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My common senses would have told me that organic vegetables are healthier and less harmful compared to vegetables treated with pesticides. A recent post on Quora made me think otherwise.

Organic vegetables are much higher in carcinogens than are vegetables grown sprayed with pesticides. This fact was discovered by Bruce Ames who was, at that time, the chair of biochemistry at UC Berkeley.

The discovery was easily understood. To grow foods without pesticides, you have to pick those subspecies that are "naturally resistant" to insects and fungus. That invariably means that they have higher levels of "natural" poisons in their skin and in their flesh. So those farmers who picked the plants that didn't need pesticides were picking plants that (to use Ames' terminology) were surviving by engaging in chemical warfare.

To support his argument, the author cites an article Science, Volume 236, Issue 4799 (Apr. 17, 1987), 271-280, which unfortunately I do not have access as I am not a member of AAAS. Moreover the author is a verified Quora user, and he claims to be a Prof of Physics, UCBerkeley.

Is there evidence to support or refute this argument?

  • Here is the cited paper. While it talks about natural pesticides, it doesn't talk about organic food, so I don't think it supports the Quora argument. I look forward to seeing an answer to this question. – Oddthinking Feb 12 '16 at 9:08
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    Also, please check the organic-food tag to see how your "common senses" have been leading you astray. – Oddthinking Feb 12 '16 at 9:09
  • The question implies that organic vegetables aren't treated with pesticides, but some pesticides (for example, nicotine sulfate) are allowed for organic vegetables. The question, as stated, has no meaning. – amaca Feb 18 '16 at 21:50
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Issue:

Whether organic vegetables are comparably more carcinogenic to vegetables treated with pesticides?

Evidence:

  1. Research shows that numerous chemicals present in natural products tested positive in the Ames test which is a biological test used to detect chemicals that are mutagenic. These chemicals also tested positive in tests for cancer using rodents.

But Dr. Ames began rethinking this war against synthetic chemicals after thousands of chemicals had been subjected to his test. He noticed that plenty of natural chemicals flunked the Ames test. He and Dr. 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.

  1. The carcinogenic effects of organic pesticides are not yet thoroughly studied and known when compared to synthetic chemical pesticides. Also it should not be commonly or generally assumed that all "natural products are uniformly safer, and thus more environmentally friendly, than synthetic chemicals" since research into a particular class of natural pesticides proved the opposite.

Most organic farmers (and even some conventional farmers, too) employ mechanical and cultural tools to help control pests. These include insect traps, careful crop selection (there are a growing number of disease-resistant varieties), and biological controls (such as predator insects and beneficial microorganisms). It should be noted, however, that we don't know for certain which system is more harmful. This is because we do not look at organic pesticides the same way that we look at conventional pesticides. We don't know how long these organic pesticides persist in the environment, or the full extent of their effects. When you look at lists of pesticides allowed in organic agriculture, you find warnings such as, "Use with caution. The toxicological effects of [organic pesticide X] are largely unknown," or "Its persistence in the soil is unknown." Again, researchers haven't bothered to study the effects of organic pesticides because it is assumed that "natural" chemicals are automatically safe.

Half of the pesticides listed for use by organic farmers under the EU’s Organic Regulation could not pass safety evaluation under the Europen Union review in 2009.

Furthermore, just over 1% of organic foodstuffs produced in 2007 and tested by the European Food Safety Authority were found to contain pesticide levels above the legal maximum levels - and these are of pesticides that are not organic. Similarly, when Consumer Reports purchased a thousand pounds of tomatoes, peaches, green bell peppers, and apples in five cities and tested them for more than 300 synthetic pesticides, they found traces of them in 25% of the organically-labeled foods, but between all of the organic and non-organic foods tested, only one sample of each exceeded the federal limits.

  1. A significantly high number of chemicals (99.9%) ingested by humans are naturally present in plants to fight off insects and other predators.

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.

Meaning:

  1. Research shows that the amount of natural pesticide consumption is more than the consumption of synthetic pesticides. The protection offered by natural pesticides in crops is sometimes noted to be less when compared to synthetic pesticides as found in a study comparing the effectiveness of a rotenone-pyrethrin mixture versus a synthetic pesticide, imidan which showed that up to seven applications of the rotenone-pyrethrin mixture were required to obtain the levels of protection provided by two applications of imidan.

We have estimated that on average Americans ingest roughly 5000 to 10,000 different natural pesticides and their breakdown products. Americans eat about 1500 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.

  1. No human diet is found to be free of natural rodent carcinogens. However, this does not mean that natural pesticides used in organic vegetables are dangerously carcinogenic when compared to synthetic pesticide residues since several other factors are known to influence the onset of cancer such as DNA damage through micronutrient inadequacy, type of diet and hereditary reasons.

Human exposure to naturally occurring rodent carcinogens is ubiquitous, and dwarfs the general public’s exposure to synthetic rodent carcinogens. It seems likely that a high proportion of all chemicals, whether synthetic or natural, might be ‘‘carcinogens’’ if administered in the standard rodent bioassay at the MTD, primarily due to the effects of high doses on cell division and DNA damage w x 2,8,12–14,27 . Without additional data on how a chemical causes cancer, the interpretation of a positive result in a rodent bioassay is highly uncertain.

  1. Carcinogenic pesticides whether natural or synthetic used on vegetables do not cause harm through carcinogenicity to humans because their exposure levels are low and the dose ranges given to rodents might not be applicable to humans.

The possible carcinogenic hazards from synthetic pesticides are minimal compared to the background of nature’s pesticides, though neither may be a hazard at the low doses consumed. Analysis also indicates that many ordinary foods would not pass the regulatory criteria used for synthetic chemicals. Caution is necessary in drawing conclusions from the occurrence in the diet of natural chemicals that are rodent carcinogens.

  1. Coffee contains more natural rodent carcinogens by weight than synthetic pesticide residues and there are also several unknown chemicals in coffee that have not yet been tested for carcinogenicity. However, this does not make coffee ingestion dangerous!

For example, more than a thousand chemicals have been identified in roasted coffee; more than half of those tested are rodent carcinogens. There are more natural rodent carcinogens by weight in a single cup of coffee than potentially carcinogenic synthetic pesticide residues in the average US diet in a year, and there are still a thousand known chemicals in roasted coffee that have not been tested. This does not necessarily mean that coffee is dangerous, but that animal cancer tests and worst-case risk assessments, build in enormous safety factors and should not be considered true risks.

Similarly, cooking also produces burnt material that contains many rodent carcinogens and mutagens more than synthetic pesticides but this does not make cooking dangerous for health!

Cooking foods produces about 2000 mg/person/day of burnt material that contains many rodent carcinogens and many mutagens. By contrast, the residues of 200 synthetic chemicals measured by FDA, primarily synthetic pesticides, thought to be of greatest importance, average only about 0.09 mg/person/day.

  1. Many ordinary fruits/vegetables such as lettuce, orange juice, black pepper, mushroom, carrot, potato and apple contain chemicals that are noted to be carcinogenic hazards when ingested at higher amounts and these in the real world would not pass the regulatory criteria for carcinogenicity used for synthetic chemicals. Also not surprisingly, adequate fruit and vegetable consumption is known to be associated with a lowered risk of diseases such as cancers, cardiovascular diseases, cataracts, and brain/immune dysfunction.

Decreases in physical activity, and increases in smoking, obesity, and recreational sun exposure have contributed importantly to increases in some cancers in the modern industrial world, whereas improvements in hygiene have reduced other cancers related to infection.

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In addition to @pericles316's detailed answer,

Summary: As I understand Ames (cited paper and a letter, see below), there are indeed cases where plants bred for increased resistance with the intent of avoiding man-made pesticides had to be withdrawn from the market because of acute toxicity to humans. Ames does not claim that all (nor on average, nor usually) organic plants have higher pesticide levels than conventionally grown plants,

(Slightly OT: but he argues that there is some trade-off between man-made and natural pesticides and that breeding plants for pest resistance does lead to increased levels of natural pesticides. He seems also to be worried that an irrational level of fear of man-made pesticides is going on, to an extent that resources that could be used more efficiently against other causes of cancer are blocked by this fear.
He also talks about the trade-off between pest control and harmful consequences of not having pest contol, e.g. of toxins produced by molds.

Sources: see the papers @pericles316 linked and the one below).


Long version:

The claim in question may be quite directly refering also to Ames BN, Gold LS. Pesticides, risk, and applesauce. Science. 1989 May 19;244(4906):755–757:

In response to fears about residues of man-made pesticides, plant breeders are active in developing varieties that are naturally pest-resistant. Such varieties contain increased amounts of natural pesticides.

He then gives 2 examples:

It should be no surprise, then, that a newly introduced variety of insect- resistant potato had to be withdrawn from the market, due to acute toxicity to humans caused by much higher levels of the teratogens solanine and chaconine than are normally present in potatoes (8). Similarly, a new variety of insect-resistant celery recently introduced widely in the United States is causing outbreaks of dermatitis in produce workers due to a concentration of the carcinogen 8-methoxypsoralen (and related psoralens) of 9000 ppb, rather than the usual 900 ppb (9).

Ames thinks of this as a natural consequence of the breeding effort and expects:

Many more such cases are likely to crop up.

He concludes:

Thus, there is a fundamental trade-off between nature's pesticides and man-made pesticides.

I don't have access to the primary sources, but here are the citations for the example plants:

  • potato: S. J. Jadhav, R. P. Sharma, D. K. Salunkhe, CRC Crit. Rev. Toxicol. 9, 21 (1981); J. H. Renwick et al., Teratology 30, 371 (1984).

  • celery: S. F. Berkley et al., Ann. Intern. Med. 105, 351 (1986); P. J. Seligman et al., Arch. Dermatol. 123, 1478 (1987)

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