Iodine Deficiency: 90 Years of Progress, But Much To Be Done
Iodine deficiency is one of the top three micronutrient deficiencies that are targeted by the World Health Organization. Shocking statistics estimate that 18 million infants are mentally impaired each year due to frank iodine deficiency. Salt iodization is effective in reducing rates of iodine deficiency, goiter and mental retardation. A program started in Michigan in 1924, represents the first successful eradication of a nutritional deficiency with food fortification. Nowadays, many countries are considered iodine replete, and the number of replete countries is increasing steadily (see report from Zimmermann on global progress in eliminating iodine deficiency). Despite these results, even in well-nourished populations such as the UK and Australia in which iodine deficiency is considered to be a problem solved, low but not clinically deficient iodine levels in infancy are associated with poorer educational outcomes one decade later at school (see Bath et al. for a UK study, and Hynes et al. for data from Australia). Yet, we do not often see nutrition publications on iodine coming through on the TalkingNutrition research radar. Even worse, iodine deficiency seems to be reported in a way that detracts attention away from the considerable proportion of the population that is iodine deficient.
Take the recent publication by Bath and co-workers as an example. They measured the iodine intake and status of 57 young women living in the UK. Most women were nutrition and dietetics students. While this is not a representative sample of the UK population, it does represent a risk group (women of child-bearing age) and also probably a best-case scenario, as the women had considerable knowledge of nutrition due to their studies. When looking at iodine intakes estimated from food diaries, the authors found that 60% of the women had an iodine intake below the RDA for pregnancy of 160 µg per day, and 28% were below the EAR of 95 µg per day. 30% of the group were considered to be iodine deficient based on urinary iodine concentration. The median 24-hour urinary iodine excretion value was 150 µg per 24 hours, which is considered adequate, but the median is only one measure of iodine sufficiency. Unfortunately the authors do not directly report the proportion with mild deficiency. From data on quartiles of 24-hour urinary iodine excretion, however, it seems that around one quarter of these women had mild iodine deficiency.
The authors report in the abstract that there is a low risk of iodine deficiency in this population, and conclude with the need for “revised cut-off values”. There is no explicit mention in the study that around one quarter of highly educated young women in the UK have inadequate iodine intakes and an iodine status indicative of mild deficiency. This same vague reporting of iodine deficiency mirrors a representative study of iodine deficiency in women of reproductive age done in the US, that also does not clearly mention the proportion of women with a low iodine status despite results showing that over one quarter are mildly iodine deficient. The abstract is also rather woolly, by stating that the study “suggests that as a population, we may not be meeting adequate intakes of iodine for pregnant women”.
In some circles, iodine deficiency in high-income countries is being noticed. For example, the International Council for the Control of Iodine Deficiency Disorders, and the British Thyroid Association found that the iodine status of the UK population “could be adversely affecting pregnancy outcomes and the cognitive development of offspring”, and that a salt iodization program should be implemented.
Perhaps it is due to the long history of successful salt iodization? Or perhaps iodine deficiency is not considered interesting enough compared to research into other nutrients? It seems that despite considerable progress in reducing iodine deficiency on a global level (with much work still to be done…), in countries considered to be well-nourished, a sizeable proportion of the population remains at risk of mild deficiency. Scientific reporting that omits explicitly stating the proportion of the population with mild, moderate and severe iodine deficiency, and vague conclusions in abstracts, do not help to define the problem or mobilize people to take action to solve it.
Bath SC, Sleeth ML, McKenna M, Walter A, Taylor A, Rayman MP. Iodine intake and status of UK women of childbearing age recruited at the University of Surrey in the winter. British Journal of Nutrition 2014; doi:10.1017/S0007114514002797
Bath SC, Steer CD, Golding J, Emmett P, Rayman MP. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet. 2013 Jul 27;382(9889):331-7. doi: 10.1016/S0140-6736(13)60436-5. Epub 2013 May 22. http://www.ncbi.nlm.nih.gov/pubmed/23706508
Jaime J. Gahche, Regan L. Bailey, Lisa B. Mirel, and Johanna T. Dwyer. The Prevalence of Using Iodine-Containing Supplements Is Low among Reproductive-Age Women, National Health and Nutrition Examination Survey 1999–2006 J. Nutr. 2013 jn.112.169326; first published online April 24, 2013. http://dx.doi.org/10.3945/jn.112.169326
Hynes KL, Otahal P, Hay I, Burgess JR. Mild iodine deficiency during pregnancy is associated with reduced educational outcomes in the offspring: 9-year follow-up of the gestational iodine cohort. J Clin Endocrinol Metab. 2013 May;98(5):1954-62. doi: 10.1210/jc.2012-4249. Epub 2013 Apr 30. http://www.ncbi.nlm.nih.gov/pubmed/23633204
Zimmermann MB. Iodine deficiency and excess in children: worldwide status in 2013. Endocr Pract. 2013 Sep-Oct;19(5):839-46. doi: 10.4158/EP13180.RA. http://www.ncbi.nlm.nih.gov/pubmed/23757630