Following a request from the European Commission, the EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA) was asked to deliver a scientific opinion on Dietary Reference Values (DRVs) for the European population, including iodine.
Iodine is an essential nutrient for mammals, required as a mandatory structural and functional element of thyroid hormones. Through these hormones, iodine has an important role in energy-yielding metabolism and on the expression of genes that impact many physiological functions, including embryogenesis and growth, and the development of neurological and cognitive functions.
The clinical effects of iodine deficiency, referred to as iodine deficiency disorders, are the result of insufficient intakes leading to insufficient thyroid function. Iodine deficiency disorders are seen at all stages of development and are particularly of concern in pregnancy and infancy. Chronic iodine deficiency may lead to compensatory thyroid hypertrophy with an enlargement of the thyroid gland denoted as goitre.
Intestinal absorption efficiency of ingested iodine is considered to be high (> 90 %). The thyroid is the major storage site for iodine in the body. Goitrogenous substances in foods, drinking water or cigarette smoke may inhibit the thyroidal uptake of iodide or its incorporation into the tyrosine precursors of thyroid hormones. The synthesis of normal quantities of thyroid hormones requires an adequate dietary intake of iodine. The kidney is the main route of excretion of iodine. In a steady state, the urinary iodine (UI) excretion represents more than 90 % of the dietary intake and is therefore a good indicator of recent iodine intakes. UI concentration has also been used to define the iodine status of a population. Concentrations of thyroglobulin, thyroid hormone T4 (thyroxine or 3,5,3',5'-tetraiodothyronine) and thyroid-stimulating hormone in serum are also considered useful biomarkers of iodine status depending on age and population group and their iodine status. Thyroid volume and goitre prevalence are useful long-term clinical indicators of iodine status.
Iodine occurs in food and water mainly as iodide. The iodine concentration of water and foods is highly variable. The richest iodine sources are marine products, eggs, milk, and food products derived from them, and iodised salt. Dietary assessment methods do not accurately quantify habitual iodine intakes.
Studies of iodine balance have provided highly variable results, with null balances observed at very different levels of intakes. In addition, balance studies performed in countries with a higher habitual iodine intake compared with most European countries are difficult to extrapolate to the European context. The same is true for results on iodine accumulation by the thyroid. Insufficient evidence is available to determine iodine requirements by a factorial approach taking into account iodine needs for hormone production and iodine storage in the thyroid as well as basal iodine losses in urine, faeces and sweat.
Health outcomes such as cognitive function in children, cancer and sub-clinical thyroid dysfunction in older adults were also considered but were found not to be suitable for deriving DRVs for iodine.
For the setting of DRVs for iodine it was considered that the prevalence of thyroid volume enlargement in a population can be used to define a threshold of UI excretion and UI concentration above which the prevalence of abnormal increases of thyroid volume is minimised. This threshold is based on observational studies of goitre prevalence in Central America in the 1960s and in European school-aged children. In the latter study, a prevalence of goitre below 5 % was almost systematically observed in all study areas when the UI concentration was above 100 µg/L. Even though this threshold has been established in school-aged children, the Panel also accepted it for adults. A UI concentration of 100 µg/L corresponds to an approximate iodine intake of 150 µg/day in adults. It was concluded that an Adequate Intake (AI) for iodine for adult men and women can be set at 150 µg/day. Accepting the threshold for UI concentration of 100 µg/L also for infants aged 7–11 months and for children and taking into account age-specific urinary volumes and body weights, AIs of 70 µg/day to 130 µg/day were derived.
For pregnant women, T4 production is estimated to be increased by a mean of 37 µg/day, corresponding to an additional iodine demand of 25 µg/day for hormone synthesis in the thyroid. The additional requirements due to the development of the fetus, placenta and amniotic fluid were considered very low when related to the whole pregnancy (equivalent to a net transfer of 1 µg iodine/day). Adding to this requirement the iodine needed for fetal synthesis of thyroid hormones, the total additional iodine reqirement is rounded to 50 µg/day. Provided that thyroid status and iodine stores before pregnancy are adequate, an AI of 200 µg/day is proposed for pregnant women. The Panel considered that the information available on the relationship between iodine intake or status of pregnant women and clinical outcomes, such as maternal thyroid function, infants being born small for gestational age or infants’ neurobehavioural impairment, cannot be used for setting DRVs for pregnant women.
The Panel noted that iodine concentrations in breast milk of European women vary widely and that large iodine stores exist in conditions of adequate iodine status before pregnancy and lactation. The Panel therefore considered that a full compensation for the iodine secreted in breast milk may not be justified for the derivation of DRVs for iodine for lactating women. Therefore, for lactating women the same AI is proposed as for pregnant women, i.e. 200 µg/day.