Benefit and risk assessment of iodization of household salt and salt used in bread and bakery products. Opinion of the Panel on Nutrition, Dietetic Products, Novel Food and Allergy of the Norwegian Scientific Committee for Food and Environment
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Original versionHenjum, Brantsæter, Holvik, Lillegaard, Mangschou, Parr, Starrfelt, Stea, Andersen, Dahl, Dalen, Løvik, Ulven, Strand. Benefit and risk assessment of iodization of household salt and salt used in bread and bakery products. Opinion of the Panel on Nutrition, Dietetic Products, Novel Food and Allergy of the Norwegian Scientific Committee for Food and Environment. The Norwegian Scientific Committee for Food Safety; 2020. 312 p.. VKM Report
Request from the Norwegian Food Safety Authority: Following a report from the National Nutrition Council calling for actions to secure adequate iodine intake in the population, the Norwegian Food Safety Authority requested the Norwegian Scientific Committee for Food and Environment (VKM) to conduct a benefit- and risk assessment of iodization of household salt and industrialised salt in bread. VKM addressed the request: VKM appointed a project group consisting of members of the Panel on Nutrition, Dietetic Products, Novel Food and Allergy and Panel on Contaminants to answer the request. The Panel on Nutrition, Dietetic Products, Novel Food and Allergy has reviewed and revised drafts prepared by the project group and finally approved the benefit- and risk assessment This benefit and risk assessment is based on 1) established knowledge about health consequences from severe iodine deficiency, 2) systematic literature review of the evidence for health consequences of mild to moderate iodine deficiency, 3) literature review of studies on adverse health effects from excessive iodine intakes to re-evaluate existing tolerable upper intake levels (ULs), 4) evaluation of estimated iodine intake levels in different population groups in Norway compared to established dietary reference values, specifically estimated average requirement (EAR) and UL, and finally 5) an estimation of the effect of different scenarios of increasing iodization levels in household salt and salt in bread on iodine intake levels in different population groups compared to the established dietary reference values EAR and UL. For the purpose of this benefit and risk assessment of iodization in household salt and salt in bread, risk may be understood as risk of adverse health effects related to too high or too low iodine intakes. Benefit may be understood as reduction or avoidance of adverse health effects related to too high or too low iodine intakes. For iodine, this is a challenging maneuver, as the gap between too low and too high intakes is narrow. The project group conducted two systematic literature reviews. One was performed to evaluate the evidence for the impact of mild- to moderate iodine deficiency on neurodevelopment, thyroid function and birth outcomes, the other to evaluate the evidence for adverse health effects from excessive iodine intake to possibly re-evaluate the existing tolerable upper intake levels. Current evidence for adverse health effects of iodine deficiency: Severe iodine deficiency resulting in hypothyroidism during foetal life, infancy, or early childhood may lead to permanent intellectual disability. Severe maternal iodine deficiency in pregnancy may also result in miscarriages, preterm delivery, stillbirth, and congenital abnormalities. These effects are due to low levels of thyroid hormones affecting the developing tissues. The health effects of severe iodine deficiency are well established, but for mild to moderate deficiency the effects are less known. After screening of more than 15 000 titles and abstracts and quality assessment of 131 full text scientific papers, 36 publications were included for the grading of evidence for consequences of mild to moderate iodine deficiency. We used the criteria proposed by the World Cancer Research Fund for grading of evidence. Based on our systematic literature review, the evidence for adverse effects from mild to moderate iodine deficiency and neurodevelopment was judged to meet the criteria for limited suggestive. Limited suggestive means that the evidence is too limited to permit a probable or convincing causal judgement but shows a generally consistent direction of effect. It was further concluded that there is limited (no conclusion) evidence to support that mild to moderate iodine deficiency causes thyroid dysfunction or has negative effects on birth outcomes. Limited (no conclusion) means that the evidence is so limited that no firm conclusion can be made. Established reference values for iodine requirement: Several competent bodies have established dietary reference values for iodine. We have based this benefit and risk assessment on estimated average requirements (abbreviated EAR or AR) from the Nordic Nutrition Recommendations (2012) for adults, and from the Institute of Medicine (2001) for adolescents and children. Estimated average requirement is an iodine intake that is estimated to meet the requirement of half the healthy individuals in the population. The iodine intake is considered to be adequate if 97.5% of a population has a habitual intake above the estimated average requirement. Re-evaluation of tolerable upper intake levels: As the evidence for health effects of excessive iodine intakes is far less comprehensive than for iodine deficiency, we did not aim at evaluating the literature for excess intakes with the same weight of evidence tools as for deficiency. We have only evaluated whether recent literature is in line with the existing tolerable upper intake levels or if it supports that tolerable upper intake levels should be changed. Generally, UL is the maximum level of total chronic daily intake judged to be unlikely to pose a risk of adverse health effects, and in the case of iodine the UL for adults is the maximum daily intake where changes in TSH are unlikely to occur (SCF, 2002). After screening more than 2500 titles and abstracts, five studies fulfilled the inclusion criteria and were found relevant and included for evaluation of existing ULs. The existing ULs from the Scientific Committee on Food from 2001 is maintained. However, according to findings in new studies, the lowest-observed-adverse-effect levels (LOAEL) for excessive iodine intake may be lower than previously assumed, and the uncertainty factor inherent in the established ULs is reduced from three to 1.3 for adults. The findings also indicate a reduced LOAEL for the existing ULs in children. Changes in thyroid stimulating hormone (TSH) without changes in thyroid hormones (T3 or T4), that is subclinical hypothyroidism, are observed in one randomised controlled dose-response study and four cross-sectional studies in children at iodine intakes close to the ULs. Subclinical hypothyroidism is not considered to be harmful, but may progress into overt hypothyroidism. Iodine intake in Norwegian population groups and implications of salt iodization: Iodine intakes in adults are higher in men than in women and increase with increasing age for both sexes. Women of childbearing age and 13-year-old girls have the lowest estimated iodine intakes. 26% of the women of childbearing age have intakes below the estimated average requirement of 100 µg/day, and 38% of the 13-year-old girls have intakes below the estimated average requirement of 73 µg/day. The estimated iodine intakes in the 5th percentile of women of childbearing age and 13-year-olds is 70 and 38 µg/day, respectively. All adults and 13-, 9- and 4- year-olds have individual intakes below the tolerable upper intake levels. The estimates for 2-year-olds show that 9% have intakes below the estimated average requirement, and 8% have intakes above the tolerable upper intake level. The estimated iodine intake in the 95th percentile is 215 µg/day, slightly above the tolerable upper intake level of 200 µg/day. The estimates for non-breastfed 1-year-olds show that 8% have intakes below the estimated average requirement, and 18% have intakes above the tolerable upper intake level. The estimated iodine intake in the 95th percentile for this group is 259 µg/day. We present 12 scenario tables combining three scenarios (household salt alone, salt in bread alone and both household salt and salt in bread) with four iodization levels (15, 20, 25 and 50 mg iodine per kg salt). The percentages of the population groups with intakes above the estimated average requirement increases with increasing iodization levels, but so does the percentages with intakes above tolerable upper intake levels for some groups of the population. The scenarios that seem to elevate iodine intakes in women of childbearing age and 13-yearolds up to adequate intakes, are iodization up to 15 or 20 mg iodine per kg salt, including iodization of salt in bread. Above these iodization levels, no increased benefit would be expected in women of childbearing age and 13-year-olds. For 1- and 2-year-olds all scenarios lead to an increase in the proportion of toddlers with estimated intakes above the tolerable upper intake level. Benefit- and risk characterisation of iodization of household salt and industrialised salt in bread: Low estimated iodine intakes in adolescents (13-year-olds) and women of childbearing age (18-45 years) cannot be sufficiently corrected by the proposed increased iodization levels of salt and/or bread without imposing high iodine intakes in 1- and 2-year-old children. VKM assumes that the women of childbearing age and 13-year-olds will benefit from increased iodization levels in salt and bread. This will also benefit other groups that, for various reasons, have few iodine-rich sources in their diet, e.g., people who do not eat lean fish or consume milk or other dairy products. The risk imposed on the youngest age groups is a higher proportion of the 1- and 2-year-olds with iodine intakes above the tolerable upper intake level. Based on the scientific evidence and the data presented in this benefit and risk assessment it cannot be concluded that a specific iodization level benefits all age and gender groups without posing increased risk of harm to others or that the benefits in one population group outweighs the risks in others, or that the benefits in one group outweigh the risks in others. It should be noted that recommendations for nutrients are set to secure adequate growth, development, maintenance of health, and to reduce the risk of chronic illnesses. Thus, iodine intakes below EAR or above UL will decrease the possibilities of achieving the beneficial effects of adequate intake. Current iodine intake in certain population groups is worryingly low, and trend studies indicate that consumption of milk and dairy products in Norway, the most significant iodine sources in the diet, is declining, especially among young women. Several studies show that especially adolescents and women of childbearing age have insufficient iodine intakes, which may leave some at risk of severe iodine deficiency. There are, however, to our knowledge, no data on the prevalence of severe iodine deficiency in Norway. In other words, we do not know how many, if any, there are who have clinical consequences of inadequate iodine intakes. The scenarios that seem to elevate iodine intakes in women of childbearing age and 13-year olds (the groups at highest risk of low intakes) to adequate levels are iodization up to 15 or 20 mg iodine per kg salt, including iodization of salt in bread. Above these iodization levels, no increased benefit would be expected in women of childbearing age and 13-year-olds whereas several population groups will be at risk of exceeding UL, especially in 1- and 2-year-olds. The WHO recommends salt as a vehicle for correcting iodine deficiency in a population, followed by a close monitoring of the iodization program.