There's a bogus new health theory making the rounds — the idea that kale is bad for you.
A recent article in Mother Jones warned that excess kale consumption could lead to "unhappy side effects" due to the presence of toxic heavy metals in the leaves. The Irish Times proclaimed that a "California-based molecular biologist [has] discovered a cause of serious illness in kale junkies with exceptionally 'healthy' diets." And so on.
This scare-mongering around kale all started with a single profile, in Craftsmanship Magazine, of Ernie Hubbard, a researcher at an alternative medicine center in Marin County who has been exploring a link between kale and a range of health conditions, from fatigue and heart arrhythmia to hair loss and brain fog.
There's one problem with Hubbard's hypothesis: There's just no published, peer-reviewed studies whatsoever of a link between kale and various diseases in humans. And other scientists say there's no basis for worrying about kale. So before you start throwing out all your produce, be aware that the science behind the kale scare looks awfully thin.
1) The supposed "link" between kale and various diseases is flimsy
Back in 2010, according to the profile, Hubbard was running a study on detoxing at the Preventive Medical Center of Marin, and noticed that many of his 20 participants' urine samples had higher than normal levels of the heavy metal thallium.
He began to dig around, and found a 2006 paper by Czech researchers that concluded kale can be a "hyper-accumulator" of thallium, a toxic metal that occurs naturally in soil and can be poisonous to people. In other words, kale is particularly good at absorbing heavy metals through its roots (as are related vegetables in the Brassica family, such as broccoli, cabbage, and Brussels sprouts).
As it happened, kale was also a very popular food among Hubbard's patients. "It suddenly hit me," he told Craftsmanship, "I thought, ‘Oh, my God!’"
Low-level thallium poisoning from kale, Hubbard suspected, might be causing an array of symptoms in his patients — from chronic fatigue to "foggy thinking."
Let's stop right here. This is a suggestive hypothesis. But at this point, the right thing to do would be to test it rigorously — and then see if your claims pass the scrutiny of peer review before publicizing them. There's no evidence in the profile that Hubbard has done this. Instead, the Craftsmanship article describes him doing this:
Whenever the clinic would send him someone with the kind of chronic problems associated with thallium, or any other complaints that were hard to pin down, Hubbard would scribble kale on a little note-card and turn it face-down on his desk. After a short work-up, he’d ask the patient to list his or her favorite vegetables. Over and over, people would mention the crucifers, especially kale. Hubbard would nod, say he expected as much, then show them the note-card on his desk to prove it.
The approach outlined here has obvious flaws. The fact that people who have various symptoms also happen to eat kale is in no way proof that the two are linked. What's more, the idea that poisonous kale might be the cause of conditions as varied as arrhythmias and cognitive troubles seems suspect on its face. In fact, experts are extremely skeptical that eating one type of food can explain a vast array of health problems. As William Toscano, an environmental health researcher at the University of Minnesota, told me: "Causality would be difficult to show because the diseases are complex and probably have multiple causes including genetic and epigenetic involvement."
2) Hubbard used a dubious lab to test his results
For his next step, according to the profile, Hubbard began collecting samples of local kale around California and sending them to labs to test for heavy metals. Hubbard eventually brought his samples to a lab in Illinois called Doctors Data.
Doctors Data also happens to be the subject of numerous lawsuits and court orders for misleading the public with test results that were used to peddle bogus dietary supplements and detox therapies. Not an auspicious sign.
3) There is no published research supporting the "dangerous kale" hypothesis
Even if Hubbard's kale samples did contain high levels of thallium, that's still not enough. No one has established a link between thallium and the illnesses documented in the profile — nor has anyone definitively ruled out other possible causes.
Right now, there is no published, peer-reviewed research on low-dose thallium-induced disease or the adverse effects in humans from eating kale. We've simply seen no credible evidence to back up this viral idea, and at this point, there is no reason that anyone should take these warnings about kale seriously.
So: Is kale really a health hazard?
I emailed Jiri Zbiral — one of the authors on the 2006 Czech paper that Hubbard had cited showing that kale can accumulate thallium — to see what he made of the claims that the greens make people sick.
Zbiral, an analytical chemist at the Central Institute for Supervising and Testing in Agriculture in Brno, explained that kale is indeed a good accumulator of thallium. But he also said that it's very unlikely the plants would suck up enough to poison humans.
To get even close to toxic levels, you'd need to plant the kale in soils with high levels of the heavy metal. "The final content [of thallium in the kale] is a function of thallium soil concentration," Zbiral said. Most soils only have very low levels of thallium, he told me. They'd need to accumulate a lot of the metal in their leaves — which doesn't always happen. "Thallium is differently bound in different soils and therefore from different soils with the same thallium content, different availability of thallium was reported," he wrote.
Zbiral also noted that kale shouldn't be singled out here. He has found that canola, for example, can be just as good as kale at absorbing this metal. So if kale is being implicated in making people sick, canola — a staple food around the world — should be, too.
Since there have been no kale-human studies, I asked Filip Poscic, a molecular biologist at the University of Udine in Italy who has also studied the thallium-Brassica link, to conjecture how much kale one would need to eat to get sick. He suspected it'd take kilograms of kale every day, a feat even the most ardent vegan probably couldn't accomplish. "It is close to impossible for humans to be poisoned by eating kale from normal soil," he wrote. "However, plants (of any kind) cultivated on and near toxic heavy-metal deposits, waste and so on are to be avoided in kitchen."
So the problem is less with kale, and more with growing food in contaminated areas.
I'll add: In America, it's far more likely that people are getting sick from eating too few fresh fruits and vegetables than from overdosing on kale.
* Correction: This piece originally claimed that Hubbard wasn't a scientist, based on the fact that he hasn't published in peer-reviewed journals. However, Hubbard reached out to us following publication to say he has worked in the field of science — including in biotech and at other health companies — for nearly 40 years. As such, we are retracting our characterization of him as a non-scientist.
Hubbard also said he has been running other experiments that weren't mentioned in the Craftsmanship article — experiments that shed more light on the kale-thallium poisoning hypothesis. If Hubbard does publish his findings on kale and low-level thallium poisoning in peer-reviewed journals, we will be happy to reexamine the issue.