16.1: Natural Chemicals in Plants

Many of us think of food hazards as “chemicals”—“unnatural” substances like additives and pesticide residues. We don’t give much thought to the fact that even a simple plant is made up of thousands of chemicals, most of them unidentified. As new varieties of plants appear, so do new chemicals. And as plant varieties become extinct, so do some unique and undiscovered chemicals—some of which may have been tomorrow’s “miracle drugs.”

Drug companies search constantly through this rich and constantly changing array of plant chemicals. Plants have long been sources of drugs. Chewing on the bark of a willow tree was known in ancient civilizations to relieve pain. Based on this willow-bark substance (salicylic acid), aspirin (acetylsalicylic acid) was synthesized in 1853. The heart drug digitalis comes from the leaves of the purple foxglove plant.

Many anti-cancer drugs come from plants, such as taxol (for ovarian cancer) from the bark of the Pacific yew tree, and vinblastine (for breast cancer) and vincristine (for leukemia) from the tropical flower rosy periwinkle. Then there’s nicotine in tobacco leaves, caffeine in coffee beans, and cocaine in coca leaves. Some drugs even come into being when a plant spoils, like the blood-thinner dicumarol in spoiled sweet clover (Chap. 7).

People who like to categorize substances into good or bad would have a hard time with plant chemicals (phytochemicals). Easiest to categorize might be vitamins—but then again, even too much of these can be toxic. What about caffeine in coffee? It’s bad if it makes a surgeon’s hand tremble during surgery, but good if it keeps a tired surgeon awake while driving home.

Even if we could identify and test each chemical in a plant before eating it, its chemical content is accurate only for the one part of that one plant at that one moment. Not only do apples on the same tree vary in composition, but one side of a single apple varies from the other. And content changes constantly. How else does fruit ripen, bread become stale, and wine age? How else does the taste of coffee change upon reheating, and cantaloupe give off its fragrance?

Natural Toxins in Plants

Our earliest knowledge of natural toxins came about by trial and error. If a caveman got sick from eating a plant, he wasn’t likely to eat it again. But many of the same toxins in poisonous plants are found in smaller amounts in edible ones. By eating a variety of foods (Chap. 4), we lessen the chance of eating any one toxin in toxic amounts.

In Chapter 13, we saw that goitrogens (substances that can cause goiter) are found in foods like cassava, peanuts, cabbage, soybeans, and turnips. Goitrogens are a concern only if these foods make up the bulk of a diet for a long time. Some people don’t have much else to eat but cassava. Cassava also has cyanide (yes, the same chemical used in gas chambers for capital punishment), as do some varieties of lima beans and almonds. Much of the cyanide is lost during food processing and preparation. Residual cyanide has been a problem (e.g., causing nerve damage and blindness) in parts of the world where cassava makes up the bulk of the diet.

Some toxins increase in food during storage. Solanine is a nerve toxin found naturally in potatoes; the amount goes up when stored potatoes are exposed to light. This is evident by a greenish tinge under the potato skin. This green pigment is chlorophyll, which also forms in response to light. In the olden days, potatoes were stored in the dark in a cellar. Today, they’re usually sold bagged in colored—rather than clear—plastic to reduce exposure to light.

By peeling off the greenish part, you peel away most of the solanine. Eating such a small amount isn’t a problem—don’t expect any ill effects (e.g., cramps) from eating a green-tinged potato. But now that you know about solanine, you may find yourself needlessly breaking off a green edge on a potato chip!

Some natural plant chemicals (including solanine) are pesticides—they kill pests. Plants can’t run from their enemies; they engage in chemical warfare. Natural pesticides are so abundant in edible plants that many scientists think that the amount of residue from farm pesticides is trivial by comparison. Plants don’t have nerves like insects do, so natural pesticides that affect neurotransmitters (Chap. 15) are ideal weapons against insects.

Plants typically make more of their pesticides/ toxins when injured. Mold damage to celery can cause some varieties to make 100 times more toxin—so much that it can cause a rash on the hands of celery-pickers and produce-checkers. If organic produce (grown without adding pesticides) is blemished, it can have more pesticides than unblemished non-organic produce. Also, if you are growing plants without using pesticides, you might favor varieties that are more resistant to pests (more “natural pesticides”).

Many natural food chemicals can even cause cancer, as can be shown by purifying the substance and feeding it to animals. Safrole is the major component of sassafras oil (from sassafras trees) and causes liver cancer in rats and mice. For this reason, sassafras is no longer allowed to flavor root beer; artificial flavor is used instead. Safrole also is found in much lower concentrations in cinnamon, nutmeg, black pepper, and ginger. Of course, this doesn’t mean we should avoid pumpkin pie spiced with cinnamon, nutmeg, and ginger. Remember, the dose matters.

You can buy sassafras root, leaves, and capsules by mail order and in some health-food stores. Because they’re sold as dietary supplements, they don’t have to be approved by the Food and Drug Administration (FDA) for safety and effectiveness (or even contain what they claim), as they would if they were classified as food additives or drugs.

A mail-order catalog says this about sassafras root, leaf, and capsules: Has a very useful effect on the blood system. Because it purifies the blood so well, it is excellent for all skin disorders, especially ailments such as acne. It stimulates the action of the liver to clear toxins from the body.*

^{*Vague terms like purify blood and clear toxins are commonly used in such claims. Kidneys continually purify blood and clear toxins. If they didn’t, we’d soon be poisoned to death, unless we get a kidney transplant, or undergo continual dialysis to clear the blood of toxins. As another example, we can go into a coma and die if the toxin ammonia (from surplus amino groups of amino acids) isn’t made into urea by the liver (Chap. 11).}