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Carrots against cancer?

Carrots Against Cancer?

Can vitamins prevent cancer? Every other morning, 22,000 doctors come a little closer to finding out--by taking a pill.

Half the participants in the Physicians Health Study take 50 milligrams of beta-carotene, a form of vitamin A found in sweet potatoes, carrots, kale, and dozens of other foods. The other half take a placebo--a pill with no active ingredients in it.

Neither group knows what it's getting.

And until this ten-year study and other trials are completed, no one will know for sure if beta-carotene works. But at least one group of people shouldn't wait to find out. Smokers would be smart to increase their intake--if not from food, then from a supplement.

You can't see 'em in kale, collards, or spinach. The chlorophyll drowns them out. But in sweet potatoes, red peppers, winter squash, and (of course) carrots, their true colors shine through. They're carotenes--a family of orange pigments the body converts to vitamin A. (Real vitamin A occurs only in animal foods.)

The National Cancer Institute (NCI) has urged the public to eat more fruits and vegetables rich in carotenes, but has stopped short of recommending vitamin pills. It is now funding more than a dozen trials to see whether the most prevalent carotene, beta-carotene, alone or together with other supplements, can prevent cancers of the lung, colon, skin, and esophagus.

The results of these and other studies won't be in for years. What should people do until then? The answer is clearest if you're a smoker.

Smoke(rs) Alarm. Every minute-and-a-half, tobacco kills another American. Surprisingly, most of these deaths are caused by heart attacks, not cancer. But cigarette smoking also increases the risk of emphysema, bronchitis, and cancer of the bladder, pancreas, kidney, mouth, throat, tongue, lip, larynx, and esophagus. Then there's cancer of the lung.

Lung cancer is the leading cause of cancer deaths in American men. And if breast cancer rates hadn't risen unexpectedly last year, lung cancer would have become the leading cause of cancer deaths in women as well.

Smokers are ten times more likely than non-smokers to get lung cancer. And for those who do, the chances of surviving are dismal. Ninety percent die within five years.

Because beta-carotene seems to cut the risk of lung cancer "only" by half--and since it has no apparent effect on other risks of smoking--many experts are not comfortable advising smokers to consume more of it. "I'm not going to give smokers an easy out," says Regina Ziegler of the National Cancer Institute. "You're kidding yourself if you think you can continue to smoke and it's not going to affect your cancer risk."

But others see things differently. "I may be a heretic," says George Comstock of Johns Hopkins University, "but I think that just because people won't quit doing something harmful to their health, that's no reason not to tell them to do something that might help."

Norman Temple, of the University of Alberta in Edmonton, agrees. "I'd say, first and foremost, don't smoke. But if you don't stop--and even if you do stop, since you're still at risk--you should know that the evidence is now getting quite strong. A guy who consumes few beta-carotene-rich foods typically has a two-fold higher lung cancer risk than the guy who consumes a lot."

What's Known. Two types of evidence now link carotene-rich foods to a reduced risk of lung cancer:

* Blood. Four studies show that smokers who have lower levels of beta-carotene in their blood have a higher future risk of lung cancer. [1-4] A fifth study failed to find a link between blood levels and beta-carotene, but it involved fewer people with lung cancer, and it tested for total carotenoids, which may have obscured the impact of beta-carotene. [5]

* Dietary. At least eight studies show that smokers who eat fewer foods rich in carotenes and/or vitamin A have a higher risk of lung cancer. [6]

A few dietary studies failed to find a link between carotene and cancer, but given the difficulty of getting accurate data on which foods have how much carotene and how often people eat how much of them, that comes as no surprise.

"Imprecision is a problem because beta-carotene is so concentrated in some foods," explains Temple. Underestimate your consumption of carrots, sweet potatoes, or a few other key foods, for example, and your carotene intake will appear far lower than it actually is.

What's Missing. Two kinds of evidence are missing. One (animal studies) can't be done, because there is no good animal "model" for lung cancer, and because rodents don't absorb beta-carotene very well. The other (clinical trials like the Physicians Health Study) are not yet complete.

Clinical trials are critical because they can show cause-and-effect. In other words, they can demonstrate that low carotene intakes are not just associated with--but can actually increase--the risk of cancer.

Also, clinical trials can show whether it's beta-carotene, rather than something else in carotene-rich foods, that's preventing cancer (see "Don't Bother Counting the Carotenes"). "It might be one of the other carotenoids," says Hopkins' Comstock. "Only with controlled trials will we know for sure."

That's why he recommends foods instead of supplements. "People are foolish to take supplements if they can take it in foods," he says.

On the other hand, smokers (for unknown reasons) have a tendency to consume fewer carotene-rich foods. It wouldn't hurt them--it just might not help--to take a beta-carotene supplement on days when they haven't eaten enough of the right foods.

"Whether beta-carotene accounts for vegetables' anti-cancer effect--if there is one--is unclear," says Dartmouth's Robert Greenberg. "But it makes sense, because beta-carotene seems to exist in plants to sop up free radicals and singlet oxygen that are generated during photosynthesis."

Free radicals and singlet oxygen can damage DNA, which may lead to tumors. Beta-carotene might also act by stimulating the immune system (see NAH, August 1988).

What's more, even large doses are safe. Roughly 300,000 International Units (IU) have been used to treat certain skin diseases for more than 15 years. At most, it can turn your skin a yellowish orange, a minor and reversible side effect.

Other Cancers? Smokers have another reason to consume more carotene: cancer of the mouth. Whether they smoke or chew, tobacco users are 4-to-15 times more likely to develop mouth tumors. The risk is especially high for smokers who consume alcohol--even one or two drinks a day.

Again, carotene seems to counter at least some of tobacco's hazards:

* When animals were exposed to a carcinogen, beta-carotene injections thwarted the development of mouth tumors. [7] "The evidence is very promising," says Gerald Sklar of the Harvard School of Dental Medicine. "Beta-carotene and vitamin E are the first nontoxic agents that can both prevent and reverse cancer in experimental animals."

* Beta-carotene and vitamin A reduced the number of micronucleus formations in the mouths of Filipinos who chew a mixture of tobacco and betel nuts. [8] The formations are a sign of DNA damage, and presumably also of cancer risk.

Beyond lung and mouth cancer, carotene's protective effect is punctuated by a large question mark. A few studies have found that people who eat more carotene-rich foods are less likely to have cancers of the prostate, stomach, breast, colon, and esophagus. But other studies have shown no link. In short, further research is needed.

Flawed Studies. Only with cancer of the cervix have more than a few studies been done. However, most are flawed because the researchers failed to adjust for factors that are known to increase the risk of the disease--cigarette smoking, more sexual partners, first intercourse at an early age, and, possibly, use of birth control pills.

"It's difficult to design and analyze a study on cervical cancer because there are so many other risk factors," explains the NCI's Regina Ziegler. As a result, she says, the studies are "provocative and suggestive, but not convincing."

Of the few studies that adjusted for other risk factors, one--from Italy--suggests that women who consume fewer than 100,000 IU of beta-carotene per month (3,300 IU per day) have six times the risk of cervical cancer. [9]

A second good study--from England--found lower beta-carotene levels in the blood of women who had early signs of cervical cancer. [10] However, the risk did not rise with each successive drop in blood levels, as one would expect.

What to Do. Thanks to Madison Avenue, beta-carotene is becoming a household word. Centrum and Geritol now boast that their multi-vitamin supplements contain beta-carotene instead of retinol (vitamin A). Hoffmann-LaRoche, the largest supplier of beta-carotene in the country, is running ads that "inform" readers about the National Cancer Institute's ongoing clinical trials.

"I think the people advertising beta-carotene on TV and in magazines are doing the public a disservice," says Dartmouth's Robert Greenberg, who is now running NCI-sponsored trials on colon and skin cancer. "They imply that the product is beneficial. The fact is, it's open to question."

Based on what we know so far, here's what we recommend:

* Everyone, but especially smokers, should eat more fruits and vegetables rich in carotene. "The worst it could do is give you fiber, vitamins, and potassium, and force excess calories out of your diet," says the University of Alberta's Norman Temple.

* If you want "insurance," it makes sense to take a supplement that contains beta-carotene. Just don't rely on pills instead of food.

* If you do take a supplement, we can't tell you how much is enough. Of the multi-vitamin supplements that contain beta-carotene, most have 1,250 to 5,000 IU (0.75 to 3 mg). The 22,000 doctors who are participating in the Physicians Health Study are taking 83,000 IU (50 mg) every other day.

* Don't take supplements with more than the U.S. Recommended Daily Allowance for retinol (vitamin A), which is 5,000 IU. Too much can be toxic, and large amounts may cause birth defects if taken during pregnancy. In fact, there's no need to take retinol at all. You can get all the vitamin A you need from beta-carotene.

[1] N. Eng. J. Med. 315: 1250, 1986.

[2] Br. J. Cancer 57: 428, 1988.

[3] J. Nat. Cancer Inst. 73: 1463, 1984.

[4] Cancer Res. 45: 2369, 1985.

[5] N. Eng. J. Med. 310: 430, 1984.

[6] Nutr. Res. 8: 685, 1988.

[7] Nutr. Cancer 11: 35, 1988.

[8] Lancet 1: 1204, 1984.

[9] Int. J. Cancer 34: 319, 1984.

[10] Br. J. Cancer 53: 653, 1986.
COPYRIGHT 1988 Center for Science in the Public Interest
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Title Annotation:role of beta-carotene in cancer prevention
Author:Liebman, Bonnie
Publication:Nutrition Action Healthletter
Date:Dec 1, 1988
Previous Article:More than "meats" the eye.
Next Article:Feeding the hungry ... congressmen.

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