Is There A “Dark Side” Of Broccoli?

Broccoli and other cruciferous vegetables are a rich source of glucosinolates, as well as isothiocyanates that help with phase 2 detoxification, increase antioxidant status, and can provide protection against different types of cancers.  However, these glucosinolates can form compounds that potentially have anti-thyroid activity by decreasing the uptake of iodine, which is why many natural healthcare professionals recommend for people with thyroid conditions to avoid cruciferous vegetables.  This is especially true with those who have hypothyroidism and Hashimoto’s Thyroiditis.  But does eating broccoli truly inhibit thyroid hormone production?

What I’d like to do is talk about the benefits of eating broccoli and other cruciferous vegetables, as well as any potential risks.  This way you can make an informed decision and decide what is best for you.  I must admit that overall I’m a big advocate of people eating broccoli, kale, and other cruciferous vegetables, as these are nutrient dense foods.  But at the same time I of course wouldn’t want to worsen the health of people who currently have a thyroid imbalance.

The Benefits of Eating Broccoli (and other cruciferous vegetables)

I listed some of the benefits of cruciferous vegetables in the opening paragraph, but I’d like to go ahead and expand on this.

Phase 2 detoxification.  How can eating broccoli help with detoxification?  Well, broccoli and other cruciferous vegetables have large amounts of glucosinolates, which in turn convert to isothiocyanates.  These include sulforaphane and 4-methylsulfinylbutyl isothiocyanate, which along with indoles (i.e. indole-3-carbinol) are very potent inducers of phase 2 enzymes responsible for detoxification (1) (2).  For a better understanding of the detoxification pathways I would read my blog post entitled “Understanding The Detoxification Pathways“:

High levels of antioxidants.  In addition to helping to support the detoxification pathways, brassica vegetables also contain micronutrients that may provide additional DNA protection from reactive oxygen species (R2).   Even though glucosinolates are converted into compounds which are considered to be goitrogenic, these compounds also have health promoting properties as well.  For example, glucoraphanin is a glucosinolate found in broccoli and other cruciferous vegetables, and when someone eats these foods they are converted into sulforaphane.  As I mentioned earlier, this can help to support phase two detoxification, but it has other health benefits as well, and can be protective against different types of cancers (3) (4) (5).  Isothiocyanates also have antioxidant properties (6) (7).

Improve gut health.  The compounds included in broccoli (such as glycosinolates) can be hydrolyzed by certain bacteria, and thus can alter the human gut microbiota (8).  Another study showed that broccoli (and blueberries) can alter the composition and metabolism of the microbiota of the large intestine (9).

Estrogen Metabolism.  The phytochemicals Indole-3-Carbinol (I3C) and 3,3′-diindolylmethane (DIM) are found in cruciferous vegetables, and there is plenty of research which shows that these can affect estrogen metabolism in a positive way.  One way they seem to work is by affecting the expression of the CYP19 gene that encodes the enzyme aromatase (10).  This is the primary enzyme responsible for the synthesis of estrogen.  In addition, I3C significantly increases the urinary excretion of 2-hydroxyestrone, which is known as the “good” estrogenic metabolite. (11) (12). The urinary concentrations of nearly all other estrogen metabolites, including levels of estradiol, estrone, estriol, and 16alpha-hydroxyestrone, were lower after I3C treatment, which lowers estrogenic stimulation (12).

The “Risks” of Eating Broccoli (and other cruciferous vegetables)

There is only a single “risk” I know of when it comes to eating broccoli and other cruciferous vegetables, which is that these foods include compounds which have goitrogenic properties.  This is especially true when eating large amounts of raw cruciferous vegetables.  But can cooking cruciferous vegetables reduce the goitrogenic properties?  I’ll talk about this more shortly, but I will say that in most people, eating one or two servings of raw cruciferous vegetables per day won’t cause any problems with thyroid health.

With regards to cooking broccoli, there have been a few studies about how different cooking methods affect the glucosinolates.  And as I mentioned earlier, these glucosinolates get converted into compounds (thiocyanates and isothiocyanates) that have antithyroid activity.  One study looked at broccoli cooked in three different ways: boiling with a cold water start, boiling with a hot water start, and steaming (13).  The study showed that steaming resulted in an increase in the amount of total glucosinolates, while boiling the broccoli with a cold or hot water start resulted in a significant decrease in the amount of total glucosinolates.  And so the authors concluded that steaming can actually improve the health aspects of broccoli.  Another study looked at the effects of microwave cooking on the nutrient composition of broccoli (14).  The results showed a decrease in the levels of glucosinolates, phenolic compounds, and vitamin C, although it didn’t reduce the minerals present.

Yet another study looked at the effects of five different types of cooking methods on  the nutrient composition of broccoli, including steaming, microwaving, boiling, stir-frying alone, and stir-frying followed by boiling (15).  The results showed that all cooking treatments, except for steaming, caused significant losses of chlorophyll and vitamin C and significant decreases of total soluble proteins and soluble sugars (15).  Total aliphatic and indole glucosinolates were significantly modified by all cooking treatments, with the exception of steaming.  It’s worth noting that in this study the broccoli was steamed for five minutes.

These studies would suggest that boiling, microwaving, or stir-frying broccoli and other cruciferous vegetables can make them less goitrogenic by causing a decrease in the glucosinolates.  It does this by degrading the enzyme myrosinase, which converts glucosinolates into those compounds which have antithyroid activity.  However, the evidence I came across shows that light steaming increases the glucosinolates, which might suggest that this would make these foods more goitrogenic.  On the other hand,  steaming cruciferous vegetables for seven minutes or longer seems to inactivate myrosinase activity (16).  And so if you are concerned about the impact of cruciferous vegetables on thyroid health, then you will want to avoid eating them raw, and will either want to boil, microwave, stir fry, or steam them for at least seven minutes.

What’s The Final Verdict On Broccoli and Thyroid Health?

I was only able to find one human study which looked at the impact of cruciferous vegetables on thyroid health.  This involved Brussels sprouts, which are a rich source of glucosinolates.  This was a very small study, as it involved only ten volunteer subjects, and they ate 150 grams of cooked Brussels sprouts per day for four weeks (17).  It was determined that there was no effect on thyroid hormone, which isn’t surprising since I discussed earlier how most cooking methods (except for light steaming) inactivate the enzyme myrosinase.

As I mentioned earlier in this post, I think it’s fine for most people with thyroid and autoimmune thyroid conditions to eat a few servings of broccoli and/or other cruciferous vegetables each day.  This is even true with regards to raw broccoli, although if you’re concerned you can always cook it.  Just remember that while boiling, microwaving, and stir-frying will greatly reduce the goitrogenic activity, light steaming for only a few minutes might actually increase the goitrogenic activity.