Iodine is essential for the synthesis of thyroid hormones required for normal growth – and therefore crucial for the development of newborns.
Deficiency during this critical period has been shown to cause growth delay, impaired hearing capacity and reduced cognitive function. According to the World Health Organisation (WHO), iodine deficiency affects 1.9 billion people globally and is the most preventable cause of intellectual disability.
Pregnant and lactating women are encouraged to up their intake to compensate for loss through breastfeeding.
Iodine in the EU
The European Food Safety Authority (EFSA) has authorised five health claims for iodine covering ‘normal thyroid function and production of thyroid hormones’, ‘maintenance of skin’, ‘normal cognitive and neurological function’ and ‘normal energy-yielding metabolism’ and ‘normal growth of children’.
WHO’s recommended daily intake of 250 μg/d for lactating women aims to ensure both mother and child get enough, while the Universal Salt Iodization (USI) programme has also sought to control such deficiency in the general population.
Yet according to researchers from the Harbin Medical University, Institute for Endemic Disease Prevention and Treatment of Shanxi Province and the Centre for Disease Control and Prevention in China, few have delved into the possible adverse effects of over consumption of this trace element.
Building on 2009 Korean research, the scientists looked at 343 healthy lactating women living in areas of China with either low, adequate or excessive iodine water content – excluding those taking anti-thyroid drugs or iodine supplements within a year of the study, consuming seafood at the time or those with congenital thyroid disease.
They found the prevalence of thyroid disease in lactating women, particularly subclinical hypothyroidism (or an 'underactive thyroid gland'), was significantly higher in those with excess iodine intakes compared to those living in the ‘sufficient’ areas.
Something in the water
Towns in Guangxi province were selected as the iodine-deficient region, with average water iodine of 10 microgram per litre (μg/l ) or less and with low coverage rates of iodised salt.
The control group came from towns in the Shanxi province where residents had sufficient iodine between 50 μg/l and 150 μg/l.
Other villages in Shanxi province were chosen for their high iodine water content of above 300 μg/l.
The researchers said there were currently 31 million people living in high water iodine areas in China.
Median urinary iodine values of the lactating women were 51.30, 282.42 and 822.51 μg/l for the iodine-deficient, sufficient and excess groups, respectively. For lactating women and infants younger than two years, average urinary iodine concentrations below 100 μg/l are defined as deficient.
“Excessive iodine intake may induce subclinical hypothyroidism in lactating women. Moreover, adequate iodine nutrition is essential for lactating women and infants, especially those living in iodine-deficient areas.
“Enhanced monitoring of iodine status is important to avoid adverse effects of iodine deficiency or excess, particularly in susceptible populations such as pregnant or lactating women and infants,” the researchers concluded in the British Journal of Nutrition.
Where Europe stands
Meanwhile in Europe differences remained on the amount of iodine recommended. Last year, EFSA proposed a new reference value of 200 μg per day as an adequate intake for pregnant women - 50 μg lower than the WHO. In the UK the Reference Nutrient Intake for adults was placed at 140 μg day, without any suggested increase for pregnant and lactating women.
The UK's National Health Service (NHS) warned that taking high doses of iodine for long periods of time could change the way the thyroid gland works, which could lead to symptoms such as weight gain.
According to its online advice service, NHS Choices, individuals should be able to get enough iodine from a varied and balanced diet.
"If you take iodine supplements, don't take too much, because this could be harmful. Taking 0.5 mg or less a day of iodine supplements is unlikely to cause any harm," it said.
Commenting on this UK advice, researchers from the University of Glasgow said: “Current dietary recommendations in pregnancy, and their dissemination, are found not to equip women to meet the requirements for iodine intake.”
The researchers behind the paper found awareness of the importance of iodine to be very low in the UK, something they said was a nutrition policy failure.
Source: British Journal of Nutrition
Published online ahead of print, doi:10.1017/S0007114515003128
“The relationship between iodine nutrition and thyroid disease in lactating women with different iodine intakes”
Authors: L. Liu, D. Wang, P. Liu, F. Meng, D. Wen, Q. Jia, J. Liu, X. Zhang, P.Jiang and H. Shen