It sounds like science fiction: “designer foods” that halt the growth of tumors; “chemoprevention” diets to keep malignancy at bay; potent protective agents on sale in every grocery store. Yet all these miracles are the subject of serious, sober investigation sponsored by the National Cancer Institute. Currently, more than 40 promising substances or combinations found in our diets are being studied for their ability to prevent cancer.
Although it seems too good to be true, evidence is mounting that certain substances in our diet really can shield us from cancer. We’re only beginning to learn how these compounds work, but scientists have already discovered natural chemicals that can defeat malignancy at all three stages of development: initiation, promotion, and progression. There’s even a new word for nutritional products with medicinal properties—”neutraceuticals.”
Often, the first clue to a cancer-fighting agent is a conspicuous lack of cancer in a certain group of people, leading researchers to look for something extra in their diet. In other cases, the opposite holds true; people who have developed cancer are found to be missing a particular nutritional element.
That was the story with the trace element selenium, for instance. While measuring levels of various chemicals in the blood and tissues of cancer patients, researchers discovered that patients with certain cancers had very low levels of selenium, a mineral present in fruits and vegetables grown in selenium rich soil. This led to speculation that correcting selenium deficiency might be cancer-preventive—a theory that is being borne out by current testing.
Once a cancer-fighting ingredient has been identified, it’s only logical to seek ways of boosting its presence in the food supply. Hence the “Designer Foods” program, a term coined by the National Cancer Institute in 1989.
Take that idea one step further and you have “chemoprevention,” or the use of concentrated supplements to cut the odds of developing cancer. When scientists noticed the anticancer effects of beta-carotene- rich foods, for example, they mounted major long-term tests with beta-carotene supplements in an attempt to confirm the substance’s ability to reverse, suppress, or prevent the growth of cancer.
Many such studies are currently in progress to test the cancer-fighting properties of hundreds of substances, both naturally-occurring and synthetic. Early results show that a number may indeed discourage oral, skin, colon, cervical, stomach, lung, and head and neck cancers. For instance, a variety of vitamins and minerals—among them vitamins A, B6, C, E, and zinc—have been linked with reduced rates of bladder, prostate, and kidney cancers.
And ongoing studies sponsored by the National Cancer Institute include dietary changes aimed at preventing breast and colorectal cancer (for example, following a low-fat, high-fiber diet rich in vegetables and fruits) and examination of the role of micronutrients (for example, calcium and vitamins D and E) in preventing lung and colorectal cancer.
Despite these exciting prospects, however, it’s important to remember that we are still a long way from guaranteed protection against cancer. The disease takes hold in many ways, most of them not fully understood; and what works for one individual may not work for the next. Chemoprevention promises to improve your odds of safety, but it’s not an impenetrable shield.
Additionally, scientists still have much to learn about the way nutrients interact. Even though certain nutrients can be singled out as cancer-fighters, they typically work most effectively in combination with others. In fact, nutritional researchers are learning that no nutrient acts alone. Vitamins or supplements that have no effect on disease when tested by themselves have been found to have significant impact when taken in certain combinations. For this reason, taking a supplement may not have the same effect as eating the food that contains it, and nutritionists still stress the need for a varied diet. The so-called “Mediterranean Diet,” for instance, is believed to help prevent heart disease and certain types of cancer. It combines large amounts of complex carbohydrates from vegetables, beans, and whole grain products with minimal amounts of saturated fat and red meat. But by themselves, not one of these ingredients is likely to protect you from anything. It’s only in concert—and in the right proportions—that they begin to have an impact. “Variety is the key,” says Carolyn Clifford, former chief of the diet and cancer branch of the National Cancer Institute. “There is no one perfect food that can supply all the necessary nutrients and save us from disease.”
Enough is known, however, to enable us to shape our overall diet in a way that can be expected to reduce the odds of cancer. To achieve this, the American Institute for Cancer Research and the World Cancer Research Fund recommend that your daily diet:
|•||Be based on plant products|
|•||Include at least 400 grams of vegetables and fruits, to provide more than 10 percent of your calories|
|•||Include cereals, beans (legumes), and tubers to provide 50 percent of your daily calories|
|•||Include less than 80 grams (a little less than 3 ounces) of meat, preferably fish or chicken|
The Cancer-Fighting Dividends of Fruits, Vegetables, and Grains
The data are compelling and consistent: Population studies that examine the relationship between diet and cancer link generous intake of fruits, vegetables, and whole grains to reduced risks of cancer.
A recent report sponsored by the World Cancer Research Fund and the American Institute for Cancer Study estimated that diets high in vegetables and fruits (400 or more grams daily) could prevent 20 percent or more of all cancers. Better yet, in many studies people who eat the greatest amounts of fruits and vegetables are found to have half the cancer risk of those eating the least amounts. And in the laboratory, researchers are zeroing in on the mechanisms by which the various compounds in fruits, vegetables, and grains help prevent cancer. In some cases, prevention occurs in the earliest stages, when cells first go awry and become cancerous. In others, the compounds seem to suppress the growth of malignant cells. Sometimes they work by blocking the carcinogenic effects of other dietary components such as nitrosamines (see “Cut Back on These Foods to Cut Your Cancer Risk”).
In 1991, based on the overwhelming scientific findings, the National Cancer Institute began a national “5 A Day for Better Health” campaign to encourage consumption of 5 or more servings of fruits or vegetables daily. Nevertheless, most Americans do not eat even half that amount. (One serving is measured as half a cup of fresh fruit or cooked vegetables, 1 cup of leafy vegetables, a quarter cup of dried fruit, or three quarters of a cup of juice.)
Fruits, vegetables, and grains owe their cancer-preventing benefits to a host of specific nutrients—many of them unknown until recently. The newest nutritional weapon in the battle to prevent cancer is a group of substances called monoterpenes. Monoterpenes are a non-nutritive part of the essential oils of many fruits, vegetables, and herbs. Some of them have been found to be very effective in preventing the growth of tumors in laboratory animals. D-limonene, for example, a monoterpene which makes up more than 90 percent of orange peel oil, prevents breast, skin, liver, lung, and stomach cancers in laboratory animals. Perillyl alcohol and geraniol are two other monoterpenes under study. These compounds may eventually play a role in treating existing cancers. Their preventive properties are under clinical investigation: Perillyl alcohol is being tested in the treatment of breast, prostate, liver, and ovarian cancer though early results have been somewhat mixed.
While these important new discoveries begin making their way through the pipeline, a host of other nutritional cancer-fighters remain under study. Here’s an overview of today’s other leading contenders.
Dietary Fiber Sampler
The best known of the cancer-fighters, fiber has been a favorite nutritional recommendation for nearly two decades. In the mid-1970s, researchers found that some groups of people in Africa with high fiber consumption (more than 10 times greater than American averages) had extremely low rates of colon cancer. More recent studies have linked high fiber consumption with reduced rates of breast cancer.
It is important not to draw the wrong conclusions from these studies. For example, diets high in fiber are often also very low in fat, and the low fat level may play a significant role of its own in reducing cancer rates. Nevertheless, nutritionists believe that fiber itself can help prevent malignancy, especially in the colon and rectum.
We get fiber in our diet from a variety of sources; fruits, vegetables, and grains—especially whole grains. Fiber is the indigestible (or partially indigestible) part of the food we eat: the bran in grain, the pulp of fruit, the crunchy skin of vegetables, the stringy filaments of beans. Some fiber is soluble (that is, it dissolves); while other fiber is non-soluble.
The fiber you eat affects your digestion in several ways. Fiber helps move food and digestive by-products efficiently through the large intestine (colon) and out of the body. The faster food and digestive by-products travel through the gastrointestinal tract, the less time there is for potential cancer-causing agents to do their damage. Fiber is also thought to dilute potential carcinogens, thus lessening their impact. It also helps alter the metabolism of certain bacteria in the digestive tract, promoting healthy digestion, and may play a role in altering hormone production, thus influencing breast, prostate, and other hormone-related cancers.
However, in recent years scientists have been astonished by some confusing and controversial findings about fiber. In early 1999, a 16-year study of 89,000 women concluded that dietary fiber did not play an important protective role against cancers or benign tumors of the lower digestive tract. When this report was published (in the respected New England Journal of Medicine), it caused a furor in the scientific community. A number of doctors and scientists were critical of the way the researchers had defined fiber and derived their data, and expressed concern that the public would turn its back on the value of eating high-fiber foods (which have also been found to be protective against heart disease).
A few months later another study, this one of 12,700 men followed over a period of 25 years, reported that fiber in the diet did indeed help prevent colorectal cancer. These researchers found that increasing the intake of fiber by 10 grams a day resulted in a 25 percent lower risk of dying from colorectal cancer. Yet hardly had these findings been reported before still more evidence to the contrary appeared. In Spring 2000, two new studies concluded that neither a low-fat/high-fiber diet nor a high-fiber dietary supplement was capable of stemming the growth of colon polyps, which are often a prelude to cancer. Then, later in the year, the pendulum swung again. A small study testing the effects of whole-meal rye bread found that it produced the type of bowel changes that point to decreased risk of colon cancer.
At this point, it’s evident that the experts really don’t know whether fiber is an effective cancer-fighter or not. But since fiber does have other beneficial effects—and won’t do you harm—it’s still a recommended addition to the typical American’s diet.
There are four major types of fiber to consider: cellulose, pectin, lignin, and gums. They have different qualities and they act in somewhat different ways:
Cellulose, the most prevalent type of fiber, softens the stool, prevents constipation, and dilutes bile acids in the colon that are thought to promote cancer growth. Cellulose is found in apples, whole grains, some nuts, carrots, and other fruits and vegetables.
Pectin, a gelatinous substance, also offsets bile acids. It complements the function of cellulose, encouraging healthy digestion by preventing diarrhea. Pectin comes from apples, bananas, beets, carrots, potatoes, and citrus fruit.
Lignin acts as a binder for cellulose. It does not appear to have a major role in preventing constipation or diluting bile acids. However, it has been found to have anti- cancer effects in laboratory animals. Lignin is found in whole grains, nuts, tomatoes, peas, and some fruits.
Gums are sticky fibers derived from plants. Manufacturers use them to thicken many processed foods. Gums lower cholesterol and have anti-cancer effects, although scientists are not sure why. They are found naturally in dried beans, oatmeal, and oat bran.
The National Cancer Institute estimates that Americans now eat an average of 11 grams of fiber daily and recommends doubling that amount. Consuming between 20 and 30 grams of fiber a day could cut your risk of cancer. But eating more than 35 grams daily may cause digestive problems, such as bloating and flatulence. You can avoid these problems by increasing the amount of fiber in your diet gradually. While supplements may furnish some benefits, researchers believe that fiber-rich foods pack a healthier punch.
Meanwhile, researchers continue to discover more about the way fiber works to prevent cancer, and why some fibers may be more beneficial than others. Latest interest has focused on wheat bran, which has been found in some human and animal studies to offer more protection against colon cancer than other types of fiber.
These health-boosting vitamins form one of the hottest areas in nutritional research—and fruits and vegetables are their primary dietary source. Antioxidants counteract the oxidizing (burning) effects of free radicals, harmful molecules that can cause cellular damage throughout the body. Besides leading to heart and coronary artery disease by promoting the buildup of plaque in the walls of blood vessels, oxidation may increase your risk of developing health problems as diverse as cataracts and Alzheimer’s disease. And oxidation can cause cellular damage that may eventually result in malignancy. Antioxidants can interrupt this process, potentially conferring protection from cancer. Some scientists believe that, in high enough doses, antioxidants can even reverse the growth of cancers that have already taken hold.
The body produces its own antioxidants that combine with free radicals to help keep the oxidation process in check. But a number of scientists believe that the body often cannot produce enough antioxidants on its own, especially when confronted with exposure to environmental contaminants like cigarette smoke and polluted air. They recommend antioxidant supplements—and supplementation has, in fact, become extremely popular.
When embarking on a regular supplementation program, however, it’s important to keep two points in mind. First, there’s a growing body of evidence suggesting that antioxidants in pill form may not provide the same benefits as those obtained from natural foods. Second, just as with many other substances, more of an antioxidant is not always better. Beyond certain levels, some actually become toxic. For instance, The U.S. Institute of Medicine has set an upper daily intake level of 2 grams for vitamin C and 1 gram for alpha-tocopherol (equivalent to about 1,500 IU of natural vitamin E or 1,100 IU of synthetic vitamin E).
Here’s an overview of each of the major antioxidants:
The more we discover about antioxidants, the more we realize that much remains to be learned. For example, one of the best known nutrients in our diet is beta-carotene, the pigment that gives the characteristic color to carrots, cantaloupe, and sweet potatoes and is also abundant in leafy green vegetables, such as kale and spinach. But beta-carotene is only one of a number of substances called carotenoids that work in different ways to resist cancer. There are thought to be more than 500 carotenoids. Of these, some 10 percent are converted by the body into vitamin A.
Besides their antioxidant properties, many carotenoids seem to have positive effects on immune function, which can be critical in stopping cancer. Recently, researchers have been focusing on the carotenoids alpha-carotene (in carrots), lycopene (in red fruits and vegetables, such as tomatoes and red peppers), beta cryptoxanthin (in oranges), and lutein and zeaxanthin (in broccoli and leafy green vegetables). People with deficiencies of these substances are more likely to develop certain kinds of cancer, particularly lung cancer, supporting the theory that increased consumption of these carotenoids might protect against cancer.
Although much research data gives cause for optimism about the carotenoid’s cancer-fighting ability, scientists are still getting mixed signals. For example, the National Cancer Institute announced in 1994 that a study of nearly 30,000 male cigarette smokers had found slightly higher rates of lung cancer in those who took beta-carotene supplements than in those who did not. Then, in early 2000, another study reported that long-term (12 years) beta-carotene supplementation had failed to reduce the risk of skin cancer. These unexpected results sent researchers back to the drawing board to investigate the possibility that some other compound in beta-carotene-rich food is responsible for the protective effect noted in several other studies.
Some researchers feel, too, that the lung cancer study was flawed because the length of time that participants took the supplements was too short to interfere with the development of cancer in long-term smokers. And other potential explanations abound. Some researchers reiterate that beta-carotene supplementation does not provide the same benefits as a natural supply obtained from your diet. Others theorize that protection comes not from beta-carotene alone, but from a variety of interactions among the various carotenoids—the type of synergy that, increasingly, seems to be critical for optimal nutritional benefits.
This has sparked additional study of this group of nutrients. In some animal experiments, alpha-carotene has been found more effective than beta-carotene in preventing the development of malignant cells. And there are many other carotenoids under investigation, including lutein, zeaxanthin, phytoene, fucoxanthin, peridinin, and astaxanthin. These substances may not yet be household words, but studies of their cancer-fighting potential are beginning to show promising results.
Lycopene is another carotenoid that has received so much attention lately that it is becoming a household word. Tomatoes are the primary dietary source of lycopene, and it may be that a tomato (or more) a day really could keep the doctor away. (Watermelon and pink grapefruit also contain substantial amounts of lycopene.)
There are indications that lycopene is more easily absorbed if it is cooked—thus tomato sauce or paste or canned tomatoes are better sources than fresh tomatoes. Cooking with a little fat also seems to improve absorption.
The research on lycopene and cancer is very encouraging. More than 35 studies have found that higher levels of blood lycopene or tomato intake are associated with lower risks of cancer, especially cancers of the prostate, stomach, and lung. One study found that people with the lowest levels of lycopene are three times more likely to get lung cancer than those with the highest levels. This study could be of particular interest to African Americans, who had eight times the risk of developing cancer when lycopene levels were lowest. Other studies seem to show a protective effect against a variety of additional cancers—among them, pancreas, colon, rectum, esophagus, oral cavity, breast, and cervix.
Another object of growing scientific interest is lutein, a carotenoid found in many vegetables and fruits, including spinach, broccoli, lettuce, tomatoes, oranges, carrots, celery, and greens. One study of nearly 2,000 people with colon cancer found that lutein intake in the study group was considerably lower than it was in people who were cancer-free. Also gaining increased scrutiny is resveratrol, an antioxidant found in varying concentrations in grapes and wine and some berries and tree barks. In laboratory tests, resveratrol has stopped cancer in all three stages of its formation-initiation, promotion, and progression. It has demonstrated the ability to inhibit the growth of skin, lung, breast, lymphoma, and colon cancer cells.
As we have seen, the body turns beta-carotene and some of the other carotenoids into vitamin A. This may explain some of their anti-cancer activities. Vitamin A has been found to be an effective treatment for some cancers, both in the laboratory and in humans. It is only in recent years however, that researchers have begun sorting out the individual effects of vitamin A, beta-carotene, and the other carotenoids.
Because vitamin A plays an important role in normal cell growth, its use in the prevention or treatment of cancer—which is characterized by abnormal cell growth—is quite plausible. Additionally, it’s known that vitamin A deficiency can result in decreased production of antibodies to certain diseases and in a decrease in the number of T-lymphocyte cells—the frontline defenses of the immune system.
Vitamin A is a retinoid, a category of drugs that has shown much promise in a number of current chemoprevention studies. Physicians often prescribe retinoids in ointment form to treat acne, and have noted an inhibiting effect on some skin cancers. Experiments with synthetic retinoids are showing protective action against lung, oral, cervical, and skin cancers as well as leukemia. The latest work with synthetic retinoids targets specific molecules, an approach that could yield specific treatment or prevention regimens with maximum benefit and minimal side effects.
Beef, liver, eggs, and dairy products contain vitamin A. However, since these are foods generally high in fat and cholesterol, it is probably healthier to get most of your vitamin A via the carotenoids in fruits and vegetables.
It’s Not Just Broccoli
|If you’re one of those people who can’t abide broccoli, don’t despair: It shares its beneficial features with other members of a class of plants called cruciferous vegetables. Although their protective power has yet to be firmly proven in humans, laboratory research has shown these vegetables to be richly endowed with compounds that discourage the development of cancer. One, in particular, renders estrogen (suspected of promoting breast cancer) harmless.
Though broccoli and brussels sprouts are not the favorites of many people, cabbage and cauliflower may be more to their liking. Give bok choy a try, too. This oriental vegetable has a pleasing, un-broccoli-like flavor. Try stir-frying it with mushrooms, garlic, and oyster sauce.
This vitamin—also known as ascorbic acid—has been associated with reduced risks of lung, breast, esophagus, stomach, colon, cervical, and bladder cancers. It also interferes with the formation of potent carcinogens called nitrosamines, and destroys or neutralizes a number of other substances that promote cancer. It works to regenerate vitamin E—another antioxidant—and enhances the immune system. And vitamin C is necessary for the production of collagen, which gives structure to bones, cartilage, muscles, and blood vessels and may help protect the body from cancer by forming a wall around malignant cells.
Although researchers have studied the effects of vitamin C on health for years, much remains unknown about the exact way it works. Recent studies have found that vitamin C alone may not be responsible for anti-cancer properties of certain foods. For example, tomato juice was found to inhibit the formation of nitrosamines even after the vitamin C was removed. Other studies have found that tomato, green pepper, pineapple, strawberry, and carrot juice all have a greater ability to prevent the formation of nitrosamines in the body than the amount of vitamin C they contain would suggest. Two other compounds—p-coumaric and chlorogenic acids—may play an important supporting role as anti-cancer agents. These acids are found in many fruits and vegetables—but not in vitamin C tablets.
Is There a Phytoestrogen In Your Future?
Sometimes referred to as the “premier antioxidant,” this vitamin helps protect against cell damage caused by free radicals, interferes with production of nitrosamines, and stimulates the immune system. All three of these actions are important in preventing and fighting cancer. Low levels of vitamin E have been linked to breast, colon, and lung cancer. Researchers have found it may have an especially potent effect against prostate cancer when taken in combination with Adriamycin, a drug commonly used for cancer chemotherapy. In laboratory animals, vitamin E and beta-carotene seem to enhance each other in the treatment of malignancies.
Vitamin E is found in grains, seeds, nuts, and oils made from seeds and nuts ( for example, sunflower and peanut oil). Wheat germ is an excellent source of vitamin E.
BHA, BHT, Propyl gallate: If you’re a careful reader of ingredient listings, you may have noticed these preservatives on the labels of many packaged foods. One of their primary functions is to prevent the oxidation that takes place when food spoils.
Although synthetic antioxidants are manufactured in a laboratory and do not occur naturally in food, they are still a part of our diet; and it’s good to know that these substances that preserve our food may also be preserving our health. As science learns more and more about the behavior of antioxidants and the specific chemical reactions they trigger inside the body, manufacturers may be able to incorporate ever more effective protective agents into our processed foods.
The Facts about Phytochemicals
Although the word “phytochemical” gained recognition with the discovery of certain cancer-fighting compounds, it is actually a global term for all the hundreds of chemicals that occur naturally in plants. Included among the phytochemicals are such scientific tongue-twisters as dithiolthiones, isothiocyanates, limonene, glycerrhetinic acid, phenols, and protease inhibitors.
Some phytochemicals are common to a wide range of plants. For example, flavinoids, a group of compounds with antioxidant properties, are found in carrots, tea, citrus fruits, berries, broccoli, peppers, squash, tomatoes, soybeans, eggplant, and a variety of other fruits and vegetables. Besides their antioxidant activities, flavinoids seem to be able to inhibit the action of certain hormones such as estrogen, and may help prevent hormone-based malignancies such as breast cancer and prostate cancer.
Likewise, chlorophylls—the chemicals that make green vegetables green—have shown the ability to block the action of certain cancer-causing substances, particularly those that develop when meat is cured or charred.
As research on phytochemicals intensifies, scientists are learning that they may have therapeutic as well as preventive benefits. At nearly every stage of cancer growth, phytochemicals have been found to block cancer activity. Meanwhile, research is pointing to certain vegetables and groups of vegetables that may be particularly valuable cancer fighters. Among the most significant:
No sooner had President George Bush revealed to the American public that he hated broccoli than scientists announced the discovery of a chemical in broccoli called sulforaphane that had a strong preventive effect against cancer in laboratory animals.
Meanwhile, other researchers began reporting that substances called indoles—again, isolated from broccoli and related vegetables—also showed anti-cancer effects in the lab. Specifically, a chemical called indole-3-carbinol changes the way the body processes estrogen, a hormone associated with breast cancer, causing it to break down into a more benign substance.
And that’s not all. Isothiocyanates—other chemicals present in the broccoli family—have shown the ability to trigger the formation of enzymes that can protect cells against damage from potential carcinogens. Sulforaphane is one isothiocynate and cyanohydroxybutene (CHB) is yet another in the list of chemicals in broccoli that seems to play a role in ridding the body of cancer-causing agents.
Broccoli is one of a class of vegetables called brassica. Other vegetables in this class include cabbage, brussels sprouts, cauliflower, bok choy, Swiss chard, and kale. They are known as cruciferous vegetables because of their cross-shaped flowers. Besides their rich endowment of phytochemicals, cruciferous vegetables are also excellent sources of fiber, beta-carotene, vitamin C, and other vitamins and minerals.
All cruciferous vegetables are not created equal: the amount of sulforaphane and other chemicals they contain varies from vegetable to vegetable and within different batches of the same vegetable. Growing conditions—including weather and soil composition—influence the chemical composition of the plants. Some studies have found that broccoli sprouts, little three-day-old seedlings, may pack the most potent punch despite their size. The sprouts have been found to contain greater numbers of cancer-preventing agents than mature plants. But even if you can’t be certain of the exact levels, you can be assured that cruciferous vegetables provide your body with a variety of tools to help protect you from cancer.
Soybeans and Green Tea
The cancer-fighting prowess of these two staples of the oriental diet derives from isoflavones, a form of flavinoids that occurs in a variety of vegetables, but is particularly concentrated in soybeans and green tea. In green tea they can account for about 30 percent of the dry weight of the tea leaves. Isoflavones work in two ways against cancer: as antioxidants and as antimutagens, preventing cell mutations which can become malignancies. In animal studies, some isoflavones—called phytoestrols—have also been shown to inhibit production of the estrogen that is thought to stimulate breast cancer.
In soy products, the isoflavone that may pack the greatest punch is a substance called genistein. Laboratory tests show that genistein has the ability to modify cell proliferation and to curb angiogenesis—the process by which tumors grow the new blood vessels that supply them with oxygen and nutrients. In addition to isoflavones, other types of anti-cancer components have been identified in soy products, including protease and other inhibitors.
Studies indicate that people who eat a lot of soy-based products decrease their risk of developing certain kinds of cancer. In some Asian countries where soy-based foods are common, rates of breast and prostate cancer are far below the American norm. Now, soy products are becoming popular in the American diet as well. You can find tofu (soy-bean curd) in the produce department of most supermarkets; in the baking goods aisle there is soy flour and soybean oil; soy milk is in the beverage aisle; and soy-based desserts fill the freezers.
Not all soy products may be equally protective, however. Fermented products, such as soy sauce, have a different chemical composition than non-fermented soy and we do not yet know whether this affects the food’s anti-carcinogenic properties. Likewise, researchers have yet to establish the exact isoflavone content of the various soy-based products, nor do we know what levels are needed to confer a protective effect. In fact, for the prevention of breast cancer, some experts question whether soy has any beneficial effect at all. Many researchers have suggested that the phytoestrogens in soy may bind to potential cancer cells, blocking the effects of real estrogen and preventing malignancy. But others warn that these compounds may act so much like estrogen that they could actually promote breast cancer.
Most studies have found that green tea exerts a protective effect against cancer when taken in large amounts. The beverage has been shown to reduce the risk of breast, colorectal, lung, pancreatic, and other cancers. Green tea comes from the same plant as the much more common black tea, but is processed differently. Black tea is fermented and green tea is not. The fermentation changes the chemical structure of the tea leaf, which enhances flavor but also seems to decrease the availability of the beneficial chemicals. Another type of tea shares green tea’s relative lack of processing, and seems to have similar cancer-preventing properties. Called white tea, it is rare, very expensive, and found almost exclusively in China. Buds of this variety are covered with silvery hairs, hence the name.
Garlic and Onions
Like soy, garlic seems to offer a variety of benefits. Although results are still preliminary, researchers are finding that in addition to promoting cardiovascular health, garlic and other members of the allium family (onions, scallions, chives, leeks, shallots) may offer protective benefits against cancer.
Garlic is currently one of the most widely studied foods in the field of cancer prevention. Dozens of studies are testing garlic’s protective effect against a wide variety of tumors, including stomach, colorectal, prostate, breast, laryngeal, liver, bladder, and lung cancers. Raw and cooked, whole and broken into its chemical components, in laboratory animals and real people, garlic is showing greater and greater promise as results begin to emerge. A study in a region of China where stomach cancer is unusually common found that the rate among the people who had eaten the most garlic (and onions) was less than half the rate of those who rarely consumed these foods. Laboratory studies looking at the different chemicals that comprise garlic are shedding light on the way it fights cancer.
Garlic and its close relatives contain allyl sulfides, which in laboratory tests have been found to increase the production of enzymes that help rid the body of carcinogens. Allyl sulfides also depress the growth of human cancer cells; and garlic has shown the ability to block the production of nitrosamines as well. Both garlic and onion oil have proven beneficial in treating some cases of skin cancer.
Although people have sung the praises of garlic for centuries, serious scientific research is just getting underway. The many forms of garlic supplements on the market complicate the issue because they are not regulated and vary widely in composition. In addition, depending on growing conditions and individual strains, different batches of fresh garlic differ in their chemistry.
Other Cancer-Fighting Vitamins and Minerals
Although fruits, vegetables, and grains pack the greatest anticancer potential, they are not the only protection that diet affords. Researchers are hard at work investigating several other common dietary elements.
Selenium, a mineral resembling sulfur, is essential to our health in small amounts, though potentially toxic in large doses. Since the 1960s, dozens of studies have linked selenium deficiency to cancer. Others have found that groups with high levels of selenium (as measured in blood and other tissue tests) have low cancer rates. Breast, prostate, skin, lung, and gastrointestinal cancers are all inhibited by selenium. It appears to confer the greatest protection on men, and seems most effective against gastrointestinal cancer.
The selenium in our diets comes from fish and shellfish, organ meats, such as liver, and some whole-grain cereals. It is also present in very small amounts in fruits and vegetables grown in selenium-rich soil. Unfortunately it’s hard to tell how much selenium you are getting in your diet because levels in selenium-bearing foods can vary widely. There is also a difference of opinion among nutritionists about how much selenium is necessary to protect against cancer. Scientists do agree, however, that there is a large gap between the several hundred micrograms of selenium per day that could have a role in cancer prevention and the several thousand micrograms that would cause toxicity.
Selenium works against cancer in several different ways. It is, first of all, a potent antioxidant. It also helps protect cells from damage, aids damaged cells in repairing themselves, and strengthens the immune system. In addition, selenium seems to boost the antioxidant activities of vitamins A and E. As they investigate the synergy among various anticancer nutrients, some scientists are checking the effect of high selenium broccoli and broccoli sprouts, and have found some significant tumor-protective effects in laboratory animals. Other animal research has revealed that selenium inhibits the growth of blood vessels that feed tumors.
Calcium and Vitamin D
Calcium is one of the most important minerals in our diet. It is an essential factor in processes ranging from bone formation to transmission of chemical messages within the brain, and regulation of the heartbeat. Vitamin D is necessary for the absorption of calcium. We get vitamin D from sunlight, fortified dairy products, and other food sources, such as oily fish, beef, liver, butter, and eggs. Calcium comes from milk and other dairy products, dark green leafy vegetables, broccoli, and oily fish such as salmon and sardines.
Population studies have linked high rates of colon and breast cancer to low levels of calcium and vitamin D. These cancers occur less frequently in sunny cities such as Miami and San Antonio, than in northern cities such as Boston and Chicago, where exposure to sunlight is particularly low in winter months, when tall buildings block most of the sun’s slanting rays. One recent study found that calcium supplementation significantly reduced the risk of recurring colorectal adenomas, benign growths which sometimes lead to cancer.
Scientists theorize that calcium binds bile and other fatty acids, thereby reducing irritation in the lining of the colon and impeding the rapid growth of cancer cells. Some laboratory tests have shown that vitamin D itself slows cell growth and thus may have cancer-fighting properties of its own.
Most nutritionists think that food and sunlight are the best sources of calcium and vitamin D. However, exposure to sunlight carries its own skin cancer risks. You must also be careful not to consume too much vitamin D; very high doses can be toxic. Taking antacids with calcium carbonate is one way to supplement calcium in your diet.
B Vitamins and Folic Acid
Some of the B vitamins—particularly B2 (riboflavin), B6 (pyridoxine), and pantothenic acid—strengthen the immune system and may play a role in fighting the early development of malignant cells. In some studies, people deficient in riboflavin have developed cancer of the esophagus. In laboratory tests, folic acid, another of the B vitamins, has a protective effect against precancerous cells from the colon and cervix; and women with low levels of folic acid are more likely to develop cervical cancer.
The B vitamins are available in a variety of foods. Citrus fruits are high in folic acid. Red meat, dairy products, asparagus, and broccoli contain riboflavin. Vitamin B6 comes from white meat chicken and fish, whole grain cereals, egg yolks, potatoes, and bananas.
Zinc, a metal necessary for many of the chemical reactions that take place in our bodies, helps support immune function. People with throat and prostate cancers are often deficient in zinc. Zinc supplements help fight cancer cells in laboratory animals. Meat, seafood, grains, and vegetables are good sources of this mineral.
This spice, most often used in curry powder, has an active ingredient called curcumin that is known for its anti-inflammatory properties. Now there are indications that it may have anticancer effects as well. It has proved effective in early laboratory testing against colon, breast, liver, oral, skin, and stomach tumors in animals.
Dietary fat has received a great deal of bad publicity. We are constantly told that it is bad for us, that it leads to obesity, heart disease, cancer, and a host of other health problems. Nevertheless, some fat is a necessity if our bodies are to function properly. One major recent study (the Nurse’s Health Study) cast doubt on the theory that high fat intake causes breast cancer. And now there is evidence that certain types of fats may actually have a protective effect against cancer.
Omega-3 fatty acids from fish oil have an anti-carcinogenic effect in the laboratory. These are the same fats currently under study for their role in preventing heart disease. While fatty fish, such as salmon, mackerel, sardines, herring, anchovies, and bluefish, are the best-known sources for omega-3, our bodies can also manufacture them from linolenic acid, an essential fatty acid found in rapeseed (used to make canola oil), soybeans, spinach, and mustard greens. Research on the role of omega-3s in cancer prevention is still in its early stages.
Olive oil, a monounsaturated fat, is one of the staples of the Mediterranean diet, which is linked with low rates of both heart disease and cancer. Scientists had studied oleic acid, which is a significant ingredient in olive oil, to see if it had any cancer-preventive properties, but decided that was unlikely because the same compound is also found in beef, poultry, and other oils that do not reduce the risk of cancer. Now attention has turned to an ingredient in olive oil called squalene. This substance kills or supresses tumor growth in animals. An antioxidative compound called pinoresinol may also be involved. Further investigation continues.
Conjugated Linoleic Acid (CLA), a form of an essential fatty acid that comes from dairy products and meat, has demonstrated a strong anticarcinogenic effect against breast tumors in a number of laboratory studies. These results have confounded researchers because CLA is an animal fat, ordinarily considered exactly the thing to avoid. Indeed, some scientists suggest that an interaction among various nutrients may be at work. For example, recent studies of the early signs of colon cancer have found that a combination of CLA, indole-3-carbinol, chlorophyllin, and tea polyphenols inhibit the changes in the colon that can lead to cancer.
Yogurt, like garlic, seems to have mythical health-promoting properties. Yogurt is basically milk fermented by adding bacteria that convert milk sugar (lactose) into lactic acid. Different types of yogurts vary in fat content. A cup of yogurt can contain anywhere from 0 to 11 grams of fat.
Some population studies link high yogurt consumption to reduced risks of cancer, particularly of the breast and colon. Some researchers have also observed that people whose diets include large amounts of yogurt have better immune systems. In animals, Lactobacillus acidophilus, a bacteria used to make some brands of yogurt, slows the growth of cancer-causing cells in the colon.
Like all the other cancer-fighting substances in our diet, yogurt offers no guarantees. Still, if colon or breast cancer tends to run in your family, you could do worse than to develop a taste for this increasingly popular food.