Iodine and Cognition in Young Adults: A Randomised, Placebo-Controlled Trial

Abstract

Iodine is a micronutrient which is essential to the production of thyroid hormones. Thyroid hormone functions are prevalent throughout the human body, but one of the most important functions is the influence on brain development. Central nervous system components such as myelin and neurotransmission are affected by the levels of iodine and thyroid hormone. Iodine deficiency is recognised as the world’s most prevalent preventable cause of intellectual disability. The most severe condition resulting from iodine deficiency is cretinism, which is characterised by severe cognitive impairment. However, even milder forms of iodine deficiency may result in some cognitive impairment. Iodine supplementation in individuals with iodine deficiency has been shown to result in improvements in cognitive scores. The majority of the work has investigated children, with the effect on the young adult population largely unknown. Potential mechanisms by which iodine affects cognition include enhancing neurotransmitters and assisting myelination (white matter maturation), both of which facilitate neurotransmission and are malleable in this age group. The present study investigated the effect that iodine supplementation has on the cognitive scores of mildly iodine-deficient young adults. It was hypothesised that individuals who took iodine supplements would show an improvement in cognitive scores above that of the placebo group. Participants were aged 18 to 30 years, and were given either iodine supplements or a placebo, in this double-blind, randomised-controlled trial. Seven subtests from the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) were used including: Block Design, Digit Span Backwards, Matrix Reasoning, Symbol Search, Visual Puzzles, Coding, and Letter-Number Sequencing. Cognitive scores were measured both prior to and after a 32-week period during which participants took iodine or the placebo. The results showed that at baseline, the participants were mildly iodine-deficient. The results did not support the hypothesis that iodine would facilitate cognition however, as there were consistent non-significant associations with iodine, both when comparing treatment groups and across all participants within examined time points. Thus, iodine supplements, while improving the iodine status of the supplement group, were not associated with an increase in cognitive scores. Furthermore, I speculated that this may have been because iodine facilitates cognition primarily through enhancing myelination, and the process of myelination is more complete in adulthood compared to childhood, and therefore less malleable. These speculations are in need of future research however. Future research could further investigate the differences between child and adult myelin development, as well as the impact of mild iodine deficiency in adults, to determine whether iodine does, in fact, have an effect on adults’ cognitive ability.