How Do Plants Help Prevent Cancer?

Here's how to sort out the facts

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Cancer-prevention allies. Clockwise from top: turmeric, cabbage, garlic, licorice, aloe, and ginseng.

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It’s hard to turn a magazine or newspaper page without finding a new study that touts a plant’s ability to help prevent or treat cancer. But clinical trials show that only a handful of plant products successfully do so. Why this disparity? Do herbs and foods help, or don’t they?

The answer is yes, they can. But the plants that help prevent cancer don’t necessarily help treat it. In fact, from a scientific standpoint, the methods for herbally preventing and treating cancer are nearly diametrically opposed.

Some herbalists would disagree with this statement, saying that effective treatment involves the whole person and that therapies such as boosting the immune system and detoxification are, in fact, primary—it’s thought that cancer cannot flourish in a healthy, balanced, and well-nourished system. Herbal remedies have certainly helped support immune systems, prolong life, and improve the quality of life in some cases.

For the most part, however, this sort of treatment is still theoretical or the existing research is still preliminary. When dealing with cancer, it’s important to be able to sort out what the research supports and what is a more experimental approach. So, for the purposes of this article, “treating cancer” means directly reducing tumor growth. Other benefits, such as boosting the immune system, we will call “adjunct” therapy.

Here’s our guide for helping you sort out fact from fiction and established science from theory.

Preventing cancer

Cancer prevention is the strong point of herbs and functional foods. Literally thousands of plant-derived products are associated with a lower risk for most types of cancer when used throughout your lifetime. These “associations” come from epidemiological studies, which rate the relationship between the risk of various cancers and the level of long-term consumption of a certain plant. This type of study gives clues about which foods and/or supplements one can use over many years to help prevent cancer. Here are some of the ways plants can keep you healthy. (See the chart on page 58 for some examples from each category.)

Prevent mutations. The major initial cause of cancer is mutations of our genetic material, DNA. Toxic or highly reactive chemical compounds can bond directly to DNA, changing and possibly damaging its structure. When a cell divides, the altered DNA copies itself, and this “mutation error” is reproduced in the new cells. These new cells now have a different DNA sequence than the parent cells did.

DNA mutation happens thousands of times a day in a typical human body. Almost always, the body destroys the mutations ­before they do any harm. But occasional mutations survive, with the ability to direct cells to grow out of control—and this is the beginning of cancer.

Suppose your career involves excessive exposure to pesticides, solvents, dyes, or industrial chemicals, any of which can cause mutations. If you want to protect yourself from developing cancer, medicinal herbs and some common foods may help. These plants are antimutagenic, meaning they have been shown to strongly ­reduce DNA damage caused by mutagenic chemicals.

Promote detoxification. You need a healthy liver to prevent cancer. We’re all exposed to potentially toxic chemicals every day, either by accident or as part of normal bodily functions. Many of these toxins can cause mutations. Others can promote the growth of cancer once it has started. Some do both. The liver is our major detoxifying organ, and if it’s not working well, we’re at greater risk for cancer.

Detoxification occurs as a series of biochemical reactions in the liver, gut, and ­kidneys. This process changes a harmful chemical into a relatively nontoxic chemical that the body then excretes. The process is hard work and not without risk—it’s sort of like a SWAT team removing a live bomb from an occupied building and detonating it in the street. Damage is still done, but it’s controlled and nonfatal.

Some phytochemicals are thought to protect against cancer because they aid specific detoxification reactions and therefore shoulder part of the body’s responsibility. According to research, the best bet seems to be a high intake of a wide variety of plants.

Fight free radicals. Free radicals are highly reactive molecules that can potentially damage cells. In part, they create this damage by oxidizing polyunsaturated fats in our cell membranes. When this happens, we’re more susceptible to disease, DNA mutations, and faulty liver function.

Many of the plants that prevent mutation and help detoxification also exhibit antioxidant activity, meaning that they help eliminate free radicals and therefore prevent oxidation. Over the long term, this process is thought to lower our risk for ­developing cancer.

Treating cancer

Why it’s different

Only a few herbal treatments can effectively be used to reduce or kill tumor growth. Most clinically accepted herbal cancer treatments are pure phytochemicals that are druglike in their effect. These phytochemicals work by interfering with specific aspects of the disordered metabolism of cancer cells. Here are a few examples.

Caution: Do not use these plants for self-treatment. The drugs derived from them are highly refined and concentrated, and should be administered and monitored only by a qualified physician.

Herb-derived cancer drugs

• The anticancer drugs Taxol and Etoposide have been developed from the Pacific yew (Taxus brevifolia) and the mayapple (Podophyllum peltatum), respectively, two plants known for centuries to be toxic.

• Perillyl alcohol and related phytochemicals have reversed breast, liver, skin, and pancreatic cancers in research animals with no adverse effects. Perillyl alcohol is found in perilla leaf, lemongrass, spearmint, sage, celery seed, and lemon balm.

• L-canavanine, an amino acid found in small amounts in alfalfa sprouts and other legumes, is toxic because it mimics—or substitutes for—the essential amino acid L-arginine. This can be a problem in a healthy body, but it’s great for treating cancer. L-canavanine is selectively toxic to malignant cells. Canavanine alone or in combination with standard chemotherapy is quite effective for pancreatic and colon cancers in animals, but human studies are not complete. Pancreatic cancer is nearly always fatal, so it’s high on the list of cancers needing more effective treatment.

What works and what doesn’t

Herbal medicines and functional foods are used differently to treat cancer than they are for all other major chronic diseases. For most diseases, we use a low dose of an herb, vitamin, or trace element to prevent a condition and a high dose to treat it. But this type of “sliding dosage-scale” treatment usually can’t be applied to cancer. In fact, very few clinical studies have identified herbs that are effective in such treatments. Here’s why.

Cancer treatment (killing off existing tumors) is necessarily short-term, localized, highly specific, and intrinsically brutal. Once cancer has been ­diagnosed, the primary concern is that tumor masses or circulating abnormal cells are killed or surgically removed and do not ­return.

By definition, cancer cells are abnormal—they have a different metabolism than normal cells, and they have lost their control systems for reproduction and natural death. Once abnormal cells are present, the phytochemicals that once helped maintain normalcy may no longer work.

For example, avoiding oxidation of polyunsaturated fatty acids (PUFA) plays a large role in preventing cancer. However, most cancer cells have significantly fewer PUFAs than normal cells, so they don’t oxidize as easily. Not only do cancer cells lack the “stuff” that oxidizes, they also have unusually high levels of antioxidants. This means that taking high doses of antioxidant vitamins and herbs probably won’t fight existing cancer (although normal doses can be good supportive therapy for chemotherapy and radiation treatments). The more malignant the cancer, the less the cell suffers from oxidative damage—meaning it literally lives forever. This is why cancer is so deadly—the cells keep growing out of control.

Radiation, most chemotherapy drugs, and even our own immune defense mechanisms kill tumors by increasing tissue oxidation in the cancer cells. Large doses of PUFA-rich evening primrose oil also have been used for this purpose, with preliminary successes for liver, brain, breast, bladder, and colon cancers and Hodgkin’s disease (lymphoma). Fish oil and perilla oil have been used in similar ways.

Neither do cancer-preventing phytochemicals treat existing cancer. Once cells have become abnormal, these phytochemicals cannot reverse the process. In fact, some preventive phytochemicals, such as indole-3-carbinol from cruciferous vegetables, can switch sides once cancer has been initiated and actually promote tumor growth.

Don’t let this scare you off, though—antioxidants and cruciferous vegetables such as broccoli are still effective for cancer prevention.

What about the “cancer-cure” herbs?

Currently, many herbal supplements are touted as cancer cures. But their role in actual treatment is adjunct, either to ­improve immune function or lessen medication side effects. Examples of adjunct herbs are maitake, shiitake, and reishi mushrooms, as well as echinacea, mistletoe, goldenseal, Siberian ginseng, and aloe vera. Approaches that combine ­conventional treatment with these herbs, along with special diets and vitamins to help one’s immune system overcome the cancer itself, can offer advantages over conventional treatment alone. (See page 57 for a list of sources for more ­information.)


C. Leigh Broadhurst, Ph.D., is a nutrition consultant in Clovery, Maryland, and author of Diabetes: Prevention and Cure (Kensington, 1999). She holds a doctorate in geochemistry and is a frequent contributor to Herbs for Health.