The Natural Pesticides in Plants

Being immobile and mostly lacking in physical armor to ward off threats, plants – including our food crops – have evolved some serious chemical warfare to fight back against all manner of hungry microbes, insects and animals. Over their 500 million year evolution, plants have accumulated an impressive array of natural pesticides which share similar toxic and carcinogenic profiles with synthetic pesticides. What is not widely appreciated is that these naturally evolved pesticides are present in our food crops at quantities that are thousands of times higher than tiny traces of synthetic pesticide residues.

Dr. Bruce Ames and his research team have comprehensively shown us over the years that the significance of synthetic pesticide residues in our food crops represents a minor, almost hypothetical risk. He knows what risk looks like, as earlier in his career he established that human mutagens were present in permanent hair dyes, in cigarette smoke and in the fire retardant material in children’s pajamas. His 38 national and international awards include the National Medal of Science awarded by President Bill Clinton, he is a member of the National Academy of Sciences, and he has over 555 scientific publications, and is one of the most often cited scientists in all fields.

One of his classic review articles, Dietary pesticides (99.99% all natural), contains a series of science based observations that you will not find in the popular media coverage of pesticides:

  • About half of all chemicals (natural or synthetic) examined in standard rodent cancer tests at the maximum tolerated dose (MTD) are carcinogens.
  • Twenty-seven of the 52 natural plant pesticides examined are carcinogenic based on standard rodent tests, and they are found in 57 foods including vegetables, fruits, herbs and beverages.
  • It is estimated that humans eat about 5,000 to 10,000 different natural pesticides and their breakdown products; as an example, cabbage contains 49 natural pesticides and metabolites.
  • Concentrations of natural pesticides in plants are usually measured in parts per thousand or million, compared to parts per billion of synthetic pesticide residues or of water pollutants. Thus, Americans eat about 10,000 times more natural pesticides per person per day than they eat of synthetic pesticide residues.

So, why aren’t we dead already?

We are well protected by natural enzymatic defense mechanisms in our body which are equally effective against natural and synthetic chemicals; a diet rich in fruit and vegetables is associated with lower cancer rates which may be because anti-carcinogenic vitamins and antioxidants also come from plants; and because dose matters – rodent cancer tests conducted at the MTD are being misinterpreted to mean that low doses of exposure to chemicals are relevant to human cancer (Ames and Gold, 1998). They propose that the explanation for a high percentage of all chemicals (natural and synthetic) being classified carcinogens at the MTD is that high doses themselves stimulate cell division rates, which increases rates of mutagenesis and carcinogenesis, leading to an exaggeration of risks that may not be relevant at common daily exposure levels. They point to the need to determine mechanisms of carcinogenesis and revise acceptable dose levels accordingly.

They also point out that since fruits and vegetables can help protect against cancer, efforts to reduce the already very small amount of synthetic pesticide used could actually increase cancer by making produce more expensive for people with limited income who already eat fewer fruits and vegetables.

What about the remaining 0.01%, representing the portion of total dietary pesticides we are exposed to that is synthetic? The very small amounts of synthetic pesticide residue on our produce was confirmed again in USDA’s latest Pesticide Data Program results, with 41% of the samples having no detectable residues, and the large majority of samples detected with residues were at concentrations in the parts per billion range, with over 99% of them below the EPA tolerance levels. Still, there are those with other agendas who juggle this data to assert consumers should be afraid of these very small amounts of residues, though their methodology appears to lack scientific credibility.

At the 2011 national meeting of the American Chemical Society, Dr. Ames gave a recorded news briefing on the topic ‘Unfounded Pesticide Concerns Adversely Affect the Health of Low-income Populations.’  He said that the pesticide field is now boring to him – and while they did great work he does not want to spend more time chasing a thousand minor, hypothetical risks, and said if we keep doing that then we are lost. He says opposition to synthetic pesticides is a religious movement, and distracts the public from the real health issues around disease – mainly smoking, a bad diet, obesity and lack of exercise. He saw his main contribution as putting things into perspective – why worry about one part per billion? It’s nothing at all; way too small to be of interest when there are other elephants in the room. He explained that he has moved on to studying mechanisms that are of real importance to disease prevention, focusing on healthy diets and nutrition.

The high quality publications (see below) behind this comprehensive perspective on the relevance of risks from natural and synthetic pesticides are what you would expect from a world class scientist, and they provide a rewarding education for the patient reader.

In summary, at a moderate dosage, coffee gives us courage to seize the day, and we can be unafraid of its minor ingredients:

Natural Pesticides* * *

Dr. Bruce Ames and collaborators on natural and synthetic pesticides:

Ranking Possible Carcinogenic Hazards
Chemical carcinogenesis: Too many rodent carcinogens
Dietary pesticides (99.99% all natural)
Nature’s chemicals and synthetic chemicals: Comparative toxicology
The Causes and Prevention of Cancer: The Role of Environment
Misconceptions about the Causes of Cancer
Pesticide Residues in Food and Cancer Risk: A Critical Analysis
Ranking Possible Cancer Hazards from Rodent Carcinogens, Using the Human Exposure/Rodent Potency Index

2 comments on “The Natural Pesticides in Plants

  1. Great post!
    Very interesting. Thank you.


  2. Dr. Keyser,

    Thank you for this coherent summary of Dr. Aimes’ work on endogenous pyrethrums and pesticides. Hopefully, the significance of this is basic and understandable enough, and that it provides introspective food for thought, rather than fear.

    Thank you for shining some light on this timely subject, particularly as it relates to zero tolerance and the precautionary principle.

    Is it true that endogenous pesticides in fruits and vegetables are less today, due to selection for more palatablity and higher yields, at the expense of more susceptibly to insects and other microorganisms (fungi, perhaps bacteria and viruses?), hence requiring exogenous pesticides?

    Indeed, complex issues require some complex, critical solutions and thinking.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: