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Ask Sam: Are Kidney Stones Kind of Like a Pearl in a Clam?

Flickr Creative Commons | James St. John

Every other Friday on Morning Edition, Outside/In host Sam Evans-Brown tackles a question from a listener.

Tim from Olympia, Wash., asks: “Is a kidney stone kind of like a pearl in a clam?”

The first thing to explain here is that if you have never listened to the Ask Sam radio segment, there’s a theme song which poses several – shall we say – farcical questions. One of which is the question posed by our dear Tim.

So to start, what on earth is a kidney stone?

“A kidney stone is probably more similar to calcifications that your form with age and poor diet and significantly with genetics in the vasculature of your heart,” explains Dr. David Bayne, an assistant professor of Urology at the University of California San Francisco.

Kidney stones form when you’ve got too much of something called calcium oxalate in your body. If you’ve ever made rock candy, it’s roughly a similar process. You start with a liquid that is supersaturated with a dissolved mineral (say hot water with a bunch of sugar), which then gloms onto some nucleus (like the stick you put into the glass for your rock candy lollipop), which then leads more and more mineral to build up in an ever bigger (delicious candy) crystal.

Same thing with kidney stones, except instead of sugar it’s calcium oxalate and the crystals form in the worst imaginable place. 

Terrifyingly, oxalate is contained in otherwise healthy foods: dark leafy vegetables, legumes and nuts. Which means if you're eating a healthy diet for the many other reasons why that's a good idea, the simplest thing to do is just dilute by staying very hydrated, so you don’t get all supersaturated inside.

And fortunately, according to David Goldfarb, a kidney stone researcher at NYU Langone, it doesn’t even need to be water. “Every study of beer and kidney stones have shown that beer actually prevents kidney stones,” Goldfarb says, “And I’m a kidney stone patient myself and so I start the day with coffee and end the day with beer.” 

Also, more bad news here, kidney stones are becoming more prevalent. Why? Since kidney stones are more common when you’re dehydrated and you get dehydrated when you’re hot, Goldfarb thinks this is a global warming story. “People are moving to cities, and as global warming progresses those urban heat islands become more of an issue.”

Ok, On To Pearls

Every sixth grader can tell you that pearls form because a grain of sand gets inside an oyster, which irritates the mollusk, which then coats it with layer after layer of pearl-stuff in attempt to stop the irritant. Right?

No! Wrong! A myth! 

“We’ve X-rayed thousands and thousands and thousands of pearls... natural pearls, from wild oysters... we’ve never found a grain of sand inside a pearl,” says Laurent Cartier, a pearl researcher and lecturer at the University of Lausanne in Switzerland. 

In fact, pearls are more like cancerous growths. A bit of the mantle tissue—the cells that form the inner layer of oyster’s shell (aka mother of pearl, aka nacre)—just gets out of place, “and it just… I dare say stupidly… kind of continues to produce mother of pearl,” says Cartier, “And nature has its ways, and nature likes round forms, and so it just seems to be that this kind of roundish or round form is quite common.”

Pearl aquaculture is also a deep, deep well of fascinating facts, but one of my favorite ones is that it’s not just oysters that make pearls, certain mussels do it to. Here, let me link you to this article I read that is much better than anything I could write about pearls.

Let’s just wrap this up

The only similarity I could find between pearls and kidney stones is that broadly speaking, they both form because of something called biomineralization. But to say that they are there for “kinda like” each other is a real stretch. Biomineralization is the broad term for the way that organisms build bones, eggs, teeth, and shells. Are all of those things also kind of like each other? If so, then sure! Kidney stones are kinda like a pearl in a clam.

Now… while we’re on this track I’d just like to get these all out of the way: Geese make vees so they can draft off each other and work less hard, bumblebees do not appear to be capable of sneezing as they don’t have noses per se, there are hundreds of ways to eat certain parts of certain trees too many to list here, and polar bears can die of hypothermia if they don’t eat and get too skinny.

Phew. I need to sit down.

Sam Evans-Brown, is host ofNHPR’s Outside/In which you can subscribe to where-ever you get your podcasts. If you’d like to submit a question you can record it as a voice memo on your smartphone and send it to, OR call the hotline, 1-844-GO-OTTER, OR submit it here.

Sam Evans-Brown has been working for New Hampshire Public Radio since 2010, when he began as a freelancer. He shifted gears in 2016 and began producing Outside/In, a podcast and radio show about “the natural world and how we use it.” His work has won him several awards, including two regional Edward R. Murrow awards, one national Murrow, and the Overseas Press Club of America's award for best environmental reporting in any medium. He studied Politics and Spanish at Bates College, and before reporting was variously employed as a Spanish teacher, farmer, bicycle mechanic, ski coach, research assistant, a wilderness trip leader and a technical supporter.
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