Soy and soy foods are rich in protein, fiber, phytochemicals and other nutrients. Soy and soy foods are popular in the United States, however there are some myths that effect consumers' opinion of soy. I asked Dr. Mark Messina, adjunct associate professor of nutrition at Loma Linda University and an expert on soy and nutrition, to help me dispel the myths surrounding soy.
Myth: Soy Will Damage Your Thyroid
Tofu, soy milk and other soy foods are considered a healthy addition to a diet. However, some people avoid soy, believing that its proteins will depress the function of their thyroid glands. The thyroid gland is found at the base of your neck, just in front and either side of your esophagus. It's main function is to regulate your metabolism and body temperature.
So what's the worry about soy? According to Messina, "concerns about the anti-thyroid effects of soy are based primarily on in vitro and animal studies involving the soybean isoflavones (phytoestrogens). Several cases of goiter were attributed to the use of soy infant formula but this problem was eliminated in the mid-1960s by fortifying the formula with iodine."
The problem with putting too much stock into animal studies is that lab animals such as rats are quite different physiologically from humans. Messina tells us that "in rats, isoflavones inhibit by about half, the activity of an enzyme called thyroid peroxidase (TPO), which is required for the synthesis of thyroid hormone. However, not only are rats much more sensitive to possible goitrogens than humans, but even though TPO was inhibited in this study, thyroid function remained normal in the rats."
Human studies are necessary to really understand how soy (or any other foods) effect the human body. In 2006, according to Messina, a review of 14 clinical trials concluded there was little evidence that soy foods or isoflavones had an adverse affect on thyroid function in healthy human subjects. "Studies published subsequent to this review have also found no effect on thyroid function. Thus, quite clearly in individuals with adequate iodine intake and normal thyroid function, the evidence indicates soy foods even in very large amounts do not adversely affect thyroid function. The only study that raised concern was published in a Japanese journal in 1991. The study was never repeated and was poorly designed."
Since there is no real evidence that soy foods cause hypothyroidism, there is no reason for healthy people to avoid soy foods. But are there any connections between soy and hypothyroidism? Messina says that eating soy foods may increase the amount of thyroid medications needed, but this is also the case for many other foods, herbs, drugs and fiber supplements. "Furthermore, it is not necessary for hypothyroid patients (with the exception of infants with congenital hypothyroidism) to avoid soy foods since medication dosages can easily be adjusted to compensate for any effects of soy."
More research needs to been done on hypothyroidism and diet. Messina describes two clinical situations that need to be explored. "One involves individuals with subclinical hypothyroidism, which represents about 5% of the postmenopausal women. This condition is defined as having normal levels of the two primary thyroid hormones, thyroxine and triiodothyronine, but elevated levels of thyroid stimulating hormone. The other situation involves individuals whose iodine intake is marginal or inadequate. In the United States, iodine intake is quite good; furthermore, when individuals with inadequate intake are identified the appropriate recommendation is to increase iodine intake not to avoid soy foods."
Mark Messina, Ph.D. is a Health and Wellness Expert for The United Soybean Board.
Source:
Email interview with Mark Messina, PhD. April 2009. The Soy Connection.
