Is eating too much kale dangerous?

Kale overkill. Photo: Getty

Right when you thought kale was the most perfect food known to (wo)man, a story comes along that says, “No, actually, it could poison you.”

The Mother Jones story, titled, “Sorry, Foodies: We're About to Ruin Kale,” is going gangbusters. In two days, it's already rounded up more than 35,000 Facebook shares and is taking over Twitter, suggesting that kale consumption commonly leads to too-high levels of thallium, a toxic metal, in the blood—and that that leads to chronic fatigue, skin problems, arrhythmias and gluten sensitivity, Yikes.

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But kale? It’s low kJ and rich in calcium and B vitamins. Plus, a single serving meets your daily needs of both vitamins A and B.

Research even suggests that kale extract may inhibit cancer cell growth. So before we dumped our kale in the compost pile, we ran the story past dietician Alexandra Caspero.

“This story makes my eyes roll," she says. "Articles like this can be so confusing to the general public. Unfortunately, people will see the headline and use it as a another reason not to eat kale.

Here's the thing: Anything in large amounts can be dangerous.” After all, too much water can cause dangerously low sodium levels, too much tuna can cause mercury poisoning, and in people with thyroid problems, too much broccoli can exacerbate symptoms, says Caspero.

For most people, though, it's unlikely that you're eating anywhere near a dangerous amount—and far more likely that they're adding beneficial nutrients to your diet.

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In the case of kale, its root systems are good at picking up thallium, an odorless, tasteless metal found in very trace amounts in the earth, she says.

However, that’s not unique to kale. One Journal of Plant Nutrition study, for instance, found thallium in green beans, beetroots, green cabbage, lettuce, onions, peas, radishes, spinach, tomatoes, turnips, and watercress. The heaviest hitters were watercress, radish, turnip, and green cabbage.

Like kale, these are all members of the Brassicaceae family. It’s likely these plants pass on greater levels of thallium to eaters because we eat their stems, and that’s what absorbs the contaminant—granted it’s in the ground in the first place, says Caspero.

“Thallium is primarily produced from coal-burning and smelting and then enters the air, water, and soil. It can be absorbed by plants and can also build up in fish and shellfish.”

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If you’re going to cut out kale due to thallium risk, you’re going to have to ditch a lot of other healthy foods while you’re at it, which probably isn’t the best approach to health-conscious eating.

But how much is too much? Because there's no real way to know if your kale was grown in soil with thallium—and how that translates into its concentration in your blood—it's pretty much impossible to set a defined limit of how much of the veggie you should be consuming. But as always, moderation is a good rule of thumb.

“Bottom line, I don't recommend eating too much of anything, and it's a good reminder to balance the types of fruits and vegetables,” says Caspero. “I eat two to three bunches of kale a week, and I'm not going to stop because of this article.

However, I also wouldn't promote juicing one to two bunches a day. That's overkill.”