Synthetic pesticide residues in plant food are insignificant

[Drs. Bruce Ames and Lois Swirsky Gold] took a systematic look at the chemicals that had been tested on rodents. They found that about half of natural chemicals tested positive for carcinogencity, the same proportion as the synthetic chemicals. Fruits, vegetables, herbs and spices contained their own pesticides that caused cancer in rodents. The toxins were found in apples, bananas, beets, Brussel sprouts, collard greens, grapes, melons, oranges, parsley, peaches — the list went on and on.

Then Dr. Ames and Dr. Gold estimated the prevalence of these natural pesticides in the typical diet. In a paper published in 2000 in Mutation Research, they conclude:

About 99.9 percent of the chemicals humans ingest are natural. The amounts of synthetic pesticide residues in plant food are insignificant compared to the amount of natural pesticides produced by plants themselves. Of all dietary pesticides that humans eat, 99.99 percent are natural: they are chemicals produced by plants to defend themselves against fungi, insects, and other animal predators.

We have estimated that on average Americans ingest roughly 5,000 to 10,000 different natural pesticides and their breakdown products. Americans eat about 1,500 mg of natural pesticides per person per day, which is about 10,000 times more than the 0.09 mg they consume of synthetic pesticide residues.

Even though these natural chemicals are as likely to be carcinogenic as synthetic ones, it doesn’t follow that they’re killing us. Just because natural pesticides make up 99.99 percent of the pesticides in our diet, it doesn’t follow that they’re causing human cancer — or that the .01 percent of of synthetic pesticides are causing cancer either. Dr. Ames and Dr. Gold believe most of these carcinogenic pesticides, natural or synthetic, don’t present problems because the human exposures are low and because the high doses given to rodents may not be relevant to humans.

“Everything you eat in the supermarket is absolutely chock full of carcinogens,” Dr. Ames told me. “But most cancers are not due to parts per billion of pesticides. They’re due to causes like smoking, bad diets and, obesity.”

He and Dr. Gold note that “many ordinary foods would not pass the regulatory criteria used for synthetic chemicals,” but they’re not advocating banning broccoli or avoiding natural pesticides in foods that cause cancer in rodents. Rather, they suggest that Americans stop worrying so much about synthetic chemicals:

Regulatory efforts to reduce low-level human exposures to synthetic chemicals because they are rodent carcinogens are expensive; they aim to eliminate minuscule concentrations that now can be measured with improved techniques. These efforts are distractions from the major task of improving public health through increasing scientific understanding about how to prevent cancer (e.g., what aspects of diet are important), increasing public understanding of how lifestyle influences health, and improving our ability to help individuals alter their lifestyles.

People who fear modernity to the contrary notwithstanding:

Rachel Carson was naive to assume there were “few” natural carcinogens, and she was also naive to assume that evolution had made us “accustomed” to these chemicals. Dr. Ames and Dr. Gold explain why we never had a chance to get accustomed:

Humans have not had time to evolve a “toxic harmony” with all their dietary plants. The human diet has changed markedly in the last few thousand years. Indeed, very few of the plants that humans eat today, e.g., coffee, cocoa, tea, potatoes, tomatoes, corn, avocados, mangoes, olive and kiwi fruit, would have been present in a hunter-gatherer’s diet. Natural selection works far too slowly for humans to have evolved specific resistance to the food toxins in these newly introduced plants.

Of course, our ancestors did develop defenses against toxins. But why assume these defenses work only against natural toxins? Dr. Ames and Dr. Gold don’t buy that assumption:

Humans have many natural defenses that buffer against normal exposures to toxins and these are mostly general, rather than tailored for each specific chemical. Thus they work against both natural and synthetic chemicals. Examples of general defenses include the continous shedding of cells exposed to toxins. The surface layers of the mouth, esophagus, stomach, intestine, colon skin and lungs are discarded every few days; DNA repair enzymes, which repair DNA that was damaged from many different sources; and detoxification enzymes of the liver and other organs which generally target classes of chemicals rather than individual chemicals. It makes good evolutionary sense to conclude that human defenses are usually general, rather than specific for each chemical. The reason that predators of plants evolved general defenses is presumably to be prepared to counter a diverse and ever-changing array of plant toxins in an evolving world.

Dr. Ames and Dr. Gold have also addressed the objection that synthetic pesticides have peculiarly dangerous properties:

DDT was unusual with respect to bioconcentration, and because of its chlorine substituents it takes longer to degrade in nature than most chemicals; however, these are properties of relatively few synthetic chemicals. In addition, many thousands of chlorinated chemicals are produced in nature. Natural pesticides also can bioconcentrate if they are fat-soluble. Potatoes, for example, contain solanine and chaconine, which are fat-soluble, neurotoxic, natural pesticides that can be detected in the blood of all potato eaters. High levels of these potato neurotoxins have been shown to cause birth defects in rodents, although they have not been tested for carcinogenicity.

To repeat, Dr. Ames and Dr. Gold are not suggesting that you stop eating potatoes or fear the plethora of natural pesticides in the produce department. The doses are generally too small to pose a risk. But if, as the scientists estimate, these natural pesticides are 10,000 times more plentiful in your diet than synthetic ones, why worry so much about the chemicals that don’t come from Mother Nature?