selenium-rich soil have higher
levels of selenium in their muscle. In the United States, meats and bread are
common sources of dietary selenium. Brazil nuts also contain large quantities
- Vitamin A is found in
three main forms: retinol (Vitamin A1), 3,4-didehydroretinol (Vitamin A2), and
3-hydroxy-retinol (Vitamin A3). Foods rich in vitamin A include liver, sweet
potatoes, carrots, milk, egg yolks and mozzarella cheese.
- Vitamin C is also called
ascorbic acid, and can be found in high abundance in many fruits and
vegetables and is also found in cereals, beef, poultry and fish.
- Vitamin E, also known as alpha-tocopherol, is found in almonds,
in many oils including wheat germ, safflower, corn and soybean oils, and
also found in mangos, nuts, broccoli and other foods.
Last Editorial Review: 7/8/2005