Kale Risks “Theoretical,” But In Reality Very Low To Minuscule

By Rachel Zimmerman

On January 1st, The New York Times published an opinion piece titled “Kale? Juicing? Trouble Ahead.” In the piece, a self-described “health nut” wrote a cautionary tale about her surprising discovery of a potential downside to eating large amounts of kale and other cruciferous vegetables: they may inhibit absorption of iodine and thus promote development of a goiter (enlargement of the thyroid gland) or slow down the thyroid’s function, ultimately causing a condition called hypothyroidism.

Here we present an informative follow-up article created by health and medicine journalist Rachel Zimmerman with Dr. Jeffrey R. Garber, our EmPower Magazine guest editor, as part of her co-hosting duties for Boston NPR station WBUR’s CommonHealth blog.

Editor’s Note: Ms. Zimmerman’s article and interview were first published on WBUR's CommonHealth blog at: http://commonhealth.wbur.org/2014/01/the-dark-side-of-kale-and-how-to-eat-around-it.

My blog post—“The Dark Side of Kale (And How To Eat Around It)”—went wildly viral this week, generating huge traffic and high passions over this once minor but now hotter-than-hot vegetable. Among the accusations from readers were charges that the post was “dubious and dangerous” and that I was, in effect, “discouraging Americans from eating vegetables” (my children would disagree).

Still, for a medical reality check, I turned to a doctor who specializes in treating the thyroid.

Bottom line: in the U.S, where we don’t have a big problem with iodine deficiency, it’s probably OK.

OK, back to the thyroid expert, who points out that this debate is particularly timely since January is Thyroid Awareness Month.

Dr. Jeffrey Garber is chief of endocrinology at Harvard Vanguard Medical Associates and lead author of the latest clinical practice guidelines on hypothyroidism in adults. He agreed to answer a few more questions on the kale-thyroid connection.

I’d sum up Dr. Garber’s take on the whole kale issue pretty simply: It’s probably unwise to embrace a longterm, pound-a-day raw kale habit, but even if you do, you will, in all likelihood, be fine. (Especially if you live in the U.S., where iodine deficiency isn’t a huge problem and if you don’t have a family history or predisposition to thyroid disorders.) (Editor’s Note: Cooked kale cannot interfere with the way the thyroid works).

“If one isn’t a food faddist or predisposed to a thyroid problem (family history, prior diagnosis) the risks are very low,” Garber said. And, he adds, if you have any concerns at all, check in with your doctor for a simple thyroid test.

Here, lightly edited, is our Q&A:

RZ: In plain terms, what’s the connection between kale, a cruciferous vegetable, and thyroid function?

JG: There are many substances that can interfere with the way the thyroid functions. Goitrogens, as in those that promote goiter, make up one of these categories. (There’s an enormous amount of interest in environmental goitrogens, like BPA and other substances, but that’s another story: We’re talking about dietary goitrogens here.) When you get into the way goitrogens can affect the thyroid directly, there are three general ways (and all relate to iodine, which is what thyroid hormone is made from):

1. the way the thyroid picks up the iodine;
2. the way the thyroid produces the hormone once the iodine is in the thyroid;
3. the way thyroid hormone is secreted into the bloodstream.

When you look at dietary goitrogens, they interfere with one or more of these three steps.

RZ: OK, so kale is one of these so-called “goitrogenic” foods, right?

JG: Yes. And [quoting from a section in the textbook The Thyroid, written by Michael B. Zimmerman, MSc, MD, Professor of Health Sciences and Technology at ETH Zurich in Switzerland] other cruciferous vegetables implicated as goitrogens include: cabbage, cauliflower, broccoli, turnips, rapeseeds — they contain something called glucosinolates, their metabolites compete with iodine for thyroidal update. Similarly, cassava, lima beans, linseed, sorghum and sweet potato contain cyanogenic glucosides; these may be metabolized to thiocyanates that compete with iodine for thyroidal uptake. Also, if you don’t have enough iodine, deficiencies in selenium, iron and vitamin A can make you even more vulnerable to developing an underactive thyroid. [An aside]: Iodine deficiency isn’t generally a problem in this country. It’s hard to avoid iodine in the diet here: many breads, dairy and salt (except for “designer” salts, kosher salt and sea salt) contain iodine. But in poor, developing countries about one billion people have an iodine deficiency that can cause an underactive thyroid and endemic goiter.

RZ: So, what’s the bottom line here? Green smoothies everyday or not?

JG: Basically the goitrogens are challenges to the thyroid. But in the absence of iodine deficiency, substantial or prolonged ingestion of dietary goitrogens and lastly, the absence of an underlying thyroid disorder, the risk in this country of having problems in this area are very, very low, almost minuscule. Again, that’s because the vast majority of people have adequate iodine levels to counteract the effect of goitrogens.

RZ: So, the writer of The New York Times opinion piece, Jennifer Berman, said she stopped consuming kale juice daily after she was diagnosed with hypothyroidism; when she looked up foods to avoid, kale was at the top of the list.

JG: She gave herself too much credit for what happened to her.

RZ: But if you actually drank a green smoothie with raw kale every day for a long period of time and somehow managed to be iodine deficient, could you do this to yourself?

JG: It could theoretically happen, but it would be unusual.

RZ: I’m still kind of confused by all of the information out there. If I Google “Hypothyroidism And Foods To Avoid,” several sites have some variation on the advice Berman got: steer clear of kale.

JG: I think it’s overplayed.

RZ: And what about children? It’s hard enough to get them to eat veggies at all.

JG: My strong personal opinion is to follow standard, conventional nutritional advice for kids.

We should end here by stating the obvious: vegetables are, in general, fabulous for your health; and any kind of obsessive, extreme diet that includes massive consumption of a single food can, in general, lead to trouble.