Even a mild level of iodine deficiency during pregnancy is associated with adverse effects on the resulting child's cognitive development, according to a study of 1040 mothers and their offspring in the United Kingdom.
Iodine, linked to the production of thyroid hormones, is known to be essential for a healthy fetal brain and neurological development, and the World Health Organization (WHO) in fact refers to its deficiency as "the single most important preventable cause of brain damage worldwide."
While the WHO warning refers largely to severe deficiency, the new study, published online May 22 in the Lancet, shows that even mild iodine deficiency in utero is linked to lower IQ and suboptimal reading ability in children
"Women should be aware of the need for an adequate iodine intake in pregnancy — requirements almost double at this life stage," said senior author Margaret Rayman, DPhil, codirector of the Nutritional Medicine MSc Program at the University of Surrey, Guildford, United Kingdom.
"Our sizable study adds to the sparse scientific literature describing the possible in utero effects of mild to moderate iodine deficiency," she and her colleagues write. "We have shown that risk of suboptimum cognitive scores in children is not confined to mothers with very low iodine status…but that iodine-to-creatinine ratios that would suggest mild to moderate deficiency are also associated with heightened risk."
First Trimester Is Key Stage for Adequate Iodine Intake
Just this month, doctors writing in Thyroid have drawn attention to the fact that iodine deficiency is still a problem in much of the world, including the United States.
For this study, led by Sarah C. Bath, PhD, from the University of Surrey, the researchers evaluated mother-child pairs from the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort, focusing on urinary iodine concentration, as well as creatinine to correct for urine volume, among the stored samples from 1040 first-trimester pregnant women.
Criteria for inclusion were having a singleton pregnancy and the availability of a urine sample from the first trimester, as well as a measure of IQ for the offspring at 8 years of age.
The group as a whole was found to have mild to moderate iodine deficiency, with a median urinary iodine concentration of 91.1 µg/L (iodine/creatinine ratio 110 µg/g).
Based on WHO guidelines on recommended concentrations of iodine during pregnancy, the researchers classified iodine/creatinine ratios of less than 150 µg/g as being iodine deficient and a ratio of 150 µg/g or more as iodine sufficient.
After adjustment for 21 socioeconomic, parental, and child factors, including parental education and breast-feeding, the researchers found women with iodine/creatinine ratios of less than 150 µg/g were more likely to have children with scores in the lowest quartile for verbal IQ (odds ratio [OR], 1.58; P = .02), reading accuracy (OR, 1.69; P = .007), and reading comprehension (OR 1.54; P = .02), compared with children of mothers with ratios of 150 µg/g or more.
The children's scores worsened when the deficient group (less than 150 µg/g) was further subdivided into 50 to 150 µg/g and less than 50 µg/g.
"Our study is the first to show an association between mild to moderate maternal iodine deficiency in UK pregnant women and impaired cognitive outcomes in their children at ages 8 to 9 years. Iodine deficiency in pregnant women in the United Kingdom should be treated as an important public-health issue that needs attention," Dr. Bath and colleagues assert.
Dr. Rayman said the findings underscore the importance of maintaining sufficient iodine levels, particularly in the earliest stages of pregnancy.
According to WHO guidelines, pregnant and breast-feeding women are recommended an intake of 250 µg of iodine per day, compared with the recommendation of 150 µg for adults who are not pregnant. "Women of childbearing age and especially those planning a pregnancy should enter pregnancy with good iodine stores in the thyroid from a diet rich in iodine sources prior to pregnancy. It is likely that the most important time to have good iodine intake is the first trimester," Dr. Rayman noted.
Eat Seafood, Drink Milk, Avoid Kelp Supplements
The authors note that previous research has linked low maternal seafood intake with low verbal IQ scores in children, which supports their findings because seafood has high iodine content. However, at least 1 prior study has suggested that the effects of seafood in pregnancy on child cognition may be due to long-chain omega-3 fatty acids in fish. To investigate this, they adjusted for such fatty acids in this new study; this made no difference to their findings, however. They conclude that the fatty acids "are unlikely to be solely responsible for the effects of seafood on brain development."
As well as seafood, other dietary sources of iodine include dairy products, especially cow's milk. The researchers point out, however, that organic milk has an iodine content that is 40% lower than conventional milk. Iodized salt is also a good source, but the United Kingdom has not adopted a national salt-iodization program, they say, which means "that iodine intake is left entirely to chance through individual food choices."
Pinpointing dietary sources of iodine can be challenging, said Dr. Rayman, because food product labels often do not contain iodine levels. "So if at all possible, women should check their diet in good time, and if they haven't done so, they should take a multivitamin and mineral supplement containing 140/150 µg iodine as recommended by the various US authorities," she noted.
However, she cautioned against going overboard with supplements marketing themselves as iodine sources, such as kelp and seaweed. "Women should avoid kelp supplements as a way to increase iodine intake as they are of unreliable content and may provide too much iodine."
Findings Should Be a Wake-Up Call to UK Authorities
In a commentary on the article, Alex Stagnaro-Green, MD, from George Washington University, Washington, DC, andElizabeth N. Pearce, MD, from Boston University, Massachusetts, note that the study's limitations include the use of urinary iodine as a marker for individual iodine status; the latter is difficult to assess due to regular fluctuations.
Other drawbacks include potential residual confounding and socioeconomic bias due to the fact that only 56% of the ALSPAC cohort had a measure of IQ, according to the commentary.
But Dr. Pearce said the finding that even mild iodine deficiency is important is troubling.
"I think it is both remarkable and concerning that cognitive effects of iodine deficiency were demonstrated even in children of only mildly iodine-deficient mothers," she told Medscape Medical News.
A recent study (J Clin Endocrinol Metab. 2013;98:1954-1956) "is really the only other study to date to demonstrate this,” she noted.
The findings should represent a wake-up call to public-health officials of the need to boost awareness of the issue, Drs. Stagnaro-Green and Pearce say.
"[The study] should be regarded as a call to action to public-health policy makers in the United Kingdom. Absence of a public-health policy in the face of clear documentation of moderate iodine deficiency and strong evidence of its deleterious effect on the neurodevelopment of children is ill advised," they observe. "Nor should unmonitored and adventitious dietary iodine sources continue to be relied on.
"Until measures are taken to ensure that iodine needs can be met by usual dietary sources, pregnant and breast-feeding women should insist that the prenatal vitamins they are prescribed contain iodine," Drs. Stagnaro-Green and Pearce conclude.
The authors and editorialists have reported no relevant financial relationships.
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Cite this: Iodine Key for IQ; Pregnancy Deficiency Affects Kids' Brains - Medscape - May 22, 2013.