The following is an extract from the article "Nature's chemicals and
synthetic chemicals: comparative toxicology", by Ames BN et al., Proc Natl
Acad Sci USA 87: 7782-7786 (1990) [Some material has been deleted at many
places].

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The Toxicology of Synthetic and Natural Toxins is Similar.
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It is often assumed that, because plants are part of human evolutionary
history whereas synthetic chemicals are recent, the mechanisms that
animals have developed to cope with the toxicity of natural chemicals will
fail to protect us against synthetic chemicals. We find this assumption
flawed for several reasons.
    (i) Defenses that animals have evolved are mostly of a general type:
          (a) shedding of surface layers of the mouth, esophagus, stomach,
              intestine, colon, skin, lungs;
          (b) Detoxifying mechanisms, like antioxidant defenses,
              glutathione transferase for detoxifying alkylating agents;
          (c) Excretion of planar hydrophobic molecules out of liver and
              intestinal cells;
          (d) DNA repair.
    (ii) Various natural toxins cause cancer in vertebrates.
    (iii) Very few of the plant that humans eat would have been present in
         an African hunter-gatherer diet. New foods include: coffee,
         cocoa, tea, potatoes, tomatoes, avocados, mangoes, olives and
         kiwi fruit. In addition, cruciferous vegtables such as cabbage
         were used in ancient times primarily for medicinal purposes.
    (iv) Some anticarcinogens in the diet protect as well against natural
         as against artificial carcinogens.
    (v) It has been srgued that synergism between synthetic carcinogens
        can multiply hazards; however, it's also true of natural
        chemicals.
    (vi) Natural toxins can have the same mechanisms of toxicity as
         synthetic toxins [example cited].

Trade-offs between Natural and Synthetic Pesticides.
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Cultivated foods commonly contain on average fewer natural toxins than do
their wild counterparts. For example, the wild potato Solanum acaule, the
progenitor of cultivated strains of potato, has a glycoalkaloid content
about 3 times that of cultivated strains and is more toxic. The leaves of
the wild cabbage Brassica oleracea contain about twice as many
glucosinolates as cultivated cabbage. The wild bean Phaseolus lunatus
contains about 3 times as many cyanogenic glucosides as does the
cultivated bean. Similar reductions in toxicity through agriculture have
been reported in lettuce, lima bean, mango, and cassava.

(...) The pest-resistant celery contained 6200 ppb of carcinogenic
psoralens instead of 800 ppb present in normal celery.(...)

As an alternative to synthetic pesticides, it is legal for "organic
farmers" to use the natural pesticides from one plant species against
pests that attack a different plant species, e.g. rotenone (which Indians
used to poison fish) or the pyrethrins from chrysanthemum plants. These
naturally derived pesticides have not been tested extensively for
carcinogenicity (rotenone is negative, however), mutagenicity or
teratogenicity as have synthetic pestices; therefore, their safety
compared to synthetically derived pesticides sould not be prematurely
assumed.

Best wishes,

Jean-Louis
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