Assessment of the Iodine Status of Lactating Women and Infants in Shanghai, China

There is a risk of iodine deficiency among pregnant women in China. However, research on the iodine status of lactating women and infants is scarce. In this study, we aimed to evaluate the iodine status of lactating women and their infants and explore the relationship between breast milk iodine concentrations (BMICs) and urinary iodine concentrations (UICs). In total, 257 lactating women and their infants were recruited from the Shanghai Sixth People’s Hospital East campus between May 2018 and May 2019. The BMIC and UIC were measured by inductively coupled plasma‒mass spectrometry (ICP‒MS). One-day 24-h dietary recall was used to determine the dietary intake of iodine. The mean dietary intake of iodine among the lactating women was 145.1 μg/day, and 97.83% (n = 225) of the lactating women had a dietary iodine intake below 240 μg/day. The median BMIC and UIC of the lactating women was 150.7 μg/L (interquartile range, IQR 102.9, 205.5) and 110.0 μg/L (IQR 65.8, 171.4), respectively, and the median UIC of the infants was 212.7 μg/L (IQR 142.1, 320.6). The BMIC of lactating women who consumed iodized salt was significantly higher than that of lactating women who did not consume iodized salt (p = 0.015). The infants’ UIC values were significantly correlated with the BMIC values (r = 0.597**, p < 0.001). The iodine nutritional status of lactating women and infants in Shanghai was generally sufficient according to the WHO's iodine nutritional status recommendation. The use of iodized salt was related to increasing dietary iodine intake and the BMIC. Improvements in BMICs have positive effects on the nutritional levels of iodine in infants.


Introduction
Iodine is an essential micronutrient for foetal and infant brain development from the beginning of pregnancy to 2 years after birth [1].Iodine deficiency can lead to irreversible body and brain damage during the critical period of infant brain development [2].Approximately 40% ~ 45% of the iodine consumed is transported into breast milk by breast cells through the Na + /I -symporter (NIS), and pendrin and iodine are preferentially provided to infants during lactation.It has been suggested that iodine deficiency in lactating women is caused by urinary iodine excretion levels that are below the breast milk iodine concentration (BMIC) [3].Iodine in breast milk is the only source of iodine intake for exclusively breastfed infants.Therefore, it is necessary to pay special attention to the dietary iodine intake of lactating women to ensure the growth and development of their offspring.For lactating women, the recommended nutrient intake (RNI) of iodine is 240 μg/day according to the Chinese Nutrition Society and 250 μg/day according to the World Health Organization (WHO) [4,5].In the WHOguided assessment of iodine deficiency disorders in lactating women and children < 2 years of age performed in 2007, a median urinary iodine concentration (UIC) ≥ 100 μg/L at the population level was regarded as sufficient [5].Approximately 1.9 billion people worldwide (30% of the world's population) are iodine deficient [5].Europe and Southeast Asia are most seriously affected by iodine deficiency [5].
China is representative of the strong impact of the natural distribution characteristics of iodine deficiency.Longterm iodine deficiency leads to iodine deficiency disorders (IDDs).The primary prevention and treatment measure for IDDs is salt iodization.In 1994, China implemented the Regulations on Edible Salt Iodization to eliminate IDDs.Although China implemented the Universal Salt Iodization (USI) act in 1995, by 2000, IDDs were eradicated at the national level [6].Based on statistics, the prevalence of endemic goitre among children aged 8 ~ 10 years in China decreased from 20.4% in 1995 to 2.4% in 2011 [7].Shanghai, located in China, is the largest coastal city and an iodine-sufficient area [8,9].In 1997In , 1999In , 2011In , 2014 and 2017, the prevalence rates of goitre were 3.07%, 0.40%, 0.08%, 0.86% and 1.90%, respectively, among children aged 8 to 10 years living in Shanghai, with median thyroid volumes of 2.9, 1.2, 1.0, 1.8 and 2.8 mL, respectively [10].The iodized salt coverage rate of Shanghai residents increased over 90% from 2002 to 2012; however, since 2010, the iodized salt coverage rate among Shanghai residents has decreased yearly, and the noniodized salt coverage rate of residents reached 20.91% in 2015 [10,11].A previous study showed that the median UIC of lactating women in Shanghai was 131.1 μg/L in 2009 [9].In 2011, the national monitoring of IDDs showed that the median UIC of lactating women in Shanghai was 123.1 μg/L [11], and the iodine nutritional status of these women was generally sufficient.
In our previous study, we found that the daily consumption of regular iodized salt by 349 pregnant women was only 78.0% and that pregnant women in Shanghai had mild iodine deficiency [12].Thus, determining whether lactating women and their infants in Shanghai are also iodine deficient is worth further investigation.Some studies have investigated the BMIC and UIC of lactating women and infants.However, it is unclear whether the current iodine status in Shanghai is sufficient after the decrease in the iodized salt coverage rate.Therefore, the purpose of this study was to evaluate whether the iodine nutritional status of lactating women and their infants in Shanghai could be used as an adequate iodine status index for lactating women.This work will provide new insights into the prevention of potential harm caused by iodine malnutrition and the promotion of appropriate countermeasures in a timely manner.

Study Design
This study was an embedded cross-sectional analysis.The recruited cohort included 257 lactating women (18 − 40 years old) who were examined 42 days postpartum at the Shanghai Sixth People's Hospital East campus and their infants between May 2018 and May 2019.The inclusion criteria were as follows: women who had generally healthy and singleton pregnancies with no history of clinical or subclinical thyroid disease or other autoimmune or endocrine diseases; those who resided in the local area for more than three years and had no immediate plans to leave the area; and those who were able to cooperate in the follow-up process.Women who were currently using drugs or lotions containing iodine or had a history of smoking or alcoholism were excluded.Of the 257 participants, three women taking antithyroid drugs during pregnancy were excluded.

Data and Sample Collection
During the 42-day postpartum follow-up, approximately 10 mL of breast milk and spot urine samples were collected from the lactating women, and spot urine samples (10 mL) were collected from the babies.Before the urine sample was collected, the infant's urethral orifice was cleaned; then, the urine was collected in a disposable urine cup, and 10 ml was kept in the disposable catheter.Breast milk and urine samples were stored separately at -80 °C and -20 °C, respectively, before use.Data on maternal age, height, weight (42 days after delivery, intrapartum and pregnancy), education level, gestational age and delivery mode were collected by means of questionnaires.Infant data regarding age, sex, birth, 42-day weight, type of feeding and brand of infant formula were also collected by means of questionnaires.Information on food consumption and dietary supplements on the day before urine sample collection was collected via 24-h dietary recall.Trained investigators conducted faceto-face interviews about the type and amount of food the participants ate.Seventy-two hours after delivery, blood samples were collected from the heel of the newborns, and dried blood spots (DBSs) were prepared.

Ethics
The Ethics Committee of Shanghai Sixth People's Hospital East Campus approved the study (No. 2016-003).The lactating women provided written consent and received written information about the study.

Laboratory Analysis
UICs and BMICs were analysed by inductively coupled plasma-mass spectrometry (ICP-MS) (Agilent 7700X; ICAP-Q, Thermo Fisher Scientific).Thyroid-stimulating hormone (TSH) levels in neonatal heel blood were measured by time-resolved fluoroimmunoassay (TRFIA).The Perki-nElmer genetic screening processor (GSPTM) instrument for TSH analysis is compatible for use with whole blood collected directly from a patient onto filter paper or whole blood collected into EDTA or lithium heparin tubes and then spotted onto filter paper (Children's Hospital of Shanghai).Maternal TSH levels were measured by cobas8000 (e602) electrochemiluminescence.Roche, Germany, supplied the Elecsys TSH test kits.

Evaluation Criteria of Dietary Iodine Intake
According to the WHO, the recommended cooking loss rate of iodized salt is 20% [13], and the concentration of iodized salt in Shanghai is 30 mg/kg [4].Dietary iodine intake mainly comes from food, drinking water and iodized salt and was calculated according to the following formula: Dietary iodine intake (μg/d) = ∑(intake of various foods × iodine content of various foods) + (drinking water + cooking food water) × iodine content of water + salt intake × iodine content of salt × (1-cooking loss rate).
Dietary iodine intake was estimated using the Nutrition Star program (NS-Pro) premium version (ShangHai ZhenDing Health Science Technology Co., Ltd., Shanghai, 201,306, China).The estimated average requirement (EAR) of dietary iodine for lactating women is 170 μg/day, the RNI is 240 μg/day and the tolerable upper intake level (UL) is 600 μg/day [4].

Definitions
The adequate TSH level in this study ranged from 0.1 to 4.0 mIU/L [14,15].A TSH upper reference limit (cut-off) of 4.0 mIU/L has been proposed.In addition, the thyroid function of the population was diagnosed according to guidelines on the diagnosis and management of thyroid diseases during pregnancy and the postpartum period [16].
The determination of TSH levels in neonatal heel blood is considered the most sensitive and reliable index to evaluate iodine nutrition levels and thyroid function.In 2007, the WHO/UNICEF/ICCIDD jointly recommended 5.00 mIU/L as the cut-off point for neonatal TSH and that the proportion of neonatal heel blood TSH levels > 5.00 mIU/L be less than 3% as the judgement standard for normal iodine nutritional status [5].The standard range of neonatal TSH is 0.00 ~ 9.00 mIU/L.
In 2007, the WHO put forwards an evaluation standard of iodine status based on the median UIC at a population level.The suitable standard iodine nutrition level for breastfeeding women and infants younger than 2 years old is a median UIC > 100 μg/L [5].
Full-term infants need 15 μg iodine per kg body weight per day to maintain a positive iodine balance, which is equivalent to a BMIC of 100 ~ 200 μg/L [17,18].

Statistical Analysis
All analyses were performed using SPSS (SPSS Inc., Chicago, IL, United States, version 25), and p < 0.05 indicated a significant difference.Data are presented as the mean values and standard deviations (SDs), numbers and percentages, or medians and interquartile ranges (IQRs) as appropriate.Spearman's correlation analysis was conducted examined the correlations between the BMIC and UIC, the maternal BMIC and UIC, and the maternal and infantile UIC.The Mann-Whitney U test was used to compare the differences between two data groups.

Characteristics of Participants
Data were collected from 254 lactating women and their babies (Table 1).The mean (± SD) ages of the women at delivery and their infants (when tested) were 30.1 ± 4.1 years and 46.4 ± 5.3 days, respectively.The mean body weights of the women before pregnancy and before delivery were 55.8 ± 7.9 kg and 69.9 ± 8.5 kg, respectively.The average weight gain during pregnancy was 14.1 ± 4.9 kg.The infants gained 1.9 ± 0.54 kg over the 42 days.
Among the mothers, approximately 63.4% had a tertiary education or above, and 59.1% underwent spontaneous delivery.In this survey, 61.4% of the infants were exclusively breastfed, and 6.3% were fed pure milk powder.
The mean scores for the 42-day weights of the breastfed infants, mixed-fed infants and milk powder-fed infants were 5.3 ± 0.7 kg, 5.2 ± 1.1 kg and 5.3 ± 0.7 kg, respectively.This result suggested no significant difference in the 42-day weight among infants with different feeding patterns (P = 0.840) (Kruskal-Wallis test).

The UIC and BMIC of Lactating Women and the UIC of Infants
The median UICs were 110.0 μg/L (IQR 65.8, 171.4) for lactating women and 212.7 μg/L (IQR 142.1, 320.6) for infants.However, 44.5% of the women and 10.1% of the infants had UIC values lower than the suggested adequate cut-off level of 100 μg/L.The median BMIC was 150.7 μg/L (IQR 102.9, 205.5), and 23.4% of the women had a BMIC below the suggested adequate cut-off level of 100 μg/L (Table 2, Fig. 1) [17].
The UIC of the infants was significantly higher than that of the lactating mothers when the UIC of the exclusively breastfed infants and the UIC and BMIC of the lactating mothers were compared (P < 0.001).The UIC of the infants was significantly higher than that of lactating mothers (P < 0.001).The UIC of the infants was the highest, followed by the BMIC and the UIC of lactating mothers.
In this study, we analysed the correlations among the UICs of exclusively breastfed infants and the UICs and BMICs of lactating mothers.The infants' UIC values were significantly correlated with the BMIC values (r = 0.597 ** , p < 0.001).The infants' UIC values were weakly correlated with the maternal UIC values (r = 0.182 * , p = 0.01).The maternal UIC values were significantly correlated with the BMIC values (r = 0.373 ** , p < 0.001) (Fig. 2).

Dietary Iodine Intake of Lactating Women
In the dietary study, a total of 230 lactating women completed 24-h dietary recalls, and their mean dietary iodine intake was 145.1 μg/day, which was lower than the 240 μg/ day recommended by the Chinese Nutrition Society (Table 2) [4].The dietary iodine intake of 74.35% (n = 171) of the lactating women was lower than 170 μg/day, 23.48% (n = 54) had an intake within the range of 170-240 μg/ day, and 2.17% (n = 5) had an intake within the range of 240-400 μg/day.
The correlation between the dietary iodine intake and the UICs and BMICs of lactating women and the UIC levels of exclusively breastfed infants was investigated.The dietary iodine intake of the lactating women was weakly positively correlated with their BMICs (r = 0.216 * , p = 0.014) and their infants' UICs (r = 0.205 * , p = 0.024).However, the BMIC did not correlate with the UIC.

Effect of the Third-Trimester TSH Level on the TSH Level in the Heel Blood of Offspring
The median TSH level of the neonates was 1.5 mIU/L (IQR 1.0, 2.3), and the levels were within normal limits in all cases.The proportion of TSH levels > 5 mIU/L in the heel blood of newborns was 0.5%, which was below 3%, so the iodine nutritional status of the newborns was considered normal [5].The TSH levels of the lactating mothers in the third trimester of pregnancy were weakly correlated with the TSH levels in the heel blood of their newborns (r = 0.179 * ) (Table 3).Additionally, to explore the relationship between maternal third-trimester TSH levels and neonatal TSH levels, 233 pairs of mothers and infants were divided into two groups-those with TSH levels less than 4 mIU/L and those with TSH levels greater than 4 mIU/L-based on the  maternal third-trimester TSH levels (Fig. 3).The median TSH level of infants in the first group (TSH < 4 mIU/L) was 1.4 mIU/L (IQR 1.0, 2.1), and the median TSH level of infants in the second group (TSH > 4 mIU/L) was 2.2 mIU/L (IQR 1.4, 3.3).Interestingly, the TSH level in the heel blood of infants in the group with TSH levels < 4 mIU/L in late pregnancy was significantly lower than that in the group with TSH levels > 4 mIU/L in late pregnancy (p = 0.031) (Table 4).

Discussion
In this study, the BMIC values, UIC values and dietary iodine intake of lactating women and the UIC values for infants in Shanghai, China, were reported.Based on the 2007 WHO criteria, these results indicate that the iodine nutritional status of lactating women and infants in Shanghai is sufficient.
A previous study from Spain reported that the median UIC of lactating women was 142 μg/L [19]; however, according to a cross-sectional survey conducted in 2012 in Shanghai, China, the median UIC of 380 lactating women was 131.1 μg/L, and 36.8% of the participants had iodine deficiency, with levels < 100 μg/L.
A previous study on iodine balance in term infants reported that the iodine balance was positive only when the dietary iodine intake was 15 μg/kg per day, which was equivalent to a BMIC of 100 ~ 200 μg/L [17].In our study, the median BMIC was 150.7 μg/L (IQR 102.9, 205.5), and 23.4% of the women had a BMIC below the suggested adequate cut-off level.Western Australia, which has always been regarded as an iodine-rich region, has a median BMIC of 167 μg/L (IQR 99, 248), with 26% of lactating women having a BMIC below 100 μg/L [20,21].Korea is an iodine-replete country, with median BMICs reported to be 2529 (IQR 355, 8484), 1153 (IQR 198,3791) and 822 (IQR 236, 1836) μg/L in the first, third and sixth weeks after delivery, respectively [22].In a study in Nepal, the median BMIC of 291 collected breast milk samples was 250 μg/L (IQR 130, 370), and approximately 18% of the patients had BMICs < 100 μg/L [23].Furthermore, several studies suggested that there was still a risk of low iodine intake among some mothers who adopted breastfeeding in areas with adequate iodine and that additional iodine supplementation was needed [20,24].The iodine content in breast milk is 2 ~ 3 times that in plasma, and thus, the consumption of iodine is accelerated due to the higher level of iodine obtained from breast milk.Breastfed infants rely on iodine in breast milk to meet their needs.The lack of iodine intake by mothers   decreases the iodine content in their breast milk, thus affecting the iodine nutrition of their infants [25].
In this study, a weak correlation was found between the maternal UIC and infantile UIC, in contrast to other studies in which a significant positive association was found between maternal and infantile urinary iodine levels [26,27].In addition, the infants' UIC values were significantly correlated with their mothers' BMIC values (r = 0.597 ** , p < 0.001) and weakly associated with their mothers' UIC values (r = 0.182 * , p = 0.01).Compared with the maternal urinary iodine level, the BMIC is a more sensitive and stable index in the evaluation of infant iodine nutritional status.The significant positive correlation between the BMIC and UIC in lactating women and their infants indicates that the UIC changes in parallel to the BMIC.A previous study showed a positive correlation between the BMIC and UIC of lactating women, which increased when an adjustment was made for creatinine, similar to the results reported in our study [27].Moreover, a significant positive correlation between the maternal BMIC and infantile UIC was observed by Wang et al. (2018) [28], which might indicate that improving the BMIC has a positive effect on the iodine nutritional level of infants.
The EAR is an essential index used to assess the iodine intake of groups and individuals.The iodine intake of groups is typically lower than the EAR, indicating that the proportion of people with iodine deficiency risk is up to 50%.The individual iodine intake is lower than the EAR, and the risk of iodine deficiency can reach 50%, indicating the need for improvement.When the intake increases to the RNI, the probability of insufficient iodine intake for random individuals becomes very small, and the likelihood of iodine deficiency decreases to less than 3%.When the average intake of a population reaches the RNI, only 2% ~ 3% of the individuals may be deficient; that is, the vast majority of individuals are not at risk of iodine deficiency.For lactating women, the RNI of iodine is 240 μg/day according to the Chinese Nutrition Society and 250 μg/day according to the WHO [4,5].However, in our study on lactating women, the mean dietary iodine intake in the Lingang area of Shanghai was 145.1 μg/day; the dietary iodine intake was below 240 μg/day for 97.83% (n = 225) of the lactating women, which is far lower than the suggested adequate cut-off, indicating that some lactating women might be at risk for iodine deficiency.
According to the 24-h dietary recall and dietary record evaluation, the current nutritional iodine intake of lactating women in Shanghai is insufficient.In 2017, a Spanish study showed that the median nutritional iodine intake of lactating women was 125 μg/day (IQR 104, 154) [19].The iodized salt consumption rate of Chinese residents has continued to decline, and the iodized salt coverage rate in Shanghai has decreased to 16.91% from 2010 to 2015 [11].
In this survey, 86.1% of the lactating women consumed iodized salt, and the dietary iodine intake (p < 0.001) and BMIC (p = 0.015) were significantly higher for lactating women who consumed iodized salt than for those who consumed noniodized salt.The use of iodized salt was related to increasing dietary iodine intake and the BMIC.A recent study suggested that using either iodine-containing supplements or iodized salt had similar positive effects on the median BMIC and that both increased the iodine content of breast milk [20].We also observed a weakly positive correlation between dietary iodine intake and the BMIC in lactating women (r = 0.216 * , p = 0.014).
Therefore, dietary iodine intake is a virtual indicator for improving breast milk iodine levels and infant iodine nutrition.Iodine is metabolized in the human body every day.When iodine intake stops, the iodine stored in the body is sufficient only for 2 ~ 3 months.Therefore, it is necessary to ensure sufficient dietary iodine intake every day [29].Although the iodine deficiency control program has been effectively maintained through salt iodization in China, the insufficient dietary iodine intake of lactating women is still a problem [29,30].Some reasons for this problem in lactating women may be as follows.First, mothers may be worried that a high salt content in their breast milk will affect the health of their infants in the early stage of breastfeeding; hence, they reduce their salt intake.Second, although Shanghai is a coastal city, the frequency and consumption of iodine-rich foods such as kelp and laver in local areas are low, and most of the iodine in the residents' diets comes from iodized salt in foods.Finally, most people do not know enough about the harm of iodine deficiency, and the awareness of prevention and control is gradually weakened.An increasing amount of evidence has revealed that lactating women should continue to increase their intake of dietary iodine during pregnancy.Iodized salt and iodine-rich foods such as kelp and laver can be considered as iodine supplements [31], as well as iodinecontaining nutrient supplements.
In the analysis of the influence of maternal third-trimester TSH levels on infant TSH levels, it was shown that TSH levels in the heel blood of infants in the group with TSH levels < 4 mIU/L in late pregnancy was significantly lower than that in the group with TSH levels > 4 mIU/L (p = 0.031).These results suggested that abnormal maternal TSH levels in the third trimester of pregnancy in this region increased the risk of abnormal TSH levels in offspring.According to Korevaar T [32], there is a strong correlation between maternal TSH levels and newborn TSH levels; newborn TSH levels can be predicted by maternal TSH levels, and the effect is good.In the analysis of TSH levels and the incidence of hypothyroidism in neonates in iodine-rich and iodine-deficient areas, it was found that TSH levels and the incidence of hypothyroidism were higher in iodine-deficient areas.Ji et al. studied the relationship between the TSH levels of pregnant women with different iodine nutritional statuses and their newborns and found that when the mothers were iodine deficient during pregnancy, the rate of abnormal TSH levels among their newborns was 3.28%, which was significantly higher than that among infants born to iodinesufficient women (P < 0.05) [33].These results indicate that the iodine nutritional status of newborns is closely related to the iodine nutritional status of pregnant women.The key role of adequate maternal iodine intake during pregnancy is important in reducing the incidence of TSH abnormalities in offspring.
This study showed that the iodine nutritional levels of lactating women and their infants in Shanghai is generally sufficient and that relevant data have been obtained for the Shanghai district.Our research has the following limitations that need to be further addressed.First, we collected only single-spot urine samples from each lactating woman and infant, which may not accurately reflect the iodine nutritional status of individuals.The ideal method is to use 24-h urine iodine tests, but it is not easy to collect such samples from lactating women.In future studies, we intend to overcome this problem and collect 24-h urine samples to measure urinary iodine levels.Second, in this study, we evaluated the iodine nutritional levels of only lactating women and infants in the Lingang district of Shanghai, so the results do not represent the entire city.Therefore, more attention should be given to generalizing the results of this study to other regions and populations.The sampling area and representativeness of the results should be increased.
In this study, abnormal maternal TSH levels in late pregnancy increased the risk of increased TSH in the heel blood of offspring.We will follow up on the investigation of pregnant women in the Shanghai Sixth People's Hospital East Campus to explore the cause and authenticity of this finding.We will evaluate pregnant women's iodine nutritional levels, measure their thyroid hormone levels, and explore the changing trends and relationships of these parameters during the first, second, and third trimesters and the postpartum period.

Conclusion
Our research provides evidence demonstrating that the iodine nutritional status of lactating women and infants in Shanghai is generally sufficient at the population level, but a large proportion of lactating women still have low daily dietary iodine intake.Therefore, we should strengthen prevention and monitoring for IDDs in particular populations.It is essential to advocate for scientific iodine supplementation and ensure the appropriate dietary iodine intake of lactating women to avoid the severe harm caused by IDDs.

Fig. 1
Fig. 1 Percentage distributions of the urinary iodine concentration (UIC) (a) and breast milk iodine concentration (BMIC) (b) in lactating women and the UIC (μg/L) (c) in infants

Fig. 3
Fig. 3 Correlation between the third-trimester TSH level of the lactating mothers and the TSH level in the heel blood of their newborns (r = 0.179*)

Table 1
Basic characteristics of infants and their mothers * Data are means ± SD or n (%)

Table 2
Iodine status in infants and their mothers

Table 3
UIC, BMIC and dietary iodine intake of mothers and UIC of their infants by type of salt intake UIC, urinary iodine concentration; BMIC, breast milk iodine concentration; IQR, interquartile range * Mann-Whitney U test

Table 4
The relationship between maternal TSH in the third trimester of pregnancy and neonatal TSH