Can Antioxidants Prevent Cancer?
1. What are antioxidants?
Antioxidants are substances that may protect cells from the
damage caused by unstable molecules known as free radicals.
Free radical damage may lead to cancer. Antioxidants
interact with and stabilize free radicals and may prevent
some of the damage free radicals otherwise might cause.
Examples of antioxidants include beta-carotene, lycopene,
vitamins C, E, and A, and other substances.
2. Can antioxidants prevent cancer?
Considerable laboratory evidence from chemical, cell
culture, and animal studies indicates that antioxidants may
slow or possibly prevent the development of cancer. However,
information from recent clinical trials is less clear. In
recent years, large-scale, randomized clinical trials
reached inconsistent conclusions.
3. What was shown in previously published large-scale
Five large-scale clinical trials published in the 1990s
reached differing conclusions about the effect of
antioxidants on cancer. The studies examined the effect of
beta-carotene and other antioxidants on cancer in different
patient groups. However, beta-carotene appeared to have
different effects depending upon the patient population. The
conclusions of each study are summarized below.
• The first large randomized trial on antioxidants and
cancer risk was the Chinese Cancer Prevention Study,
published in 1993. This trial investigated the effect of a
combination of beta-carotene, vitamin E, and selenium on
cancer in healthy Chinese men and women at high risk for
gastric cancer. The study showed a combination of
beta-carotene, vitamin E, and selenium significantly reduced
incidence of both gastric cancer and cancer overall.
• A 1994 cancer prevention study entitled the Alpha-Tocopherol
(vitmain E)/Beta-Carotene Cancer Prevention Study (ATBC)
demonstrated that lung cancer rates of Finnish male smokers
increased significantly with beta-carotene and were not
affected by vitamin E.
• Another 1994 study, the Beta-Carotene and Retinol (vitamin
A) Efficacy Trial (CARET), also demonstrated a possible
increase in lung cancer associated with antioxidants.
• The 1996 Physicians' Health Study I (PHS) found no change
in cancer rates associated with beta-carotene and aspirin
taken by U.S. male physicians.
• The 1999 Women's Health Study (WHS) tested effects of
vitamin E and beta-carotene in the prevention of cancer and
cardiovascular disease among women age 45 years or older.
Among apparently healthy women, there was no benefit or harm
from beta-carotene supplementation. Investigation of the
effect of vitamin E is ongoing.
4. Are antioxidants under investigation in current
large-scale clinical trials?
Three large-scale clinical trials continue to investigate
the effect of antioxidants on cancer. The objective of each
of these studies is described below. More information about
clinical trails can be obtained using cancer.gov/clinicaltrials,
www.clinicaltrials.gov, or the CRISP database at
• The Women's Health Study (WHS) is currently evaluating the
effect of vitamin E in the primary prevention of cancer
among U.S. female health professionals age 45 and older. The
WHS is expected to conclude in August 2004.
• The Selenium and Vitamin E Cancer Prevention Trial
(SELECT) is taking place in the United States, Puerto Rico,
and Canada. SELECT is trying to find out if taking selenium
and/or vitamin E supplements can prevent prostate cancer in
men age 50 or older. The SELECT trial is expected to stop
recruiting patients in May 2006.
• The Physicians' Health Study II (PHS II) is a follow up to
the earlier clinical trial by the same name. The study is
investigating the effects of vitamin E, C, and multivitamins
on prostate cancer and total cancer incidence. The PHS II is
expected to conclude in August 2007.
5. Will NCI continue to investigate the effect of
beta-carotene on cancer?
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
6. 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 molecules.
7. 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
• 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
• 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 of selenium.
• 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
• 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.
Source: National Cancer Institute