Stop worrying so much and eat your vegetables. That's the advice Bruce
Ames gives to those concerned that pesticide residues in their food
along with other chemical exposures might be causing them cancer.
Ames, biochemist and toxicologist at the
University of California, Berkeley,
is a pioneer in cancer research whose articles are among the
most cited by other scientists largely thanks to his work on the Ames
mutagen test, an inexpensive predictor of a substance's ability to
mutate certain bacteria cells. Though Ames once advocated the
"one-molecule" theory of carcinogenicity, which implied no safe
threshold for potential toxins, he is now at once more relaxed and
more iconoclastic in appraising the risks of cancer, which he calls
"fundamentally a degenerative disease of old age," though outside
factors such as smoking and poor eating habits can contribute
significantly.
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.]
†