The vegetable du jour kale has been getting negative attention in the media recently. Despite kale’s many nutritional benefits—including calcium, vitamin C, iron, fiber, and antioxidants—there is speculation that kale might be a contributor to hypothyroidism (an underactive thyroid). To uncover whether there is truth to this theory or the media is making much ado about nothing, EndocrineWeb spoke with Angela M. Leung, MD, who is an endocrinologist and Clinical Assistant Professor of Medicine at UCLA David Geffen School of Medicine, Los Angeles, CA, and Chair of the Public Health Committee at the American Thyroid Association.

Kale is considered a goitrogenic food, meaning that it contains substances (goitrogens) that may contribute to an enlarged thyroid. In fact, all cruciferous vegetables like kale are considered goitrogenic, including arugula, bok choy, broccoli, brussel sprouts, cabbage, cauliflower, collard greens, mustard greens, turnips, and watercress.

These vegetables “contain the substance thiocyanate, which in very high concentrations, can interfere with adequate iodine nutrition,” Dr. Leung said. The thyroid needs iodine to produce thyroid hormone, and “thus exposure to very high amounts of thiocyanate can potentially result in hypothyroidism (an underactive thyroid) and compensatory growth of the thyroid (goiter),” Dr. Leung explained.

“For the general population, the many health benefits of eating kale and other cruciferous vegetables in usual amounts far outweigh any potential adverse risks to the thyroid,” Dr. Leung said. “In usual amounts, kale consumption is healthy and should not be avoided, in contrast to some media reports which caution against all ingestion of cruciferous vegetables to promote thyroid health,” she said.

In people who already have a thyroid condition, “The risks of worsening a preexisting thyroid condition are likely minimal if goitrogenic foods are consumed in their usual amounts,” Dr. Leung said. “Patients should seek the advice of their physician for the proper management of their specific thyroid conditions,” she added.

How Much Kale Can You Safely Eat?

“The thiocyanate content of kale has not been well-studied, and there are only sparse measurement data from limited reports,” Dr. Leung said. “As such, the exact amount of kale ingestion that may be associated with an adverse thyroid health risk is unknown. However, ingestion of kale—including raw kale—in usual amounts should be fine,” Dr. Leung said.

In some reports in the press, juicing kale is the real culprit. “Although it has not been specifically studied, juicing kale concentrates the vegetable and thus potentially poses a greater risk toward iodine deficiency and hypothyroidism if ingested in large amounts on a very frequent basis,” Dr. Leung said. “These risks may be exacerbated in individuals who are already iodine deficient, and these may include those with restricted diets, such as vegetarians and vegans,” she said.

The United States is considered generally iodine sufficient, as most people get enough iodine through their diet, from table salt (of which most, but not all, formulations have iodine added to it), and from iodine-containing multivitamins and supplements, Dr. Leung noted. “However, adequate iodine nutrition is particularly important in women of child-bearing age and their children, given the importance of iodine and normal thyroid function on the developing brain in young infants. The daily recommended intake for iodine is 150 mcg (micrograms), and women who are contemplating pregnancy, pregnant, or lactating should supplement with a multivitamin containing 150 mcg daily. It should be noted that iodine excess is also potentially dangerous, as it can similarly induce thyroid dysfunction, and ingestion of any supplement containing more than 500 mcg iodine per day should be avoided,” she said.

Thus, media reports of kale causing hypothyroidism does not seem likely in most cases. Eating greens in their usual amounts will not be a significant contributor toward thyroid disorders. As always, it is important to talk to your doctor regarding your individual risk for a thyroid disorder and what types of food are right for you.

Sources

1. Paxman PJ and Hill R. The goitrogenicity of kale and its relation to thiocyanate content. J Sci Food Agric. 1974;25(3):329-337.

2. Chandra AK, Singh LH, Ghosh S, Pearce EN. Role of bamboo-shoot in the pathogenesis of endemic goiter in Manipur, northeast India. Endocr Pract. 2013;19(1):36-45.

3. Public Health Committee of the American Thyroid Association, Becker DV, Braverman LE, Delange F, et al. Iodine supplementation for pregnancy and lactation – United States and Canada: Recommendations of the American Thyroid Association. Thyroid. 2006;16(10):949-951.

4. ATA Statement on the Potential Risks of Excess Iodine Ingestion and Exposure. Accessed July 7, 2014. Available at: http://www.thyroid.org/ata-statement-on-the-potential-risks-of-excess-iodine-ingestion-and-exposure/

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