7 October 2019

Cancer Risk of Plant-Made vs. Man-Made Pesticides

Bruce N. Ames, PhD

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

The tenet of toxicology, “the dose makes the poison,” means that every chemical at some level can become toxic as well as safe. This applies to both natural and synthetic chemicals, such as plant-made and man-made pesticides.

Given about 391,000 plant species in the world compared to 70,000 synthetic chemicals in use, our exposure to plant-made chemicals is far greater than to man-made. This is especially true for pesticides as there are 20,000 species of edible plants in the world (note fewer than 20 species provide 90 percent of our food) and only four types of synthetic pesticides (insecticides, fungicides, herbicides and rodenticides). All plants have evolved to contain natural toxins against pests, to which we are exposed in the diet in parts per thousand or million, while synthetic pesticides may leave residues on food crops in typically parts per billion if at all. Ingesting plants exposes people to natural toxins in significantly higher amounts than synthetic pesticide residues.

As a result, higher human and animal exposures to natural pesticides cast doubt on the health relevance of far lower levels of exposure to synthetic ones. For example, man-made pesticides are not more likely to be cancer-causing (carcinogenic) in rodents than plant-made pesticides. Yet, the former are much more highly scrutinized.

Moreover, several natural pesticides that have been shown to cause cancer in rodents at high doses have been shown to be anti-carcinogens in humans at lower doses, such as coffee, fruits and vegetables. Therefore, the dose and context of dietary chemical exposure is critical.

Finally, many foods from orange juice to cooked dishes contain substances that are rodent carcinogens. Exaggerating the risks from them to humans, especially from man-made substances, while ignoring the natural world and blaming the pesticide industry, does not advance public health efforts.

All plants have evolved to contain natural toxins against pests, to which we are exposed in the diet in parts per thousand or million, while synthetic pesticides may leave residues on food crops in typically parts per billion if at all.

Natural vs. Synthetic Pesticides

Surprisingly, few plant-made pesticides have been tested in animals for cancer unlike man-made pesticides. Among those tested (52), half are carcinogenic in rodents and present in many common foods in parts per 30,000 to 4 million. Such foods include brewed coffee, orange juice, apples, potatoes, carrots, celery, black pepper, cabbage, Brussels sprouts, fennel, lettuce, basil, nutmeg and mustard.

The finding that in high-dose rodent tests, a high proportion of both natural and synthetic chemicals are cancer-causing, birth defect-inducing and/or DNA-damaging (30-50 percent for each group), undermines regulatory efforts to protect public health from synthetic chemicals alone. Both natural and synthetic chemicals are equally positive in rodent tests. Yet compared to natural dietary carcinogens, the number and levels of cancer-causing synthetic pesticide residues are minimal.

Whether there is any risk at all from very low doses of human exposure to these natural pesticides that cause tumors in rodents at very high doses has been debated by toxicologists and regulators for years, precisely because low-dose effects cannot be measured. However, at the low doses of most human exposures to both, risks are negligible and the comparative hazards of synthetic pesticide residues insignificant.

Natural Toxins

Various natural toxins, some of which have been present throughout vertebrate evolutionary history, nevertheless cause cancer in living beings. Eleven mold aflatoxins, for example, have been shown to cause cancer in trout, rats, mice, monkeys and possibly humans. An additional 19 are DNA-damaging. Similarly, many of the common elements, such as salts of lead, arsenic, nickel, selenium and chromium, are harmful at high doses despite their long-time presence. Ironically, selenium and chromium are essential trace elements in human and animal nutrition.

Sometimes the form of such substances matters as well. For example, chromium comes in two forms – trivalent and hexavalent – with different properties. Trivalent chromium or chromium(III) is essential to human health. Hexavalent chromium or chromium(VI), on the other hand, is unequivocally toxic.

Similarly, cultivated plant foods commonly contain fewer natural toxins than their wild counterparts. For example, wild potato contains three times the amount of a toxic substance than cultivated cousins, rendering it more “wild.”

Along these lines, the secretion of plant toxins in the milk of foraging animals was quite common in previous centuries. Cow and goat milk were contaminated by natural plant toxins eaten by animals in agrarian societies because these toxins were absorbed in their guts. Since the plants foraged by animals varied from place to place and were usually inedible for humans, their toxins in milk were not ones to which humans could have easily adapted. Abraham Lincoln’s mother, for example, died from drinking cow’s milk contaminated with toxins from the snakeroot plant.

Cooked food is also a major dietary source of rodent carcinogens. Cooking produces about 2,000 milligrams per person per day of mostly untested, burned material. Roasted coffee, for example, is known to contain 826 volatile chemicals, 16 of which cause cancer in rodents. A typical cup of coffee contains at least 10 milligrams of these carcinogens. Yet, there is no evidence that coffee, nor cooked food, is a risk for human cancer. In fact, health benefits have been documented in coffee drinkers and cooking food is a sure-fire way to ensure microbial safety.

Synthetic pesticides have markedly lowered the cost of plant foods, thus increasing their consumption. Besides not smoking, eating more vegetables and fruits may be the best way for people to lower cancer risk.

Human Cancer Risk

While incidence rates for some human cancers have been rising, they may be biased due to improved registration and diagnosis. Nonetheless, establishing causes of cancer remains difficult given the many changing aspects of human lifestyle. Most importantly, our life expectancy continues to increase every year.

Epidemiologic data point to major risk factors for human cancer as smoking (30%), dietary and hormonal imbalances, chronic inflammation and infections, not synthetic pesticide residues. For example, salt consumption is a risk factor for stomach cancer. This is Japan’s major type of cancer, where high-sodium diets are common.

Caution is necessary when interpreting the implications of dietary exposure to plant-made pesticides that cause cancer in rodents. Human exposures are likely of little relevance health-wise because consumption of vegetables and fruits is strongly associated with lower cancer rates.

Moreover, how can carcinogenic effects of pesticides in short-life species such as rodents be used to estimate effects in a long-life species like humans? In order to achieve a long lifespan, humans have developed many types of defenses to ensure they are orders of magnitude more resistant to spontaneous cancer at a particular age than rats.

These defenses do not distinguish between natural and synthetic toxins. Thus, the tiny exposure to synthetic pesticide residues compared to the enormous exposure to natural pesticides should not pose greater risk. In fact, synthetic pesticides have markedly lowered the cost of plant foods, thus increasing their consumption. Besides not smoking, eating more vegetables and fruits may be the best way for people to lower cancer risk.

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, and 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 and 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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