Phytochemicals: plants against cancer.

They're not vitamins or minerals. They're not fiber or, complex carbohydrates.

Yet they could explain--at least in part--why eating more broccoli or soybeans or dtrus fruits may help prevent cancer.

They're the hundreds--perhaps thousands--ofphytochemicals that are found in plant foods. You've probably heard some of them mentioned in the news: genistein in soybeans, flayones in dried beans, indoles and isothiocyanates in broccoli.

Well, if we were you, we'd start memorizing those names today. Phytochemicals could be our vitamins of the future, as researcher John Potter of the University of Minnesota explains in an interview beginning on page 9.

Okay, ready? Repeat after us: I-so-THIGH-oh-SIGH-uh-NATES.

Q: Why the hubbub over vegetables and fruits?

A: For the past 20 years, scientists all over the world have consistently found that people who eat greater amounts of vegetables and fruits have lower rates of most cancers. Some other plant foods, like grains, nuts, and seeds, may also protect us. But the clearest evidence is for vegetables and fruits.

Q: Which cancers are they most effective against?

A: Mostly ones that involve epithelial cells, which line the body's organs. That means cancer of the lung, bladder, and cervix, as well as the mouth, larynx (voicebox), throat, esophagus, stomach, pancreas, colon, and rectum.

Q: What about breast, ovarian, and prostate cancer?

A: There is less evidence for those. Their development is much more strongly influenced by hormones like estrogen and testosterone. But, at least for breast cancer, there appears to be a lower risk associated with eating more vegetables.

Q: Are some vegetables and fruits more potent cancer-fighters than others?

A: Some do stand out in the research. It's just not clear whether this merely reflects the fact that it's easier for us to measure the consumption of some foods than others.

For lung cancer, the ones that pop out are carrots and green leafy vegetables. For colon cancer, it's cruciferous vegetables and perhaps carrots.

For cancers of the larynx, throat, mouth, and esophagus, fruit seems to be consistently important. For stomach cancer, it's lettuce and onions.

Although these fruits and vegetables are associated with a lower risk of some cancers, the conclusion I draw after looking at all the evidence is that the lower risk is really associated with vegetables and fruits in general.

Q: How much can a person expect to reduce the risk of cancer by eating more vegetables and fruits?

A: Quite a bit. Researchers have compared the eating habits of people with and without lung, esophageal, and other cancers. Those who ate the most fruits and vegetables were about half as likely to have cancer as those who ate the least?

There are now at least 150 of these kinds of human studies. In almost every one of them, some plant food is associated with a lower risk of some type of cancer.

Q: But the studies don't prove that vegetables and fruits are responsible.

A: That's true. These people could be doing other things that make them less susceptible--eating less fat or exercising more, for instance. But there is other evidence to support a vegetablecancer connection. When animals are given vegetables or fruits before or after being exposed to carcinogens, they are less likely to develop cancer?

And when they are given compounds extracted from the same vegetables and fruits, we see similar results?

Q: What compounds?

A: Hundreds. There are the nutrients such as beta-carotene, vitamin C, and folic acid. There's fiber. And there are at least a dozen groups of chemicals that are not strictly nutrients. We call them "phytochemicals," which simply means chemicals found in plants.

Q: Are they in all vegetables and fruits?

A: Yes, although certain families of plants are riddled with intriguing compounds. The cruciferous vegetables have a whole heap of them, and the sulfur-containing compounds in the onion family are particularly potent in some experiments (see "Good Phyto").

Q: Experiments in animals. What about in humans?

A: We don't have clear evidence that the compounds that are potent in animals and in the laboratory really explain why vegetables and fruits appear to prevent cancer in humans. It's plausible, but we don't have the evidence that makes it absolutely clear.

Q: How would you obtain the evidence?

A: By adding one or another of these compounds to people's diets and seeing whether it lowers their risk of cancer. That's most often done with people who are already at fairly high risk of cancer.

For example, some people who are prone to develop benign polyps in their large intestines are also more likely to develop colon or rectal cancer. We can give these people one or another phytochemical and see if fewer polyps develop.

We can do the same for people suffering from benign breast disease or dysplasia in the cervix. Both are also stages on the path to cancer.

If we can slow down, stop, or even reverse these stages, it would tell us that we are probably affecting the development of the cancer itself.

Q: How many vegetables and fruits should we be eating?

A: More is better, but I don't know how much more. It's extraordinary how little we eat, though, and how many people don't eat any at all.

In the late 1980s, the average American was eating about 1 1/2 servings of vegetables, less than one serving of fruit, and less than a third of a serving of legumes or nuts per day. That comes to about 2Vz servings a day, not counting potatoes. And a serving is pretty tiny--only about half a cup.

Q: Can you eat too much vegetables and fruit?

A: I doubt it. The National Cancer Institute and other authorities advocate five servings a day. But for the last year, my research group has been studying a few hundred people who have been told to eat up to eight servings a day.

While people can initially suffer from considerable inconvenience when they increase their consumption-they may get more bloating, wind, and so on--our preliminary data suggest very strongly that not only do people eventually tolerate it, they actually like it and feel better on it.

Q: Is raw better than cooked?

A: If you cook carotenoids you're going to lose some of their effect because they will absorb more oxygen. And vitamin C is water-soluble, so you'll flush a lot of it away in the cooking water. It's also affected by heat. But a lot of other compounds are heat-stable, so they wouldn't be affected much.

And even if cooking slightly lowers the effect, it certainly isn't a reason to eat only raw vegetables and fruits.

As a matter of fact, cooking may actually free up some of the compounds. When you cook broccoli, for example, you're likely to produce more indoles. That's because there's a particular enzyme that needs to get from one set of cells to another in order to turn broccoli's glucosinolates into indoles.

A lot of chewing will do this, but cooking greatly improves the likelihood that indoles form.

Q: Does it matter if the vegetables are canned, frozen, juiced, or peeled?

A: No. That's just fine-tuning.

Q: Why would plant foods protect against cancer?

A: For most of our evolutionary history we have been gatherers and hunters, but mostly gatherers. I think people tend to fantasize much more about the hunting than the gathering. What happened, I think, is that our metabolisms became dependent upon the plant foods we gathered. And these foods provide us with important substances our bodies don't make.

Now that's a well-established concept when it comes to vitamin C, for example. I propose that we are equally dependent upon a lot of other substances in vegetables and fruits that we've not yet called vitamins. In some sense, they could be regarded as just as essential as vitamins in maintaining us in a healthy, cancer-free state.

Q: Do you think we'll eventually see these phytochemicals in pills?

A: Yes, probably over the next ten years or so. And while I think that they may have some value, what they'll mostly do is produce expensive urine.

That's bemuse we're still discovering more of these compounds, and a pill may lull people into thinking that a single compound---or even a small number of compounds--will do the job of hundreds.

And if we're going to end up with a "multi-vitamin-and-bioactive-compound" pill, it will have to be a gigantic one. So why not eat the food and enjoy yourself at the same time?

Q: Do you think vegetables will be bioengineered to have higher levels of phytochemicals?

A: Yes. I think we'll see companies use genetic engineering to boost the levels of some compounds in vegetables and fruits. But it may well be that if you increase your intake of such and such, for it to do any good you also need to increase something else that hasn't yet been discovered in vegetables and fruits.

You don't have that trouble when you bite into an orange or a piece of steamed broccoli.

1. Cancer Causes and Control 2: 325, 1991.

2. Cancer Causes and Control2: 427, 1991.

Good Phyto

There are hundreds, perhaps thousands, of phytochemicals. Here are some of the more widely studied ones. lt's far from a comprehensive list, though.

Keep in mind that most of the evidence comes from animal studies, and that some is simply the result of seeing what a chemical does to cells in a test tube.

CITRUS FRUITS

Limonene steps up the production of enzymes that may help dispose of potential carcinogens.

GARLIC

ONIONS

LEEKS,

& CHIVES 

Allyl sulfides increase the production of glutathione S-transferase, which may make carcinogens easier to excrete.

Other alllure compounds may decrease reproduction of tumor cells.

BROCCOLI

Dithiolthiones trigger the formation of glutathione S-transferase and other enzymes that may block a cell's DNA.

GRAPES

Ellagic acid scavenges cardnogens and may prevent them from altering a cell's DNA.

SOYBEANS & DRIED BEANS

Protease inhibitors suppress the production of enzymes in cancer cells, which may slow tumor growth.

Phytosterols slow down the reproduction of cells in the large intestine, which may prevent colon cancer.

Isoflavones block the entry of estrogen into cells, which may reduce the risk of breast or ovarian cancer.

Saponins interfere with the process by which DNA reproduces, which may prevent cancer cells from multiplying.

FRUITS

Caffeic acid beefs up the production of enzymes that make carcinogens more soluble in water, which may make it easier to rid them from the body.

Ferulic acid binds to nitrates in the stomach, which may prevent them from being converted into carcinogenic nitrosamines.

GRAINS

Phytic acid binds to iron, which may prevent the mineral from creating cancercausing free radicals.

CRUCIFEROUS 

VEGETABLES

(Bok Choy, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Mustard Greens, Rutabaga, Turnip Greens, and Turnips)

Indoles stimulate enzymes that make the hormone estrogen less effective, which could reduce the risk of breast cancer.

Isothiocyanates trigger the formation of glutathione S-transferase and other enzymes that may block carcinogens from damaging a cell's DNA.