Phytochemicals and the American Cancer Society



Other common name(s): certain phytochemicals may be called antioxidants, flavonoids, flavanols, flavanones, isoflavones, catechins, epicatechins, anthocyanins, anthocyanidins, proanthocyanidins, isothiocyanates, carotenoids, allyl sulfides, polyphenols, phenolic acids, and many other names

Scientific/medical name(s): various names


The term “phytochemicals” refers to a wide variety of compounds made by plants, but is mainly used to describe those compounds that may affect human health. Phytochemicals are found in plant-based foods such as fruits, vegetables, beans, and grains. Scientists have identified thousands of phytochemicals, although only a small fraction have been studied closely. Some of the better-known phytochemicals include beta carotene and other carotenoids, ascorbic acid (vitamin C), folic acid, and vitamin E.


Some phytochemicals have either antioxidant or hormone-like actions. There is some evidence that a diet rich in fruits, vegetables, and whole grains reduces the risk of certain types of cancer and other diseases. Researchers are looking for specific compounds in these foods that may account for these healthful effects in humans. Available scientific evidence does not support claims that taking phytochemical supplements is as good for long-term health as consuming the fruits, vegetables, beans, and grains from which they are taken.

How are they promoted for use?

Phytochemicals are promoted for the prevention and treatment of many health conditions, including cancer, heart disease, diabetes, and high blood pressure. There is some evidence that certain phytochemicals may help prevent the formation of potential carcinogens (substances that cause cancer), block the action of carcinogens on their target organs or tissue, or act on cells to suppress cancer development. Many experts suggest that people can reduce their risk of cancer significantly by eating more fruits, vegetables, and other foods from plants that contain phytochemicals.

There are several major groups of phytochemicals.

The polyphenols include a large subgroup of chemicals called flavonoids. Flavonoids are plant chemicals found in a broad range of fruits, grains, and vegetables. They are being studied to find out whether they can prevent chronic diseases such as cancer and heart disease. The isoflavones found in foods and supplements such as soy products, red clover, garbanzo beans, and licorice and the lignans found in flaxseed and whole grains might mimic the actions of the female hormone estrogen (see Licorice and Soybean). These seemingly estrogen-like substances from these plant sources are called phytoestrogens. They may play a role in the development of and protection against some hormone-dependent cancers such as some types of breast and prostate cancer.

Other polyphenols (including some flavonoids) act as antioxidants. These are thought to rid the body of harmful molecules known as free radicals, which can damage a cell’s DNA and may trigger some forms of cancer and other diseases. These compounds are commonly found in teas and in vegetables such as broccoli, Brussels sprouts, cabbage, and cauliflower. Grapes, eggplant, red cabbage, and radishes all contain anthocyanidins — flavonoids that are thought to act as antioxidants and may protect against some cancers and heart disease. Quercetin, another flavonoid that appears to have antioxidant properties, is found in apples, onions, teas, and red wine. Ellagic acid, found in raspberries, blackberries, cranberries, strawberries, and walnuts, is being studied to see if it has anti-cancer effects (see Ellagic Acid).

Carotenoids, which give carrots, yams, cantaloupe, butternut squash, and apricots their orange color, are promoted as anti-cancer agents (see Vitamin A, Retinoids, and Provitamin A Carotenoids). Tomatoes, red peppers, and pink grapefruit contain lycopene, which proponents claim is a powerful antioxidant (see Lycopene). The phytochemicals lutein and zeaxanthin are carotenoids found in spinach, kale, and turnip greens that may reduce the risk of some cancers.

Another group of phytochemicals, called allyl sulfides, are found in garlic and onions (see Garlic). These compounds may stimulate enzymes that help the body get rid of harmful chemicals. They may also help strengthen the immune system.

What does it involve?

Phytochemicals are present in virtually all of the fruits, vegetables, legumes (beans and peas), and grains we eat, so it is quite easy for most people to include them in their diet. For instance, a carrot contains more than a hundred phytochemicals. There are thousands of known phytochemicals, but only a few have been studied in detail.

Many of the better-known phytochemicals can be purchased as dietary supplements. However, most available evidence suggests that these single supplements are not as good for you as the foods from which they are derived.

What is the history behind it?

Only a few years ago, the term “phytochemical” was barely known. But doctors, nutritionists, and other health care practitioners have long advocated a low-fat diet that includes a variety of fruits, vegetables, legumes, and whole grains. Historically, cultures that consume such a diet have lower rates of certain cancers and heart disease.

Since the passage of the Dietary Supplement Health and Education Act (DSHEA) in the United States in 1994, a growing number of phytochemicals are being sold as dietary supplements.

What is the evidence?

The idea that a diet rich in fruits, vegetables, legumes, and grains reduces the risk of cancer, heart disease, and other illnesses is widely accepted. But only recently have researchers begun to try to learn the effects of specific phytochemicals contained in those foods.

Because of the number of phytochemicals and the complexity of the chemical processes in which they are involved, it is difficult for researchers to find out which phytochemicals in foods may fight cancer and other diseases, which may have no effect, and which may even be harmful.

Much of the evidence so far has come from observations of cultures in which the diet comes mainly from plant sources, and which seem to have lower rates of certain types of cancer and heart disease. For instance, the relatively low rates of breast and endometrial cancers in some Asian cultures are credited at least in part to dietary habits. These cancers are much more common in the United States, possibly because the typical American diet is higher in fat and lower in fruits, vegetables, legumes, and grains. Part of the lower risk in Asian cultures is likely due to other factors such as lower obesity rates and more exercise.

Many studies have looked at the relationship between cancer risk and eating fruits and vegetables, legumes, and whole grains. Most of the evidence indicates that eating a diet high in these foods seems to lower the risk of some cancers and other illnesses.

Some of the links between individual phytochemicals and cancer risk found in laboratory studies are compelling and make a strong case for further research. So far, however, none of the findings are conclusive. It is still uncertain which of the many phytochemicals in fruits and vegetables actively helps the body fight disease.

Researchers have also shown much interest in phytochemical supplements. Some laboratory studies in cell cultures and animals have shown that certain phytochemicals have some activity against cancer cells or tumors. But at this time there have been no strong studies in humans showing that any phytochemical supplement can prevent or treat cancer.

Until conclusive research findings emerge, the American Cancer Society’s 2012 nutrition guidelines recommend choosing what you eat and drink in amounts that help you get to and stay at a healthy weight. Eating a balanced diet that includes 2½ cups of vegetables and fruits each day and choosing whole grains over refined grains and sugar-sweetened products should be part of this plan. Limiting intake of red meats and processed meats such as bacon, sausage, lunch meats, and hot dogs is also recommended in order to help reduce cancer risk. A good way to do this is to choose fish, poultry, or beans for some meals rather than beef, pork, lamb, or processed meats. The guidelines note that although eating fish is linked to a lower risk of heart disease, the evidence regarding cancer in humans is limited. A balanced diet with foods from a variety of plant sources is likely to be more effective in reducing cancer risk than consuming large amounts of a few phytochemicals.

Are there any possible problems or complications?

These products are sold as dietary supplements in the United States. Unlike companies that produce drugs (which must be tested before being sold), the companies that make supplements are not required to prove to the Food and Drug Administration that their supplements are safe or effective, as long as they don’t claim the supplements can prevent, treat, or cure any specific disease.
Some such products may not contain the amount of the herb or substance that is on the label, and some may include other substances (contaminants). Actual amounts per dose may vary between brands or even between different batches of the same brand. In 2007, the FDA wrote new rules to improve the quality of manufacturing for dietary supplements and the proper listing of supplement ingredients. But these rules do not address the safety of the ingredients or their effects on health.
Most such supplements have not been tested to find out if they interact with medicines, foods, or other herbs and supplements. Even though some reports of interactions and harmful effects may be published, full studies of interactions and effects are not often available. Because of these limitations, any information on ill effects and interactions below should be considered incomplete.

Phytochemicals, in the amounts consumed in a healthy diet, are likely to be helpful and are unlikely to cause any major problems. Some people assume that because phytochemical supplements come from “natural” sources, they must be safe and free from side effects, but this is not always true. Many phytochemical supplements, especially when taken in large amounts, have side effects and may interact with some drugs. Some of these interactions may be dangerous. Before taking a phytochemical in supplement form, consider talking to your doctor and pharmacist to be sure it will not interact with other medicines or herbs you may be taking. Relying on the use of phytochemicals alone and avoiding or delaying conventional medical care for cancer may have serious health consequences


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