Bee, thanks for posting that. On my way to work this AM, I heard NPR news mention something about McCandless, but did not hear the piece from "All Things Considered". So I read the article linked above with great interest. What a detective story!
Then I looked up the NPR site, and they have an interview with Jon Krakauer, who literally "wrote the book on McCandless". It pretty much summarizes the New Yorker article, but is good to listen: Did Jon Krakauer Finally Solve 'Into The Wild' Mystery?
To summarize: McCandless was subsisting on small game, roots, berries and seeds. He collected and ate the seeds of a "wild potato" plant, Hedysarum alpinum, which is described as nontoxic in books and scientific literature. But they are wrong, in some situations -- one being McCandless' age and condition: young male living on a subsistence diet. Jon Krakaur found a paper published by Ron Hamilton, who had knowledge of a WWII prison camp, Vapniarca, where lack of food led to giving the prisoners horse feed ("pea fodder"), which led to many prisoners' slow paralysis, a disease called Lathyrus.
Hamilton's paper, The Silent Fire: ODAP and the Death of Christopher McCandless
describes sampling and experimenting with the Alaskan wild potato seeds, and finding they contain higher concentrations of ODAP, the toxic protein that causes lathyrus, than the other plants known to be toxic. While the disease is known to permanently paralyze thousands in the poorest third-world areas, until Hamilton's discovery, the wild potato was considered safe and edible.
The only curious question that comes to mind: Why didn't Hamilton contact Krakauer when he made the discovery or posted his paper? It seems that months passed before Krakauer discovered the paper on his own.
It is all quite an interesting chemistry detective story.