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Green tea: a potential pain in the neck

Though it may not be as effective for fat loss as early studies suggested, green tea is still seen as extremely healthy. But animal evidence has pointed to possible thyroid side effects from excessive green tea consumption. How convincing is this evidence?

Study under review: Effect of Excessive Green Tea versus Fluoride and Caffeine on Body Weight and Serum Thyroid Hormones in Male Mice

Introduction

Tea is the most popular beverage[1] in the world after water. While there are many kinds of herbal infusions, actual teas are made from the leaf of Camellia sinensis. Based on the level of processing, teas are classified principally as black (fermented), oolong (semi-fermented), or green (unfermented). In recent years, there has been a growing demand for tea, with worldwide consumption increasing to 4.84 million tons in 2013.

In addition to the surge of people looking to quench their thirst, there has been growing interest in the potential health benefits[2] of tea, especially green tea. For instance, it has been estimated (from observational studies) that the risk of dying from any cause is reduced by 4% with each additional cup of green tea consumed daily. The leaves of Camellia sinensis are a complex mix of proteins and free amino acids, carbohydrates, fats, minerals, and most importantly, polyphenols. Most of those polyphenols are catechins, the major ones being epicatechin, epigallocatechin, epicatechin-3-gallate, and epigallocatechin-3-gallate (EGCG). All have been studied extensively[3] for their potential role in the prevention of some chronic diseases and for their contribution to general health.

However, it is not far-fetched to believe that benefits only exist up to a certain dose, above which adverse effects might occur. For example, previous research[4] has suggested that daily green tea consumption can induce hypothyroidism in rats by interfering with the thyroid gland’s use of iodine to make thyroid hormones (T4 and T3). Studies in northern Europe, Japan, and the U.S. have found the prevalence of spontaneous hypothyroidism to be between 1% and 2% in iodine-replete humans. Green tea is one of the most popular beverages in the world. Could it be contributing to this phenomenon?

The potential link between green tea and thyroid function could be owed to any number of compounds within green tea. In addition to the catechins, green tea contains variable amounts of fluoride and 30-60 milligrams of caffeine per cup (eight ounces), both of which are dependent on the plant’s growing conditions. An association between hypothyroidism and the fluoride content of drinking water has been established, and animal research has shown that excessive fluoride inhibits thyroid hormone production. Animal research has also shown that excessive caffeine consumption induces hypothyroid effects that may be synergistic[5] with iodine deficiency. In the current study, the potential effects of green tea on thyroid function were studied and compared to those of two of its components, fluoride and caffeine, to determine potential adverse effects.

Green tea had been studied extensively for its health benefits, but sometimes there can be too much of a good thing. The purpose of this study was to determine if higher doses of green tea could adversely affect thyroid function in mice. Fluoride and caffeine, both found in green tea, were also examined in parallel to see how their effects compared to green tea.

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