Via Metafilter today comes a thoroughly engrossing article about the cure for scurvy, which was found, lost, and found again, but not without a shocking amount of human suffering in the interim.  As the author points out, this is not just an interesting scientific and medical detective story - it is also a sort of parable about why the scientific process is important, and what happens when people go off half-cocked with incomplete evidence.

As everyone surely knows, scurvy was the scourge of long-distance travelers for hundreds of years.  In particular, the sailors crossing the seas during the expansion of the Western empire.  Arr!  

Scurvy causes a long and painful death, but you have to really work to get it in the first place.  And it is ridiculously easy to cure.  It takes real determination to die from scurvy, although plenty of people did.

Scurvy is caused by a lack of vitamin C.  We and guinea pigs are the only mammals who cannot synthesize our own vitamin C.  Thus, eating the fresh meat of another mammal - say, a bear or seal - will provide a scurvy-reducing dose of vitamin C.  Vitamin C is also present in almost every fresh fruit and vegetable, in varying amounts.

However, vitamin C is also very fragile.  Cooking too long, or storing too long, or cooking in a copper pot - these things will all destroy the vitamin.  Therefore it's the freshies that are important.  Fresh meat, or fresh fruit, or fresh vegetables.  Even fresh unpasteurized milk will do the trick.

The juice of lemons was found to be particularly useful for this, and lemons stored fairly well.  But things started to go wrong for the British navy when they decided that it must be the acidity of the lemons that was doing the trick.  And if it's the acidity, why then the more acidic the better!  Then J. Random Guy asserted - through nothing more than observation of the evidence - that preserved meat contained ptomaine, and that scurvy was a result of ptomaine poisoning.  Acidic fruits helped, because they killed the ptomaine.

Several high profile Antarctic expeditions proved this theory wrong repeatedly.  But it wasn't until someone accidentally switched to guinea pigs for an unrelated experiment that the truth was revealed.  As the article points out, "They had a theory of the disease that made sense, fit the evidence, but was utterly wrong."

This is the danger of coming up with theories based simply on having observed events.  It's our old friend, "correlation does not equal causation" again.  The ptomaine theory was not just completely wrong, it was also easily debunked.  But few people bothered to debunk it - certainly the man primarily responsible for spreading it never thought to actually test his theory.

We see this problem over and over, mostly with people (like Jenny McCarthy) whose scientific literacy is poor.  We also saw it recently in talking about "the Paleolithic diet," which relies on the theory that Paleolithic humans were healthier than modern humans.  Which is not only unobservable, unrecordable, and unknowable - it's also unprovable and completely untested!