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Soy and the Thyroid

The Controversy Over Soy and Thyroid Health


Updated June 30, 2014

Soy and the Thyroid

There is a continuing controversy over whether soy foods can be harmful to thyroid health.

For at a past decade, the issue of whether soy has a negative effect on the thyroid has been a continuing question. The potential effects of soy on the thyroid remains a controversial issue, one that shows no signs of being resolved in the near future.

On one side, we have health and nutrition magazines touting the benefits of soy as a cure-all for menopause, cancer prevention, heart disease, weight loss, and many other health concerns. And behind the many soy food products and supplements is a multi-billion-dollar industry that make profits hugely from soy. Some nutritionists tout soy as a key part of a healthy diet. And rounding out the pro-soy contingent are some nutritionists and doctors who believe soy is a wonder food, even for thyroid patients. (Menopause "guru" Christiane Northrup, MD is, for example, a huge proponent of soy. Northrup even recommended that Oprah Winfrey incorporate a great deal of soy into her diet. Coincidentally -- or not -- both women are now hypothyroid.)

On the other side of the issue are the opponents of soy, who believe that soy is a toxin, and is particularly toxic to thyroid patients. Various experts and organizations, ranging from Dr. Joseph Mercola to the Weston Price Foundation, are vocally opposed to soy.

In the middle, there are experts who suggest that some soy -- as long as it is in unprocessed food form, or fermented forms, and not genetically modified -- may be safe for thyroid patients, as long as it's eaten only in moderation.

As a thyroid patient, how can you decide what to do? Here is a look at some of the issues to consider.

About Soy

Soy (or soybeans) are a type of legume that have been used for 5,000 years in China for food -- i.e., tofu, tempeh, and edamame beans -- and medicinal purposes. Soybeans are considered a source of protein, and are processed into many meat and dairy substitutes. The main producers of soy are the United States, Brazil, Argentina, China and India.

Soy and many soy products contains isoflavones, which are phytoestrogens -- plant-based estrogens. It is soy's weak estrogenic properties that are often touted as a health benefit of soy.

Soy is a highly profitable for some of the the world's largest multinational agribusinesses. These include Cargill, Archer Daniels Midland and Solae (a joint venture of DuPont and Bunge). (These companies are collectively sometimes referred to as "Big Soy.") In the past decade, the market for soy has exploded, and soy is now being incorporated into a variety of processed foods, and included in various nutritional supplements.

Does Soy Have Health Benefits?

While soy is enjoying popularity, it's inconclusive whether soy has much in the way to offer, health-wise. A U.S. government-sponsored review of 200 different studies on soy, published in 2005, found very limited evidence of health benefits from soy: primarily a small reduction in "bad" LDL cholesterol, and a small percentage of women who have a minor reduction in hot flashes when using soy during menopause. The Journal of the American Medical Association has reported that isoflavones do not improve cholesterol levels, cognitive function or bone mineral density. The American Heart Association backtracked on its earlier support of soy, and is now saying that there is no evidence that soy has specific benefits for heart health or for lowering cholesterol. Research on the use of soy and isoflavones for cancer prevention is also inconclusive. And there is no evidence that soy can "cause" weight loss -- except as part of the simple equation of substituting a lower-fat, lower-calorie protein source for a fattier, higher-calorie protein as part of a weight loss effort. In general, at present, there is insufficient data to suggest that soy has a protective role against any medical conditions or diseases.

Soy and the Thyroid

Apart from the question as to whether soy even has demonstrable health benefits, there are long-standing concerns that soy may have negative effects on thyroid function and hormonal health. Soy falls into a category of foods known as goitrogens -- vegetables, grains and foods that promote formation of goiter -- an enlarged thyroid. Some goitrogens also have a definite antithyroid effect, and appear to be able to slow thyroid function, and in some cases, trigger thyroid disease. These concerns have been studied for years, but were raised specifically by Food and Drug Administration (FDA) researchers Daniel Doerge and Daniel Sheehan. Doerge and Sheehan were the FDA's key experts on soy. In 2000, Doerge and Sheehan wrote a controversial letter of protest (PDF) to their own employer, protesting the positive health claims for soy that the FDA was approving at the time. They wrote:
...there is abundant evidence that some of the isoflavones found in soy, including genistein and equol, a metabolize of daidzen, demonstrate toxicity in estrogen sensitive tissues and in the thyroid. This is true for a number of species, including humans. Additionally, isoflavones are inhibitors of the thyroid peroxidase which makes T3 and T4. Inhibition can be expected to generate thyroid abnormalities, including goiter and autoimmune thyroiditis. There exists a significant body of animal data that demonstrates goitrogenic and even carcinogenic effects of soy products. Moreover, there are significant reports of goitrogenic effects from soy consumption in human infants and adults.
Since publication of their letter, Doerge and Sheehan have refined their concerns, and in the journal Environmental Health Perspectives, suggested that for soy to cause toxicity, there need to be several factors, including iodine deficiency, defects of hormone synthesis, or additional goitrogens in the diet. They also stated that: "Although safety testing of natural products, including soy products, is not required, the possibility that widely consumed soy products may cause harm in the human population via either or both estrogenic and goitrogenic activities is of concern. Rigorous, high-quality experimental and human research into soy toxicity is the best way to address these concerns."

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