Plants in the foxglove and milk- weed family make glycosides (e.g., digitalis), which interfere with the transmission of electrical impulses needed for maintaining normal heart rhythm. Lectins cause blood cells to clump and block capillar- ies. Many plants make substances that interfere with the nervous sys- tem-opioids in poppies, caffeine in coffee beans, cocaine in the coca leaf. Are such medically useful substances really toxins? The dose of caffeine contained in a few coffee beans may give us a pleasant buzz, but imagine the effect of the same dose on a mouse! Potatoes contain diazepam (Valium), but in amounts too small even to cause relax- ation in humans. Other plants have toxins that cause cancer or
genetic damage, sun sensitivity, liver damage-you name it. The plant-herbivore arms race has created weapons and defenses of enor- mous power and diversity.
What happens if we overload our bodies with so many toxin mol- ecules that all the processing sites in the liver are occupied? Unlike the orderly queues of shoppers in the supermarket, these molecules do not just wait their turn to be processed. The excess toxins circu- late through the body, doing damage wherever they can. While our bodies cannot instantly make additional detoxification enzymes, many toxins stimulate increased enzyme production in preparation for the next challenge. When medications induce these enzymes, this may hasten the destruction of other medications in the body, thus necessitating dose adjustments. Timothy Johns’s book notes the interesting possibility that inadequate exposure to everyday toxins may leave our enzyme systems unprepared to handle a normal toxic load when one occurs. Perhaps with toxins, as with sun exposure, our bodies can adapt to chronic threats but not to occasional ones.