Over the past 35 years, some 400 people in southwestern China have died mysterious, sudden deaths. Many researchers have concluded that they died from eating a toxic mushroom.

Now a McMaster University biologist has changed all that.

Jianping Xu has spent the last four summers climbing mountains and slogging through forests in search of the Trogia venenata, a mushroom once suspected of causing fatal heart attacks with its high concentration of the metal barium.

But after trekking up to 30 kilometres per day collecting samples for analysis, Xu has found that the mushroom may have been a minor factor in the deaths, but not the primary cause.

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Jianping Xu plans to continue studying the wild mushrooms of southwestern China. (Chantall VanRaay/McMaster University)

Xu found that the barium concentration in the mushrooms is so low that it mirrors the level in common foods such as fresh poultry. In fact, a 150-pound person would have to eat at least 35 kg of the dried mushroom for it to be lethal, Xu said.

That means we still don't know what caused Yunnan Sudden Unexplained Death (SUD) in 400 otherwise healthy Chinese citizens during the rainy seasons from June to August, he said.

A 2010 article in the journal Science said the mushrooms were at fault, and that's been the common hypothesis.

Now "it is sort of back to the drawing board," Xu said, who published his findings in Applied and Environmental Biology.

It wasn't easy work collecting samples for the study. Often led by local guides, Xu walked and climbed for hours a day collecting the fruiting bodies of mushrooms. One day, the trek was so strenuous that his companion was hospitalized.

Xu thinks genetics have something to do with the deaths, which occurred in remote villages and often claimed female victims. SUD claimed as many as seven lives in some families, and as many as one-third of the residents of some villages.

Because the victims were residents of remote villages, few researchers have actually seen someone die from SUD, Xu said.

"The story I've heard is the person would be eating and talking, stand up and have a sudden cardiac arrest," he said. "Most of them were otherwise young and healthy."

Barium can't be ruled out as a cause of death given that high levels were found in some of the victims. But the mushrooms were likely not the primary cause.

Xu plans to continue analyzing mushrooms in the region to determine if any of them could have contributed to the deaths.