Noting that plants create their own pesticides to fend off insects and other predators, Ames found in the course of setting priorities for carcinogen tests that Americans are exposed to about 10,000 times as many natural chemical toxins, by weight, than synthetic toxins—99.99 percent. When tested on rodents using the same "maximum tolerated dose" standard—in which near-lethal amounts of a substance are repeatedly fed to a rodent to see if it develops cancer—about half of the natural chemicals tested positive as carcinogenic, the same percentage as synthetic chemicals. In essence, researchers had tricked themselves into believing synthetic chemicals were a major cause of cancer because, as Ames says, "we were only testing synthetic chemicals," and because rodent tests were conducted in such a way as to make it more likely to find tested chemicals to be dangerous. But Ames insists that it is the dose that makes the poison—that cancer in rodents is likely caused by chronic tissue damage from injesting poisonously large doses of otherwise benign substances, rather than the chemical per se—and also rejects the idea that risks from smaller doses can be extrapolated in a linear fashion from larger ones.
When testing various foods, Ames found that apple juice contains about
137 varieties of natural toxin, only five of which have been subjected
to animal testing, and three of which tested positive as carcinogens.
Environmental reporter Gregg Easterbrook comments that these three
chemicals—alcohol, acetate, and acetaldehyde—"would inspire
supermarket panic if injected into foods by processing companies."
By contrast, the risk of Alar—an apple preservative that was banned
in 1989 following just such a supermarket panic—in a daily glass of
apple juice is one-tenth that from naturally occurring carcinogenic
hydrazines from consuming a daily mushroom or from the aflatoxin in a
daily peanut butter sandwich. It is also lower than the hazard from
bacon, which contains the sort of carcinogenic nitrosamines (burnt
material) that makes much cooked food a potential health hazard.
According to Ames and his colleague Lois Gold, "The total amount of
browned and burnt material consumed per person in a typical day is at
least several hundred times more than that inhaled in a day from
severe outdoor air pollution."
Cabbage and broccoli also contain a chemical whose breakdown products appear to act upon the body like dioxin, a feared industrial contaminant. Ames estimates that "eating a portion of broccoli daily poses a possible hazard one thousand times that of being exposed to the EPA's allowable dose of dioxin." Compared with alcohol's propensity to cause cancer and reproductive damage, the EPA's allowable daily dose of dioxins would be equivalent to drinking 1/3,000,000 of a beer, or a single glass of beer over a period of 8,000 years. Likewise, the risk from the long-feared pesticide DDT is about a third that from the chloroform found in ordinary tap water, and one third of one percent the risk from the estragole found in a single basil leaf.
As it turns out, surprisingly few natural plant toxins have been through animal cancer tests, simply because no approval process is required to market them. Chemicals from one species of plant were tested, and about half its component chemicals (27 of 52) tested positive as carcinogenic. Plant foods that contained just those 27 chemicals include anise, apple, banana, basil, broccoli, Brussels sprouts, cabbage, cantaloupe, caraway, carrot, cauliflower, celery, cherry, cinnamon, cloves, cocoa, coffee, comfrey tea, dill, eggplant, endive, fennel, grape, grapefruit juice, honey, honeydew melon, horseradish, kale, lettuce, mace, mango, mushroom, brown mustard, nutmeg, orange juice, parsley, parsnip, peach, pear, black pepper, pineapple, plum, potato, radish, raspberry, rosemary, sage, sesame seeds (toasted), strawberry, tarragon, thyme, and turnip. Clearly, there are "too many rodent carcinogens" for the rodent tests to be credible, or one would expect unsustainably high rates of cancer as a result of routine exposure to the environment.
Ames notes that the dangers of natural chemicals are no different than from synthetics, countering the widely cherished belief that man has fallen out of balance with nature by introducing new chemicals, and that nature has not had evolutionary time to catch up by developing natural defenses against them. If it were true that humans had to develop specific evolutionary defenses against foodstuffs, Ames notes that they would not have had time to adapt to dramatically different and relatively recent dietary additions such as potatoes, tomatoes, corn, olives, coffee, cocoa, tea, avocados, mangoes, and kiwi fruit. In fact, animals' biological defenses against toxins—such as the constant shedding of exposed cells and mucous membranes—are generalized rather than targeted towards a specific chemical, and simply can't tell the difference between natural and synthetic chemicals. It is these general defenses that protect people against the low doses of synthetic toxins and the plethora of natural toxins in their food, which is why they can stop worrying and eat their vegetables. Indeed, studies consistently show that people who eat a diet rich in fruits and vegetables are less likely to contract cancer.
[Ed.: Regulatory scientists critical of Ames's work correctly note that he has proposed no alternative to maximum-threshold rodent tests that would gauge cancer risks more accurately. For his part, Ames objects to much regulation of pesticides and preservatives, using the economic argument that raising the cost of fruits and vegetables will, on balance, increase the incidence of cancer as consumers seek less healthy alternative foods.]
