All About Cruciferous Vegetables Christina Badaracco, MPH, RD
Updated: Nov 14, 2019
At a recent family celebration, an in-law asked for my opinion about her doctor’s recommendation to avoid cruciferous vegetables after diagnosing her with Hashimoto’s thyroiditis. As an enthusiastic advocate for nutrition, I appreciate when doctors address the impact of patients’ diets on their health. Yet, I hesitated to respond because I also realized that cruciferous vegetables are some of the most nutrient-dense foods we can eat and are typically beneficial to include in the diet. I was eager to read further about the effects of these potential "goitrogens," or compounds that interfere with the function of the thyroid gland. What are the various cruciferous vegetables? Crucifers belong to the taxonomic family of Cruciferae, named for the characteristic cruciform pattern (meaning in the shape of a cross) of their petals. They are also called Brassicaceae, providing the alternate name of brassicas. Cultivated in temperate climates throughout the Northern Hemisphere, they have been incorporated into very diverse cuisines. While the family includes more than 3,000 species, humans typically eat those in the species B. oleracea, which includes cabbage and broccoli. Unexpected members of the family include canola, brown mustard and horseradish, which are typically used in small quantities for their cooking properties or flavors. See below for some of the most commonly eaten crucifers, including the leaves, roots and flowers of different plants.
Why are cruciferous vegetables so healthy? Crucifers are rich sources of isothiocyanates and indole-3-carbinol, which are derivatives of molecules called glucosinolates (seen in the equation below). Isothiocyanates impart a spicy taste, making them bitter and unpalatable to some people. But bitterness can be a sign of the healthiest and wildest plants.
Chopping or massaging crucifers releases an enzyme called myrosinase, which is stored separately from glucosinolates in plant cells. Heat typically inactivates this enzyme and prevents it from converting glucosinolates into the bioactive isothiocyanates. Studies of glucosinolate concentrations after various methods of cooking suggest that boiling leads to the greatest breakdown and leaching of glucosinolates out of vegetables. Indeed, a recent review by Dr. Francisco Barba and others indeed suggested that these healthful compounds are most bioavailable in raw vegetables. Interestingly, a recent review article suggested that our gut bacteria actually produce and supply their own myrosinase, helping to metabolize glucosinolates even from cooked vegetables. This further indicates the importance of supporting a healthy gut microbiome for its role in properly digesting and metabolizing our food.
Health benefits of crucifers: Studies of the link between cruciferous vegetable consumption and cardiovascular health have yielded inconsistent results. However, a high intake of cruciferous vegetables has consistently been shown to be associated with lower risk of many types of cancer. A number of mechanisms may be responsible, such as inactivating carcinogens and inhibiting tumor cell migration. They are also good sources of both soluble fiber, which fosters healthy gut bacteria and lowers cholesterol and blood glucose, and insoluble fiber, which supports regular bowel movements. These vegetables provide good amounts of vitamins C, E and K; folate (a B vitamin); and the minerals selenium, calcium and potassium. They also provide carotenoids, such as lutein and lycopene, which promote good vision and prevent harmful chemical oxidation and protect against cancer. Finally, they are anti-inflammatory, further helping to prevent the development of cancer, heart disease and many other chronic diseases. What are the causes of hypothyroidism and how does it affect our bodies? The thyroid gland is located in the front of the neck and produces hormones that regulate our metabolism. An underactive thyroid, known as hypothyroidism, is commonly caused by autoimmune diseases like Hashimoto’s thyroiditis or Graves’ disease. It may also be caused by an iodine deficiency. Without iodine supplementation, it is difficult to get adequate iodine in the diet because soils are commonly depleted, and Americans rarely eat iodine-rich sea vegetables. The thyroid’s ability to produce hormones, thyroxine (T4) and the active form triiodothyronine (T3), diminishes when it lacks iodine. This can cause goiter, or an enlarged thyroid gland, as well as many other symptoms, such as obesity, infertility and fatigue. How do cruciferous vegetables impact thyroid function? High intake of cruciferous vegetables has been shown to cause hypothyroidism in animal studies by two mechanisms. First, crucifers contain goitrin molecules, which interfere with thyroid hormone synthesis. These molecules are activated upon ingestion. Also, one type of glucosinolates releases thiocyanate ions, which impair the thyroid’s ability to take up and use iodine, also reducing thyroid hormone production. While these effects have not been fully proven in humans, people with hypothyroidism who are more sensitive to goitrogens may be advised to avoid them. Doctors’ recommendations vary: the University of Maryland Medical Center suggests limiting portion sizes, while the Mayo Clinic suggests that the portion size necessary to inhibit absorption would be so large that it is not necessary to avoid them. Thus, it is important to follow the advice of your doctor who knows your symptoms, history and medication regimen.
What if I dislike cruciferous vegetables or don’t digest them well? Cruciferous vegetables contain a complex carbohydrate called raffinose. In the large intestine, bacteria ferment raffinose and may produce noticeable methane gas. If you experience bloating or gas after eating these vegetables, try eating smaller portion sizes—particularly if you’re not used to eating such fibrous foods. You may also try eating fermented forms of cruciferous vegetables, like sauerkraut or kimchi. If you dislike their bitter taste, try adding a creamy or sweet sauce (such as with dried fruit, goat or feta cheese, or honey). Finally, cooking with a little sea vegetable, such as kombu, provides a source of alpha-galactosidase, which is the enzyme found in Beano that breaks down raffinose, easing digestion and reducing gas production.
Tips for incorporating cruciferous vegetables into your diet:
Try preparing recipes of crucifers that incorporate probiotics to support maximal production of myrosinase. A few examples include a massaged kale salad with a yogurt-based dressing, roasted cauliflower with ginger miso, or collard greens sautéed with smoky tempeh.
If you suffer from gas and bloating when eating these vegetables (or beans), try reducing your portion size, adding enzymes or eating a fermented form.
If you don’t suffer from hypothyroidism or struggle with digesting cruciferous vegetables, then consider incorporating some forms of them regularly in your diet. When cooking, choose a quick-cooking method, such as steaming or stir-frying, to avoid depleting the nutrient content. Try to minimize storage time or buying pre-chopped vegetables, as well, to avoid excessive nutrient loss. Of course, it’s important to prepare them in a way that your family will enjoy eating them.
Look for iodized and unrefined sea salt in your local grocery store, whether or not you suffer from hypothyroidism.
Be sure to follow your doctor’s instructions regarding consumption of crucifers. Also, make sure he or she knows whether you regularly consume them so your thyroid medication dosage can be adjusted accordingly. If you suspect you have an undiagnosed case of hypothyroidism, see your doctor for an evaluation.
You may want to avoid other foods that may exacerbate hypothyroidism. For example, coffee may block absorption of thyroid replacement hormones, as may gluten, which may also irritate the small intestinal lining in some people.
Because heat can partially break down goitrins, people with hypothyroidism should focus on avoiding raw cruciferous vegetables. If your doctor agrees, you can consume small portions of cooked crucifers to benefit from these nutritional powerhouses.
Barba FJ, Nikmaram N, Roohinejad S, Khelfa A, Zhu Z, Koubaa M. "Bioavailability of glucosinolates and their breakdown products: impact of processing." Front Nutr. 2016;3:24.Cruciferous Vegetables. Linus Pauling Institute Micronutrient Information Center. lpi.oregonstate.edu/mic/food-beverages/cruciferous-vegetables#reference2. Updated 2018. “Hypothyroidism.” University of Maryland Medical Center. umms.org/ummc/patients-visitors/health-library/medical-encyclopedia/articles/hypothyroidism. Updated 2015. International Agency for Research on Cancer. Handbook of Cancer Prevention, Volume 9. Chapter 1: Cruciferous Vegetables, Isothiocyanates and Indoles. 2004.Melby AE, Linder T. Thyroid Disorders. Conjoint 401—403 Review Topics. Seattle, WA: University of Washington Pharmacy Doctoral Program. courses.washington.edu/conj/bess/thyroid/thyroid.htmlTian S, Liu X, Lei P, Zhang X, Shan Y. "Microbiota: a mediator to transform glucosinolate precursors in cruciferous vegetables to the active isothiocyanates." Journal of the Science of Food and Agriculture. October 2017.Torborg L. “Mayo Clinic Q and A: Hypothyroidism, spinach and kale.” Mayo Clinic. newsnetwork.mayoclinic.org/discussion/mayo-clinic-q-and-a-hypothyroidism-spinach-and-kale/. December 2016.
Christina Badaracco, MPH, RD
Christina is a registered dietitian and author who aims to improve access to healthy and sustainable food and educate Americans about the connections between food and health. She loves to experiment with healthy recipes in the kitchen and share her creations to inspire others to cook. Christina completed her dietetic internship at Massachusetts General Hospital and earned her Master of Public Health degree from the University of California, Berkeley. Previously, she graduated with a degree in Ecology and Evolutionary Biology from Princeton University, after conducting her thesis on sustainable agriculture and energy in Kenya. She has done clinical nutrition research at the National Institutes of Health, menu planning and nutrition education at the Oakland Unified School District and communications at the Environmental Protection Agency’s Office of Water. She has also enjoyed contributing to children’s gardens, farmers’ markets and a number of organic farms.
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