With commentary by Margaret Rayman, DPhil (Oxon), RNutr, professor of nutritional medicine at the University of Surrey, Guildford, UK.
In the study—the first large epidemiological one of its kind—researchers at The First Affiliated Hospital of Xi’an Jiaotong University Health Science Center set out to investigate the prevalence of thyroid disease in two counties in Shaanxi Province—counties considered similar in virtually every way except one: The amount of selenium in their soil.
In rural areas in China, daily selenium intake among residents differs according to the amount of selenium in locally produced foods, which is a direct function of how much selenium the soil contains. Researchers deemed one of the study counties “adequate selenium” and the other “low selenium” based on their average soil selenium levels.
A total of 6,152 adults from the two counties between the ages of 18 and 70 participated in the observational study, answering a food-frequency questionnaire covering intake of tea, meat, eggs, salt (iodized or not) and seaweed. Participants also underwent a clinical examination, a thyroid ultrasound and a fasting blood draw.
Results: The risk of thyroid disease was 69% higher for those living in the low-selenium county than for those in the adequate-selenium county. More than 30% of subjects in the low-selenium area had thyroid disease compared with 18% of those in the adequate-selenium area.
“Selenium is well-known to protect the thyroid,” said study author Margaret Rayman, DPhil (Oxon), RNutr, professor of nutritional medicine at the University of Surrey, Guildford, UK. “The importance is that we have shown that low selenium is associated with an increased risk of thyroid disease.”
Among those living in the adequate-selenium county, thyroid disorders including hypothyroidism, subclinical hypothyroidism, autoimmune thyroiditis and enlarged thyroid were significantly less common than for those living in the low-selenium county. The biggest difference was for subclinical hypothyroidism: 11.7% versus 21.4%.
Overall, median blood levels of selenium were nearly twice as high in people living in the adequate-selenium area. And higher blood levels of selenium were significantly associated with reduced risk of autoimmune thyroiditis, hypothyroidism, subclinical hypothyroidism and thyroid volume.
In general, those with the lowest levels of serum selenium had the highest risk for thyroid disorders including overt hypothyroidism, subclinical hypothyroidism, autoimmune thyroiditis and enlarged thyroid.
The authors noted that consuming meat significantly protected against hypothyroidism and subclinical hypothyroidism. But the biggest selenium-boosting effect came from drinking high-selenium green tea, which cut the risk of hypothyroidism by 38% and of subclinical hypothyroidism by 45%.
The implications for other areas of the world are significant, particularly in countries and areas with low soil selenium, Rayman said, like parts of Eastern Europe.
Getting enough dietary selenium is not considered a problem in the U.S. Still, claims have been made that taking supplemental selenium, which acts as an antioxidant in the body, may prevent or benefit a number of conditions ranging from asthma and heart disease to a variety of cancers. Conversely, there is research that doesn’t support these claims.
The National Institutes of Health lists selenium supplements as possibly effective for autoimmune thyroiditis (Hashimoto’s thyroiditis), but the agency deems the supplements possibly unsafe when taken in high doses or long-term. Low-dose, long-term use has been shown to increase the risk of developing diabetes. The supplements might also interact with medications including immunosuppressants, anticoagulants and antiplatelet drugs, statins and blood thinners.
“The idea that everyone should be taking selenium supplements in the off-chance they could help is not warranted and potentially dangerous,” said Jason Baker, M.D., assistant professor of clinical medicine at Weill Cornell Medical College in New York. “People should focus on getting selenium from their diets, which isn’t difficult in places like the U.S.”
Selenium is easily absorbed from food. Its content can vary considerably depending on the selenium content of the soil where the animal was raised or the plant was grown, as the recent study in China demonstrated. In general, top selenium sources include organ meats, seafood, poultry, wheat and Brazil nuts.
Wayne Hawkes, PhD., adjunct clinical professor of nutrition at University of California, Davis, and a retired research chemist from the U.S. Department of Agriculture, says that because grains grown in the U.S. contain selenium, it's really difficult—even for vegans—to get inadequate selenium from diet.
Selenium, like any other nutrient, should be consumed in moderation, said Dr. Baker. The Institute of Medicine of the National Academies lists the Recommended Daily Allowances for adults and those 14 years of age or older as 55 mcg of selenium daily. People with additional questions should consult their health care providers.