How Chris McCandless Died: An Update

On September 12, 2013, I posted an article titled “How Chris McCandless Died” on this Web site, in which I reported that McCandless, the subject of my book “Into the Wild,” seemed to have inadvertently poisoned himself in 1992, in the Alaska bush, by eating seeds from a plant known colloquially as wild potato, and known to botanists as Hedysarum alpinum. In the first edition of my book, I had speculated that a toxic alkaloid in the seeds had significantly contributed to, if not caused, his death.

The debate over how McCandless died has persisted for two decades now, and many people have insisted that he simply starved to death. But I concluded that, were it not for these seeds, he might very well have survived. After reading an essay by a man named Ronald Hamilton on the apparent toxicity of the seeds that McCandless had eaten, I’d sent a hundred and fifty grams of H. alpinum seeds to Avomeen Analytical Services, in Ann Arbor, Michigan. There, by means of a process called high-pressure liquid chromatography (HPLC), an Avomeen chemist determined that the seeds contained a potentially lethal concentration of a neurotoxin called beta-N-oxalyl-L-alpha-beta diaminopropionic acid, a non-protein amino acid commonly referred to as ODAP.

Five days after my piece was published, Dermot Cole, a journalist in Fairbanks, Alaska, posted an article called “Krakauer’s Wild Theory on McCandless Gives Short Shrift to Science” on the Web site Alaska Dispatch. Cole wrote:

Krakauer should take the advice of Tom Clausen, the retired organic chemist from UAF who has spent much of his career studying plants in Alaska and their properties.

Clausen said that absent peer-reviewed scientific research he would not make any conclusions about what amounts to a highly technical and complicated scientific question.

The difference between a popular account for a general audience and a peer-reviewed journal is that an editor or two may check the former, while the latter will be subject to critical examination aimed at uncovering sloppy work.

Clausen said he has nothing to refute the conclusion, reached by both [Ron Hamilton] and Krakauer, that ODAP was present.

“With that said, let me follow with the comment that I am very skeptical about the entire story,” Clausen wrote in an e-mail….

“I would be much more convinced if I was reading the report from a credible peer-reviewed professional.”

Clausen, I realized, was right: I couldn’t be absolutely certain that the seeds were toxic until I did additional, more sophisticated analysis, and published the results in a reputable peer-reviewed journal. So I embarked on another round of testing.

I began by asking Avomeen to analyze the seeds using a method called liquid chromatography-mass spectrometry (LC-MS), which would give us a crucial piece of information not provided by the HPLC analysis: mass spectrometry would determine the molecular mass, or weight, of the compound in question. This analysis detected a prominent seed component with a molecular weight of 176—the molecular weight of ODAP is 176.13—which appeared to corroborate the earlier HPLC results.

Next, Avomeen suggested that we take the analysis to an even higher resolution, by using liquid chromatography-tandem mass spectrometry (LC-MS/MS). A sample of seeds was analyzed using two mass spectrometers connected by a chamber called a collision cell, in which the seeds’ molecules were shattered into fragmentation ions. The second mass spectrometer sorted and weighed these fragments, producing a fragmentation-ion pattern that is unique for any given compound, and therefore functions like a molecular fingerprint. The results confirmed that the molecular weight of the component we were interested in is 176. But the fingerprint for this component didn’t match the definitive fingerprint of a sample of pure ODAP that was also analyzed. The results were unequivocal: ODAP was not present in H. alpinum seeds. The LC-MS/MS analysis conclusively disproved Hamilton’s hypothesis.

The analysis nevertheless suggested that a significant concentration of a compound structurally similar to ODAP might be present in the seeds. So I combed the scientific literature again, even more exhaustively this time, reading every paper I could find about toxic non-protein amino acids with a molecular mass of 176. Eventually, I discovered an article by a scientist named B. A. Birdsong, published in the Canadian Journal of Botany in 1960, which reported that H. alpinum seeds contain a toxic amino acid called L-canavanine. The mass of L-canavanine, it so happens, is 176.17.

I had missed this article in my earlier searches because I had been looking for a toxic alkaloid instead of a toxic amino acid. Clausen had been looking for a toxic alkaloid as well, when he and Edward Treadwell reported, in a peer-reviewed paper published in the journal Ethnobotany Research & Applications, that “no chemical basis for toxicity could be found” in H. alpinum seeds.

Birdsong and his co-authors had determined the presence of L-canavanine in the seeds using paper chromatography and a spraying agent called trisodium pentacyanoammonioferrate, or PCAF. Given the controversy over the potential toxicity of H. alpinum, and because methods for plant-constituent analysis have advanced significantly in the fifty-four years since Birdsong’s investigation, I asked Avomeen to evaluate the presence of L-canavanine in the seeds using LC-MS/MS, the same technique that had disproved the presence of ODAP. When the Avomeen scientists completed their work, they determined that H. alpinum seeds do indeed contain a significant concentration of L-canavanine: 1.2 per cent by weight.

Canavanine, it turns out, is an antimetabolite stored in the seeds of many leguminous species to ward off predators, and its toxicity in animals is well documented in scientific literature. Numerous cases have been observed of cattle being poisoned after foraging on jack beans (Canavalia ensiformis), the seeds of which contain about 2.5 per cent canavanine by dry weight; symptoms include stiffness of the hindquarters, progressive weakness, emphysema, and hemorrhages of the lymph glands. Although there have been few clinical or epidemiological studies of canavanine-induced illness in humans, there have been anecdotal reports of toxic effects in people who have ingested jack-bean seeds. An article published in the prestigious German journal Die Pharmazie observed that “the scattered reports about poisoning by this plant probably stand in no relation to the actual number of incidences that are caused by it in agricultural practice, because the cause is so difficult to recognize.”

Dr. Jonathan Southard, Dr. Ying Long, Dr. Andrew Kolbert, Dr. Shri Thanedar, and I co-authored a paper titled “Presence of L-canavanine in Hedysarum Alpinum Seeds and its Potential Role in the Death of Chris McCandless,” which will be published in the peer-reviewed journal Wilderness and Environmental Medicine in March, 2015. In the conclusion of our paper, we write:

Our results confirmed the presence of L-canavanine (an antimetabolite with demonstrated toxicity in mammals) as a significant component of H. alpinum seeds…. In the case of Christopher McCandless, there is evidence that H. alpinum seeds constituted a significant portion of his meager diet during a period before his death. Based on this and what is known about the toxic effects of L-canavanine, we make the logical conclusion that under these conditions, it is highly likely that the ingestion of relatively large amounts of this antimetabolite was a contributing factor to his death.

The death of Chris McCandless should serve as a caveat to other foragers: Even when some parts of a plant are known to be edible, other parts of the same species may contain dangerous concentrations of toxic compounds. Additionally, there may be seasonal, as well as ecotypic, variations in the concentrations of L-canavanine between various communities of H. alpinum. Further studies are needed to determine the range of L-canavanine concentrations among different populations of the plant. Given the known toxic properties of L-canavanine and its established presence in H. alpinum seeds, it seems prudent to use caution before ingesting these seeds, especially as a significant part of the diet.

Although Ron Hamilton was wrong about ODAP’s role in the death of McCandless, he was correct that H. alpinum seeds can be poisonous, and that an amino acid is the toxic constituent. But it happens to be L-canavanine instead of ODAP. I am tremendously grateful to Hamilton for publishing “The Silent Fire: ODAP and the Death of Christopher McCandless”; if he hadn’t, it is likely that Birdsong’s article would have escaped my notice, and I would never have learned about the presence of L-canavanine in H. alpinum seeds. And because many people—both admirers of McCandless and his detractors—regard “Into the Wild” as a cautionary tale, it’s important to know as much as possible about how McCandless actually may have died.