Allison Swift, a urology nurse practitioner in Cooperstown, New York, is no stranger to kidney stones. She often sees patients with obesity, insulin resistance or dehydration (mostly men, mostly over 40) complaining of some combination of the abdominal or back pain, fever, chills, painful urination, persistent urge to go and discolored urine that are the hallmarks of the small, hard deposits that build up in kidneys. But lately she’s been seeing healthy, fit young women coming into the hospital with stones. It had her puzzled, until she figured out the common denominator: kale.

Lately healthy, fit young women have been going to the hospital with kidney stones. It had one nurse puzzled, until she figured out the common denominator: kale.

“I’ve seen the same problem,” says Roger Sur, M.D., director of the UC San Diego Comprehensive Kidney Stone Center. “I just had two young females who had never had stones and suddenly they’re coming in with them. They were both doing these kale smoothies.”Most kidney stones are formed when calcium and oxalate bind together in the body and crystallize. Those crystals can build up into hard masses in the kidney and get stuck there or elsewhere in the urinary tract until you “pass” them—that is, pee them out. (It’s as uncomfortable as it sounds.) Kale and other leafy greens are high in oxalate. Ergo, downing kale like it’s your job can lead to kidney stones.

When to Worry About Your Kale Consumption

Let’s get one thing straight: Kale is great. It’s low in calories, high in vitamins—including K and A for smooth skin, and C for immunity and anti-aging—and packed with iron, which is hard to get from plants. There’s a reason people are pounding kale smoothies, kale salads and kale chips. But you know what they say about too much of a good thing.


Here are some things to consider to make sure you don’t overdose on special K:

Do you have a kidney stone? If you have one now, or have had one in the past, you shouldn’t be eating a ton of kale.

How often do you eat kale? There’s no rule on how often is too often, but if you’re eating kale on a daily basis, that’s pretty darn often. Think about it: How many things do you really eat every single day?

How much are you eating? A single kale smoothie may contain 2 cups of kale. That’s a lot of kale right there, without counting that kale salad you picked up for lunch or the side of sautéed greens you’ll order at dinner.

What kind of kale are you eating? The data are sketchy on just how much oxalate is released into the body when you eat kale, but it’s generally agreed that flat leaf kale (aka dinosaur, black, lacinato or Tuscan kale) has less oxalate than curly kale. And cooked kale should have less oxalate than raw. Curly kale is higher in stone-causing oxalate than flat kale.

What else are you eating? Pay attention to whether you’re eating other oxalate-laden foods and limit those on kale smoothie days. Some examples: nuts, wheat, blueberries, raspberries, beets, spinach and other dark, leafy greens. On the other hand, increasing your dairy intake could help shuttle excess oxalate out of your system. The calcium binds to oxalate in the gut so it’s excreted when you poop. But at a certain point you can tip the scale too far and end up with an excess of calcium, so it’s safer to turn down the kale than turn up the Gouda. (Plus, dairy’s got its own issues.)

Do you have risk factors for kidney stones? These include obesity, insulin resistance, a family history of stones and gut absorption problems, which may be caused by any number of things, such as bowel surgery, Crohn’s disease, ulcerative colitis or even IBS. Certain medications—Topamax for migraines and high-dose vitamin C, for instance—carry the side effect of stone risk. Extreme diets, such as Atkins and Paleo, can also potentially predispose you to developing kidney stones.If you answered “uh oh” to any of the questions above, it might be time to take a kale sabbatical—or at least institute a couple kale-free days a week.

MORE: There’s Sugar Hiding in These “Healthy” Foods