Friday Favorites: Is It Safe to Eat Raw Mushrooms?

Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

There is a toxin in plain white button mushrooms called agaritine, which may be carcinogenic. And plain white button mushrooms grow up to be cremini mushrooms (the brown mushrooms), and cremini mushrooms grow up to be portobello mushrooms—they’re all the exact same mushroom. It’s like how green bell peppers are just unripe red bell peppers. But you can reduce the amount of agaritine in these shrooms through cooking.  Frying, microwaving, boiling, or even just freezing and thawing lowers the levels. It is therefore recommended to cook mushrooms before consumption, something I noted in a video that’s now more than a decade old.

But if you look at the various cooking methods the agaritine isn’t completely destroyed. Take dry baking for example: 10 minutes at about 400 degrees Fahrenheit (basically how you’d make a pizza), only cuts the agaritine levels by about a quarter, so 77 percent still remains.

Boiling looks better, appearing to wipe out over half of the toxin after just five minutes.  But it’s not actually wiped out; instead, it’s just transferred to the cooking water. So yes, levels within the mushrooms drop by about half at five minutes and 90 percent after an hour, but that’s mostly because it’s leaching into the broth. So, if you’re making soup or something, five minutes of boiling is no better than the pizza, and even after an hour about half remains.

Frying for 5 to 10 minutes wipes out a lot, but microwaving is a more healthful way to cook, and it works even better. Just one minute in the microwave “reduce[s] the agaritine content of the [fresh sliced] mushrooms by 65 percent,” and only 30 seconds wipes out about half. So, microwaving is probably the easiest way to reduce agaritine levels in fresh mushrooms. What I do with dried mushrooms is throw them into pasta water when I’m making spaghetti and between the 20-ish percent drop from the drying and the 60 percent-ish drop from boiling for 10 minutes and straining, more than 90 percent is wiped out.

Should we be concerned about the residual agaritine? Should we skip mushroom pizza? If you’re eating pizza, mushrooms are probably the last thing you need to worry about, but seriously should we be concerned about agaritine? According to a review paid for by the mushroom industry, not at all. “The available evidence to date suggests that agaritine from consumption of . . . mushrooms poses no known toxicological risk to healthy humans.” The researchers acknowledge it’s considered a potential carcinogen in mice, but then you have to extrapolate that data to human health outcomes.

For example, the Swiss Institute of Technology estimated the average mushroom consumption in the country would be expected to cause about two cases of cancer per 100,000 people. That’s actually similar to U.S. consumption, so one could theoretically expect about 20 cancer deaths per million lives from mushroom consumption. Now typically with a new chemical, pesticide, or food additive we’d like to see less than one in a million cancer risk. By this approach, “the average mushroom consumption … would be 20-fold too high to be acceptable.” To get it down to one in a million you could only eat about a half cup serving once every 250 days or something to remain under the quote-unquote “tolerable” limit. But to put that into perspective, even if you were eating a single serving every single day, the resulting additional cancer risk would only be about 1 in 10,000. In other words, “if 10,000 people consumed a mushroom meal daily for 70 year[s], then in addition to the 3,000 cancer cases arising from other factors, one more case could be attributed to consuming mushrooms.”

But again, this is all based on the presumption that results in mouse models are valid in humans; this is all just extrapolating from mice. What we need is a huge prospective study to examine the association between mushroom consumption and cancer risk in people, but there weren’t any such studies… until now.

“Mushroom consumption and risk of … cancer in [the] two large [Harvard] cohorts”, and… “no association between mushroom [intake] and [cancer]”.

Eating raw or undercooked shiitake mushrooms can cause something else, though: shiitake mushroom flagellate dermatitis. Flagellate as in flagellation, whipping, flogging. Check out this crazy rash. You break out in a rash that makes it look as if you’ve been whipped. Doesn’t it look wild? That’s just how you break out. It’s thought to be caused by a compound in shiitake mushrooms called lentinan, but because heat denatures it, it only seems to be a problem with raw or undercooked mushrooms.

Now it’s rare; only about 1 in 50 people are even susceptible. And it goes away on its own in a week or two. Interestingly, it can strike as many as ten days after you eat them, which is why people may not make the connection. One poor guy suffered on and off for 16 years before a diagnosis. Hopefully a lot of doctors will watch this video, and if they ever see a rash like this, they’ll tell their patients to cook their shiitakes.

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Motion graphics by Avo Media