- Do You Really Need to Give Up Kale, Cauliflower, and Other Cruciferous Vegetables When You Have Hypothyroidism?
- How Cruciferous Vegetables Affect Your Thyroid
- Can You Safely Eat Cruciferous Vegetables?
- Weighing the Risks and Benefits of Cruciferous Veggies
- Do Cruciferous Vegetables Cause Hashimoto’s?
- What Are Cruciferous Vegetables?
- What Is A Goitrogen?
- How Can You Minimize the Effect of Goitrogenic Foods?
- The Final Takeaway.
- Micronutrient Information Center
- Metabolism and Bioavailability of Glucosinolates
- Disease Prevention
- Nutrient Interactions
- Intake Recommendations
- Authors and Reviewers
- Is Kale Bad for My Thyroid Health?
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- Being high in powerful antioxidants like sulforaphane and vitamin C, fiber, carotenoids, vitamin E and K, and folate, there is quite a case for consumption of these foods as a big contribution to enjoying greater health.
- Many clinicians believe that one would have to eat a ton of raw cruciferous vegetables, in excess of 1-2 pounds daily to have an adverse effect on the thyroid.
- How Cooking and Fermentation Help:
- What About Green Smoothies?
- The Bottom Line
- Getting Iodine From Your Diet
- Thyroid Testing
- Can You Eat Raw Kale, and Should You?
- 90.9 WBUR wbur
- The Thyroid Controls Metabolism
- Goitrogenic Substances Interfere with Iodine Uptake in the Thyroid
- Kale and Broccoli: Health Foods or Thyroid Toxins?
- Patients Who Are Susceptible to Goitrogenic Foods
- Further Dietary Considerations for Thyroid Health
- Other Strategies to Improve Thyroid Function
Do You Really Need to Give Up Kale, Cauliflower, and Other Cruciferous Vegetables When You Have Hypothyroidism?
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If you have hypothyroidism (underactive thyroid), you may have been told to avoid cruciferous vegetables — such as kale, cauliflower, broccoli, cabbage, and Brussels sprouts.
These vegetables have been shown, in certain situations, to interfere with how your thyroid gland uses iodine. Using iodine effectively is necessary for normal thyroid function.
It’s not clear, though, if consuming cruciferous vegetables in moderation has any harmful effects on thyroid function in people without thyroid disease. And many people with hypothyroidism can safely incorporate these foods into their diet.
Here’s what you need to know about cruciferous vegetables and hypothyroidism — the effects these veggies can have on your thyroid, whether you should avoid them, and how to weigh the risks and benefits of eating them.
How Cruciferous Vegetables Affect Your Thyroid
Cruciferous vegetables belong to a group of related plants, and include the following produce:
- Bok choy
- Brussels sprouts
- Collard greens
- Mustard greens
- Turnips and turnip greens
These vegetables are chemically unique in many ways, including their high level of sulfur-containing compounds called glucosinolates — which gives them their distinctive smell.
But glucosinolates and other chemicals in these veggies are considered goitrogens, which means they may interfere with the production of thyroid hormone — mostly by disrupting your thyroid’s use of iodine. Your thyroid uses iodine for normal production of thyroid hormone, and if it can’t access iodine properly or if there isn’t enough iodine in your body, your thyroid hormone level will drop.
For most people, goitrogens shouldn’t be a concern and consuming cruciferous vegetables can lead to a number of health benefits, including a reduced risk of several types of cancer.
“They’re not necessarily harmful if you have a normally functioning thyroid,” says Luis O. Rustveld, PhD, RD, a dietitian and assistant professor at the Baylor College of Medicine in Houston.
In fact, Rustveld says, many concerns that people have about cruciferous vegetables stem from animal studies showing that they can trigger hypothyroidism. One such study, in which researchers observed thyroid gland enlargement in rabbits eating cabbage, was published in 1928 and led to the discovery of dietary goitrogens.
According to researchers at Oregon State University, normal consumption of foods containing goitrogens doesn’t seem to increase the risk of hypothyroidism in humans — unless someone is deficient in iodine. Iodine deficiency, Rustveld adds, is extremely rare in the United States, largely due to iodine supplementation in salt and other food items.
Researchers have found that collard greens, Brussels sprouts, and certain varieties of Russian kale contain enough goitrogens to potentially interfere with iodine uptake in healthy people when eaten in large quantities, according to a study published in March 2016 in the journal Nutrition Reviews. But other cruciferous vegetables — including broccoli, turnips, and most types of kale — didn’t contain enough goitrogens to pose a significant risk to people with normal thyroid function.
Cruciferous vegetables aren’t the only dietary source of goitrogens, Rustveld notes. They’re also present in soy, spinach, strawberries, peaches, and peanuts.
Can You Safely Eat Cruciferous Vegetables?
If you have hypothyroidism, you may want to consider limiting your intake of cruciferous vegetables, says Rustveld.
“If you have an underactive thyroid or iodine deficiency, you should avoid juicing cruciferous vegetables,” he adds, since juicing makes it easy to ingest large quantities of produce.
You should also be sure to cook cruciferous vegetables if you have hypothyroidism. “Cooking cruciferous vegetables lessens their goitrogenic properties,” says Rustveld. “It reduces the effect of goitrogens on the thyroid, so they don’t cause any problem” in normal quantities.
But you don’t have to overcook your veggies until they’re mushy; normal cooking or steaming should do the trick.
Another good rule of thumb: People with hypothyroidism shouldn’t consume more than half a cup of cooked cruciferous vegetables each day, Rustveld says.
Weighing the Risks and Benefits of Cruciferous Veggies
When deciding whether you should eat cruciferous vegetables, the first thing to consider is the advice of your doctor, says Rustveld.
It’s also important to keep tabs on how well your treatment for hypothyroidism is working, he says. If your thyroid levels are normal, you may have more room for choices in your diet.
If your doctor leaves the decision to you, you should consider the potential benefits of these vegetables along with their risks.
Don’t underestimate the nutritional value of these vegetables, urges Rustveld. “Cruciferous vegetables are a rich source of nutrients — fiber, all kinds of vitamins, phytonutirents, antioxidants,” he says.
The bottom line: “Check with your doctor, but if you want to consume them because you like them, and if you cook them well, then you should be OK.”
Do Cruciferous Vegetables Cause Hashimoto’s?
September 10th, 2019
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When it comes to thyroid dysfunction, one of the questions I hear most often is: are cruciferous vegetables bad for thyroid health? As with any controversial topic, it’s best to evaluate the potential risks and benefits.
I believe the vitamins, minerals, and antioxidants found in cruciferous vegetables make them an indispensable part of a healthy diet. In fact, I’m such a big fan of veggies that I usually start my day with a smoothie made with my Organic Greens powder. It’s an easy way to get an extra serving of vegetables in your diet each day. This is so important because the nutrient levels in our produce are far less than they were for our grandparents. That’s a result of our modern growing methods.
Getting the right nutrients for your body is critical for thyroid health. Hashimoto’s, a disease of the thyroid, generally results in enlargement of that gland, and underproduction of thyroid hormones. Keeping your levels of thyroid hormones in balance is crucial for your body. Your metabolism, moods, and even aging processes are all affected by your thyroid gland. Because an imbalance in one bodily system causes imbalances in the others. This means there is no part of you that functions optimally when you have a thyroid problem.
Except for rare instances, conventional doctors mainly take the position that thyroid disease is of unknown cause. However, in functional medicine, we know the root cause. It’s one or a combination of these factors: leaky gut, nutrient deficiencies, an inflammatory diet (particularly gluten), your toxic burden, infections, and stress.
If your thyroid is out of balance, it’s important to identify which foods promote thyroid health, and which ones worsen the problem. So, is there any basis for the suggestion that you should avoid cruciferous vegetables and skip their health benefits? Can they cause thyroid disorders?
What Are Cruciferous Vegetables?
Cruciferous vegetables are vegetables in the family Brassica, and they are some of the most nutritionally dense foods we eat. Among others, this family includes:
- Brussels sprouts
Cruciferous vegetables are rich in antioxidants and phytonutrients that support cardiovascular health and overall cell growth. In addition to the health-promoting vitamins and minerals, these vegetables also contain goitrogens.
What Is A Goitrogen?
A goitrogen is a compound that suppresses the thyroid gland by inhibiting the uptake of iodine. It gets its name from the word “goiter” an enlargement of the thyroid gland. Iodine is one of the two building blocks of thyroid hormone. If you are deficient in it, you can develop hypothyroidism (an underactive thyroid). Hashimoto’s disease, the most common form of hypothyroidism, is an autoimmune disease in which your body attacks your thyroid gland.
Cruciferous vegetables are rich in two different goitrogens: isothiocyanates and thiocyanates. Animal studies suggest that consumption of cruciferous vegetables, including broccoli, kale, and cabbage, can lead to suppressed thyroid function. However, evidence that the same thing happens in humans is lacking. Unless a person is deficient in iodine, the consumption of cruciferous vegetables has little or no effect on the thyroid. Fortunately, iodine deficiency is very rare as most people consume table salt fortified with iodine.
If you have a functioning thyroid, there is no reason to believe that cruciferous vegetables will cause hypothyroidism. Consumption of these foods could suppress thyroid function in individuals who are already deficient in minerals such as iodine and selenium. Instead of avoiding these incredibly nutritious, beneficial foods, remedy the deficiencies. Or, if you take supplemental thyroid hormone, adjust your medication so that you may reap their nutritive benefits.
How Can You Minimize the Effect of Goitrogenic Foods?
If you are concerned about goitrogenic foods, there are steps you can take to minimize their potential harm to your thyroid.
Cook Your Vegetables
Cooking cruciferous vegetables lessens their goitrogenic properties. However, it also destroys some of their beneficial phytonutrients.
Eat foods rich in iodine.
Sea vegetables are a great source of dietary iodine. However, too much iodine can be harmful. It’s important to eat a diet rich in a variety of other thyroid-balancing minerals. Calcium may be a good example.
Juicing cruciferous vegetables allows their chemical components to be absorbed in your body in much greater concentrations. For this reason, juicing can be a great way to get vitamins, minerals, and antioxidants.
However, for those of you who are concerned about a low-functioning thyroid, you may want to avoid drinking concentrated amounts of goitrogenic foods. Instead, support your thyroid with my Organic Greens powder. It’s ideal for anyone looking for a quick blast of nutrients.
It’s rich in the superfoods and phytonutrients that are missing in our modern diets. It includes spirulina and chlorella, which are among the richest nutrient sources on the planet. My powder is also packed with antioxidants from alkaline greens. I included Ashwagandha to promote ideal immune function and Peruvian maca root for optimal hormonal health. Finally, I made sure that turmeric and ginger were mixed in to support a balanced inflammatory response.
The Final Takeaway.
In the end, we still aren’t sure how harmful the goitrogens in cruciferous vegetables may be. We do know for certain that these antioxidant-rich foods contain lots of phytonutrients to promote your optimal health.
The benefits of a diet rich in cruciferous vegetables far outweigh any danger. Problematic mineral deficiencies can be helped by supplementation. Making sure your body has what it needs to optimally function will protect you and your thyroid.
Micronutrient Information Center
- Cruciferous vegetables are unique in that they are rich sources of sulfur-containing compounds known as glucosinolates. (More information)
- Chopping or chewing raw cruciferous vegetables results in the formation of bioactive glucosinolate hydrolysis products, such as isothiocyanates and indole-3-carbinol. These metabolites are also generated by colonic bacteria, following the ingestion of cooked cruciferous vegetables. (More information)
- The biological activities of glucosinolate-derived isothiocyanates and indole-3-carbinol likely contribute to the potential health-promoting effects of cruciferous vegetables. (More information)
- Variations in the sequence of genes coding for Phase II detoxification enzymes, glutathione S-transferases (GSTs), may influence the potential health benefits of consuming cruciferous vegetables. (More information)
- A few observational studies have examined the link between cruciferous vegetable consumption and cardiovascular health, but the results have been largely inconsistent. (More information)
- High intakes of cruciferous vegetables have been associated with lower risk of bladder, breast, colorectal, endometrial, gastric, lung, ovarian, pancreatic, prostate, and renal cancer. However, evidence of associations remains largely limited to case-control studies. (More information)
- The 2015-2020 Dietary Guidelines for Americans recommend for adults to consume 1½-2½ cup-equivalents of dark-green vegetables per week, including cruciferous vegetables, as part of healthy meals. (More information)
Cruciferous or Brassica vegetables are so named because they come from plants in the family known to botanists and biologists as Cruciferae or alternately, Brassicaceae. The Brassicaceae family, which includes the model plant Arabidopsis thaliana, comprises approximately 375 genera and over 3,000 species (1). Many, but not all, commonly consumed cruciferous vegetables come from the Brassica genus; examples include broccoli, Brussels sprouts, cabbage, cauliflower, collard greens, kale, kohlrabi, mustard, rutabaga, turnips, bok choy, and Chinese cabbage (2). Examples of other edible crucifers include radish (Raphanus sativus), horseradish (Armoracia rusticana), watercress (Nasturtium officinale), wasabi (Wasabia japonica), and Swiss chard (Beta vulgaris flavescens) (2).
Cruciferous vegetables are unique in that they are a rich source of sulfur-containing compounds called glucosinolates (β-thioglucoside N-hydroxysulfates) that impart a pungent aroma and spicy (some say bitter) taste (Figure 1). Glucosinolates can be classified into three categories based on the chemical structure of their amino acid precursors: aliphatic glucosinolates (e.g., glucoraphanin), indole glucosinolates (e.g., glucobrassicin), and aromatic glucosinolates (e.g., gluconasturtiin) (Figure 1) (1). Around 130 glucosinolate structures have been described to date (3), but only a subset can be found in the human diet. In a cohort of 2,121 German participants in the European Prospective Investigation into Cancer and Nutrition (EPIC study), glucobrassicin, sinigrin, glucoraphasatin (dehydroerucin), glucoraphanin, and glucoiberin were found to contribute most to total glucosinolate intake (4).
Glucosinolates and their breakdown derivatives (metabolites), especially isothiocyanates and indole-3-carbinol, exert a variety of biological activities that may be relevant to health promotion and disease prevention in humans (see the MIC articles on Indole-3-Carbinol and Isothiocyanates).
Metabolism and Bioavailability of Glucosinolates
The hydrolysis of glucosinolates, which is catalyzed by a class of enzymes called myrosinases (β-thioglucosidases), leads to the formation of breakdown compounds, such as thiocyanates, isothiocyanates, indoles, oxazolidine-2-thiones (e.g., goitrin), epithionitrile, and nitrile (Figure 2). In intact plant cells, myrosinase is physically separated from glucosinolates. Yet, when plant cells are damaged, myrosinase is released and comes in contact with glucosinolates, catalyzing their conversion into highly reactive metabolites. In plants, thiocyanates, isothiocyanates, epithionitrile, and nitrile are defensive compounds against pathogens, insects, and herbivores (1). When raw cruciferous vegetables are chopped during the cooking process, glucosinolates are rapidly hydrolyzed by myrosinase, generating metabolites that are then absorbed in the proximal intestine. In contrast, boiling cruciferous vegetables before consumption inactivates myrosinase, thus preventing the breakdown of glucosinolates. A small fraction of intact glucosinolates may be absorbed in the small intestine, but a large proportion reaches the colon (5). Of note, boiling cruciferous vegetables has also been found to reduce their glucosinolate content to a much greater extent than steam cooking, microwaving, and stir-frying do (5). Nonetheless, when cruciferous vegetables are cooked, bacterial myrosinase-like activity in the colon is mainly responsible for glucosinolate degradation, generating a wide range of metabolites (5, 6).
A neutral pH may favor the formation of isothiocyanates from glucosinolates (Figure 2). Once absorbed, isothiocyanates, such as glucoraphanin-derived sulforaphane, are conjugated to glutathione in the liver, and then sequentially metabolized in the mercapturic acid pathway (Figure 3). Sulforaphane metabolites — sulforaphane-glutathione, sulforaphane-cysteine-glycine, sulforaphane-cysteine, and sulforaphane N-acetylcysteine (Figure 3) — collectively known as dithiocarbamates, are ultimately excreted in the urine (5).
The composition and content of glucosinolates in cruciferous vegetables are relatively stable, yet depend on the genus and species and can vary with plant growing and post-harvest storage conditions and culinary processing (7, 8). Since most cruciferous vegetables are cooked prior to eating, bacterial myrosinase-like activity in the gut rather than plant myrosinase is responsible for the initial step in glucosinolate degradation (Figure 2). In a feeding study involving 45 healthy subjects, the mean conversion rate of glucosinolates (of which 85% was glucoraphanin) to dithiocarbamates over a 24-hour period was estimated to be around 12% with wide variations among participants (range, 1.1 to 40.7%) (7). In contrast, 70%-75% of ingested isothiocyanates were found to be metabolized to dithiocarbamates. Therefore, following the ingestion of cooked cruciferous vegetables, the conversion of glucosinolates into isothiocyanates by gut bacteria appears to be a limiting step in the generation of dithiocarbamates (7). However, differences in individuals’ capacity to metabolize glucosinolates have not been linked to differences in gut microbiota composition (9).
Like most other vegetables, cruciferous vegetables are good sources of a variety of nutrients and phytochemicals that synergistically contribute to health promotion (see Bioactive compounds in cruciferous vegetables) (10). One challenge in studying the relationships between cruciferous vegetable intake and disease risk in humans is dissociating the benefits of whole diets that are generally rich in vegetables from those that are specifically rich in cruciferous vegetables (11). One characteristic that sets cruciferous vegetables apart from other vegetables is their high glucosinolate content (see Introduction). Glucosinolate hydrolysis products may play important roles in disease prevention by triggering antioxidant and anti-inflammatory response and contributing to the maintenance of cell homeostasis (see the MIC articles on Isothiocyanates and Indole-3-Carbinol).
Once absorbed, glucosinolate-derived isothiocyanates (like sulforaphane) are promptly conjugated to glutathione by a class of phase II detoxification enzymes known as glutathione S-transferases (GSTs) (Figure 3). This mechanism is meant to increase the solubility of isothiocyanates, thereby promoting a rapid excretion in the urine. Isothiocyanates are thought to play a prominent role in the potential anticancer and cardiovascular benefits associated with cruciferous vegetable consumption (12, 13). Genetic variations in the sequence of genes coding for GSTs may affect the activity of these enzymes. Such variations have been identified in humans. Specifically, null variants of the GSTM1 and GSTT1 alleles contain large deletions, and individuals who inherit two copies of the GSTM1-null or GSTT1-null alleles cannot produce the corresponding GST enzymes (14). It has been proposed that a reduced GST activity in these individuals would slow the rate of excretion of isothiocyanates, thereby increasing tissue exposure to isothiocyanates after cruciferous vegetable consumption (15). However, human interventional studies with watercress report there is no difference in the isothiocyanate excretion rate between positive (+/+) and null (-/-) genotypes (16). Similar studies with broccoli have shown that GSTM1-/- individuals excreted a greater proportion of ingested sulforaphane via mercapturic acid metabolism than GSTM1+/+ individuals (17, 18). In addition, GSTs are involved in “detoxifying” potentially harmful substances like carcinogens, suggesting that individuals with reduced GST activity might also be more susceptible to cancer (19-21). Finally, induction of the expression and activity of GSTs and other phase II detoxification/antioxidant enzymes by isothiocyanates is an important defense mechanism against oxidative stress and damage associated with the development of diseases like cancer and cardiovascular disease (22). The ability of sulforaphane (glucoraphanin-derived isothiocyanate) to reduce oxidative stress in different settings is linked to activation of the nuclear factor E2-related factor 2 (Nrf2)-dependent pathway. Yet, whether potential protection conferred by isothiocyanates via the Nrf2-dependent pathway is diminished in individuals carrying GST-/- variants is currently unknown.
Some, but not all, observational studies have found that GST genotypes could influence the associations between isothiocyanate intake from cruciferous vegetables and risk of disease (23).
High intakes of fruit and vegetables have been consistently associated with a reduced risk of cardiovascular disease (CVD) (24, 25). Yet, few observational studies have specifically examined the potential benefits of cruciferous vegetable consumption. In the Shanghai Women’s Health Study (mean follow-up, 10.2 years) and the Shanghai Men’s Health Study (mean follow-up, 4.6 years), which included a total of 134,796 Chinese adults, participants in the highest versus lowest quintile of cruciferous vegetable intakes had a 22% reduced risk of all cause-mortality and a 31% reduced risk of CVD-related mortality (26). In contrast, a pooled analysis of two large US prospective cohort studies, the Nurses’ Health Study (70,870 women) and the Health Professionals’ Follow-Up Study (38,918 men), found no significant association between cruciferous vegetable intake and combined risk of myocardial infarction (MI) and ischemic stroke (27). A case-control study conducted in 2,042 subjects (ages, <75 years) who survived a first acute myocardial infarction (MI), and matched healthy controls with no CVD history found that the individuals in the highest versus lowest tertile of cruciferous vegetable intakes (6 times/week versus <1 time/week) had 27% lower odds of MI (28). However, further analyses showed that the association between cruciferous vegetable intake and MI events was significant in individuals with two functional GSTT1 alleles but not in carriers of two alleles of the GSTT1 null variant (-/-) (28).
Analysis of data from two 12-week randomized controlled trials in 130 participants with mild or moderate CVD risk found that the consumption of 400 g/week of high-glucosinolate broccoli (containing 3 to 6 times more glucoraphanin and glucoiberin than standard broccoli) resulted in a significant reduction in low-density lipoprotein (LDL)-cholesterol concentration in plasma compared with standard broccoli (29). Whether the effect of glucosinolates on cholesterol metabolism might be beneficial in the prevention of CVD needs further investigation.
A recent intervention study demonstrated that cruciferous vegetables could increase the detoxification of carcinogens and other xenobiotics in humans. In this 12-week randomized controlled trial in 391 healthy Chinese adults exposed to high levels of air pollution, daily consumption of a broccoli sprout-rich beverage (providing 600 µmol/day of glucoraphanin and 40 µmol/day of sulforaphane) significantly increased the urinary excretion of a known carcinogen, benzene, and a toxicant, acrolein, compared to placebo (20). The biological activities of glucosinolate derivatives, isothiocyanates and indole-3-carbinol, which include modulation of xenobiotic metabolism, but also antioxidant and anti-inflammatory properties, induction of cell cycle arrest and apoptosis, and inhibition of angiogenesis, likely contribute to the potential benefits of cruciferous vegetables in the prevention of cancer (see the MIC articles on Isothiocyanates and Indole-3-Carbinol) (23).
Evidence from observational studies
Numerous observational studies have examined the relationship between cruciferous vegetable intake and cancer risk. Results from recent published meta-analyses of observational studies are reported in Table 1 (adapted from 23).
Most meta-analyses found inverse associations between cruciferous vegetable intake and risk of bladder, breast, colorectal, endometrial, gastric, lung, ovarian, pancreatic, prostate, and renal cancer. Subgroup analyses showed that inverse associations remained significant in pooled analyses of case-control studies but not in pooled analyses of prospective cohort studies (see Table 1). Retrospective case-control studies are susceptible to bias in the selection of participants (cases and controls) and prone to dietary recall bias compared to prospective cohort studies, which collect dietary information from participants before they are diagnosed with cancer (47). The method of cooking cruciferous vegetables, which strongly affects the bioavailability and potential anticancer benefits of isothiocyanates (see Metabolism and Bioavailability of Glucosinolates) may be a source of bias and explain variation in the results of the studies (heterogeneity among studies). The lack of information regarding cooking methods prevented data adjustment to reduce bias.
In the past decades, some observational studies have examined the effect of individuals’ genetic variations on the relationship between cruciferous vegetable intake and the risk of different cancer types. For example, a pooled analysis of two prospective cohort studies and six case-control studies found an inverse association between cruciferous vegetable consumption and risk of colorectal neoplasm in carriers of the GSTT1 null variant but not in individuals with the GSTM1 null variant or those with both the GSTT1and GSTM1 null variants (-/-) (36). The results of a pooled analysis of five case-control studies also suggested a stronger association between cruciferous vegetable intake and lung cancer in carriers of both the GSTT1-/- and GSTM1-/- variants compared to carriers of wild-type alleles (+/+); however, it was not reported whether results from these two groups of individuals were significantly different (39). There is also a significant body of evidence suggesting that GSTM1+/+ individuals gain greater cancer protection from consumption total cruciferous vegetables or broccoli compared to GSTM1-/- variant carriers (25, 48, 49). Current evidence is scarce, and adequately powered, well-designed studies are required to assess and explain potential interactions between cruciferous vegetable intake and GST genotypes.
A few observational studies have looked at whether cruciferous vegetable intake could be associated with reduced risks of disease progression and mortality. The highest versus lowest intake of cruciferous vegetables (assessed before diagnosis) was associated with a better survival rate over 72 months after diagnosis in 547 women with lung cancer (50). A prospective study in 29,361 men who underwent a prostate-specific antigen (PSA) test found that intake of cruciferous vegetables was inversely associated with risk of metastatic prostate cancer — cancer that has spread beyond the prostate (i.e., late-stage prostate cancer) — during a mean follow-up of 4.2 years (51). Another prospective study in 1,560 men diagnosed with non-metastatic prostate cancer reported that higher post-diagnosis intake of cruciferous vegetables was associated with a 59% lower risk of prostate cancer progression during a two-year period after completion of the dietary assessment (52). In contrast, cruciferous vegetable consumption in a cohort of 11,390 women with stage I-III invasive breast cancer (from four US and Chinese prospective studies), assessed about two years after diagnosis, was not found to be associated with risk of cancer recurrence or total mortality (53).
Iodine and thyroid function
Very high intakes of cruciferous vegetables, such as cabbage and turnips, have been found to cause hypothyroidism (insufficient production of thyroid hormones) in animals (54). Two mechanisms can potentially explain this effect. The hydrolysis of progoitrin, found in cruciferous vegetables (see Figure 1), may yield a compound known as goitrin, which may interfere with thyroid hormone synthesis. The hydrolysis of another class of glucosinolates, known as indole glucosinolates, results in the release of thiocyanate ions (see Figure 2) that can compete with iodine for uptake by the thyroid gland (55). However, increased exposure to thiocyanate ions from cruciferous vegetable consumption or, more commonly, from cigarette smoking, does not appear to increase the risk of hypothyroidism unless accompanied by iodine deficiency. One study in humans found that the consumption of 150 g/day (5 oz/day) of cooked Brussels sprouts for four weeks had no adverse effects on thyroid function (56). Similarly, consumption of high amounts of cruciferous vegetables has been associated with increased thyroid cancer risk only in iodine-deficient areas (57).
The 2015-2020 Dietary Guidelines for Americans recommend eating a variety of vegetables daily (2½ cup-equivalents/day for a 2,000 calorie diet) from all of the five vegetable subgroups (dark green, red and orange, legumes, starchy, and other; see 58). No separate recommendations have been established for cruciferous vegetables, yet the 2015-2020 Dietary Guidelines for Americans recommend that adults consume 1½-2½ cup-equivalents of dark-green vegetables (which include cruciferous vegetables) per week (58).
Bioactive compounds in cruciferous vegetables
Cruciferous vegetables are important sources of some vitamins and minerals, fiber, and various phytochemicals other than glucosinolates (Table 2). Many of these compounds likely contribute to the potential health-promoting benefits of cruciferous vegetables.
|Sulfur bioactives (other than glucosinolates) (59)|
Authors and Reviewers
Originally written in 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in December 2008 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University
Updated in December 2016 by:
Barbara Delage, Ph.D.
Linus Pauling Institute
Oregon State University
Reviewed in April 2017 by:
Maria Traka, Ph.D.
Senior Research Scientist
Chair of the Athena SWAN SAT
Food and Health Programme
Institute of Food Research
Norwich, United Kingdom
Copyright 2005-2020 Linus Pauling Institute
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49. Spitz MR, Duphorne CM, Detry MA, et al. Dietary intake of isothiocyanates: evidence of a joint effect with glutathione S-transferase polymorphisms in lung cancer risk. Cancer Epidemiol Biomarkers Prev. 2000;9(10):1017-1020. (PubMed)
53. Nechuta S, Caan BJ, Chen WY, et al. Postdiagnosis cruciferous vegetable consumption and breast cancer outcomes: a report from the After Breast Cancer Pooling Project. Cancer Epidemiol Biomarkers Prev. 2013;22(8):1451-1456. (PubMed)
55. Felker P, Bunch R, Leung AM. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism. Nutr Rev. 2016;74(4):248-258. (PubMed)
57. Cho YA, Kim J. Dietary factors affecting thyroid cancer risk: a meta-analysis. Nutr Cancer. 2015;67(5):811-817. (PubMed)
58. US Department of Health and Human Services and US Department of Agriculture. 2015-2020 Dietary Guidelines for Americans. 8th ed.; 2015.
Is Kale Bad for My Thyroid Health?
See How to Make Healthy Kale Chips
Kale’s über-healthy reputation is in part thanks to the cancer-fighting compounds it boasts called glucosinolates. But there’s a compound within glucosinolates that interferes with your thyroid function-and some may worry that eating too much kale could hurt their thyroid and possibly even cause hypothyroidism (underactive thyroid). (Why does that matter, you ask? Your thyroid regulates many body functions-and top on the list is metabolism.)
But there’s more to the story.
Dig a little deeper and you’ll learn a chain of reactions has to happen for those thyroid-interfering compounds-called thiocyanates-to be released. Cooking kale stops that chain of reactions from happening. However, chopping raw kale for a salad or chewing it does allow thiocyanates to form. But the quantity of thiocyanates in a few ounces of raw kale (what you’d probably eat) is minimal. The odds are low that even true kale lovers will harm themselves. “You’d have to eat a couple of large bunches of kale a day to get enough to have an impact-and you’d have to eat it raw,” explained Jeffrey Garber, M.D., past president of the American Association of Clinical Endocrinologists and associate professor of medicine at Harvard Medical School.
Thiocyanates are found in all cruciferous vegetables, not just kale, and rutabaga is the vegetable with the most. Runners-up-alongside kale-are turnips, cabbage and Brussels sprouts, but the rest of the cruciferous family has very little.
If you’re still worried, ask your doctor to check your thyroid. It’s a simple blood test. If you’re taking thyroid hormone for an underactive thyroid or have a predisposition to thyroid problems, you are more susceptible to the effects of thiocyanates. Garber’s advice: “Eat as many cruciferous vegetables as you want-go to your limit-and do this for a number of weeks; then ask your doctor to check and see if it’s had an effect.”
The Bottom Line
Don’t banish kale. It delivers healthy amounts of bone-strengthening vitamin K, eye-healthy vitamin A, plus some fiber and cancer-fighting compounds. Any concern about your thyroid is truly tiny.
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This is a guest post written by Christa Orecchio, C.N., a clinical and holistic nutritionist and a member of my Advisory Council. As the founder of The Whole Journey, she uses food as medicine wherever possible to help others heal and thrive and takes a mind, body, spirit approach in order to address the whole person. I reached out to Christa to write a post about the consumption of kale because it’s one of my favorite foods and one that I get asked about often. I want to provide you with the most cutting-edge health information backed by nutrition research. Here’s what Christa has to say about how cruciferous vegetables – like kale – can affect the thyroid:
As a nutritionist, I often get asked if cruciferous vegetables can cause or worsen thyroid issues. As is the answer to most controversial and confusing nutrition topics, it depends on a myriad of factors such as your current state of thyroid, digestive, and brain health, how much energy you are able to produce (mitochondrial production), the quality of the vegetable (soil it was grown in and whether pesticides were used), your cooking method, and how often you consume these foods.
Cruciferous vegetables are vegetables that belong to the Brassica family. Examples include:
- Bok Choy
- Broccoli Rabe
- Brussels Sprouts
- Cabbage (all kinds)
- Collard Greens
- Radish (includes Daikon)
Being high in powerful antioxidants like sulforaphane and vitamin C, fiber, carotenoids, vitamin E and K, and folate, there is quite a case for consumption of these foods as a big contribution to enjoying greater health.
They have long since been touted as supportive medicinal foods that help prevent cancer, support the liver, and contribute to improved immune and neurological health.
However, cruciferous vegetables offer a unique dichotomy because they also contain compounds called glucosinolates which are connected to exacerbating iodine-deficiency related hypothyroidism that can lead to thyroid swelling called a goiter. Hence these foods are referred to as goitrogenic foods.
They can do this by blocking the body’s ability to uptake iodine, which every cell of the body needs, but especially the thyroid, which contains the highest concentration of iodine in the human body. Those with pre-existing hypothyroidism may find that excessive consumption of raw cruciferous vegetables, could further suppress thyroid activity.
However, you may have heard the saying “the dose makes the poison” and it takes a lot of brassica to be clinically significant.
Many clinicians believe that one would have to eat a ton of raw cruciferous vegetables, in excess of 1-2 pounds daily to have an adverse effect on the thyroid.
You can start off with a smaller amount and work up from there. According to a study in Human Toxicology, no effect on thyroid function was observed in people who ate almost 1 cup of cooked Brussels sprouts daily for 4 weeks.
How Cooking and Fermentation Help:
Steaming your veggies until they are fully cooked reduces the goitrogens to one-third of the original content while boiling for thirty minutes is known to be a reliable way to destroy 90% of the goitrogens (which are released into the water and discarded).
Cooking also reduces other goitrogenic compounds in foods called nitriles.
Fermentation is another way to make them healthier for you. When you ferment something like cabbage, it actually increases the goitrogen content but truly reduces the nitriles by 50% or more, making the benefits of fermented vegetables far outweigh the goitrogenic risk to the thyroid that raw cabbage could potentially propose in large amounts.
What About Green Smoothies?
If you are worried about goitrogens or have been diagnosed with hypothyroidism, then add steamed greens (instead of raw) like kale and Collards to your morning green smoothie or only add raw kale to your green juice once or twice a week versus daily.
You may have seen the recent article in Mother Jones entitled “Sorry Foodies: We’re About to Ruin Kale” where they discussed how kale can uptake thallium, a toxic heavy metal from the soil. They also included other cruciferous vegetables in the article like cabbage, watercress, radishes, and turnips.
Again, there are many factors involved here and this doesn’t mean that you should not eat these foods. The quality of the soil is the key. The majority of organic farms (which test their soil) likely do not have dangerous levels of thallium. Because thallium contamination comes from nearby oil drilling, cement plants, and coal burning, it’s good to find out if the farm you purchase your kale from has exposure to these things. Or better yet, grow your own organic veggies in clean, nutrient rich soil.
The Bottom Line
It’s worth repeating not to go overboard with any one food so don’t eat a diet of just kale or any other vegetable for that matter.
If you are concerned about this and have hypothyroidism, then steam or boil your cruciferous vegetables and limit them to 1.5 – 2 cups/day.
This way you can still benefit from their high micronutrient profile and their anticancer benefits while supporting your immune function and brain health. These health benefits will support cellular energy and mitochondrial health, which is highly supportive to the thyroid gland.
I often operate by the mantra, “don’t guess at it, test it” so be sure to ask your doctor about testing your iodine levels and get adequate iodine from your diet.
Getting Iodine From Your Diet
Being in the nutrition field I am not a fan of iodized salt because it contains 2.5% chemicals in the form of anticaking agents and is exposed to extremely high heat during processing. This process removes much of the minerals, which in turn can create a mineral deficiency and therefore more salt cravings (because a craving for salt is a craving for minerals) as the body intelligently seeks to fill in its nutritional gaps.
I prefer to use Himalayan or Celtic sea salt, which does not contain added iodine like traditional salt but it does contain all 84 essential trace minerals to support the nervous system.
You can obtain adequate iodine from foods such as kelp, wild cod, wild shrimp and other shellfish, pasture-raised eggs, free-range turkey and organic raw milk.
If you have symptoms of hypothyroidism like constipation, cold hands and feet, hair loss, exhaustion, or dry skin, it’s always a good idea to get tested because this knowledge gives you great power to create balance and choose your food and supplement intake wisely.
I always test four thyroid markers to get a complete understanding of what is going on with a person’s thyroid. The four markers are TSH (thyroid stimulating hormone), T4 (inactive thyroid hormone) T3 which is the active thyroid hormone that truly translates to how well you are able to convert oxygen and calories to energy, and TPO or thyroid peroxidase which will tell you if you are building up antibodies and forming an autoimmune thyroid condition called Hashimoto’s Disease.
And lastly, let’s not forget to support the thyroid in other ways by minimizing or eliminating all of the truly thyroid destructive foods like canola, unfermented soy, an excess of polyunsaturated fats, gluten, and processed and refined sugar and flour. This alone will have a more positive impact on your thyroid health than whether or not you eat cruciferous vegetables.
- “Cruciferous Vegetables and Cancer Prevention”. Fact Sheet. National Cancer Institute, U.S. Department of Health and Human Services. 06/07/2012
- Masterjohn C (15 February 2008). “Bearers of the Cross: Crucifers in the Context of Traditional Diets and Modern Science”. The Weston A. Price Foundation for Wise Traditions in Food, Farming, and the Healing Arts
- McMillan M, Spinks EA, Fenwick GR: “Preliminary observations on the effect of dietary Brussels sprouts on thyroid function”. Hum Toxicol 1986; 5:15-19.
Note from Vani:
What do you think about Christa’s post? Please share your thoughts in the comments below. Are there other topics you’d like to see her address? For more information on thyroid health from Christa, you may want to read her blog called The Thyroid Uncovered which gets deeper into the science and lab work of thyroid testing and balancing or watch her TV episode on How To Optimize Thyroid Function.
I’m now on Periscope!!!
Sorry my recent playback to the grocery store is no longer available – but be sure to follow me on Periscope for future broadcasts! I’m “thefoodbabe”. See you on the flip side.
Kale is king, right? Ever since Whole Foods gave this leafy green a perfect score of 1,000 on the ANDI, or Aggregate Nutrient Density Index scale, kale has been the darling of health advocates. Hormone guru Sara Gottfried MD says, “Kale is a lifestyle,” and I agree.
But lately kale, a member of the cruciferous or brassica family, has been under some attack. Should we be concerned?
Under the rubric of “The Dark Side of Kale,” the web has been abuzz for more than a year with the idea that kale and its cruciferous mates like broccoli, Brussels sprouts, mustard greens, bok choy, and cauliflower may be harmful to our thyroid glands. This started in part when an opinion piece suggesting a risk was published in the New York Times last year.
And yet if you search the National Library of Medicine, you won’t find much to support this concern. One 2002 study showed no relationship between cruciferous vegetable consumption and thyroid cancer, while another suggested some relationship only in those with low dietary iodine intake.
In addition to the NYT piece, kale and its cruciferous cousins got a bad rap thanks to one overzealous 88-year-old woman. This Chinese woman ate up to 3.3 pounds of raw bok choy daily for several months and developed a severe case of low thyroid function. The authors wrote, “when eaten raw, brassica vegetables release the enzyme myrosinase, which accelerates the hydrolysis of glucosinolates; the cooking process largely deactivates the myrosinase in these vegetables.”
So before you cut out kale based on this limited data, I think it’s important to consider the demonstrated health benefits of eating cruciferous vegetables versus the very low risk to our thyroids. Here are seven science-backed reasons to keep eating kale:
1. In the 2013 Adventist Health Study that tracked thousands of people, a vegan diet — presumably rich in crucifers — was associated with a lower, not higher, risk of hypothyroidism.
2. In one study, healthy volunteers who consumed 150 grams of cooked Brussels sprouts daily experienced no change in thyroid function.
3. In more than 134,000 Chinese adults, increased cruciferous vegetable intake was associated with a lower total and cardiovascular death rate.
4. In a study of the Dutch population, eating more brassica vegetables was associated with a lower risk of colorectal cancer.
5. Research shows that eating cruciferous vegetables is associated with a lower risk of breast cancer.
6. Cruciferous vegetables can lower the risk of a heart attack, but only if you have a functioning gene that detoxifies the chemicals in this plant family.
7. A 2013 study found that broccoli sprouts may lessen the complications of diabetes.
The Bottom Line: Why I’m Still Eating Kale
Overall, the health benefits of kale and related vegetables far outweigh the minor concern over thyroid function. Of course, prudence suggests that eating pounds of raw cruciferous veggies daily may be unwise. For optimal thyroid function, it’s also important to pay attention to adequate dietary iodine intake (sea vegetables are a particularly healthy way to consume enough iodine).
For now, I am going to continue to juice, blend, and cook kale and other cruciferous vegetables on a regular basis, knowing that the benefit to my health is high and the risk is very low.
Can You Eat Raw Kale, and Should You?
Raw kale may be more nutritious, but it may also harm your thyroid function.
Kale, along with other cruciferous vegetables, contains a high amount of goitrogens, which are compounds that can interfere with thyroid function (8).
Specifically, raw kale contains a type of goitrogen called goitrins.
There are some concerns about eating raw kale, as goitrins can decrease the uptake of iodine, which is essential for the production of thyroid hormones (8).
This is worrisome, as thyroid hormones help regulate your metabolism. As a result, thyroid dysfunction can lead to reduced energy levels, weight gain, sensitivity to cold, and irregularities in heart rate (9).
One review of goitrin concentrations in cruciferous vegetables found that only an excessive intake of 2.2 pounds (1 kg) of kale per day for several months significantly impaired thyroid function in otherwise healthy adults (8).
However, research has shown that a moderate intake of goitrin-rich vegetables, including kale, is likely safe for most individuals.
Additionally, animal and human studies indicate that eating broccoli and Brussels sprouts doesn’t significantly affect thyroid hormone levels or functioning, suggesting that moderate amounts may even be safe for those with thyroid issues (10, 11).
Furthermore, regular intake of cruciferous vegetables has only been associated with an increased risk of thyroid cancer in women with very low iodine intake (12, 13).
Still, given that cooking vegetables deactivates the enzyme responsible for releasing goitrin, those with thyroid problems may benefit from cooking kale before eating it, as well as ensuring adequate intake of iodine from foods like seafood and dairy (14, 15).
Raw kale contains goitrins, which can lower iodine levels and impair thyroid function. Yet, research shows that a moderate intake of kale is unlikely to have any serious effects on thyroid health.
The headline in The New York Times made my heart sink: “Kale? Juicing? Trouble Ahead.”
Confession: I’m one of those Vitamix-loving, green-smoothie worshipping, kale bandwagoners. I brim with holier-than-thou pride when my kids eat raw kale (no dressing, even!) and thick kale-laden shakes while other children snack on sugary GoGURT squeezes and suck on juice boxes.
I am not alone. Presidents and stars are kale-lovers too.
Kale, in case you haven’t noticed, is health-conscious America’s “it” vegetable. Raw, blended, sauteed or in chip or “crunch” form, it appears to be the manna of celebrities: Gwyneth and Jennifer devour it while Kevin Bacon recently declared “the age of kale.” In an astutely reported feature called “Stars Who Love Kale,” US Weekly quotes Bette Midler saying: “Kale is burning up the veggisphere.”
Even the Obamas dined on kale salad at their Thanksgiving feast, notes The Washington Post.
But apparently there’s trouble in cruciferous paradise.
Writing for The Times, Jennifer Berman reports on the dark side of kale, and how the health-infused, veggie Eden she’d carefully built over years began to crumble:
Imagine my shock, then, at my last physical, when my doctor told me I had hypothyroidism, common in women over 40. When I got home I looked up the condition on the Internet and found a list of foods to avoid. Kale, which I juiced every morning, tops the list, followed by broccoli, cauliflower, cabbage, Brussels sprouts and collard greens — the cruciferous vegetables I consumed in large quantities because they are thought to prevent cancer, which runs in my family. And flax — as in the seeds — high in omega 3’s, that I sprinkled on cereal and blended in strawberry almond milk smoothies. Also forbidden: almonds and strawberries, not to mention soy, peaches, peanuts, corn, radishes, rutabaga and spinach.
My first reaction: Berman must be misguided, mistaken. Can kale possibly be bad?
Well, yes, possibly. Here’s the science-y lowdown on the kale-thyroid connection from the Oregon State University Micronutrient Information site:
Very high intakes of cruciferous vegetables…have been found to cause hypothyroidism (insufficient thyroid hormone) in animals (68). There has been one case report of an 88-year-old woman developing severe hypothyroidism and coma following consumption of an estimated 1.0 to 1.5 kg/day of raw bok choy for several months. Two mechanisms have been identified to explain this effect. The hydrolysis of some glucosinolates found in cruciferous vegetables (e.g., progoitrin) may yield a compound known as goitrin, which has been found to interfere with thyroid hormone synthesis. The hydrolysis of another class of glucosinolates, known as indole glucosinolates, results in the release of thiocyanate ions, which can compete with iodine for uptake by the thyroid gland. Increased exposure to thiocyanate ions from cruciferous vegetable consumption or, more commonly, from cigarette smoking, does not appear to increase the risk of hypothyroidism unless accompanied by iodine deficiency. One study in humans found that the consumption of 150 g/day (5 oz/day) of cooked Brussels sprouts for four weeks had no adverse effects on thyroid function.
(For an updated reality check on kale consumption and the thyroid, see our Q and A with endocrinologist and thyroid expert Dr. Jeffrey Garber here. Bottom line: in the U.S, where we don’t have a big problem with iodine deficiency, it’s probably OK.)
Teresa Fung, Sc.D., M.S.. an adjunct professor of nutrition at the Harvard School of Public Health and a professor at Simmons College in Boston, confirms the kale-thyroid link. But, she says, “normal, reasonable amounts of eating should not be a problem. A regular person who eats several servings of cruciferous vegetables a week should not have problems.”
Fung adds: “It’s the dose that makes a poison. If people have hypothyroidism or they’re taking thyroid medication, then they should check with their doctor. But even in this case, reasonable amounts shouldn’t be a problem. Now, if people have a tall glass of kale juice every single day, then it gets into the unknown territory.”
So, what are still-anxious kale-lovers to do? I asked Somerville, Mass. health coach and psychology of eating coach Nina Manolson to offer some guidance. (She’s not a doctor, but she knows a lot about food, so keep that in mind and always check with a professional if you make major changes in your diet.)
Nina reiterated that kale is a goitrogenic food, meaning that it can contribute to an enlarged thyroid — a goiter. A goiter indicates that the thyroid gland is not functioning optimally. But, she says, there are ways to have our kale and eat it too. Here, lightly edited, are her suggestions:
1. Cook Your Kale
The goitrogenic properties of kale become dramatically lessened when kale — or any other cruciferous vegetable — is cooked. (Other veggies in this category include: broccoli, brussel sprouts, cabbage, cauliflower, collard greens, kohlrabi, mustard, rutabaga, turnips, bok choy and Chinese cabbage. Arugula, horseradish, radish, wasabi and watercress are also cruciferous vegetables.)
2. Eat Seaweed
Kale on its own does not increase the risk of thyroid problems. It’s a combination of factors; including potential iodine deficiency. (One of the most common causes of goiters is iodine deficiency.) Adding seaweed or another iodine rich food to your diet may, in some cases, help you get adequate iodine.
3. Throw A Brazil Nut Into Your Smoothie
Selenium can support normal iodine levels which in turn may support a healthy thyroid. A Brazil nut or two in your daily smoothie or as a topping to any dish might help keep selenium levels strong.
4. Switch Up Your Greens
Vary your greens. If you’re going to eat kale one day choose a non-cruciferous, non-goitrogenic veggie dish the next, like a simple cucumber and tomato salad, or beets. There are many highly nutritious vegetables that aren’t goitrogenic, including celery, parsley, zucchini, carrots and more. Our bodies need many nutrients and by eating a variety of vegetables you’ll ensure that you don’t overload on one and skip another.
If you don’t have a thyroid issue, kale can and should be a delicious and healthy part of your diet because it is, indeed, a nutritional superstar with excellent credentials:
•It supports strong bones because of its high calcium content.
•It’s a potential immune booster, rich in Vitamin C.
•It may protect us against cancer because it’s packed with antioxidants and as mentioned, it’s a cruciferous vegetable.
•It’s high in iron which can support blood and energy levels.
•And of course, it’s packed with fiber so it’s great for digestion.
All of this leaves me wishing that eating were just, well, simpler. With no arsenic in the rice and no flip-flopping guidance on milk, soy, whatever, from the experts. It reminds me (and several commenters on The Times’ kale story) of that looming prophet of health, Woody Allen, and this scene from his film, “Sleeper”:
Dr. Melik: This morning for breakfast he requested something called “wheat germ, organic honey and tiger’s milk.”
Dr. Aragon: Oh, yes. Those are the charmed substances that some years ago were thought to contain life-preserving properties.
Dr. Melik: You mean there was no deep fat? No steak or cream pies or… hot fudge?
Dr. Aragon: Those were thought to be unhealthy… precisely the opposite of what we now know to be true.
Dr. Melik: Incredible.
Warnings about goitrogenic foods are popping up on alternative and conventional healthcare sites alike. The truth is, goitrogens can be a problem, especially for patients with thyroid problems. Read on to learn what foods are goitrogenic, how food can be prepared to limit them, and which patients should be extra concerned about goitrogens.
The Thyroid Controls Metabolism
The small, butterfly-shaped thyroid gland has big responsibilities. The thyroid and its hormones control metabolism throughout the body, affecting the brain, GI tract, cardiovascular system, lipid and cholesterol metabolism, hormone synthesis, gallbladder and liver function, and more.
Unfortunately, 20 million Americans suffer from some form of thyroid disease, and 60 percent of those who have it may not even be aware. One in eight women will develop a thyroid disorder in her lifetime. If something disrupts thyroid function, the health consequences can be widespread.
Goitrogenic Substances Interfere with Iodine Uptake in the Thyroid
The term “goitrogenic” means something that causes “goiter,” or swelling of the thyroid gland. Goitrogens accomplish this by interfering with iodine uptake in the thyroid gland. When not enough iodine is available, the thyroid cannot produce sufficient levels of thyroid hormones T4 and T3. The hypothalamus senses low T4 and releases TSH-releasing hormone, which triggers the pituitary gland to produce TSH. The thyroid gland responds to TSH by making more hormones. If it can’t keep up with demand, it grows bigger trying.
Goitrogens, found in many vegetables, can be problematic for patients with thyroid disorders.
Goitrins, thiocyanates, and nitriles are all goitrogenic chemicals derived from natural plant pesticides called glucosinolates. During digestion, an enzyme breaks down glucosinolates into both goitrogenic and non-goitrogenic byproducts (1).
Foods that have been identified as goitrogenic include cruciferous vegetables such as bok choy, broccoli, brussels sprouts, cabbage, canola, cauliflower, Chinese cabbage, coy sum, collard greens, horseradish, kai-lan, kale, kohlrabi, mizuna, mustard greens, radishes, rapeseed, rapini, rutabagas, and turnips. The Rosaceae family of fruits, which includes almonds, apricots, cherries, peaches, pears, plums, raspberries, and strawberries, is also goitrogenic. Other examples are bamboo shoots, millet, soy, spinach, sweet potato, tapioca, and yuca (cassava or manioc).
Many chemicals from the environment and medications are also classified as goitrogenic:
- Amiodarone (medication for irregular heart beat)
- Bromides (from pesticides, plastic, brominated vegetable oils, medications)
- Dioxins (toxic industrial byproducts)
- Heavy metals
- Lithium and benzodiazepines (depression and anxiety drugs)
- Oxazolidines (from paint)
- Perchlorates (from jet fuel, water)
- Thiocyanate (in cigarettes)
At relatively low concentrations, goitrogens decrease the uptake of iodine by the thyroid gland. This effect can often be offset by supplementing with iodine. However, exposure to large amounts of goitrogens impairs the incorporation of iodine into thyroid hormone itself, which means that the thyroid gland can’t properly utilize the iodine. In this case, no amount of supplemental iodine would be able to overcome a large intake of goitrogenic substances.
Kale and Broccoli: Health Foods or Thyroid Toxins?
Goitrogens are not an outright death sentence for kale and broccoli. Fruit and vegetable consumption in general lowers the risk of chronic diseases (2). Before you stop recommending antioxidant-rich fruits and vegetables to your patients, keep reading. The goitrogen content of foods varies widely and can be modified.
Crucifers are the biggest goitrogenic offenders, with certain varieties of kale, collard greens, and brussels sprouts at the top of the list. Others have significantly lower goitrogen levels. For example, the progoitrin (one of the harmful downstream products of glucosinolates) content per dry weight of Russian kale is approximately 150 times higher than that of Chinese cabbage (1).
Fortunately, cooking lowers the goitrogenic content of foods. Steaming crucifers until fully cooked reduces goitrogens by two-thirds. Boiling crucifers for 30 minutes destroys 90 percent of the goitrogens by stimulating the production of myrosinase, an enzyme that helps deactivate goitrogenic glucosinolates (3).
On the flip side, when green vegetables are boiled and the water is discarded, some beneficial nutrients are lost as well. About 45 percent of the vitamin C, 20 percent of the thiamin, and 40 percent of the folate are lost (4). Minerals (calcium, iron, etc.), vitamin B12, vitamin A, and others are very well retained. Steaming versus boiling will retain more nutrients. Although some nutrients leach out, cooking goitrogenic foods is generally beneficial.
In contrast to cooking, fermenting increases the goitrogen content of cabbage, but it simultaneously decreases the level of nitriles (5). Because nitriles are more harmful than goitrogens, the overall effect of fermentation is probably positive.
In a given week, if a patient is enjoying a couple of sides of steamed broccoli, a few servings of sauerkraut, and several small salads containing spinach and kale, that shouldn’t be a problem. On the other hand, if a patient is downing green smoothies every day, each with two or more cups of raw kale or spinach, then I would be worried about how that’s affecting thyroid function.
Bottom line: encourage your patients to steam or boil goitrogenic foods and not to consume them in excess.
Patients Who Are Susceptible to Goitrogenic Foods
Certain groups of patients need to take special considerations when it comes to goitrogenic foods:
Patients at Risk for Iodine Deficiency
As stated earlier, goitrogens reduce the uptake of iodine in the thyroid. If someone is already iodine-deficient, then goitrogens are more likely to cause issues. Despite iodized salt supplementation programs, iodine deficiency is on the rise. In Europe, it’s estimated that up to 44 percent of the population maybe be iodine-deficient (6).
The best food sources of iodine come from the sea: seaweeds, cod, shrimp, and tuna. Eggs and iodized salt are also options. Have your patients start with lower-iodine-containing foods and work their way up to higher levels. Simultaneously, ensure proper selenium intake. Selenium is best obtained from foods such as Brazil nuts, cremini mushrooms, cod, shrimp, tuna, halibut, salmon, scallops, chicken, eggs, shiitake mushrooms, lamb, and turkey.
Patients with Thyroid Problems
For patients already experiencing thyroid problems, especially hypothyroidism, goitrogens will exacerbate the condition (1). These patients should be limiting their cruciferous vegetables to one cooked serving per day. And take it easy on the green smoothies!
If you suspect a patient has thyroid issues, make sure you run a full thyroid blood work panel, which should include the standard TSH and T4, but also T3, free T3, free T4, and thyroid antibodies.
Pregnant and Nursing Women
Pregnant and nursing women require 50 percent more iodine than the average adult, making them extra susceptible to iodine deficiency (7, 8, 9). Goitrogens can inhibit the transfer of iodine into a mother’s breast milk. In a study of Boston mothers, 47 percent of breast milk samples did not have sufficient levels of iodine (10). I recommend only three to five servings per week of cooked cruciferous vegetables and other highly goitrogenic foods for these patients.
Further Dietary Considerations for Thyroid Health
Patients with thyroid disorders may also want to consider other dietary choices.
Oxalates protect plants from being eaten by critters and are found in most plants, nuts, and seeds. Spinach, okra, sweet potato, elderberry, figs, leeks, buckwheat, celery, other leafy greens, and dandelions are some sources. For frame of reference, ingesting 250 mg of oxalates is considered high, and one cup of raw spinach contains a whopping 656 mg (11).
Oxalic acid binds minerals like calcium and potassium, making them insoluble and less bioavailable (12). Oxalate salts increase the risk of kidney stones, especially in patients with gut dysbiosis. Gut bacteria are responsible for breaking down oxalates, but when the microbiota are compromised, oxalates can enter the blood, turn into crystals, and get stored in tissues like the kidney. In one study, 65 percent of kidney stones contained calcium oxalate (13). Unfortunately, cooking doesn’t decrease the oxalate content much.
Foods that May Trigger an Immune Response
For patients with an autoimmune thyroid disorder, eggs, nightshades, and dairy products are common offenders.
Industrial Seed Oils
Chronic inflammation can worsen thyroid disorders. The high content of omega-6 fatty acids in industrial seed oils drives inflammation.
Very-Low-Carb or Low-Protein Diets
Carbs and proteins both promote the release of insulin, which is required to convert T4 to T3. Have your patients aim their macros to at least 20 percent carbs and 10 percent protein. The increasingly popular keto diet may not be appropriate.
Other Strategies to Improve Thyroid Function
I deal with thyroid disorders often in my practice, and they can be very complex conditions. For more information, check out my articles about the thyroid’s connection with cardiovascular disease, the gut microbiome, blood sugar, and more. Beyond limiting goitrogenic foods, other strategies I recommend to improve thyroid function include the following:
- Manage stress – Stress impairs thyroid function (14, 15)
- Heal the gut – Gut bacteria assist in converting T4 to T3 (16)
- Monitor vitamin D status – Vitamin D deficiency is associated with numerous autoimmune diseases
- Avoid bromide and other iodine toxins
- Minimize exposure to toxins
For the past few years, kale has enjoyed a meteoric rise in the superfood world akin to Jennifer Lawrence’s ascendency in Hollywood. Just when we’d resolved that this is the year we make healthy, nutritious green juices every morning; however, total buzzkill The New York Times published an essay in which writer Jennifer Berman says kale—with which she religiously juiced every morning for years—was to blame for her hypothyroidism. Plus, thanks to all the fruit-and-vegetable-based juices she drank, she had five new cavities during a recent trip to the dentist.
Apparently, researchers have known about the deleterious effects of very high intakes of cruciferous vegetables like kale, cabbage, and turnips for years. And it’s easier to slip into “very high intake” level than you think. “It’s the dose that makes a poison,” Teresa Fung, a nutrition professor at Harvard, told WBUR’s CommonHealth blog. “Reasonable amounts shouldn’t be a problem. Now, if people have a tall glass of kale juice every single day, then it gets into the unknown territory.”
Do not take this as a sign you should forsake kale entirely, though. Studies have found that cooking cruciferous vegetables dramatically reduces their “goitrogenic properties” (read: ability to enlarge your thyroid). Plus, kale alone doesn’t increase your risk of developing thyroid problems, there have to be other factors, such as iodine deficiency. Fung recommends eating foods like seaweed, a good source of iodine, to counteract this.
She also recommends eating foods that contain selenium, like Brazil nuts, which support normal iodine levels, in turn aiding your thyroid. You should also keep a lot of variety in your choice of greens. If you eat kale one day, “choose a non-cruciferous, non-goitrogenic veggie dish the next, like a simple cucumber and tomato salad, or beets,” Fung says. Other non-goitrogenic veggies include parsley, zucchini, carrots, and celery.
As for the cavities, Berman’s dentist said to always wait half an hour after eating or drinking anything before brushing her teeth. Also, there’s no need to brush more than two times a day. If you really want to spare your choppers, eat fruits and vegetables in solid form. The natural sugars in juices can lead to decay, and the acid can erode enamel. In conclusion, there’s no need to turn your back on kale just yet. Phew.
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