Adding iodine to household salt and to salt used in bread can compensate for low iodine intake in adolescents and in women of childbearing age, but can at the same time cause excessive intake in toddlers.
This is the key message in a benefit and risk assessment VKM has done at the request of the Norwegian Food Safety Authority.
Terms of reference
The Norwegian Food Safety Authority requested an assessment of health consequences of iodization of household salt, of salt in bread, and in plant-based alternatives to cow’s milk. They asked VKM to assess the benefit and risk of adding 15, 20, 25 or 50 mg of iodine per kg of salt. Currently, adding 5 mg of iodine per kg of household salt is permitted, but not for salt used in bread.
The background for the request from the Norwegian Food Safety Authority is the National Nutritional Council, in their report from 2016, pointed out that in Norway iodine intake among women of childbearing age, and of pregnant and breastfeeding women was low. Norwegian 13-year-olds also had low iodine intake levels.
VKM has conducted a systematic review of negative health effects of mild to moderate iodine deficiency. We looked at three categories of health outcomes: neurodevelopment in children, and thyroid function, fertility and birth outcomes for women.
VKM has also reviewed literature to assess the negative health effects of high iodine intake, and whether new literature findings justify an amendment of the existing tolerable upper intake level for iodine.
The project group estimated iodine intake in different population groups in Norway and compared these to established values for iodine requirements and tolerable upper intake levels.
The project group also estimated what the iodine intake would be in different population groups after addition of iodine to household salt and salt in bread, and compared these estimates with established values for iodine requirements, and tolerable upper intake levels.
The negative health effects of severe iodine deficiency are well documented. Severe iodine deficiency in pregnancy, infancy or early childhood can lead to permanent brain damage. Severe iodine deficiency also increases the risk of miscarriage, premature birth, stillbirth and congenital malformations. All these negative health effects are due to changes in thyroid hormone levels, which are crucial for growth and development.
Regarding the association between mild to moderate iodine deficiency and impaired neurodevelopment, the systematic literature review revealed limited documentation. In professional terms, this is called limited suggestive evidence. 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.
"This means that the documentation points in one direction, but is too weak for us to conclude that the causal relationship is probable or convincing," explains Sigrun Henjum. She is the academic leader for the work.
VKM concluded the evidence is so limited that no firm conclusion can be made about whether mild to moderate iodine deficiency leads to thyroid dysfunction, or has negative effects on fertility or birth outcomes. In professional terms, this is called limited, non-conclusive evidence.
Excessive intake of iodine can lead to subclinical hypothyroidism. This condition is not harmful, but can progress to hypothyroidism, which is harmful.
After reviewing the literature documenting high iodine intake levels and effects, VKM proposed to maintain the tolerable upper intake levels set by the European Scientific Committee for Food (SCF) in 2002.
Addition of iodine: both favorable and unfavorable
“Our estimates show that 13-year-olds and women of childbearing age are likely to benefit from adding more iodine to salt than currently permitted, e.g. 15 or 20 mg of iodine per kilo of salt for both household salt and salt in bread. Other population groups, whom for various reasons do not eat lean fish, or use milk or other dairy products, would also benefit from such an increased iodization,” says Henjum.
"At the same time, the addition of more iodine in salt and bread can pose a risk to younger children, because an increasing number of 1- to 2-year-olds would get too high an intake of iodine," continues Henjum.
“Our estimates show that the 1- and 2-year-olds may be a group at risk for high iodine intake. This also generally applies to users of kelp and seaweed-based products. Based on scientific studies and data, we cannot conclude that women of childbearing age and that youths will benefit from a certain level of iodine supplementation, without at the same time increasing the risk of high iodine intake in 1- and 2-year-olds," Henjum states.
The benefit and risk assessment has been assessed and approved by the Panel on Nutrition, Dietetic products, Novel Foods and Allergy.