Intakes of dairy products and dietary supplements are positively associated with iodine status among U.S. children

Dairy food consumption is beneficially linked with iodine status in US children and adults: National Health and Nutrition Examination Surveys 2001-2018

OBJECTIVES

The objective of this study was to determine the association between the consumption of dairy foods with urinary iodine concentration (UIC) and iodine deficiency risk in a nationally representative sample of the US population.

DESIGN, SETTING AND PARTICIPANTS

24-hour dietary recall data and laboratory data for UIC (μg/l) from subjects 2+ years old US population participating in National Health and Nutrition Examination Surveys 2001-2018 were used (n 26 838) for analyses after adjusting for demographic covariates. Significant associations were assessed at P < 0·05.

RESULTS

Mean intakes of total dairy were 2·21, 2·17 and 1·70 cup equivalents (cup eq) among those 2-8, 9-18 and 19+ years, respectively. Of the dairy components, intake of milk was highest followed by cheese and yogurt for all age groups. Total dairy intakes were positively associated with UIC among those 2-8 years (β = 29·9 ± 9·9 μg/l urine/cup eq dairy) and 9-18 years (β = 26·0 ± 4·8 μg/l urine/cup eq dairy) but not associated among those 19+ years. Total dairy intakes were associated with lowered risks (30 %, 21 % and 20 % for among 2-8, 9-18 and 19+ years, respectively) of being classified as iodine insufficient (UIC < 100 μg/l) or lowered risk (47 %, 30 % and 26 % among 2-8, 9-18 and 19+ years, respectively) of being classified as iodine severely deficient (UIC < 20 μg/l).

CONCLUSIONS

The results indicate that dairy foods are beneficially associated with UIC and lowered iodine deficiency risk.

Food sources of iodine in schoolchildren and relationship with 24-h urinary iodine excretion in Victoria, Australia

Dietary recalls have been used previously to identify food sources of iodine in Australian schoolchildren. Dietary assessment can provide information on the relative contributions of individual food groups which can be related to a robust objective measure of daily intake (24-h urinary iodine excretion (UIE)). In Australia, the government has mandated the use of iodised salt in breadmaking to address iodine deficiency. The aim of this study was to determine the dietary intake and food sources of iodine to assess their contribution to iodine excretion (UIE) in a sample of Australian schoolchildren. In 2011-2013, UIE was assessed using a single 24-h urine sample and dietary intake was assessed using one 24-h dietary recall in a convenience sample of primary schoolchildren from schools in Victoria, Australia. Of the 454 children with a valid recall and urine sample, 55 % were male (average age 10·1 (1·3 (sd) years). Mean UIE and dietary iodine intake were 108 (sd 54) and 172 (sd 74) μg/d, respectively. Dietary assessment indicated that bread and milk were the main food sources of iodine, contributing 27 and 25 %, respectively, to dietary iodine. Milk but not bread intake was positively associated with UIE. Multiple regression (adjusted for school cluster, age and sex) indicated that for every 100 g increase in milk consumption, there was a 3 μg/d increase in UIE (β = 4·0 (se 0·9), P < 0·001). In conclusion, both bread and milk were important contributors to dietary iodine intake; however, consumption of bread was not associated with daily iodine excretion in this group of Australian schoolchildren.

Consequences of Iodine Deficiency and Excess in Pregnancy and Neonatal Outcomes: A Prospective Cohort Study in Rio de Janeiro, Brazil

Background: Insufficient or excessive iodine intake during gestation may compromise adaptive mechanisms in maternal thyroid function and lead to adverse pregnancy outcomes. In this context, we aimed to study the effects of maternal iodine status in the first and third trimesters of gestation on obstetric and neonatal outcomes in an iodine-sufficient population in Rio de Janeiro, Brazil. Methods: A total of 214 pregnant women in the first trimester of gestation were enrolled and prospectively followed until delivery between 2014 and 2017. All participants were ≥18 and ≤35 years, had a spontaneous single pregnancy, and had no history of thyroid or other chronic diseases, nor were they taking iodine-containing supplements at enrollment. In the first trimester, we obtained clinical information and determined thyroid function and the urinary iodine concentration (UIC) of the participants. Thyroid function and UIC were reassessed in the third trimester. Iodine status was determined by the median of UIC obtained from six urine spot samples by the inductively coupled plasma mass spectrometry method. Pregnancy and neonatal outcomes and delivery information were obtained from medical records. Results: The median UIC in the whole population was 219.7 μg/L. The prevalence of UIC <150 μg/L was 17.2%, and 38.7% had UIC ≥250 μg/L. Gestational diabetes (GDM) was higher in the group with UIC 250-499 μg/L (n = 77) compared with the group with UIC 150-249 μg/L (n = 94) (20.3% vs. 9.7%, p < 0.05). Ultimately, UIC ≥250 μg/L was an independent risk factors for GDM (relative risk [RR] = 2.9 [confidence interval, CI = 1.1-7.46], p = 0.027) and hypertensive disorders of pregnancy (HDP) (RR = 4.6 [CI = 1.1-18.0], p = 0.029). Among 196 live-born newborns, lower birth length was observed in infants whose mothers had UIC <150μg/L (n = 37) in the first trimester compared with those with UIC 150-249 μg/L (n = 86) (median interquartile range: 48.0 [2.2] vs. 49.0 [4.0] cm, p = 0.01). Maternal UIC <150 μg/L was negatively associated with birth length of newborns (Exp (B) = 0.33 [CI = 0.1-0.9], p = 0.03). Conclusions: In a population whose median iodine intake is sufficient, extensive individual variation occurs. Such abnormalities are associated with increased GDM and HDP when UIC is ≥250 μg/L, and lower infant birth length when UIC is <150 μg/L.

Consequences of Severe Iodine Deficiency in Pregnancy: Evidence in Humans

Iodine is a necessary micronutrient for the production of thyroid hormones and normal human development. Despite the significant worldwide strategies for the prevention and control of iodine deficiency, it is still a prevalent public health issue, especially in pregnant women. Severe iodine deficiency during pregnancy and neonatal period is associated with many major and irreversible adverse effects, including an increased risk of pregnancy loss and infant mortality, neonatal hypothyroidism, cretinism, and neuropsychomotor retardation. We will review the impact of severe iodine deficiency on maternofetal, neonatal, and offspring outcomes. We will also discuss its epidemiology, classification of iodine deficiency severity, and current recommendations to prevent iodine deficiency in childbearing age and pregnant women.

Iodine bioavailability from cow milk: a randomized, crossover balance study in healthy iodine-replete adults

BACKGROUND

Milk and dairy products are considered important dietary sources of iodine in many countries. However, to our knowledge, iodine bioavailability from milk has not been directly measured in humans.

OBJECTIVE

The aim of this study was to compare iodine bioavailability in iodine-replete adults from: 1) cow milk containing a high concentration of native iodine; 2) milk containing a low concentration of native iodine, with the addition of potassium iodide (KI) to assess a potential matrix effect; and 3) an aqueous solution of KI as a comparator; with all 3 containing equal amounts of total iodine (263 µg/250 mL). We also speciated iodine in milk.

DESIGN

We conducted a 3-wk, randomized, crossover balance study in adults (n = 12) consuming directly analyzed, standardized diets. During the 3 test conditions - high intrinsic iodine milk (IIM), extrinsically added iodine in milk (EIM), and aqueous iodine solution (AIS) - subjects collected 24-h urine over 3 d and consumed the test drink on the second day, with 3- or 4-d wash-out periods prior to each treatment. Iodine absorption was calculated as the ratio of urinary iodine excretion (UIE) to total iodine intake. Milk iodine speciation was performed using ion chromatography-mass spectrometry.

RESULTS

Iodine intake from the standardized diet was 195 ± 6 µg/d for males and 107 ± 6 µg/d for females; the test drinks provided an additional 263 µg. Eleven subjects completed the protocol. There was a linear relation between iodine intake and UIE (β = 0.89, SE = 0.04, P < 0.001). There were no significant differences in UIE among the 3 conditions (P = 0.24). Median (range) fractional iodine absorption across the 3 conditions was 91 (51-145), 72 (48-95), and 98 (51-143)% on days 1, 2, and 3, respectively, with day 2 significantly lower compared with days 1 and 3 (P < 0.001). In milk, 80-93% of the total iodine was inorganic iodide.

CONCLUSION

Nearly all of the iodine in cow milk is iodide and although fractional iodine absorption from milk decreases with increasing dose, its bioavailability is high. The trial was registered at clinicaltrials.gov as NCT03590431.

Iodine Status and Consumption of Key Iodine Sources in the U.S. Population with Special Attention to Reproductive Age Women

We estimated iodine status (median urinary iodine concentration (mUIC (µg/L))) for the US population (6 years and over; n = 4613) and women of reproductive age (WRA) (15⁻44 years; n = 901). We estimated mean intake of key iodine sources by race and Hispanic origin. We present the first national estimates of mUIC for non-Hispanic Asian persons and examine the intake of soy products, a potential source of goitrogens. One-third of National Health and Nutrition Examination Survey (NHANES) participants in 2011⁻2014 provided casual urine samples; UIC was measured in these samples. We assessed dietary intake with one 24-h recall and created food groups using the USDA’s food/beverage coding scheme. For WRA, mUIC was 110 µg/L. For both non-Hispanic white (106 µg/L) and non-Hispanic Asian (81 µg/L) WRA mUIC was significantly lower than mUIC among Hispanic WRA (133 µg/L). Non-Hispanic black WRA had a mUIC of 124 µg/L. Dairy consumption was significantly higher among non-Hispanic white (162 g) compared to non-Hispanic black WRA (113 g). Soy consumption was also higher among non-Hispanic Asian WRA (18 g compared to non-Hispanic black WRA (1 g). Differences in the consumption pattern of key sources of iodine and goitrogens may put subgroups of individuals at risk of mild iodine deficiency. Continued monitoring of iodine status and variations in consumption patterns is needed.

Determination of Iodine Content in Dairy Products by Inductively Coupled Plasma Mass Spectrometry

A probing study to establish a reliable and robust method for determining the iodine concentration using the ELAN^® DRC^™ II ICP-MS was performed in combination with a sample digestion and filtration step. Dairy products from locally available sources were evaluated to help determine the possibility and need for further evaluations in relation to the U.S. population's iodine intake. Prior to analysis, the samples were aliquoted and digested for 3 hours at 90±3 °C. Dilution and filtration were performed, following the digestion. The sample extract was analyzed, and the results were confirmed with NIST SRM 1549a Whole Milk Powder. Further experimentation will need to be performed to optimize the method for projected sample concentration and throughput.

Variation in iodine food composition data has a major impact on estimates of iodine intake in young children

BACKGROUND/OBJECTIVES

The reliability of an estimate of iodine intake is largely dependent on the quality of the food composition data. We aimed to assess the impact of variations in food composition data for iodine and season on the estimates of iodine intake in young children.

SUBJECTS/METHODS

Cross-sectional dietary intake study of Irish 2-year-olds participating in the Cork BASELINE Birth Cohort Study (n=468; 30% of the cohort at the 2-year follow-up) were used to assess the impact of variation in iodine food composition data on estimates of iodine intake, dietary adequacy and risk of exceeding the tolerable upper intake level (UL).

RESULTS

Mean (SD) iodine intakes calculated using UK (147 (71)) and Irish (177 (93)) food composition data were significantly different (P < 0.001) (mean difference (95% confidence interval) = 30 (26-33) µg/day) and largely adequate (7-14% below the estimated average requirement). Intakes at the 95th percentile were 138% and 173% of the UL using UK and Irish food composition data, respectively, of which milk accounted for 106% and 150% of the UL. This translated into 22% and 35% of toddlers exceeding the UL, using UK and Irish composition data, respectively. The mean (SD) daily intake of cow's milk among the 91% of consumers was 309 (208) ml. Intakes of cow's milk at the 75th and 95th percentiles were 452 and 706 ml, respectively. Using Irish composition data for iodine in cows' milk, a daily intake of 450 ml could result in a toddler exceeding the UL from milk alone.

CONCLUSIONS

Variability in food composition has a large impact on assessments of iodine intake, particularly among young children for whom milk contributes a large proportion of their daily nutrient intake. Although this is unlikely to result in long-term adverse effects, our study highlights the need for development of valid biomarkers of individual iodine status.

Consequences of iodine deficiency and excess in pregnant women: an overview of current knowns and unknowns

Severe iodine deficiency during development results in maternal and fetal hypothyroidism and associated serious adverse health effects, including cretinism and growth retardation. Universal salt iodization is the first-line strategy for the elimination of severe iodine deficiency. Iodine supplementation is recommended for vulnerable groups in severely iodine-deficient regions where salt iodization is infeasible or insufficient. A recent clinical trial has informed best practices for iodine supplementation of severely iodine-deficient lactating mothers. Because of successful programs of universal salt iodization in formerly severely iodine-deficient regions around the world, public health concern has shifted toward mild to moderate iodine deficiency, which remains prevalent in many regions, especially among pregnant women. Observational studies have shown associations between both mild maternal iodine deficiency and mild maternal thyroid hypofunction and decreased child cognition. Iodine supplementation has been shown to improve indexes of maternal thyroid function, even in marginally iodine-deficient areas. However, no data are yet available from randomized controlled trials in regions of mild to moderate iodine insufficiency on the relation between maternal iodine supplementation and neurobehavioral development in the offspring; thus, the long-term benefits and safety of such supplementation are uncertain. Although it is clear that excessive iodine intake can cause alterations in thyroid function in susceptible individuals, safe upper limits for iodine intake in pregnancy have not been well defined. Well-designed, prospective, randomized controlled trials that examine the effects of iodine supplementation on maternal thyroid function and infant neurobehavioral development in mildly to moderately iodine-deficient pregnant women are urgently needed. In addition, clinical data on the effects of iodine excess in pregnant and lactating women are needed to inform current recommendations for safe upper limits on chronic iodine ingestion in general and on iodine supplementation in particular.

High Iodine and Salt Intakes and Obesity do not Modify the Thyroid Function in Mexican Schoolchildren

Mexico is considered as a nutritional transition country with a high prevalence of overweight and obesity, and recent studies have reported a high iodine intake in children. Both high iodine intake and obesity have been associated with thyroid dysfunction. Our aim was to assess iodine and salt intake and thyroid function in Mexican schoolchildren with normal weight and obesity. A cross-sectional study was performed during 2012-2013 in schoolchildren from Queretaro, Mexico. Six hundred seventy-eight schoolchildren were evaluated to obtain nutrition status, urinary iodine concentration (UIC) and thyroid volume (TVol). The prevalence of overweight and obesity was 47.3 %, the median UIC was 428 μg/L and TVol was normal in all schoolchildren; however, obese girls had a higher TVol than normal weight at the age of 8, 10 and 12 years. A subsample of schoolchildren was divided in 6-8 and 9-12-year-old groups, in order to compare thyroid function (thyrotropin, free T4, and anti-thyroid antibodies); iodine and salt intake were estimated with 24-h urinary samples. No differences in thyroid function were observed in both age groups. In the 6-8-year-old group, obese schoolchildren had higher iodine intake than normal-weight children (415.5 vs. 269.1 μg/day, p < 0.05), but no differences in salt intake. In contrast, in the 9-12-year-old group, obese schoolchildren had higher salt intake than normal-weight children (6.2 vs. 3.8 g/day, p < 0.05), but no differences in iodine intake. Dietary patterns could explain the differences between both age groups. Further studies are needed to identify the main sources of iodine intake in Mexican populations.

Iodine Deficiency and Hypothyroidism From Voluntary Diet Restrictions in the US: Case Reports

Iodine deficiency is rare in the United States today, and this is largely due to the effectiveness of iodization in the general food supply. Recent trends among specific populations of children in the United States include adopting food restrictions, such casein-free and gluten-free diets. Although the effect of these types of diets on overall nutrition status and certain micronutrients has been studied in children with autism spectrum disorder, the effect of these limitations on iodine levels in children has not been assessed. We present here 2 cases of iodine deficiency resulting from severe food restriction and associated primary hypothyroidism. In 1 case a classic presentation with a goiter was seen. These children were able to discontinue thyroid hormone treatment once iodine levels were normalized. There were no adverse events or unanticipated outcomes. The occurrence of these cases of iodine deficiency in the United States points to the need for thyroid function testing in children with severe food restrictions, especially those who have limited exposure to dairy, baked goods, and table salt.

Food Group Intakes as Determinants of Iodine Status among US Adult Population

Adequate intake of iodine is essential for proper thyroid function. Although dietary reference intakes for iodine have been established, iodine intake cannot be estimated due to the lack of data on iodine contents in foods. We aimed to determine if food group intakes can predict iodine status assessed by urinary iodine concentration (UIC) from spot urine samples of 5967 US adults in the National Health and Nutrition Examination Survey (NHANES) 2007–2012. From an in-person 24-h dietary recall, all foods consumed were aggregated into 12 main food groups using the individual food code of the US Department of Agriculture (USDA); dairy products, meat/poultry, fish/seaweed, eggs, legumes/nuts/seeds, breads, other grain products, fruits, vegetables, fats/oils, sugars/sweets, and beverages. Chi-square test, Spearman correlation, and multiple linear regression analyses were conducted to investigate the predictability of food group intakes in iodine status assessed by UIC. From the multiple linear regressions, the consumption of dairy products, eggs, and breads, and iodine-containing supplement use were positively associated with UIC, whereas beverage consumption was negatively associated with UIC. Among various food group intakes, dairy product intake was the most important determinant of iodine status in both US men and women. Subpopulation groups with a high risk of iodine deficiency may need nutritional education regarding the consumption of dairy products, eggs, and breads to maintain an adequate iodine status. Efforts toward a better understanding of iodine content in each food and a continued monitoring of iodine status within US adults are both warranted.

Changes in iodine status among US adults, 2001-2012

Urinary iodine concentrations (UICs) in the US have been reported to be stable since 1988-1994, although those in selected subgroups remained low. We aimed to investigate iodine status among adults (≥20 years) by two different criteria of assessing iodine deficiency in population. Utilizing National Health and Nutrition Examination Surveys 2001-2012, we conducted linear logistic regressions adjusting for covariates. The prevalence of <50 μg/L UIC was higher in women than in men; increased from 11.6% (2001-2004) to 13.2% (2009-2012) at the national level and in young adults, non-Hispanic blacks (NHBs) and non-users of iodine-containing supplements (all, p <0.05); the adjusted odds ratios (95%CI) in young adults (1.54 [1.11-2.15], p = 0.0007) and NHBs (1.70 [1.15-2.52], p = 0.0078). Median UICs confirm women and NHBs being in borderline iodine status. Recognizing the critical consequence of iodine deficiency particularly in women and NHBs, regular monitoring of iodine status is important for public health in the US.

Iodine intake as a risk factor for thyroid cancer: a comprehensive review of animal and human studies

Thyroid cancer (TC) is the most common endocrine malignancy and in most countries, incidence rates are increasing. Although differences in population iodine intake are a determinant of benign thyroid disorders, the role of iodine intake in TC remains uncertain. We review the evidence linking iodine intake and TC from animal studies, ecological studies of iodine intake and differentiated and undifferentiated TC, iodine intake and mortality from TC and occult TC at autopsy, as well as the case–control and cohort studies of TC and intake of seafood and milk products. We perform a new meta-analysis of pooled measures of effect from case–control studies of total iodine intake and TC. Finally, we examine the post-Chernobyl studies linking iodine status and risk of TC after radiation exposure. The available evidence suggests iodine deficiency is a risk factor for TC, particularly for follicular TC and possibly, for anaplastic TC. This conclusion is based on: a) consistent data showing an increase in TC (mainly follicular) in iodine deficient animals; b) a plausible mechanism (chronic TSH stimulation induced by iodine deficiency); c) consistent data from before and after studies of iodine prophylaxis showing a decrease in follicular TC and anaplastic TC; d) the indirect association between changes in iodine intake and TC mortality in the decade from 2000 to 2010; e) the autopsy studies of occult TC showing higher microcarcinoma rates with lower iodine intakes; and f) the case control studies suggesting lower risk of TC with higher total iodine intakes.

Dietary ratio of animal:plant protein is associated with 24-h urinary iodine excretion in healthy school children

Adequate dietary iodine intake in children is essential for optimal physical and neurological development. Whether lower dietary animal food and salt intake may adversely affect iodine status is under discussion. We examined the association between dietary animal:plant protein ratio with 24-h urinary iodine excretion (24-h UI, μg/d), and whether this is modified by salt intake. A 24-h UI was measured in 1959 24-h urine samples from 516 6- to 12-year-old participants of the Dortmund Nutritional and Anthropometric Longitudinally Designed Study. Parallel 3 d weighed food records were used to estimate dietary intakes. Protein sources were classified as dairy, animal and plant. A repeated-measures regression model (PROC MIXED) was used to analyse the effect of animal:plant protein ratios on 24-h UI.

ANIMAL

plant protein ratios ranged from 0.5 (95 % CI 0.4, 0.6) to 1.6 (95 % CI 1.4, 1.9) (lowest and highest quartile). After adjustment for total energy intake, main dietary iodine sources (dairy and salt intake), and further covariates, the inter-individual variation in animal:plant protein ratio was significantly associated with variation in 24-h UI. One unit higher animal:plant protein ratio predicted 6 μg/d higher 24-h UI (P= 0.002) in boys and 5 μg/d (P= 0.03) in girls. This relationship was partially mediated by a higher salt intake at higher animal:plant protein ratios. These results suggest that lower consumption of animal protein is associated with a small decline in iodine excretion, partially mediated by decreased salt intake. Because limited salt and increased intake of plant-based foods are part of a preferable healthy food pattern, effective nutrition political strategies will be required in the future to ensure appropriate iodine nutrition in adherent populations.

Iodized salt sales in the United States

Iodized salt has been an important source of dietary iodine, a trace element important for regulating human growth, development, and metabolic functions. This analysis identified iodized table salt sales as a percentage of retail salt sales using Nielsen ScanTrack. We identified 1117 salt products, including 701 salt blends and 416 other salt products, 57 of which were iodized. When weighted by sales volume in ounces or per item, 53% contained iodized salt. These findings may provide a baseline for future monitoring of sales of iodized salt.

Iodine excess as an environmental risk factor for autoimmune thyroid disease

The global effort to prevent iodine deficiency disorders through iodine supplementation, such as universal salt iodization, has achieved impressive progress during the last few decades. However, iodine excess, due to extensive environmental iodine exposure in addition to poor monitoring, is currently a more frequent occurrence than iodine deficiency. Iodine excess is a precipitating environmental factor in the development of autoimmune thyroid disease. Excessive amounts of iodide have been linked to the development of autoimmune thyroiditis in humans and animals, while intrathyroidal depletion of iodine prevents disease in animal strains susceptible to severe thyroiditis. Although the mechanisms by which iodide induces thyroiditis are still unclear, several mechanisms have been proposed: (1) excess iodine induces the production of cytokines and chemokines that can recruit immunocompetent cells to the thyroid; (2) processing excess iodine in thyroid epithelial cells may result in elevated levels of oxidative stress, leading to harmful lipid oxidation and thyroid tissue injuries; and (3) iodine incorporation in the protein chain of thyroglobulin may augment the antigenicity of this molecule. This review will summarize the current knowledge regarding excess iodide as an environmental toxicant and relate it to the development of autoimmune thyroid disease.

Consequences of excess iodine

Iodine is a micronutrient that is essential for the production of thyroid hormones. The primary source of iodine is the diet via consumption of foods that have been fortified with iodine, including salt, dairy products and bread, or that are naturally abundant in the micronutrient, such as seafood. Recommended daily iodine intake is 150 µg in adults who are not pregnant or lactating. Ingestion of iodine or exposure above this threshold is generally well-tolerated. However, in certain susceptible individuals, including those with pre-existing thyroid disease, the elderly, fetuses and neonates, or patients with other risk factors, the risk of developing iodine-induced thyroid dysfunction might be increased. Hypothyroidism or hyperthyroidism as a result of supraphysiologic iodine exposure might be either subclinical or overt, and the source of the excess iodine might not be readily apparent.