4 Dietary reference values for iodine
4.1 Iodine requirement and recommended intakes
Institute of Medicine, USA (2001)
In 2001, the US Institute of Medicine established a recommended dietary allowance (RDA) for iodine at 150 µg/day for adults. Thyroid iodine accumulation and turnover were used to set the estimated average requirement (EAR). The US Institute of Medicine (IOM) has recently been renamed and incorporated into the National Academies of Science,
Engineering and Medicine (NASEM). Thus, all DRI reports through 2011 were published by IOM, while all subsequent reports are published by NASEM.
IOM (2001) proposed the following equation to calculate daily iodine intake from UIC:
Daily iodine intake = UIC (μg/L) × 0.0235 × bodyweight (kg)
The equation assumes that 92% of dietary iodine is absorbed. Although body weight is poorly correlated with urine volume in adults, the equation is a good approximation
considering an average 24-h urine volume of 1.5 L/day in adults. Alternatively, daily iodine intake can be estimated from UICs by estimating the daily urinary iodine excretion by means of the urinary creatinine concentration.
The EAR for adolescents ≥14 years and adults was set to be 95 µg/day, assuming a coefficient of variation of 40%. The EAR for pregnant and lactating women was set to be 160 and 209 µg/day, respectively, with a coefficient of variation of 20%. The EARs for
children 1-8 years an 9-13 years were set to 65 and 73 µg/day, respectively. For children 1-8 years, the EAR is based on iodine balance studies in children (Malvaux, 1969; Ingenbleek and Malvaux, 1974 both cited in IOM, 2001). For adolescents aged 9-13 years, the EAR was extrapolated from adult EARs on a metabolic body weight basis allowing for growth needs (kg 0.75).
The RDA was defined as equal to the EAR plus twice the coefficient of variation to cover the needs of 97 to 98 percent of the individuals in the group. The calculated values for RDA were rounded up to the nearest 50 µg. The suggested RDA values for various age groups and pregnant and lactating women are provided in Table 4.1.1-1.
48 In collaboration with WHO and FAO, NASEM has proposed harmonised nutrient reference values for populations. Instead of the term EAR, AR should be used, and for iodine the harmonised AR (h-AR) is the previous EAR for all age groups (Allen et al., 2019).
Table 4.1.1-1 Suggested recommended dietary allowances (RDAs) for iodine from IOM, µg/day (2001).
Adults Pregnant Lactating Children and adolescents
≥19 y 1-8 y 9-13 y 14-18 y
Recommended dietary
allowances (RDAs)
150 220 290 90 120 130
EAR* 95 160 209 65 73 95
*The definition of EAR by IOM corresponds to the term AR used by NNR and the updated proposed harmonised nutrient reference values from NASEM (previously IOM).
WHO (2005 and 2007)
Based on recommendations from the Food and Nutrition Board of the United States National Academy of Sciences from 1989 and previous WHO-reports, WHO (2005) recommended a daily iodine intake of 40 µg/day for young infants (0–6 months), 50 µg/day for older infants (7–12 months), 60–100 µg/day for children (1–10 years), and 150 µg/day for adolescents and adults (WHO, 2005). According to WHO, these recommended values will ensure normal T4 production without stressing the thyroid iodide trapping mechanism or raising TSH levels.
The recommended nutrient intake (RNI) at 150 µg iodine/day for adolescents and adults is justified by the fact that it corresponds to the daily urinary excretion of iodine and to the iodine content of food in non-endemic areas (i.e. in areas where iodine intake is adequate).
Furthermore, it represents the intake of iodine necessary to maintain the plasma iodide concentration above the critical limit of 0.10 µg/dL, which is the average level likely to be associated with the onset of goiter. Moreover, 150 µg iodine/day is required to maintain iodine stores in the thyroid above the critical threshold of 10 µg, to avoid disorders in the production of thyroid hormones (WHO, 2005).
In 2007, WHO/UNICEF/ICCIDD increased the recommended nutrient intake (RNI) for iodine during pregnancy and lactation from 200 to 250 µg/day (WHO, 2007). During lactation, thyroid hormone production and UIC return to normal, but iodine is concentrated in the mammary gland for excretion into breast milk. Thus, using the UIC to estimate intake may lead to an underestimate of requirements. To ensure sufficient iodine from breast milk to build reserves in the thyroid gland in the infant, WHO increased the RNI for iodine for lactating women to 250 µg/day. On the other hand, the rationale for increasing the RNI for pregnant women was not well described. The consensus reached by the
WHO/UNICEF/ICCIDD in 2007 emphasised that in countries without universal salt iodization, women of childbearing age should be given oral iodine supplementation to ensure that the total iodine intake meets the recommendation of 150 µg iodine per day. However, pregnant women should not be recommended to take iodine-containing supplements if the population in general had been iodine sufficient for at least 2 years (Andersson et al., 2007).
49 Table 4.1.2-1 Recommended nutrient intakes (RNIs) of iodine from FAO/WHO (20051and 20072), µg/day.
Adults1 Pregnant2 Lactating2 Children 1-10 y1 Recommended
nutrient intake (RNI)
150 250 250 90-120
Nordic and Norwegian Nutrition recommendations (2012)
Prior to the 5th revision of the Nordic Nutrition recommendations (2012), an expert group conducted a systematic literature review aiming to summarise the scientific basis for the previous iodine recommendation in the Nordic countries (Gunnarsdottir and Dahl, 2012). The recommended intake of iodine from 2004 remained unchanged for adults and children because no new data supported any changes in the 2012 revision. The iodine requirement to prevent goiter was estimated to be 50-75 µg/day for adult women and men, and the AR was estimated to be 100 µg/day, an intake level at which the iodine concentration in the thyroid gland reaches a plateau. The recommended intake (RI) for adults and adolescents therefore remained 150 µg/day and this amount includes a safety margin for goitrogenic substances (foods containing substances interfering with uptake of iodine in the thyroid gland, e.g. soy and cabbage).
The NNR expert group also evaluated the scientific rationale for the recommended increased iodine intake during pregnancy and lactation from WHO (Gunnarsdottir and Dahl, 2012). In pregnancy, a higher iodine intake is recommended to cover for the higher thyroid hormone production and simultaneously increased excretion in the urine (Andersen and Laurberg, 2016). A higher iodine intake is also recommended during lactation to ensure sufficient iodine in the breast milk. Although WHO decided to increase the recommended intake of iodine for pregnant and lactating women from 200 to 250 µg/day in 2007, these changes were not adopted in NNR (2012). The experts responsible for NNR (2012) based this decision on lack of new data to support changes in recommended iodine intake for this particular group, and that women in the Nordic countries were considered iodine replete before pregnancy and having easy access to iodine rich food, such as milk, seafood, and dietary supplements. The Norwegian recommendations were based on the Nordic
recommendations, and the Nordic and Norwegian recommendations for intake of iodine in various age groups and pregnant and lactating women are shown in Table 4.1.3-1.
Table 4.1.3-1 Nordic and Norwegian dietary reference values for iodine, µg/day (NNR Project Group, 2012).
≥ 10 years and adults
Pregnant Lactating Children
12-23 mo 2-5 y 6-9 y
Recommended
intake(RI) 150 175 200 70 90 120
Average requirement (AR)*
100
50
≥ 10 years and adults
Pregnant Lactating Children
12-23 mo 2-5 y 6-9 y
Lower intake
level (LI) 70
*the definition of AR corresponds to the term EAR used by IOM (USA).
Dietary Reference Values from EFSA (2014)
The most recent dietary reference values (DRVs) for iodine were set by EFSA in 2014. The EFSA Panel concluded that there was insufficient evidence to derive an average requirement (AR) or a population reference intake (PRI) for iodine and decided to set an adequate intake (AI). The AIs from EFSA were based on studies exploring the relationship between iodine intake/status and thyroid gland volumes/prevalence of goiter as markers of mid to long-term iodine intakes (EFSA, 2014).
For adults and children, the AI was based on iodine intakes ensuring a UIC which had been associated with the lowest prevalence of goiter in school-aged children (≥100 μg/L). An UIC of 100 µg/L corresponds to an approximate daily intake of 150 µg iodine in older adolescents and adults. An average urinary volume of 1.5 L/day in adults derived from the water intake recommendations, was used to estimate the intake based on UIC at 100 µg/day.
For infants and young children the proposed AI was also based on the UIC, which had been associated with the lowest prevalence of goiter in school-aged children. For children, age-specific urinary volumes and absorption efficiency of dietary iodine were taken into account to calculate the AI.
For pregnant women, iodine intake was assumed to be adequate before conception. The AI for pregnant women at 200 µg/day takes into account the additional needs because of increased thyroid hormone production, and iodine uptake by the foetus, placenta and amniotic fluid accounting for an additional iodine requirement of 50 µg/day.
The proposed AI for lactating women of 200 µg/day takes into account the existence of large iodine stores in conditions of adequate iodine status before pregnancy and considers that a full compensation of the transitory loss of iodine secreted in breast milk is not justified for the derivation of an increased AI for iodine for lactating women. Thus, the same AI was set for pregnant and lactating women.
Table 4.1.4-1 Adequate intakes (AIs) for iodine from EFSA (2014), µg/day.
Adults Pregnant Lactating Children and adolescents
≥18 y 1-10 y 11-14 y 15-17 y
Adequate
Intakes (AI) 150 200 200 90 120 130
51 Overview of dietary reference values from previous reports
Table 4.1.5-1 gives an overview of the recommended dietary iodine intake in adults, whereas Table 4.1.5-2 gives reference values for recommended iodine intake in pregnant and
lactating women.
Table 4.1.5-1 Overview of dietary reference values for iodine in adults.
Authority µg/day Critical endpoint
EFSA, 2014 AI1=150 Thyroid volume enlargement NNR, 2012 RI2=150 Thyroid volume enlargement WHO, 2005 RNI3=150 Maintain normal T4 production and
prevention of goiter IOM, 2001/ NASEM
(Allen et al., 2019)
RDA4=150 Thyroid iodine accumulation and turnover
1AI=Adequate Intake, 2RI=Recommended intake, 3RNI=Recommended nutrient intake, 4RDA=Recommended Dietary Allowances.
Table 4.1.5-2 Overview of DRVs for iodine in pregnant and lactating women and the rationale for higher recommendations than in non-pregnant adults.
Authority Pregnant µg/day
Lactating µg/day
Rationale for higher intakes than adults
EFSA, 2014 AI1=200 AI=200 Pregnancy: Increased thyroid hormone production, iodine uptake by foetus, placenta and amniotic fluid. Additional need approx. 50 µg/day, assuming adequate intakes before pregnancy.
Lactation: Daily iodine losses in breast milk= 60-90 µg. However, to further increase iodine intake to cover full compensation for iodine losses in breast milk were not considered justified.
NNR, 2012 RI2=175 RI=200 Pregnancy: To cover the needs of the foetus and to maintain maternal thyroid gland function. Assuming adequate intakes before pregnancy.
Lactation: To provide sufficient iodine in the breast milk.
WHO, 2007 RNI3=250 RNI=250 Pregnancy: Increased glomerular filtration in pregnant women.
Lactation: To ensure sufficient iodine from breast milk to build reserves in the thyroid gland in the infant.
IOM, 2001 RDA4=220 RDA=290 Pregnancy: Thyroid iodine uptake by the foetus approx 75 μg/day Lactation: Average daily loss of iodine in breast milk approx 114 μg.
1AI=Adequate Intake, 2RI=Recommended intake, 3RNI=Recommended nutrient intake, 4RDA=Recommended Dietary Allowances.
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