Mixed Results for Vitamin A
A poll released by the American Institute for Cancer Research found that 54 percent of Americans above the age of 65 are taking vitamins with the specific aim of reducing their risk of cancer, (as are 24 percent of those aged 55 to 64.) However, while eating plenty of fresh fruits and vegetables has been shown in hundreds of studies to lower the risk of developing cancer, there is no strong evidence that taking just vitamin supplements provides the same benefits.
And it can also be dangerous.
Take, for example, vitamin A, which has recently been touted in the news as a preventive measure for breast and other cancers. The National Academy of Sciences Institute of Medicine has just issued recommendations for how much vitamin A is safe to take—no more than 10,000 international units (IU), or 3,000 micrograms per day.
Yet many vitamin A nutritional supplements sold in retail outlets or over the Internet contain up to 25,000 IU. Too much of the vitamin can lead to liver and nerve damage, bone and joint pain, bone loss and birth defects.
Furthermore, three different studies appearing in the Journal of the National Cancer Institute alone have come to three different conclusions about this particular vitamin.
A well-publicized study in the May 17, 2000 issue of the Journal of the National Cancer Institute found that a gene believed to suppress the growth of breast cancer tumors appears to be switched off in the early stages of the disease. But a two-pronged treatment that includes a form of vitamin A (called retinol) may help to reactivate this tumor-suppressing gene and thus prevent tumor growth.
Dr. Martin Widschwendter from the University of Innsbruck, Austria and colleagues identified a gene called RAR-beta-2 that is believed to stop the growth of several types of tumors, including breast cancer tumors. But when this gene is inexplicably turned off, the tumors appear to grow unencumbered.
The Austrian researchers found that a substance identical to vitamin A (a retinol-like substance called ATRA), when mixed with a demethylating agent, can turn RAR-beta-2 back on. In theory, this would then have a suppressing effect on the growth of cancerous tumors.
However, while their studies on laboratory animals have shown promise, they acknowledge that ATRA is still far too toxic for use in humans. Nonetheless, the researchers suggest that retinol-like substances such as ATRA have significant potential for preventing and eliminating breast and other cancers.
Conversely, another study published in the June 21, 2000 issue of the Journal of the National Cancer Institute found that a combination of vitamin A and an antioxidant called N-acetylcysteine does not halt the spread of smoking-related cancers. It was hoped that taking supplements of the two agents, which work differently but have shown individual promise in smaller studies, would help prevent cancer in the lungs, head and neck.
Over a two-year period, Dr. Nico van Zandwijk of The Netherlands Cancer Institute in Amsterdam and colleagues provided vitamin A and N-acetylcysteine supplements to more than 2,500 cancer patients who smoked or had a history of smoking. After a follow-up period of four years, they found that neither supplement, whether taken alone or in combination with each other, had any significant effect on survival or cancer recurrence.
A third study published in late 1999 in the Journal of the National Cancer Institute came to two different conclusions. Dr. Umberto Veronesi from the Istuto Nazionale Tumori in Milan and colleagues found that another substance that is molecularly similar to vitamin A, fenretinide, has no significant effect in preventing a second breast tumor from occurring in postmenopausal women with breast cancer. (In fact, it may actually increase the risk.) However, fenretinide in premenopausal women appeared to lower their risk of a second breast cancer.
"We have been in certain ways surprised because we didn’t expect such a difference in the sense that this drug is very active in very young women," said Veronesi. But after menopause, he noted, "it appears to exert the opposite effect." In fact, his team’s findings suggest that postmenopausal women had a slightly greater risk of a second breast cancer when fenretinide was taken.
A study published in the Archives of Family Medicine found that 40 percent of Americans had taken a vitamin or mineral supplement within the past month. The authors, led by Lina Balluz of the Centers for Disease Control and Prevention in Atlanta, found that the highest use of any type of supplement was among non-Hispanic whites (42.6 percent), with more women (57 percent) than men (43 percent) taking supplements. They calculated that Americans spend up to $1.7 billion each year on vitamin and mineral supplements.
Unfortunately, Balluz’ team also found that more than 300 non-vitamin and non-mineral products were being taken as well—some of which had proven toxic side effects.
The bottom line is that there is simply no substitute for a healthy, well-balanced diet that includes plenty of fruits and vegetables. While most vitamin and mineral supplements are not harmful in appropriate doses—and many may certainly have specific, albeit limited benefits—claims that a particular vitamin or mineral supplement can single-handedly prevent a cancer from occurring are clearly not warranted. And possibly even dangerous.
SOURCES:
Archives of Family Medicine, March 2000; 9:258-262
American Institute for Cancer Research (http://www.aicr.org)
The National Academy of Sciences Institute of Medicine (http://www4.nationalacademies.org/IOM)
Journal of the National Cancer Institute, May 17, 2000; 92:780-781, 826-832
Journal of the National Cancer Institute, June 21, 2000; 92:977-986
Journal of the National Cancer Institute, November 3, 1999; 91:1847-1856
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