Fit Fare

Although it is underreported in conventional media, health problems are plaguing some of those who have extensively consumed soy products. Many of these health concerns are being hotly debated, while the scientific evidence is contradictory. Yet the least contested of these negative impacts is that of soy on the thyroid. Even those who would proudly wave the pro-soy flag, admit that soy can impair the thyroid in a diet deficient of iodine. Well folks, here’s the alarming reality – many of those who rely on soy to replace meat and dairy products in their diet are the exact same ones concurrently risking an iodine deficiency. Vegetarians and vegans beware.

Arguably, the thyroid gland is one of the most important organs in the human body. The thyroid controls the metabolism of every cell in the body, so an imbalance can lead to dysfunction of other organ systems. There are two forms of imbalance — hyperthyroidism and hypothyroidism. Hyperthyroidism, or overactive thyroid, results in symptoms such as high metabolism, increased heart rate, mental fogginess, and anxiety. Extreme hyperthyroidism can lead to heart failure. Hyothyroidism, or under-active thyroid, results in a general lack of energy, slow metabolism, memory loss, depression, and moodiness. In a severe state it can lead to heart failure or coma. One study suggests that nearly 12% of Americans have a thyroid imbalance, yet many are unaware of it.

Iodine is necessary for thyroid function, as it is a raw material incorporated into two of the thyroid hormones. Reliable dietary sources of iodine are seafood, sea vegetables, and iodized salt. As most vegetarians and vegans shun seafood, and often times manufactured salt, and since few incorporate sea vegetables in their diets, this group is vulnerable to iodine deficiency. In the absence of iodine, hypothyroidism usually follows, potentially with impaired mental acuity and/or an enlarged thyroid – a condition known as goiter.

Soy foods magnify the impact of low iodine diets or of latent hypothyroidism on thyroid hormone production. Soy isoflavones, genistein and daidzein, appear to interfere with thyroid peroxidase (TPO), the enzyme responsible for incorporating iodine into thyroid hormones. Genistein and diadzein bind and inactivate TPO, and they will only release TPO in the presence of iodine. There is also speculation that this conglomeration of TPO with genistein could incite the immune system to form antibodies, leading to autoimmune thyroiditis – a condition where the body attacks healthy thyroid cells. In adults without histories of thyroid conditions, one study found that as little as 2 tablespoons of soybeans per day, that is 30 grams, can impair thyroid function. 30 grams of soybeans translates to a daily dose of about 65 mg of isoflavones, which would be the equivalent of consuming about two servings of soymilk, tofu, tempeh, or miso.* As further evidence of soy’s anti-thyroid potential, soy consumption antagonizes administered synthetic thyroid hormone.

Unfortunately finding processed foods without added soy is becoming difficult. We are all being regularly bombarded by soy through the foods we eat. Long term vegetarians and vegans may wish to ask their doctor to check their iodine status and thyroid function, especially if they feel low on energy or have a family history of thyroid troubles.

* Calculated from information provided here.