Can Kale REALLY Damage Your Thyroid? A Cardiologist Explains

Kale is king, right? Ever since Whole Foods gave this leafy green a perfect score of 1,000 on the ANDI, or Aggregate Nutrient Density Index scale, kale has been the darling of health advocates. Hormone guru Sara Gottfried MD says, “Kale is a lifestyle,” and I agree.

But lately kale, a member of the cruciferous or brassica family, has been under some attack. Should we be concerned?

Under the rubric of “The Dark Side of Kale," the web has been abuzz for more than a year with the idea that kale and its cruciferous mates like broccoli, Brussels sprouts, mustard greens, bok choy, and cauliflower may be harmful to our thyroid glands. This started in part when an opinion piece suggesting a risk was published in the New York Times last year.

And yet if you search the National Library of Medicine, you won't find much to support this concern. One 2002 study showed no relationship between cruciferous vegetable consumption and thyroid cancer, while another suggested some relationship only in those with low dietary iodine intake.

In addition to the NYT piece, kale and its cruciferous cousins got a bad rap thanks to one overzealous 88-year-old woman. This Chinese woman ate up to 3.3 pounds of raw bok choy daily for several months and developed a severe case of low thyroid function. The authors wrote, “when eaten raw, brassica vegetables release the enzyme myrosinase, which accelerates the hydrolysis of glucosinolates; the cooking process largely deactivates the myrosinase in these vegetables."

So before you cut out kale based on this limited data, I think it's important to consider the demonstrated health benefits of eating cruciferous vegetables versus the very low risk to our thyroids. Here are seven science-backed reasons to keep eating kale:

1. In the 2013 Adventist Health Study that tracked thousands of people, a vegan diet — presumably rich in crucifers — was associated with a lower, not higher, risk of hypothyroidism.

2. In one study, healthy volunteers who consumed 150 grams of cooked Brussels sprouts daily experienced no change in thyroid function.

3. In more than 134,000 Chinese adults, increased cruciferous vegetable intake was associated with a lower total and cardiovascular death rate.

4. In a study of the Dutch population, eating more brassica vegetables was associated with a lower risk of colorectal cancer.

5. Research shows that eating cruciferous vegetables is associated with a lower risk of breast cancer.

6. Cruciferous vegetables can lower the risk of a heart attack, but only if you have a functioning gene that detoxifies the chemicals in this plant family.

7. A 2013 study found that broccoli sprouts may lessen the complications of diabetes.

The Bottom Line: Why I'm Still Eating Kale

Overall, the health benefits of kale and related vegetables far outweigh the minor concern over thyroid function. Of course, prudence suggests that eating pounds of raw cruciferous veggies daily may be unwise. For optimal thyroid function, it's also important to pay attention to adequate dietary iodine intake (sea vegetables are a particularly healthy way to consume enough iodine).

For now, I am going to continue to juice, blend, and cook kale and other cruciferous vegetables on a regular basis, knowing that the benefit to my health is high and the risk is very low.

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