Browsing a thread on the JABS forum recently I came across a man pointing to some research indicating that thimerosal, a vaccine ingredient, may be a genotoxin. I'm not going to get into the whole antivaccine nonsense here because others have done a far better job (in doing so prompting the very weird thread I was looking at). But I was reminded what potatoes can teach us about faulty logic.

The thread reminded me of a quote from the self-important 16th century physician[1], Phillip von Hohenheim[2], who said: "Tout est poison. Rien n'est poison. Le poison c'est la dose." Everything is poison. Nothing is not poison. The poison is the dose.

Hohenheim died in the 1540s, which meant that he never lived to see the arrival of an exotic new plant in Europe some 30 years later [3], one that quickly established itself as a core part of our diet - the potato. Originating somewhere near Lake Titicaca [4], the potato quickly spread across the world once Europeans established contact with the Americas, but the humble spud contains a deadly secret - it contains potentially lethal neurotoxic chemicals.

The chemicals in potatoes that are particularly worth noting are scopolamine , atropine, solanine, nicotine. Scopolamine has been used as a "truth drug"; atropine is a hallucinogenic drug also used to treat cardiac arrest; solanine can be used as a sedative, but a few hundred milligrams can kill you in as little as 30 minutes; and nicotine of course is an addictive insecticide. All four are common to members of the nightshade family of plants, which includes potatoes, tobacco, capsicums and tomatoes.

But it's not just potatoes that can kill you with their toxins. Apple seeds contain cyanogenic glycosides, which can release hydrogen cyanide gas when acted on by enzymes; chocolate containes theobromine (as does tea and cola), which can stimulate the heart to the point of cardiac arrest; and caffeine is a fatal neurotoxin that can also cause miscarriages in pregnant women.

These chemicals have one thing in common - they are naturally occuring pesticides, designed to protect the plant from damage by insects or herbivores. These defenses can be surprisingly dynamic too - it's hard to think of the potatoes in your kitchen as living things, but their defenses are still active. Potatoes exposed to sunlight "recognize" that they are no longer protected by the earth and start shifting large quantities of solanine toxin to the tissue just under their skins. We can see this as "greening" - the greener a potato gets, the more poisonous it is.

Fortunately we've evolved various defenses of our own - our liver being the front-line. Every day we consumed a wide variety of toxic natural pesticides, and every day our liver filters them out, keeping us safe, as long as we don't consume too much. Being a general-purpose organ, the liver is effective across a range of natural pesticides, and many artificial ones too.

The point I'm getting to is that when people label something "genotoxic" or "neurotoxic", it means basically nothing. Yes, Thimerosal is a genotoxin, but while obviously it's worthy of further research, it doesn't automatically mean that it's unsafe, in the same way that potatoes and chocolate aren't unsafe.

Likewise, it's amusing to see organic-fanatics preaching about the lack of pesticides in their food, when in fact they consume a dose of pesticides on a daily basis. The fact that they are natural rather than artificial makes, in practical terms, little difference. You can't argue that artificial pesticides are worse because we haven't evolved to tolerate them because we didn't evolve to tolerate potatoes or chocolate either - they were only cultivated in the last couple of hundred years or so. If anything, the unpredictability of the dose of natural pesticides might be greater cause for concern.

So if some pseudoscientist tries to scare you with stories about neurotoxins and genotoxins, remember that these are just words. The poison is the dose. And the dose of thimerosal in the latest vaccines is generally tiny to zero.

[1] ...and astrologer, alchemist and occultist. See http://en.wikipedia.org/wiki/Paracelsus.
[2] Philip von Hohenheim seems to have had a bit of a Rome fetish (common in the era), and took to calling himself first "Philippus Theophrastus Aureolus Bombastus von Hohenheim" and then simply Paracelsus, the name meaning "Greater than/equal to Celsus", a Roman medical expert. See http://en.wikipedia.org/wiki/Paracelsus.
[3] Hawkes, J. G. 1992. History of the Potato. In: P.M. Harris, Ed. The Potato Crop: The Scientific Basis for Improvement. Second Edition. Chapman and Hall. London. pp. 1-12.
[4] Spooner, D.M. (2005). A single domestication for potato based on multilocus amplified fragment length polymorphism genotyping. Proceedings of the National Academy of Sciences, 102(41), 14694-14699.