A new report, using iodine data from the National Health and Nutrition Examination Survey (NHANES), and iodine data from pregnant women at sites around the country, was just published. The study found that the median urinary iodine concentration in 2009-2010 --144 µg/L -- was significantly lower than in 2007-2008, which was 164 µg/L. Non-Hispanic blacks had the lowest urinary iodine concentrations (131 µg/L), compared to non- Hispanic whites or Hispanics (147 and 148 µg/L). The median for pregnant women in the 2005-2010 period studied was 129 µg/L, which is considered inadequate during pregnancy.
According to the report, the Institute of Medicine's Recommended Dietary Allowance (RDA) for iodine in adult men and women is 150 µg per day. (FYI: One teaspoon of iodized table salt contains approximately 400 µg iodine.) For pregnant women, the RDA is 220 µg iodine per day, and 290 µg iodine per day during breastfeeding.
While we often assume that most of our dietary iodine comes from salt, the researchers found that intake of dairy foods -- not salt, seafood or grains -- was significantly and positively associated with urinary iodine levels in women of childbearing age. According to the study:
"Dairy, grain, seafood, and to a lesser degree, iodized salt are the major sources of iodine in the U.S...In the U.S. approximately 60% of iodine consumed comes from dairy products. The iodine is added to dairy products as a consequence of iodine added to cattle feed, or use of iodophor disinfectants in the milking process. Iodized salt accounts for a small amount of dietary iodine. Approximately 70% of salt consumed in the US comes from processed and restaurant foods, which generally do not use iodized salt."They also concluded that the pregnant women evaluated tended to have inadequate urinary iodine levels during the first and second trimesters of pregnancy. To prevent iodine deficiency, during pregnancy and breastfeeding, the American Thyroid Association recommends that women of childbearing age supplement with 150 µg of iodine daily, particularly when planning pregnancy, during pregnancy, and when nursing.
The report concluded: "Prevention of unnecessary self-imposed dairy restriction and improved iodine intake may be achieved by medical evaluation and education of individuals who perceive themselves to be lactose intolerant but who are not. Individuals with confirmed lactose intolerance, particularly women who are pregnant or who are of childbearing age, should be counseled about iodine supplementation. Public health intervention may include continued efforts by the medical community and public health officials to promote the use of iodine- containing prenatal vitamins and to promote continued efforts to monitor actual iodine content in prenatal vitamins."
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Source: Caldwell, KL et. al. "Iodine Status in Pregnant Women in the National Children's Study and in U.S. Women (15-44), NHANES 2005-2010," Thyroid, March, 2013, Instant Online, ahead of print.