Cruciferous vegetables, fermentation, and your thyroid

People with low thyroid function (hypothyroidism) or autoimmune thyroid diseases, such as Graves’ or Hashimoto’s Disease, are often told to limit or avoid cruciferous vegetables. Nevertheless, crucifers contain powerful plant compounds (phytochemicals) that prevent disease, especially cancer. Additionally, crucifers are main ingredients in many staple fermented foods, such as sauerkraut and kimchi, that help heal the gut and strengthen the immune system – two therapeutic targets in managing thyroid dysfunction.

This calls into question whether avoidance of crucifers is really warranted? Do the potential risks really outweigh the benefits of eating crucifers? And what can we do about it? 

Before we can answer these questions, let’s first learn

  • what are cruciferous vegetable
  • the known health benefits
  • possible risks associated with crucifers
  • whether scientific research validates these potential risks 
  • what effect fermentation has on cruciferous vegetables, and
  • the essential nutrients help you avoid negative effects of crucifers

What are cruciferous vegetables?

Cruciferous vegetables belong to the Cruciferae (formerly Brassicaceae) family that include those vegetables with a sharp bitter taste and pungent sulfur smell – characteristics that have children turning their noses up and mothers responding, “Eat your broccoli; it’s good for you.” Broccoli, cabbage, cauliflower, Brussel sprouts, and kale are among the most commonly grown crucifers, but there are many other vegetables that fall within this plant family (see full listed below). 

arugula (rocket)
bok choy*
broccoli*
broccolini*
canola/rapeseed*
cauliflower (white, purple, green)*
cabbage*
Chinese cabbage
napa cabbage*
collard greens* 
daikon
field pepperweed
garden cress
horseradish
kale*
kohlrabi*
komatsuna
mizuna (mustard greens)
mustard seedsradish 
rapini (broccoli rabe)
rutabaga*
savoy cabbage 
tatsoi
turnip root and greens*
wasabi
watercress
wild broccoli
sprouts of any of these vegetable seeds* 

*Cruciferous vegetables in the Brassica genus are particularly high in health-promoting glucosinolates. 

Glucosinolates in crucifers

Crucifers have outstanding health benefits, yet not exclusively related to their high source of vitamins and minerals. High levels of health-promoting glucosinolates, are the reason cruciferous vegetables take the health spotlight among other plants. 

Glucosinolates are sulfur-containing plant compounds that repel pest but attract many health-conscious eaters due to the well-documented health benefits. Of the more than 120 glucosinolates identified across 16 plant families (1), a subset of glucosinolates in cruciferous vegetables are well researched for prevention and treatment of disease, especial cancer. 

Glucosinolate in themselves are not health-promoting. They are actually inactive precursor chemicals to bioactive breakdown products – isothiocyanates, indoles, nitriles, and thiocyanates. Activation requires degradation of glucosinolates by myrosinase, a plant enzyme, upon handling, processing, or chewing cruciferous vegetables. Bacteria in the colon also contain myrosinase (bacterial myrosinase), but studies show that bacterial myrosinase is no more than 20% efficient; whereas, plant myrosinase is 17-88% efficient in activating precursors (2,3).

Health benefits of cruciferous vegetables

The health benefits highlighted below are owing to the breakdown products of glucosinolates:

  • Anticarcinogenic: Isothiocyanates, such as sulforaphane, and indoles, such as indole-3-carbionol and ascorbigen stimulate detoxification enzymes in the liver, particularly phase II enzymes, to clear toxins, some of which cause mutations to DNA and carcinogenesis (4,5). Sulforaphane has been described as “the most potent inducer of phase II enzymes identified to date” (6). Inducing phase II enzymes also combats carcinogens by enhancing antioxidant capacity (7,8). Furthermore, isothiocyanates act directly on cancer cells by inducing apoptosis, cell death (9,10). 
  • Detoxification: The same detoxifying mechanism described above also promotes rapid and sustainable detoxification of airborne pollutants such as those in industrialized areas of China, which represent some of the most toxic air quality (11).
  • Anti-inflammatory: Stimulating phase II enzymes lowers oxidative stress and reduces inflammation. The imbalance of oxidative stress and antioxidants can lead to multiple chronic conditions including allergic respiratory inflammation, degenerative diseases, and autoimmune diseases, such as Hashimoto’s Thyroiditis and Graves’ Disease. Cruciferous vegetables increase antioxidant function. As a result, inflammation decreases, and many diseases improve. 
  • Estrogen balance: Indole-3-carbinol targets estrogen dominance in three ways. It favours the conversion of estrogen to forms with weak estrogenic activity (i.e. 2-hydroxyestrone) rather than forms with potent estrogenic activity (i.e. 16α-hydroxyestrogens and estrogen) (12,13). It stimulates phase II enzymes to clear and eliminate excess estrogen from the body (13). Lastly, it blocks estrogen receptors on estrogen-sensitive cells, such as breast cell, which prevents circulating estrogen from binding and exerting estrogenic action (14). 

If crucifers yield such remarkable anticarcinogenic, detoxifying, anti-inflammatory, and hormonal balancing benefits (these are only some of many other benefits), what is the basis for recommending that thyroid patients avoid cruciferous vegetables?

One word: goitrogens.

What are goitrogens?

Goitrogens are substances that interfere with thyroid function by disrupting thyroid hormone production, inhibiting its release, or impairing iodine uptake, which is a mineral incorporated into thyroid hormones. High amounts of goitrogens can cause an abnormal enlargement of the thyroid gland known as goiter. Non-cruciferous and cruciferous plants, environmental chemicals, and medications are sources of goitrogens.

Goitrogens in crucifers

Glucosinolates in raw cruciferous vegetables yield two classes of goitrogenic compounds: isothiocyanates (e.g. goitrin) and thiocyanates. These goitrogens block iodine uptake into the thyroid and impair an enzyme involved in thyroid hormone production (thyroid peroxidase). Cruciferous vegetables yield varying amounts of goitrogenic compounds. For example broccoli, Chinese cabbage, and bok choy have low levels of goitrogenic compounds, while Russian/Siberian kale, collards, and Brussel sprouts contain high amounts that could impact thyroid function (15). 

It is well known that cooking deactivates myrosinase, the enzyme needed to convert glucosinolates into active goitrogens. Since bacterial myrosinase in the colon cannot efficiently degrade glucosinolate precursors into goitrogens, eating cooked cruciferous vegetables does not affect thyroid function. This was confirmed by a 1986 study that provided participants with a portion of cooked Brussel sprouts, a particularly rich source of glucosinolates, for 4 weeks (16). 

Still, reports of goiter in iodine-deficient animals that ate high amounts of crucifers raised concerns about human consumption of goitrogens in cruciferous vegetables. Until recently, there lacked substantial evidence from human studies to clarify whether raw cruciferous vegetables actually affect thyroid function. Let’s look at those new studies.

Scientific research 

Recent studies have isolated extracts from raw cruciferous vegetables to determine whether various doses of inactive glucosinolate precursors or activate products (isothiocyanates) of raw cruciferous vegetables impact thyroid function. 

Human studies

In a randomized control trail in 2019, 300 participants blindly received either a juice with no broccoli sprout extract (placebo), a sulforaphane-concentrated broccoli sprout extract, or an untreated broccoli sprout extract (contains only glucosinolates) for 12 weeks. All extract doses reflected portions of broccoli sprouts that people would realistically eat. At the end of the trial, TSH, free thyroxine (T4), and thyroglobulin were unaffected. Of the participants with antibodies that met the diagnostic criteria for Hashimoto’s thyroiditis, there was no worsening of autoimmune markers. Of people with subclinical hypothyroidism, there was also no worsening (increase in TSH). In both thyroid disorders, participants actually saw improvements in these markers, although it was not statistically significant (17).

A smaller 2006 study gave extracts of glucosinolates and isothiocyanates from broccoli sprouts to 12 healthy participants for seven days. The extract doses reflected what would be in portions of broccoli sprouts that people realistically consume in a day (i.e. roughly ¼ cup or 1½ cup). Participants blindly received the control (no broccoli sprout extract) or broccoli sprout extracts in a low dose of isothiocyanates (active) or high dose of glucosinolates (inactive), and in the activate or inactive precursor form. No significant changes were observed in terms of TSH, free thyroxine (T4), or total triiodothyronine (T3) (18).

Animal Study

An animal study in 2018 gave rutabaga sprouts, which contain known goitrogens, to one of three types of rats: rats with normal thyroid, rats with iodine deficiency, and rats with thyroid damage induced by sulfadimethoxine (the latter two are models of hypothyroidism). The sprouts had no negative thyroid effects in healthy rats, but enhanced the negative effects of iodine deficiency and sulfadimethoxine (19). 

These three studies emphasize that moderate portions of raw crucifers do not affect the thyroid function in humans, provided iodine levels are adequate. Higher portions of glucosinolates from cooked crucifers are also of no concern. The question still remains: Does fermentation of raw cruciferous vegetables change the goitrogen level? 

Effect of fermentation on goitrogens levels in crucifers

Indeed, fermentation does change goitrogen level in crucifer-containing ferments. Unfortunately, it increases it. A 2016 study tracked how glucosinolate levels changed over the entire fermentation process of raw cabbage shreds to finished sauerkraut. Complete degradation of glucosinolates corresponded with the highest count of lactic acid bacteria, showing that bacteria contribute to degrading 100% of glucosinolates into active forms, some of which are goitrogenic (20). This is much more efficient than the degradation of raw, non-fermented crucifers that have been shredded, sliced, or chopped (2,3). 

These results shouldn’t suggest eliminating fermented cruciferous vegetables, like sauerkraut and kimchi, from your diet, especially if you have thyroid dysorders. The 2016 study also noted that the levels of breakdown products (Isothiocyanates, indoles, nitriles, thiocyates – some of which are goitrigenic) decrease during storage. This one of many reasons why fermentation of kraut and all other brine-pickled cruciferous vegetables should be fermented for 11-13 weeks before consuming, rather than seven days, as many internet websites recommend. 

Gut health for thyroid function

Other immune-balancing and gut-healing benefits of fermented foods may outweigh the goitrogenic risks of fermented cruciferous vegetables. Looking at the larger picture of healing and health is essential when determining what foods to include in a therapeutic diet. As long as sauerkraut isn’t your main and only ferment that you are using for gut health, you’ll be fine. 

In addition to eating moderate amounts of fermented cruciferous vegetables, aim for a variety of non-cruciferous fermented foods, such as fermented onions, turmeric, carrots, cucumbers, zucchini, jalapeno peppers, lemons, garlic, asparagus, tomatoes, beets, or beet kvass. Each of these ferments have tons of healthy benefits, not to mention lots of flavour. 

Even though patients with hypothyroidism and autoimmune thyroid disease can include moderate amount of raw cruciferous vegetables in their diet and larger portions of cooked crucifers, essential nutrients that support thyroid hormone production must be adequate in the diet to avoid negative effects.

Essential nutrients to support the thyroid

Idoine

Iodine is a component of thyroid hormones. In iodine deficiency, thyroid hormone production reduces due to inadequacy of materials to build it. This can lead to hypothyroidism. Cruciferous vegetables and cassava inhibit the uptake of iodine from the bloodstream into thyroidal cells; however, several studies confirm that cruciferous vegetables consumption does not affect thyroid status when iodine is adequate, yet may have negative effects when coupled with iodine deficiency.

Aim to meet your requirements, 150 mcg/day – no more, no less. Marine animals can concentrate iodine from seawater, making fish, shellfish, and seaweed rich sources of iodine. Moreover iodized salt is also a rich source at 76 mcg of iodine per one gram (¼ teaspoon) compared to 27 mcg in non-iodized salt (i.e. Redmond Real Salt). Diary products, namely milk and cottage cheese, are also high sources. Plant-based sources are variable related to iodine-depleted soils in Canada and large areas of the northern United States. People most susceptible to deficiency are those who exclude seafood, dairy products, and iodized salt. People who eat moderate portions of fermented foods made with non-iodized salt – the recommended type of salt for fermentation – need not worry about low iodine. 

More is not better, in the case of iodine. Over-emphasizing iodine-rich food sources to avoid deficiency can inhibit thyroid hormone production. This can elevate TSH and induce hypothyroidism or contribute to autoimmune thyroid diseases. The upper limit for all adults regardless of pregnancy or lactation is 1100 mcg/day. Above this amount, iodine inhibits thyroid hormone synthesis and can elevate TSH as seen in iodine deficiencies. Excessive iodine is of concern for people who consume high amounts of seaweed and of processed food made with iodized salt. 

Selenium

Selenium is another essential mineral for thyroid hormone production, though its contribution is often overlooked. It is a component of enzymes (5’-iodinase) that convert T4 (thyroxine) into T3 (triiodothyronine), the relatively inactive thyroid hormone into the biological active form. It is also part of enzymes (thioredoxin reductase) that support other enzymes in clearing excess iodine so thyroid function returns to normal. 

The daily recommended amount of selenium for adults, 55 mcg/day, is easily met by consuming two Brazil nuts, the richest source of selenium. Meat and seafood are also good sources of selenium to ensure thyroid activity via selenium-dependent enzymes.

Iron and Zinc

Other thyroid-supportive nutrients are iron and zinc. Iron is needed for an enzyme involved in thyroid hormone production (thyroid peroxidase). Research shows that iron deficiency anemia sharply reduces the activity of this enzyme (21). Zinc is believed to facilitate conversion of convert T4 into T3. Both of these minerals are sourced from red meat and some legumes. Fermentation and sprouting help to increase the bioavailability of these minerals from plant food sources.

Populations with special consideration

Pregnant or breastfeeding women are at higher risk of iodine deficiency, because their iodine requirements are 1.5 times higher in pregnancy (220 mcg/day) and nearly double during lactation (290 mcg/day) compared to male and female adults (150 mcg/day). The fetus relies exclusively on maternal thyroid hormone via the placenta during early gestation, after which fetal requirements are met by a combination of maternal and fetal sources. Maternal supplies continue to contribute iodine during lactation. Brain development occurs from gestation through the first two years of life, and is thyroid hormone-dependent. Iodine deficiency compromises both maternal and fetal thyroid hormone. Mild to moderate iodine deficiency during pregnancy, therefore, has negative effects on child neurodevelopment (22,23), emphasizing the need to ensure optimal iodine status for the health of the mother and offspring.

What should we do going forward?

Considering all the evidence and information above, here are some big-picture thoughts as you ease your way back into including cruciferous vegetables into your diet.

1. Choose low goitrogen cruciferous vegetables. When wanting to eat raw crucifers, choose ones that are low in goitrogens, such as broccoli, Chinese cabbage, bok choy, and broccoli rather than Russian/Siberian kale, collards, and Brussel sprouts. Cooked forms of the latter crucifers become low goitrogens. 

2. Ensure optimal intake of essential nutrients that support thyroid function, namely iodine, selenium, iron, and zinc.

3. Beware of non-cruciferous food sources of goitrogens, and eat in moderation.

  • millet
  • soybeans*
  • canola oil*
  • cassava
  • sweet potatoes
  • spinach
  • peaches
  • strawberries
  • green tea 

As with cruciferous vegetables, goitrogenic effects from other high-goitrogen foods become concerning when consumed in high doses. For example, catechins from green tea are potent antioxidants with significant anti-inflammatory properties. Having a daily cup of green tea is more helpful than harmful; however, the problem arises when people go overboard on a good thing. Use moderation.

*Completely exclude soybeans and canola oil, because these are highly genetically modified and often found in refined forms in processed foods.  

4. Reduce chemical exposure. Often overlooked, many environmental toxins are goitrogens (24). Aim to live a low-toxin lifestyle, especially avoiding the following thyroid-disrupting toxins:

  • cadmium and lead
  • pesticides (organichlorine)
  • solvents and plastics (organophosphates)
  • hand sanitizer (triclosan)
  • non-stick coatings, water-resistant materials, ScotchgardTM (perfluorinated chemicals) 
  • cosmetics with UV filters
  • large fish (polychlorinated biphenyls)
  • BPA-lined cans, plastic bottles and food containers (bisphenol-A)
  • flame retardants in furniture, electronics, mattresses, and circuit boards (polybrominated diphenyl ethers)

5. Support your gut. Inflammation contributes to autoimmune diseases, and the gut is a major regulator of inflammation. Even though the thyroid is quite a distance from the gut, inflammatory signals can spread from the gut to the thyroid. Daily prebiotics and fermented foods with live culture within a diet of whole foods support your gut. Processed foods, refined sugar, and food additives do not. Removal of foods that irritate the gut can reduce inflammatory signals.

6. Go easy on green smoothies. Blending kale or spinach (high source goitrogens) with other items in a green smoothie can amount to a concentrated cocktail of goitrogenic compounds. It’s easy to double or triple your portion of kale in a smoothie compared to what you would consume whole. Use moderation, as with all goitrogenic-containing foods, and vary the greens you use in smoothies.

7. Trial a short-term avoidance of cruciferous vegetables. If you are still feeling off from eating cruciferous vegetables while implementing the recommendation above, consider a short-term avoidance. There are other components in cruciferous vegetables, such as FODMOPS, that may be affecting you. Talk to a knowledgeable health professional to identify your triggers.

Take home

Mother are right – broccoli (and other cruciferous vegetables) are good for health. As long as essential nutrients that support thyroid function are optimal, cruciferous vegetables in moderate portions are not concerning for healthy people or for people with hypothyroidism and autoimmune thyroid diseases. Fermented cruciferous foods can also support thyroid function by supporting gut health, although portion needs to be in check. The benefits of crucifers really do outweigh the potential risks. 

References

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