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?
Leena writes:
“I am a healthy 28-year-old with no medical issues but three of my siblings have hypothyroidism. I recently became aware that certain foods can have a negative effect on thyroid function, and I am a little concerned! Some of my favorite foods are on that list (broccoli, cabbage, millet). Can I still enjoy broccoli, cabbage, millet, and other veggies, without inducing hypothyroidism? Should I tell my siblings to stop eating these foods?”
Broccoli and cabbage are some of your favorite foods? Be still my beating heart!! Fortunately, Leena, I don’t think you or your siblings necessarily need to cut broccoli and other so-called goitrogenic foods out of your diet.
What Is a Goitrogen?
First, let me quickly explain why broccoli and other foods in the cabbage or brassica family are known as goitrogens. These vegetables contain a natural compound which, in large amounts, can interfere with your body’s ability to metabolize the mineral iodine.
Your body needs iodine in order to make thyroid hormone and if iodine is lacking—either because your diet is deficient in the nutrient or because something is inhibiting its uptake—your thyroid hormone levels can sink. When this happens, your thyroid gland can become enlarged; that’s your body’s way of trying to increase thyroid hormone production. An enlarged thyroid, which looks like a swelling in the neck, is known as a goiter.
The good news is that goiters caused by iodine deficiency can be easily reversed: Once iodine is added to the diet, the thyroid can produce enough thyroid hormone and the gland shrinks back to its normal size.
Goiters used to be much more common than they are today—and not because people ate more broccoli back then! Most goiters are caused by diets that are too low in iodine. Fortunately, since the advent of iodized salt, iodine deficiency is pretty rare these days.
What Causes Hypothyroidism?
Hypothyroid disease, especially the kind the runs in families, is not caused by iodine deficiency. It’s usually caused by an auto-immune condition where the body attacks and slowly destroys the thyroid gland. Once that happens, no amount of iodine is going to fix it. Instead, people with this type of thyroid disease take thyroid replacement hormone.
Even if you have thyroid disease in the family, eating goitrogenic vegetables does not increase your risk of getting it. And being treated for thyroid disease does not mean that these otherwise nutritious vegetables need to be off the menu. Assuming that your diet contains a sufficient amount of iodine, you—and your siblings—can eat these foods on a daily basis without worrying about interfering with your thyroid function.
But is it really possible to overdo it?
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 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.
