By : Apeksha Thakkar Clinical Dietitian, Certified Diabetes Educator and Nutrigenomic Counselor

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Soy Protein and Thyroid Health

Soy products are enjoying immense popularity as it is praised as a healthy food by many. Soy foods are rich in nutrients including B vitamins, fibre, potassium, magnesium, and high-quality protein. Soy protein ,unlike other plant proteins are considered a complete protein, as they contain all nine essential amino acids which the body cannot make and must be obtained from the diet.

Soybeans can be consumed as whole or made into a variety of products, including soy milk, tofu, tempeh and other dairy and meat alternatives. It can also be made into soy protein powder such as soy protein isolate used in protein supplements, soy protein concentrate used in infant formulas, baked goods etc, and textured soy protein used as a substitute in meat based curries, soups etc.

Health Benefits associated with Soy 

Several studies have shown the effect of soy in lowering incidences of heart diseases, diabetes, warding off osteoporosis, and protecting against cancers like breast and prostate and providing relief from menopausal symptoms. Several animal and human  studies have also shown that consumption of soy protein or associated isoflavones  has positive impacts on the risk factors for cardiovascular disease including lowering  cholesterol levels, blood triglyceride and  increasing HDL cholesterol.

The two main components present in soy thought to be responsible for the various health benefits are soy protein and soy isoflavones. Each gram of soy protein in soybeans and traditional soy products contains approximately 3.5 mg of isoflavones. 

The Other Side of the Story!

At the same time, there is an increasing concern that soy and its products may have an increasing role in thyroid problems, endocrine disruption, and cancer development.

Soy And Thyroid

Thyroid hormones are essential for normal growth, development and function of nearly all tissues. They also play a crucial role in the development of the central nervous system, as a vital regulator of oxygen consumption, energy balance and basal metabolic rate. 
The most serious effect of thyroid hormone deficiency in children is permanent brain damage and dwarfism.
Soy has been linked to adverse effects of thyroid functions in susceptible individuals by interfering with the absorption of  thyroid hormone. 
Many studies have reported the negative effects of soy consumption leading to thyroid disorders such as hypothyroidism (low production of thyroid hormones), goitre (enlargement of thyroid gland), and autoimmune thyroid diseases.

The Mechanism Causing Thyroid Disorder.

Oestrogenic Effect

Oestrogen is a hormone produced by the endocrine system and is  responsible for the normal development of sexual characteristics and reproductive systems. 
It binds to its receptor in a lock and key manner and carries out its function. 
Since oestrogen receptors are really flexible, any substance that resembles oestrogen can bind to the receptor.
Soy, which has isoflavones that resemble oestrogen, bind to oestrogen receptors and activate some while inhibiting others. 
High oestrogen levels cause the thyroid function to slow down.
Thus, soy indirectly affects the thyroid adversely by influencing oestrogen levels.

Goitrogenic Effect

Goitre is a condition that causes enlargement of the thyroid gland.
Goitrogens are compounds that interfere with the uptake of iodine by the thyroid gland. Iodine is necessary for synthesis of thyroid hormones.
As soy is classified as goitrogenic food, it decreases thyroid functioning and may trigger hypothyroidism. 
In a study of infants fed soy formula, goitre was observed; which was reversed when the formula was replaced by cow milk or iodine-supplemented diets.
In another study on teenage children diagnosed with autoimmune thyroid diseases (Hashimoto’s thyroiditis or Graves’ disease), it was observed that those consuming soy formula as infants had twice the prevalence of autoimmune disease compared to healthy subjects.

Inactivation Of Thyroid Peroxidase

Thyroid peroxidase is an essential enzyme for the production of thyroid hormones.
If the enzyme is inactivated, it leads to decreased production of thyroid hormone, which can cause hypothyroidism (low thyroid function).
Isoflavones, present in soy products, inhibit the activity of thyroid peroxidase (TPO), an enzyme essential for the synthesis of thyroid hormones, triiodothyronine (T3) and thyroxine (T4).

Recommended Soy Intake

Soy intake recommendation would majorly depend on the amount of soy protein or isoflavones to be consumed as all soy foods have different quantities of these two components. A reasonable intake for adults is 15 gms soy protein and about 50 mg total isoflavones per day which are provided by approximately two servings of traditional soyfoods.

The Takeaway

Soy is a high quality source of protein that provides multiple health benefits like decreasing heart diseases, osteoporosis and cancer, especially when consumed as a substitute for red meat. But recent studies have emphasised the possible adverse outcomes of soy on thyroid health . It has been observed that too much soy consumption has negative effects on the thyroid due to the presence of isoflavones in soy products. Nutritionists often classify soy as a beneficial food for good health. However, contrary research regarding soy and its effect on certain health conditions  has led to hesitancy to wholeheartedly promote and accept soy as part of our diet.