Nutrition

Foods that can help cancer patients be a cancer survivor

Green Revolution — Cancer-Fighting Foods

by Kristine Napier, Harvard Health Letter, April 1995

No one wants to be one of the 1.2 million Americans diagnosed with cancer each year. In an effort to avoid this all too common fate, people may fill up on fiber, obsess about antioxidants, or shun red meat and fat. In recent years, however, scientists have realized that these dietary elements are only the tip of the iceberg when it comes to reducing cancer risk.

A previously hidden world of natural chemicals in edible plants is unfolding, and the more researchers learn, the more certain they are that mom was right: we should eat our vegetables, and lots of them.

"There's an explosion of compelling and consistent data associating diets rich in fruits and vegetables with a lower cancer risk," said epidemiologist Tim Byers, who studies the relationship between diet and chronic disease at the Centers for Disease Control and Prevention in Atlanta.

One analysis of data from 23 epidemiologic studies found that a diet rich in vegetables and grains slashed colon cancer risk by 40%. Another study found that women who ate few vegetables had an incidence of breast cancer that was about 25% higher than those who consumed more produce. All in all, at least 200 epidemiologic studies from around the world have found a link between a plant-rich diet and a lower risk for many types of tumors.

Findings such as these have inspired laboratory scientists to try and analyze just what it is about fruits and vegetables that might fend off cancer. "There's more to food than vitamins, minerals, fiber, calories, and protein," said cancer epidemiologist John D. Potter of Seattle's Fred Hutchinson Cancer Research Center.

"We're discovering a plethora of bioactive substances in plant foods." Called functional components, these include a large class of naturally occurring compounds known as phytochemicals. Meanwhile, "many traditional nutrients, including folic acid and selenium, have functions that are becoming clearer — including an ability to fight cancer," said Dr. Potter.

This powerful epidemiological evidence is being bolstered by newer laboratory studies showing how functional components interfere with carcinogenesis. "These compounds seem to interact with every step in the cancer process, mostly slowing, stopping, or reversing them," Dr. Potter said. Most functional components appear to boost the production or activity of enzymes that act as

• blocking agents, detoxifying carcinogens or keeping them from reaching or penetrating cells, or
• suppressing agents, restraining malignant changes in cells that have been exposed to carcinogens.

Hotter than the Internet?

Anyone who hasn't yet heard about functional foods soon will — the term is well on its way to becoming the latest nutrition buzzword. Before settling on this appellation, researchers tossed around at least 20 different names, including "designer foods," "nutriceuticals," "pharmafoods," and "chemopreventers." But the functional foods label won out in 1994 when it was endorsed by the food and nutrition board of the Institute of Medicine. It simply means "foods with ingredients thought to prevent disease."

Although the conventional wisdom is that new trends take hold first on the coasts, in this case a midwestern institution appears to be out in front. The University of Illinois has the nation's first (and only) full-scale scientific program devoted to the study of phytochemicals and other functional components. The Functional Foods for Health Program (FFH) involves 63 faculty members from more than 20 disciplines and represents both the Chicago and Urbana-Champaign campuses of the university.

"The program combines expertise from agriculture and medicine to study how naturally occurring components in foods may protect people from disease," said FFH director Clare Hasler. In related work, the department of medicinal chemistry and pharmacognosy on the Chicago campus maintains the world's largest database on the chemical constituents and pharmacology of plant extracts.

Phytochemicals

When life began, plants were anaerobic — they lived in a world devoid of oxygen. As they evolved and began turning carbon dioxide into oxygen, however, they gradually polluted their own environment. In order to survive, plants were forced to develop defenses against unstable forms of oxygen, explained researcher David Heber, head of the clinical nutrition research unit at the University of California-Los Angeles.

Phytochemicals, many of which are brightly colored and help give plants their vivid hues, are key parts of this antioxidant defense system. In addition to resisting oxidation, these substances guard against an array of adversities including viral attack, harsh weather, and the insults of handling.

Eons later, it appears that humans can now benefit from eating plants that contain these disease-fighting substances. Unlike other minor constituents in food, however, phytochemicals have no calories and no known nutritional value. In other words, they are not necessary for normal physiologic function.

There are literally hundreds of phytochemicals, only a sprinkling of which have been studied. They can be categorized in several ways: by chemical name, by primary food source, and by anti-cancer action.

Many foods contain numerous phytochemicals, each acting via one or several mechanisms. And because new data are being published almost daily, constant updating is needed to keep any list of phytochemicals current. In this section, they are organized by chemical name.

As exciting as scientists find this area of inquiry, they don't pretend to have all the answers yet. "While there's no doubt that diets rich in fruits and vegetables are cancer-protective, much of what we know about individual phytochemicals is still speculative," cautioned nutrition researcher Phyllis Bowen, co-director of the University of Illinois' functional foods program.

Flavonoids are an array of chemicals widely found in fruits, vegetables, and wine. They may reduce cancer risk by acting as antioxidants: blocking the access of carcinogens to cells, suppressing malignant changes in cells, or a combination of these. "Flavonoids may also interfere with the binding of hormones to cells and thus may inhibit cancer development," said nutrition researcher Diane F. Birt of the University of Nebraska Medical Center's Eppley Institute.

Compare Organic Food, particularly ”Flavonoids”.

Indoles and isothiocyanates (also called mustard oils) are largely responsible for putting broccoli on the cancer prevention map. Both account for some of the "bite" in the taste of cruciferous vegetables, and both are breakdown products of complex plant compounds called glucosinolates.

They are formed when these compounds are altered by processing, cooking, or chewing. Scientists believe that indoles and isothiocyanates act mainly by blocking cancercausing substances before they reach their cellular targets. Isothiocyanates may also suppress tumor growth. (See below, "Beyond Broccoli.")

Compare Cruciferous vegetables and cancer risk, “Brassica family prevents pre-cancerous cells turning into potentially deadly cancer cells” and Broccoli (sprouts or extract) may halt growth of breast cancer cells.

Isoflavones are prominent in soy beans and everything that's made from them. Some scientists believe that differences in soy consumption explain why the incidence of breast cancer in Asian women is 5-8 times lower than in American women, as well as why prostate cancer is lower in Asian men. (See "Diet and the Prostate," Harvard Health Letter, July 1994.) Isoflavones can act as antioxidants, carcinogen blockers, or tumor suppressors. Plants contain many forms, including genistein, biochanin A, and daidzein.

See caveats in On Soy & Soy Risks, however.

Other cruciferous chemicals that are thought to have anti-cancer properties include dithiolthiones, chlorophylline (chlorophyll combined with sodium and copper), and organonitriles. In addition to these functional components, cruciferous vegetables are also rich in fiber and in vitamin C and selenium. "No doubt these substances work in some complex synergy to fight cancer," said Matthew A. Wallig, an investigator in functional-foods research at the University of Illinois.

Compare Brassica vegetables and cancer risk, “Brassica family prevents pre-cancerous cells turning into potentially deadly cancer cells” and Broccoli (sprouts or extract) may halt growth of breast cancer cells.

Lignans occur in many foods, but are especially concentrated in linseed (which are seeds from flax, the same plant that is woven into linen cloth). Lignans may have an antioxidant effect and may block or suppress cancerous changes.

"Although flax hasn't been used much in this country, an increasing number of health food stores and bakeries are adding it to bread products," said Dr. Bowen of the University of Illinois. This practice started because flax is also high in omega-3 fatty acids, which are thought to protect against colon cancer and heart disease.

Compare Johanna Budwig diet & protocol.

Organosulfur compounds are found in plants from the genus Allium which includes garlic, onions, leeks, and shallots. Diallyl disulfide is the most potent of these chemicals, which may act as blocking or suppressing agents.

Addendum: In fact, allium vegetables (or some of their other constituents such as flavonoids — quercetin, fisetin etc. — and selenomethionine) have a range of anticarcinogenic effects, see e.g. Garlic, onion and other Allium vegetables against cancer.

Monoterpenes occur naturally in citrus fruits (one variety is D-limonene) and in caraway seeds (in the form of D-carvone). Scientists think they act by interfering with the action of carcinogens.

Saponins are a large family of modified carbohydrates found in many vegetables and herbs. So far, researchers have identified 11 different saponins in soybeans alone. In addition to having anti-cancer activity, there is evidence that some of these substances break down red blood cells, deactivate sperm, or lower circulating levels of certain lipids.

See caveats in On Soy & Soy Risks, however.

Carotenoids

Although phytochemicals have been hogging the spotlight of late, the red and yellow plant pigments known collectively as carotenoids are also thought to be potent cancer fighters. This is still true, even though beta carotene supplements lost some of their luster after several large studies failed to demonstrate the kind of anticancer activity that many people had hoped for. (See "Second Thoughts About Antioxidants," Harvard Health Letter, February 1995.)

Researchers may have been too quick to assume that beta carotene by itself deserved credit for lower cancer rates, according to nutritional epidemiologist Regina G. Ziegler of the National Cancer Institute. People who ate diets rich in fruits and vegetables or who had high circulating levels of beta carotene showed a reduced risk of cancer. But, she pointed out, "blood levels of beta carotene may simply be a good marker for fruit and vegetable intake."

Beta carotene may be beneficial in its natural form, bound up with other constituents of food, but not when it is isolated as a supplement. It is also possible that other carotenoids may be the real cancer inhibitors, and that they may be more efficacious against some types of carcinogens and tumors than others.

In addition to having antioxidant properties, carotenoids may work in several other ways, Dr. Bowen said. They may enhance normal communication among healthy cells, a buzz of biochemical conversation that scientists think helps keep cancer cells from running amok. It's also possible that beta carotene is transformed into retinoic acid, a substance that some researchers say can turn on and off genes that may play a role in cancer development.

New roles for old nutrients

The phytochemicals and carotenoids are just two dietary defenses in the war against cancer. As scientists learn more about how cancer progresses they are finding that some traditional vitamins and minerals also show protective promise.

Folate (also known as folic acid) is best known for its role in the formation of healthy red blood cells. Now there's compelling epidemiologic evidence that people with higher folic acid levels are less likely than others to develop colon cancer and precancerous colon polyps, according to researcher Joel B. Mason, an assistant professor of medicine and nutrition at Tufts University.

"This relationship has been uncovered just in the last few years," said Dr. Mason, who is coordinating a multicenter trial probing folic acid's ability to modify colon cancer risk.

Other researchers have shown that folate contributes to normal tissue formation by guarding the integrity of the genetic messages encoded in DNA, and Dr. Mason speculates that this protective effect may thwart carcinogens that would ordinarily cause colon cancer. But he isn't ready to recommend that people who are worried about colon cancer take large doses of folic acid.

"It's definitely a good idea, though, to get the RDA of 200 micrograms (mcg), or perhaps up to 400 mcg, the level to which some experts recommend raising it." In his experiments, Dr. Mason uses doses 20-40 times greater than the RDA, which would not be safe for everyone. High folate intake can increase seizure risk for people with epilepsy, for example, or may mask B12 deficiency, which can lead to serious neurological troubles, especially in older people.

Calcium appears to have some preventive value where colon cancer is concerned. Researchers propose several mechanisms to explain how calcium acts as an anti-cancer warrior in the colon; these include inhibiting cell growth and/or disarming potential toxins by binding them to fatty acids.

Selenium is being actively studied by epidemiologists and basic scientists with mixed results. Interest was sparked by epidemiologic evidence that population groups with higher selenium intakes have less cancer than those who consume little of this trace mineral.

Although these findings were supported by results from animal experiments, further epidemiologic investigations have found little or no protective effect in humans. Some researchers believe that selenium may work best as an anti-cancer agent in concert with phytochemicals or antioxidants such as beta carotene and vitamin C. A large scale clinical trial now underway is expected eventually to shed more light on this mineral's possible benefits.

But what to eat?

Will Americans soon be slurping down an elixir of lignans, flavonoids, saponins, and folic acid? Most likely not, agree the experts consulted for this article. "I don't condone emphasizing one or even several functional components," said Dr. Hasler from the University of Illinois. "Phytochemicals and other dietary substances no doubt work in concert to fight cancer and other diseases. In addition, isolated phytochemicals may actually be harmful at high doses."

"Simply put," said nutrition and cancer expert Cheryl Rock, an assistant professor at the University of Michigan, “the best advice is to eat real food instead of relying on supplements. If you just take supplements, you simply don't get all of the compounds in foods we're still learning about. We don't know yet if we should combine an indole with an isoflavone, or folic acid with selenium. Right now, only nature knows best."

Recommendations from the National Cancer Institute (NCI) and others emphasize the importance of eating at least five to nine servings of fruits and vegetables each day, aiming for a wide variety. This isn't as hard as it seems: try for one or two fruits at breakfast, one fruit and two vegetables at lunch and dinner, and a snack to make a total of nine.

Although the NCI doesn't fine-tune its advice about exactly what to eat, many experts believe that people can best attain a balance of beneficial substances by making sure that their diet includes foods from each of the following categories:

• cruciferous vegetables
• citrus fruits
• dark green leafy vegetables
• dark yellow/orange/red vegetables

A good rule of thumb is to eat at least three different colors of fruits and vegetables every day. "We know, for example, that the red pigment in tomatoes has completely different bioactive ingredients than the orange pigment in carrots; the same is true for the bioactive ingredients in citrus fruits versus those in the cruciferous vegetables," said Dr. Hasler.

And be sure to eat other plant foods as well, said Dr. Potter. "Grains, nuts, seeds, and legumes also contain a wide variety of bioactive compounds."

Humans got their start as gatherers, probably eating little bits of many different fruits and vegetables every day, and we should strive for such variety and quantity again, said Dr. Potter. Vegetables and fruits contain the anticarcinogenic cocktail to which we are adapted. We abandon it at our peril," he said.

Beyond Broccoli

There are a lot more cruciferous vegetables than most people realize. They include: bok choy, collards, broccoli, kale, Brussels sprouts, kohlrabi, mustard greens, cabbage, rutabaga, cauliflower, turnips.

Malignancy and Macrobiotics

Some people claim that the macrobiotics enthusiasts were on to something long before mainstream nutrition researchers discovered phytochemicals and other functional components in foods. Were they?

Only to a certain extent, according to researchers Cheryl Rock, a University of Michigan expert on nutrition and cancer. "It's a big stretch to call the macrobiotic diet an anticancer diet. While the diet does call for a lot of vegetables, it's low in fruit and also low in calcium and vitamin D."

It's also inadequate in many key nutrients and low in energy and protein, she noted. "Over time, protein and calorie deficits can harm the immune system, which may impair the body's ability to fight cancer."

Compare On macrobiotics & cancer healing.

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