Phytochemicals

Overview

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, and researchers are looking for specific compounds in these foods that may account for the beneficial effects in humans. Available scientific evidence does not support claims that taking phytochemical supplements is as helpful 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 may mimic the actions of the female hormone estrogen (see Licorice and Soybean). These 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 vegetables such as broccoli, brussels sprouts, cabbage, and cauliflower and in teas. Grapes, eggplant, red cabbage, and radishes all contain anthocyanidins -- flavonoids that act as antioxidants and may protect against some cancers and heart disease. Quercetin, another flavonoid with antioxidant properties, is found in apples, onions, teas, and red wine. Ellagic acid, found in raspberries, blackberries, cranberries, strawberries, and walnuts, also is said to have anti-cancer effects (see Ellagic Acid).

Carotenoids, which give carrots, yams, cantaloupe, squash, and apricots their orange color, are also promoted as anticancer agents. Tomatoes, red peppers, and pink grapefruit contain lycopene, which proponents claim is a powerful antioxidant (see Lycopene). The phytochemicals lutein and zeaxanthin, found in spinach, kale, and turnip greens, 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 are now available as dietary supplements. However, most available evidence suggests that these single supplements are not as beneficial 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 large number of phytochemicals are being sold as dietary supplements.

What is the evidence?

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

Much of the evidence so far has come from observations of cultures in which the diet consists mainly of 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.

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

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, health care professionals advise a balanced diet with an emphasis on fruits, vegetables, legumes, and whole grains. The interaction between certain phytochemicals and the other compounds in foods is not well understood, but it is unlikely that any single compound offers the best protection against cancer. A balanced diet that includes 5 or more servings a day of fruits and vegetables, along with foods from a variety of other plant sources such as nuts, seeds, whole grain cereals, and beans is likely to be more  effective in reducing cancer risk than consuming one particular phytochemical in large amounts.

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 provide the FDA with results of detailed testing showing their product is safe and effective before the drug is approved for sale), the companies that make supplements do not have to show evidence of safety or health benefits to the FDA before selling their products. Supplement products without any reliable scientific evidence of health benefits may still be sold as long as the companies selling them do not 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 written on the label, and some may include other substances (contaminants). Though the FDA has written new rules to improve the quality of manufacturing processes for dietary supplements and the accurate listing of supplement ingredients, these rules do not take full effect until 2010. And, the new rules do not address the safety of supplement ingredients or their effects on health when proper manufacturing techniques are used.

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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