Iodine deficiency is a major health problem worldwide, but a new study points to the potential downsides of too much iodine.
Iodine is a mineral found in iodized salt, seafood, eggs, dairy and some breads. It is used by the thyroid gland to help regulate metabolism and development, especially in babies and children.
Iodine deficiency during fetal and early-childhood development is a leading cause of brain impairments in much of the world. Most research has been directed at the effects of inadequate iodine. Less is known about how much iodine is too much.
In the new study, Chinese researchers randomly assigned 256 healthy adults to take various doses of iodine supplements for four weeks.
They found that at relatively higher doses -- 400 micrograms a day and up -- study participants began developing what's called subclinical hypothyroidism.
That refers to a dip in the body's thyroid hormone levels, but with no obvious symptoms of hypothyroidism -- which include problems like fatigue, depression, dry skin and weight gain. Subclinical hypothyroidism has no obvious symptoms, but studies have linked it to an increased risk of heart disease over the long term.
In this study, people taking 400-microgram supplements were getting around 800 micrograms of iodine per day when diet was factored in.
The findings suggest that people -- at least in China -- should get no more than 800 micrograms a day.
That's different from what's recommended in the U.S. and Canada, where guidelines say the safe upper limit for adults is 1100 micrograms of iodine per day.
Experts caution against taking iodine supplements with more than 150 micrograms in a daily dose. And most North Americans could skip supplements altogether.
However, there are certain people who may need supplements, including pregnant women.
In the North America adults are advised to get 150 micrograms of iodine each day; pregnant women should get 220 micrograms, while breastfeeding moms are told to get 290 micrograms.
The American Thyroid Association recommends that pregnant and breastfeeding women take a vitamin with iodine because low iodine can increase the risk of miscarriage and thyroid problems in moms, in addition to mental disabilities in babies.
Vegans may also want to take a supplement. In a recent study, scientists found that the average iodine level in a group of 63 vegans was lower than what's recommended -- though their thyroid hormone levels were in the normal range.
Vegans avoid all animal products, including dairy and eggs, so their iodine sources may be few.
It's thought that the effects of your iodine intake may depend on who you are and where you live. In certain parts of the world, where the soil is low in iodine and people who eat mainly local foods, the risk of deficiency is high. In other parts of the world -- Japan, for example -- people have a high iodine intake starting early in life, and they seem to "tolerate" that high level.
In China, natural iodine levels vary by region. The country introduced universal salt iodization in 1996, so the problem of iodine deficiency has been controlled in most areas. It's not clear if the adults in the current study had adequate iodine intake early in life. If not, that could be a factor in their response to iodine supplements.
SOURCE: American Journal of Clinical Nutrition, online December 28, 2011
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