6 September 2019

Dietary Pesticides 99.99 Percent Natural

Bruce N. Ames, PhD

Senior Scientist & Director
of Nutrition and Metabolism
Childrens Hospital
Oakland Research Institute

Exposure to Plant-Made Pesticides Far Greater Than Man-Made

By Bruce N. Ames, Ph.D.

Since no plot of land is immune to attack by insects, diseases or animal predators, plants need chemical defenses – either natural (plant-made) or synthetic (man-made) – to survive. This includes food crops.

In contrast to public perception, dietary pesticides are 99.99 percent natural as they are chemicals that plants produce to defend themselves. Tens of thousands of these natural pesticides have been discovered and every plant species has perhaps two dozen. For example, cabbage alone has 49 (including breakdown products).

The concentration of natural pesticides is usually measured in parts per thousand or million rather than parts per billion (ppb) which is used for synthetic pesticides. Americans likely consume 5,000 to 10,000 natural pesticides and breakdown products in their lifetime at a rate of about 1,500 milligrams (mg) a day – 10,000 times more than man-made pesticide residues, which the U.S. Food and Drug Administration estimates at a low 0.09 mg/day. The estimated intake of natural pesticides is based on the content of toxins in major plant foods, such as phenolics, flavonoids, glucosinolates, saponins and aflatoxins.

Evolution of Plants vs. Humans

Because plants are part of human evolutionary history and man-made chemicals are recent, it is often assumed that humans have evolved to cope with the toxicity of natural chemicals but not to synthetic ones. This assumption is flawed for several reasons.

First, defenses that humans have developed are mostly of a general type since the number of natural pesticides is so large. General defenses offer protection against both natural and synthetic pesticides, making humans well buffered against food toxins in low doses.

Second, plants have been evolving and refining their chemical weapons for at least 500 million years and incur large fitness costs producing them. If these substances were not effective in deterring predators, plants would not have been naturally selected to produce them, nor would certain plant species still exist!

On the other hand, humans, who evolved less than a million years ago, have not had time to evolve into a “toxic harmony” with all of the plants in their diet. In fact, very few of the plants that we eat today would have existed in the hunter-gatherer days. Our diet has changed drastically in the last few thousand years; most of us eat recently introduced plants that our ancestors did not such as coffee, cocoa, tea, potatoes, tomatoes, corn, avocadoes, mangoes, olives and kiwi fruit. In addition, cruciferous vegetables such as cabbage, broccoli, kale and cauliflower, were not popularized as foods in Europe until the Middles Ages; used primarily for medicinal purposes in ancient times. Natural selection works far too slowly for humans to have developed resistance to the toxins in all of these food crops.

Natural vs. Man-Made Pesticides

We can easily breed out many of nature’s pesticides, but then we need more man-made chemicals to protect crops from pests. On the other hand, we can also increase levels of natural pesticides.

Crops that are more insect-resistant are also higher in natural toxins, which occasionally can impact people. For example, a new variety of highly insect-resistant celery led to rashes and burns on handlers who were exposed to sunlight afterwards. That’s because the new celery contained 6,200 ppb of a certain toxin compared to 800 ppb in normal celery. Worse, a potato variety with higher levels of two natural toxins had to be withdrawn from the market because of its toxicity to people. In addition, when plants are stressed or damaged, as during a pest attack, they increase their natural pesticide levels many-fold, but rarely to levels that are toxic to humans.

Some crops are popular in developing countries because they thrive without costly synthetic pesticides. However, the trade-off is that they are highly toxic and require extensive processing to detoxify them. Cassava root, for example, a major food crop in Africa and South America, is quite resistant to pests and diseases but it contains cyanide at such high levels that only a laborious process of washing, grinding, fermenting and heating can make it edible. Not surprisingly, chronic cyanide poisoning is endemic in many cassava-eating areas of Africa.

Natural and Organic Not Safer

There is a tendency for non-scientists to think of pesticides as being only synthetic and to characterize them as toxic, ignoring the fact that every natural pesticide is also toxic at some dose. For instance, the toxin tomatine is present at 36 mg per 100-gram of tomato (360 ppm), which is a concentration closer to a toxic level in humans than trace amounts of synthetic pesticide residues.

Natural toxins can also have the same mechanisms of toxicity as synthetic toxins like dioxin. For example, cabbage and broccoli contain a chemical that breaks down into substances that mimic dioxin’s behavior in the body. Dioxin causes cancer and birth defects in rodents at extremely low doses. However, the doses humans ingest are far lower. In addition, dioxin pales by comparison to alcoholic beverages in effect due to differences in exposure. Alcoholic beverages are the most birth defect-causing human chemical, which also increases risk of cancer. The birth defect risk of normal exposure to dioxin is equivalent to drinking a single beer (15 grams) over 8,000 years and its cancer-causing potential is the same as ingesting one beer every 345 years.

Ironically, organic farmers are legally allowed to use natural (bio-based) pesticides, such as rotenone, which Indians used to poison fish, or pyrethrins from chrysanthemum plants. These pesticides have not been tested as extensively as synthetic pesticides, therefore, their safety by comparison should not be assumed.

In summary, plant-made pesticides represent the vast majority of dietary pesticides to which humans are exposed and carry equal if not greater risk than man-made ones as a result. Meanwhile, synthetic pesticides have markedly lowered the cost of food crops, which is a major advance in nutrition and thus, health.

Bruce N. Ames, Ph.D., is senior scientist and director of the Nutrition & Metabolism Center at Childrens Hospital Oakland Research Institute in Oakland, Calif., USA. He is also professor emeritus of biochemistry and molecular biology at the University of California at Berkeley.

This editorial was adapted by Angela Dansby, consultant to CropLife International, from the following scientific journal articles with permission from Ames:

  • Ames BN, Profet M, Swirsky Gold L. July 19, 1990. Nature’s chemicals and synthetic chemicals: Comparative toxicology. Natl. Acad. Sci. USA, Vol. 87, pp. 7782-7786.
  • Ames BN, Profet M and Swirsky Gold L. October 1990. Dietary pesticides (99.99% all natural). Natl. Acad. Sci. USA, Vol. 87, pp. 7777-7781.
  • Ames BN and Swirsky Gold L. October 1990. Chemical carcinogenesis: Too many rodent carcinogens. Natl. Acad. Sci. USA, Vol. 87, pp. 7772-7776.

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