Kale sheds bum rap on kidney stones

By Angela Barnes

Kale is one of America’s current health food crazes. Studies show the trendy veggie is exceptionally rich in nutrients, antioxidants and other cancer fighting properties.

But now some critics are warning the ubiquitous leafy green super food may pose a risk. Website articles and health gurus are warning that eating too much kale can cause kidney stones.

Fear not! Those claims are baseless, according to University of Chicago kidney specialist Dr. Fredric Coe, a professor of medicine who built and has run the university’s kidney stone prevention center for 45 years.

“Kale offers all the benefits of dark green vegetable but it ranks really low in oxalate,” said Coe. “There’s only 17 milligrams of oxalate in a hundred grams (about 3 ounces) of kale.”

“So in every three ounces of kale you get nothing” in terms of oxalate, added Coe. “Essentially, it’s about impossible to eat enough of kale to cause kidney stones.”

Oxalate in higher concentrations is linked to a chemical cascade that can result in kidney stones.


So what is oxalate? Oxalate is a molecule that’s a waste product of metabolism. Plants use it to store energy.  All the physical and chemical processes in the body that use energy –  for example breathing or digestion – need oxalate, according to the National Institutes of Health website.

So why is oxalate a problem? Even though our bodies need oxalate, the foods we eat that are high in it can cause kidney stones.

But kale is not one of those foods. According to Coe, it just resembles another green food that’s dark in color. “Kale gets a bad rep because of the way it looks,” said Coe. “It looks like spinach and spinach has an enormous amount of oxalate.”

So how much oxalate is in spinach? “Spinach has hundreds of milligrams…hundreds in a single serving,” said Coe. “If you eat a lot of it, yes, you can get kidney stones.”

Spinach Concerns

So kale is off the hook: it only contains 17 milligrams of oxalate to spinach’s hundreds of milligrams of oxalate. Spinach and other dark green leafy vegetables like Swiss chard are oxalate dense vegetables. These green foods, although with healthy benefits, are the ones to be concerned about over-consuming. Coe says you can find a list of vegetables that’s high in oxalate at Harvard Medical School.

So why is kale so good for us? It contains micronutrients, vitamins and lots of fiber. Jill Harris, certified health coach, says fiber is really important.

“A diet high in fiber is going to really help prevent disease,” said Harris. “It makes us feel full so we’re not eating mindlessly throughout the day.”

You can even eat the stem if you like, according to Harris. Some people, like myself, don’t like the stem because it’s fibrous and somewhat bitter tasting. But it’s definitely safe to eat.

Juicing Problems

There are many ways you can eat kale and still reap all of its rich benefits: in salads, soups, raw and smoothies. But there’s one way you may want to reconsider—juicing. Harris says you’re losing some important nutrients when you juice.

“The difference with juicing is you’re stripping away all the fiber,” said Harris. “So you get the benefits of the micro-nutrients in juicing but you lose all the fiber.”

So is there a difference between juicing and smoothies? It turns out, there is.  With smoothies, “you’re not breaking down all the fiber,” Harris said. You’re still getting your fiber. But with juicing, a lot of the skin (that contains the fiber) is left in the blender and not in your drink.

Fiber is so important for our diet and many of us fail to consume the required amount.

“Most of us are getting 12 grams a day and we should be getting 25-30,” said Harris. “This would avoid a few things: colon cancer and bowel disease.”

There you have it – kale is still one of the number one super foods. We can eat it in various ways and not worry that too much of it will cause kidney stones.

A few tips:

  • If you’re lacking in fiber, try not to juice as often.
  • For those who love spinach, Swiss chard or other vegetables high in oxalates, just be judicious with your consumption.
  • Always check out the health benefits when introducing any food into your diet.
Photo at top: Healthy eaters stock up on kale but are worried about health reports linking the trendy veggie to kidney stones. (Angela Barnes/Medill)