Do Cruciferous Vegetables Affect Your Thyroid?

Broccoli, cauliflower, and other cruciferous vegetables are sometimes said to interfere with thyroid function. Could eating these veggies cause low thyroid function?

Monica Reinagel, MS, LD/N, CNS
4-minute read
Episode #368

How to Kill Yourself with Kale

Of course, it is possible to overdo just about anything. In fact, these days, the minute something gets a reputation for being good for you, there seems to be a race to see who can overdo it most. Although eating several servings of broccoli, kale, and other cruciferous vegetables a week is a great idea, eating a couple dozen servings every single day isn’t.

If eaten in very large quantities, these vegetables could hypothetically impair iodine metabolism to the extent that thyroid function is disturbed. Fortunately, the effect would be completely reversible. But other effects of overconsumption may not be so easily outdone.

As I recently discussed on the Nutrition Over Easy blog, kale and other leafy greens in the brassica family are particularly good at absorbing minerals from the soil. As a result, they may contain trace amounts of cadmium, thallium, or arsenic—which are naturally present in some soils. Unlike calcium and magnesium, thallium and arsenic are not particularly good for humans.

The amount of heavy metals you might absorb from leafy green vegetables is not a concern if you’re eating them in normal quantities. But if you’re eating them in extravagant amounts day after day, it could actually pose a problem.

See also: Kale craze raises heavy metal concerns

The Questionable Value of Juicing

One very common culprit in these too-much-of-a-good-thing scenarios is juicing. One of the reasons people get excited about juicing is that it extracts and concentrates the nutrients from vegetables. So, instead of eating five servings of vegetables a day, you can down twenty servings in a single glass!

But here’s the thing: while research shows that eating five to nine servings of vegetables a day can reduce your risk of obesity and disease, there’s no evidence that eating twenty or thirty servings per day (or the juice or supplement equivalent) offers any additional benefit.

See also: Can I Get My Vegetables in a Pill?

Maybe the fact that whole vegetables take time to chew and contain fiber that fill up our tummies is nature’s way of regulating our consumption? This is why I recommend that you eat (rather than drink) the majority of your fruits and vegetables.

Vegetables are usually juiced in their raw state and while raw vegetables can be higher in certain nutrients, cooking vegetables deactivates several compounds that interfere with nutrient absorption, including the compounds that inhibit iodine uptake. This is why I recommend including both raw and cooked vegetables in your diet.

See also: What Are the Benefits of a Raw Food Diet?

So, Leena, while I certainly don’t think that a ban on brassicas is necessary, I would suggest eating them along with a wide variety of vegetables from lots of different plant families. Not only does this give you a wider variety of nutrients, but it also helps prevent accidental over-exposure to compounds that are only a concern when consumed in excessive quantities.

See also: How Important is a Varied Diet?

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About the Author

Monica Reinagel, MS, LD/N, CNS

Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes' most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation's leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.