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While bok choy offers a number of health benefits, it may also come with downsides for some people when consumed in large amounts. In their raw, uncooked form, cruciferous vegetables contain an enzyme called myrosinase that breaks down the glucosinolates that these vegetables also contain. Some of the breakdown products of glucosinolates may inhibit your body from absorbing iodine, which is critical for thyroid function.
In one case study, an older woman was hospitalized in intensive care for severe hypothyroidism and related coma. She had been eating 14–21 cups (1,000–1,500 grams) of raw bok choy daily for several months. Though this effect may seem severe, keep in mind this is a single, extreme case. This effect may really only be a concern if you eat an unusually large amount of raw cruciferous vegetables over an extended period.
One review notes that there is little research on the potential thyroid-inhibiting effect of bok choy. However, the researchers state that typical serving sizes of bok choy probably do not impair thyroid function. Furthermore, cooking bok choy deactivates the enzyme myrosinase, so thyroid inhibition isn’t a concern with cooked bok choy.
Additionally, because bok choy is a rich source of vitamin K, you may not want to consume it in larger amounts if you take blood thinning medication. Significant changes in your vitamin K intake can interfere with the effectiveness of these medications. This is because vitamin K plays a role in blood clotting. A 1-cup (70-gram) serving of loosely packed, chopped bok choy provides about 27% of your daily vitamin K needs.
If you are taking blood clotting medications, such as warfarin, you can ask your healthcare provider how much bok choy is safe to consume. *
