While women often have iodine intakes above what is considered adequate, a new analysis of the Canadian Health Measures Survey suggests that women in Canada may actually have intakes well below what is considered essential for the support of a healthy pregnancy and breastfeeding.
 
Dietary assessment of iodine intake is challenging due to the fact that iodized salt consumption is difficult to accurately assess in large surveys and iodine content of similar foods can differ widely, the authors of the review, appearing in the Journal of Nutrition, said. For these reasons, they relied upon data from cycle five of the Canadian Health Measures Study (CHMS) which included multiple urine samples.
 
“Estimation of iodine intakes in a population using a single spot urine sample would reflect the day-to-day variation in intakes and lead to overestimations of iodine inadequacy or excess in populations. If over two urine samples are collected from a representative subset of the population, iodine intakes can be adjusted for within-person variation using statistical procedures. The resulting usual intakes more accurately reflect long-term average intakes of the population that are more relevant to nutritionists and policy makers for assessing and planning intakes,” the authors said.
 
Contrary to previous evidence based on food intake questionnaires, the authors of the study found that in 1,875 children, 557 adolescents, and 567 women of childbearing age, biomarkers suggested that the median iodine intakes for adolescent females (14-18) and women between the ages of 19 and 39 were insufficient – only 68% of women and adolescent girls involved in the study had measured iodine concentrations which were above the estimated average requirement (EAR) listed by the World Health Organization.
 
“Iodine deficiency in utero and early life can cause serious cognitive and motor impairment, making children and women of childbearing age vulnerable subpopulations for iodine deficiency,” the authors of the study said, noting that this deficiency runs contrary to the estimates achieved through less reliable means of measurement. “The Institute of Medicine (now the National Academy of Medicine) developed a formula to estimate iodine intakes. It is based on the relation between 24-hour urine volume and body weight derived from a study in children and adolescents aged 7-15 years. The formula has been used but it is overly simplistic and does not take into account urine dilution, and may result in inaccurate estimates of 24-hour [urinary iodine excretion] and iodine intakes.”
 
The substantial increase in iodine requirements during pregnancy and lactation highlight the importance that all women who could become pregnant, are pregnant, or are breastfeeding supplement their diets with iodine, the authors concluded.