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An Up-to-Date Look at Goitrogenic Substances in Food

If you've wondered exactly what is meant by the terms "goitrogen" and "goitrogenic food," you are definitely not alone. These widely used terms often raise red flags in relationship to our health. Yet, as it turns out, many people often use these terms in the wrong context.

In the early 1950s, scientists wondered whether there might be identifiable chemical compounds in certain foods - such as cruciferous vegetables like broccoli and cauliflower - that were not known to play a supportive role in nourishment but instead worked only to compromise health and, in particular, the health of the thyroid gland. They used the term "goitrogen" to describe the activity of these chemical compounds. Since then, scientists have learned more and more about food substances with goitrogenic effects and how they work.

Over the past 50 years, however, researchers have determined that there are no such "negative" substances in food, but only health-supportive nutrients that are not a good match for certain individuals because of their unique health history and health status. Five decades of research have also determined that certain nutrients - like tyrosine, iodine, and selenium - play a unique role in thyroid health. (The exact nature of deiodinase enzymes, enzymes that are the key to production of thyroid hormones, was only discovered during this 50-year time period.) What has emerged from this scientific work on diet and health is a shift from a focus on "negative" foods that might be "bad" for the thyroid to a new focus on the need to create a right fit between each person's diet and the nutrients needed by his or her thyroid for optimal function.

Goitrogens

From a science perspective, "goitrogen" is a term used to describe any substance that can cause enlargement of the thyroid gland. That's because "goiter" is an enlarged thyroid gland. (Sometimes a person with an enlarged thyroid gland is described as "having a goiter," and sometimes the condition itself is simply referred to as "goiter.") A "goitrogen" is simply any substance that causes or contributes to goiter.

There are many different chemical substances that can contribute to goiter. From a chemical standpoint, they usually have nothing in common except for their ability to cause thyroid enlargement.

Many drugs act as goitrogens. These drugs include lithium (a drug used to treat bipolar disorder) and phenylbutazone (a non-steroidal anti-inflammatory drug). Sometimes these drugs are described as having "goitrogenic effects" instead of being called goitrogens themselves. Most of the time, drugs with goitrogenic effects interfere with iodine metabolism. Since the thyroid needs iodine to produce thyroid hormones, drugs that interfere with iodine metabolism often prevent the thyroid from making its iodine-containing hormones properly. (Without the proper amount of iodine, the thyroid cannot make sufficient amounts of thyroid hormone. When the thyroid fails to make enough of its hormones, the pituitary gland in the brain sends a message to the thyroid to increase in size and make more thyroid hormone. That increase in thyroid size is what scientists call goiter.) Nutritional deficiency of iodine is the most common dietary cause of goiter, and, in this sense, iodine deficiency could be accurately described as goitrogenic.

Goitrogenic Foods

Contrary to popular belief, foods themselves are not "goitrogenic" in the sense of causing goiter whenever they are consumed, or even when they are consumed in excess. In fact, most foods that are commonly called goitrogenic - such as the cruciferous vegetables (including broccoli, cabbage, kale, and cauliflower) and soyfoods - do not interfere with thyroid function in healthy persons even when they are consumed on a daily basis. Nor is it scientifically correct to say that foods "contain goitrogens," at least not if you are thinking about goitrogens as a category of substances like proteins, carbohydrates, or vitamins. With respect to the health of our thyroid gland, all that can be contained in a food are nutrients that provide us with a variety of health benefits but which, under certain circumstances, can also interfere with thyroid function. The term "goitrogenic food" makes it sound as if something is wrong with the food, but that is simply not the case. What causes problems for certain individuals is not the food itself but the mismatched nature of certain substances within the food to their unique health circumstances.

Goiter, Thyroid Function, and Diet

With respect to diet, the most common cause of goiter is iodine deficiency from lack of iodine-containing foods. However, other food factors can come into play under certain unique circumstances.

For example, since the mineral selenium is required for certain activities within the thyroid gland, selenium deficiency can combine with borderline iodine deficiency to produce goiter. The selenium content of foods is highly variable and depends to a large extent upon soil conditions. For this reason, in parts of Asia, Africa, and Latin America where soils are particularly low in selenium, thyroid problems related to borderline iodine intake may be worsened by this added problem of soil selenium deficiency and inadequate intake of selenium-rich foods.

Thiocyanates and the Thyroid

Other dietary circumstances can contribute to problems with thyroid function, but the circumstances here are not common. One circumstance involves excessive intake of phytonutrients called thiocyanates. When an imbalanced diet overloads the body with excessive amounts of thiocyanates - as might happen, for example, in parts of Central Africa where cassava root and cassava flour may be relied on for too great a percentage of total carbohydrates - risk to thyroid function has been documented, especially when accompanied by deficiency of dietary selenium.

Since isothiocyanates - one category of thiocyanates - are commonly made in the body from other phytonutrients (called glucosinolates) that are especially concentrated in cruciferous vegetables, some people have wondered whether daily intake of cruciferous vegetables can increase their risk of thyroid problems. The answer here is no; there is no research to show that daily intake of cruciferous vegetables in ordinary dietary amounts poses any unwanted risk to the thyroid. In fact, there are many well-documented health benefits from daily consumption of cruciferous vegetables, including decreased risk of colorectal cancer. One study has even found a weak association between high intake of cruciferous vegetables and slightly decreased risk of thyroid cancer.

From a practical standpoint, a food like broccoli (with its thiocyanates) is not "bad" for your thyroid. The only time you should be concerned about broccoli and your thyroid is if you are an individual with a history of thyroid problems who regularly eats large amounts of raw broccoli (or other cruciferous vegetables). In that case, it would be worthwhile for you to talk with your healthcare provider about the best dietary steps to take.

Isoflavones and the Thyroid

Another circumstance that may contribute to risk of thyroid problems involves excessive intake of phytonutrients called isoflavones. Isoflavones are phytonutrients that belong to the much larger phytonutrient family called flavonoids. (Most foods contain flavonoids, and many vegetables are especially rich in this family of phytonutrients.) At a molecular and biochemical level, isoflavones have the ability to change thyroid cell events in at least two ways. First, isoflavones can interfere with activity of an enzyme called thyroid peroxidase (TPO). TPO is an enzyme that helps attach iodine to an amino acid called tyrosine. This iodine-tyrosine combination forms the basis for production of thyroid hormones. How isoflavones are able to interfere with TPO activity is not fully understood. But it's clear that they have the ability to do so. (In fact, it's clear that most flavonoids - and not just isoflavones - have the ability to interfere with TPO.)

A second type of change involves uptake of iodine into the thyroid cells by a protein referred to as the sodium-iodide symporter (and abbreviated as "NIS," where the "N" comes from "Na," which is the scientific abbreviation for the element sodium). Like TPO activity, uptake of iodine into the thyroid cells by NIS can be interfered with by isoflavones. Once again, it's not 100% clear how isoflavones are able to interfere with NIS activity, but it's clear that they can.

The effects of isoflavones on thyroid function have been studied with isoflavones obtained from soybeans, including the isoflavones genistein, daidzein, malonylgenistin, and malonyldaidzin. Research findings on these soy isoflavones show a limited impact on thyroid function, even when the soy isoflavones are consumed in supplement form at levels higher than expected from food. For example, one small-scale study looked at the impact of 141 milligrams of dietary isoflavones from soy protein isolate each day over the course of one week and found no impact on thyroid measurements. Since isoflavone-rich soy foods like tofu typically provide, at most, about 40 mg of isoflavones per 4 ounces, it's reassuring to see a lack of thyroid symptoms even at larger levels of isoflavone intake. We've also seen additional studies that resulted in a lack of thyroid symptoms (for example, no increase in the weight of the thyroid gland) even when there was evidence of decreased TPO and NIS activity. In other words, despite the fact that biochemical changes can be triggered in the thyroid by soy isoflavones, the health consequences for adults do not appear to be automatic or guaranteed, and seem to be contingent on the involvement of additional factors like iodine deficiency, selenium deficiency, or other health problems.

Soy-Based Infant Formulas (SBIF) and Thyroid: A Special Case

Several studies on infant feeding show greater rates of autoimmune thyroid problems in infants who are exclusively fed soy formula. In these studies, some of the infants may have had thyroid problems (or have been predisposed to thyroid problems) even before consuming soy formula. Others may have had problems with digestive function, liver function, or immune function that combined with soy formula intake to compromise the health of their thyroid. But even if these other factors were involved, the association between thyroid problems and exclusive intake of soy formula by infants seems like an issue of concern and reason to treat intake of soy formula as a different situation than intake of similarly processed soy food by adults.

Part of our concern in this area involves the use of soy protein isolate (SPI) in most infant soy formulas. SPI is a highly processed form of soybean, and it is likely to expose an infant's digestive tract to unexpected proteins and protein components. Since the infant's immune system is functioning at a very early stage of development, this exposure to unexpected proteins and protein fragments would logically be expected to increase risk of adverse reactions, including allergic reactions.

At present, the American Academy of Pediatrics has established no special precautions for the use of soy-based infant formula (SBIF), except in the case of preterm infants or infants with digestive tract problems (specifically enteropathy or enterocolitis). Additionally, the Canadian Paediatric Association recommends the monitoring of thyroid hormone levels when using SBIF with infants who are born with hypothyroidism. If you are considering exclusive soy-based feeding for an infant, we believe a consult with your pediatrician or healthcare provider is worthwhile, even if the none of the above problems have been identified.

Goitrogens, Diet, and Health

Given the research evidence described above, we believe there is no good basis for the vast majority of individuals to avoid any of the World's Healthiest Foods - including cruciferous vegetables like broccoli or soy foods like tofu - in a healthy everyday diet. A healthy mix of the World's Healthiest Foods will help protect all of your organ systems, including your thyroid gland. There are just far too many health benefits from these foods (in addition to their flavors and textures) to consider omitting them from a health-supportive diet. However, in the context of known dietary deficiency for either iodine or selenium or both, and especially when coupled with a previously diagnosed thyroid problem, it would be worth discussing the role of these foods in your diet with your healthcare provider.

Impact of Cooking on Isoflavones and Isothiocyanates

Although research studies are limited in this area, cooking does appear to help inactivate both isoflavones (commonly found in soy foods) and isothiocyanates (commonly found in cruciferous vegetables). These compounds appear to be heat-sensitive, and cooking appears to lower their availability. In the case of isothiocyanates in cruciferous vegetables like broccoli, as much as one third of this goitrogenic substance may be deactivated when broccoli is boiled in water.

WHFoods Recommendations

For the vast majority of individuals, foods containing potentially goitrogenic substances like thiocyanates and isoflavones pose no documented health risk to the thyroid and can be included on a daily basis in a balanced diet. Most of us can simply enjoy the delicious taste of these foods and the amazing variety of nutrients they provide. For individuals with known dietary deficiency of iodine or selenium or both, especially when combined with previously diagnosed thyroid problems, intake of soy foods (high in isoflavones) and cruciferous vegetables (high in glucosinolates that can be converted into thiocyanates) may need to be discussed with a healthcare practitioner. (If you are a person who belongs to this latter group, you can still work together with your healthcare provider to determine what food combinations are the right fit for you and your thyroid, and these food combinations may end up containing both soy foods and cruciferous vegetables.) Since cooking may help to inactivate some of the potentially goitrogenic substances found in food, any individual wishing to lower his or her intake of these substances can consume soy foods and cruciferous vegetable in cooked rather than raw form.

References

Brent GA. Environmental Exposures and Autoimmune Thyroid Disease. Thyroid 2010 July; 20(7): 755—761. doi: 10.1089/thy.2010.1636

Dal Maso L, Bosetti C, La Vecchia C et al. Risk factors for thyroid cancer: an epidemiological review focused on nutritional factors. Cancer Causes Control. 2009 Feb;20(1):75-86.

de Souza dos Santos MC, Goncalves CFL, Vaisman M et al. Impact of flavonoids on thyroid function. Food and Chemical Toxicology, Volume 49, Issue 10, October 2011, Pages 2495-2502.

Doerge DR and Chang HC. Inactivation of thyroid peroxidase by soy isoflavones, in vitro and in vivo. Journal of Chromatography B, Volume 777, Issues 1—2, 25 September 2002, Pages 269-279.

Hampl R, Ostatnikova D, Celec P et al. Short-term effect of soy consumption on thyroid hormone levels and correlation with phytoestrogen level in healthy subjects. Endocr Regul. 2008 Jun;42(2-3):53-61.

Messina M and Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review of the relevant literature. Thyroid. 2006 Mar;16(3):249-58.

Milerov J, Cerovsk J, Zamrazil V et al. Actual levels of soy phytoestrogens in children correlate with thyroid laboratory parameters. Clin Chem Lab Med. 2006;44(2):171-4.

Teas J, Braverman LE, Kurzer MS, et al. Seaweed and soy: companion foods in Asian cuisine and their effects on thyroid function in American women. J Med Food. 2007 Mar;10(1):90-100.

Vanderpas J. Nutritional epidemiology and thyroid hormone metabolism. Annu Rev Nutr. 2006;26:293-322.

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