Urinary iodine excretion: A valuable tool for monitoring salt iodisation programme

INTRODUCTION

Globally, goitre and iodine-deficiency disorders (IDDs) have been recognized as major public health problems.¹ Iodisation of salt has been the time-tested intervention to prevent IDDs.¹ The Kangra valley trials demonstrated the effectiveness of salt iodisation for goitre control in India.² India initiated the Goitre Control Programme in 1962 to provide iodised salt (IS) to the population in the sub-Himalayan region with high goitre prevalence, but the progress was tardy.³ Surveys carried out by the Directorate General of Health Services showed that every state had pockets of IDD; Central Council of Health in 1984 decided to ensure universal iodisation of salt; the programme commenced in 1986 and covered the entire country by 1992;⁴ the Government of India issued a notification proposing the ban on sale of non-IS for human consumption in May 2005 and on 17th May 2006, the sale of non-IS was banned under the Food Adulteration Act.⁵ Data from National Family Health Surveys 4 and 5 indicate that in the past 5 years, over 90% of the population accessed IS.^6,7

The WHO^8–11 had recommended that urinary iodine (UI) estimation is a valuable tool to monitor the progress under the universal salt iodisation programme. India had been monitoring progress in the iodisation programme by assessing household access to IS, testing the adequacy of iodisation of salt at the household level, and iodine excretion in spot samples of urine, mostly in school children. UI surveys conducted in India a decade ago had shown that median UI levels were above 100 µg/L in over 80% of the districts, suggesting that the iodisation levels were adequate.^12,13 However, UI was over 200 µg/L in 10% of the districts; occasionally, median UI levels above 300 µg/L were reported.^12,13 Delhi was one of the states in which high median UI had been reported. High median UI is a matter of concern; excess iodine intake has been shown to be associated with hyperthyroidism and autoimmune thyroiditis.^14–20

Most of the available global and Indian data on UI excretion had been based on spot urine collection in school-age children,^12,13 perhaps because it was convenient to combine UI with goitre surveys. Women of reproductive age (WRA) are vulnerable to IDD; IDD in pregnancy has adverse consequences on the mother-child dyad.^21–25 There is a need to document the UI status of women. IS is purchased and used by families. So far, there had been no studies reporting the adequacy of iodine intake using UI in available members of the families. This option needs to be explored because it may help in overcoming the problems of wide variation in UI in spot sample collection and also provide information on UI in all age and sex groups.

The prevalence of anaemia in India is high. In an attempt to improve iron intake and reduce the prevalence of anaemia in children, iron-fortified IS (DFS) is being used for preparing hot cooked meals in Integrated Child Development Services (ICDS) and Mid-day Meal (MDM) programmes. In some states, DFS is available through Public Distribution System (PDS) or the open market. There are few research publications on UI in Indians using DFS.

We estimated UI from two community-based studies undertaken between 2017 and 2021 to document UI in available and consenting members of the families and the impact of study provided salt formulations on the UI in both studies.

METHODS

Our institution had undertaken two community-based open randomized studies comparing the impact of the IS and DFS between 2017 and 2021: Study A was in families of Anganwadi helpers and workers and Study B was in urban low-middle-income group families. Both studies were carried out in South Delhi blocks where our institution had been working for over 15 years and had built up a good rapport with the community, ICDS and health functionaries.

Before the initiation of these studies, the families had regularly purchased powdered IS in polythene packets from the market and used it for cooking. All families had used IS for more than 1 year; some had used IS for a decade or longer. The family members habitually consumed food cooked at home; school children ate a midday meal in schools and men ate mostly home-cooked food but occasionally ate outside food.

All families were provided with the study information sheet about the study on UI excretion (UIE). The details of the study were discussed with the family members, and consent was obtained from those who were willing to provide a urine sample. They were given a container for collecting an early morning sample of urine. The study team visited their homes in the forenoon and collected urine samples (Table 1). In both these studies, urine samples were collected from a sub-sample of the participants before initiation of the randomized studies (Study A, 2017 and Study B, 2019). Samples were stored in a deep freeze until analysis. The UI data from these samples were analysed to assess the UI in families and individuals in these families who had purchased IS from the market.

Both the salts for study A were procured from the same manufacturer. For study B, IS and two formulations of DFS (ferrous sulphate and ferrous fumarate) were procured from a manufacturer who produced all three types of salt.

Families in Study A were provided with either IS or DFS every month according to random allocation and were followed up to assess use of the salt provided. Urine samples were collected from a sub-sample (10%) of families who were using IS/DFS for a year or longer (2019) (Table 1).

Families in study B were provided with IS or one of the two formulations of DFS approved by the Food Safety and Standards Authority of India (FSSAI). They were followed up to assess use of the salt provided; 1-year use of IS or DFS was completed in March 2020. The study was interrupted for 6 months due to the Covid-19-related lockdown. During lockdown, all the families purchased and used packaged IS. The randomized study was restarted in October 2020. Urine samples were collected in a sub-sample of users in 2021 after they had continuously used DFS/IS for 1 year or more (Table 1). All urine samples were coded and stored in a deep freeze at –20°C.

The number of families, the total number of persons in the family and the number of WRA (15–49 years) are given in Table 1.

UI estimation was done in coded samples using the microplate method based on the Sandell–Kolthoff reaction.²⁵

The method consists of urine digestion with ammonium per sulphate to eliminate interfering substances and to release iodide, which is followed by estimation of iodide by the Sandell–Kolthoff reaction. Briefly, iodide catalyses the reduction of the yellow-coloured ceric (IV) ions by arsenic to colourless ceric (III) ions and elemental iodine, allowing the spectrophotometric detection of the colour disappearance. An in-house pooled sample was prepared and iodine estimation was carried out 20 times, and the mean and SD were established. The pooled sample was run with each batch of samples as a quality control measure.

RESULTS

Sociodemographic profile

Nearly two-thirds of the study families were nuclear families. Over three-fourths of the fathers and over half of the mothers had a secondary school education. Nearly two-thirds of the fathers were working in semi-skilled jobs or were self-employed. Over 80% of the families lived in brick-and-mortar buildings with two or three small rooms; about a third owned their house. Piped water supply at home was available in three-fourths of the houses. Over 80% had access to flush toilets either in their own home or shared with other households. Almost all used liquefied petroleum gas and stainless-steel utensils for cooking. Over 95% of households owned a colour TV, which was their main source of entertainment. The families stated that they earned enough to meet their families’ food, education and healthcare needs; they stayed in unhygienic localities and overcrowded tenement houses because of urban housing constraints.

The mean and median UI before enrolment (when they were buying IS from the market) in both Study A (2017) and Study B (2019) were over 230 µg/L. In study A, the mean and median UI levels at follow-up (2019) in both DFS and IS groups were significantly lower compared to the UI in 2017. These differences were statistically significant. In study B, mean and median UI levels at follow-up (2021) were significantly lower both in DFS and IS groups compared to the UI in 2019. These differences were statistically significant. The differences in the mean or median UI values in the DFS and IS group, either in study A or study B, were small and not significant (Table 2).

TABLE 2. Urinary iodine excretion (µg/L) in iodised salt (IS) and iron fortified iodised salt (DFS) users

	IS purchased from market and used for ≥ one year
	Study A prior to enrolment	Study B prior to enrolment
	2017	2019
Mean (SD)	233.4 (87.17) [192]	232.8 (89.38) [241]
Median	234	235
	IS or DFS given by research team and used for one year
	Study A use of salt for ≥1 year	Study B use of salt for ≥1 year
IS	2019	2021
Mean (SD)	202.1 (±98.18) [125]	159.7 (92.88) [100]
Median	183	139
DFS	2019	2021
Mean (SD)	194.3 (102.17) [172]	145 (79.19) [208]
Median	182	125

Numbers in square parenthesis indicate number of persons in whom urinary iodine was estimated

Student t test two-tailed

Study A 2017 vs 2019: IS p value 0.003; Study B 2019 vs 2021 IS and DFS p<0.0001

Study A IS vs DFS Not significant (NS);Study B IS vs DFS NS

As there were no differences in the mean or median UI between samples collected in 2017 and samples collected in 2019 among families who had used IS for 1 year or longer, the data were pooled to assess the median UI in boys, girls, women and men from these families. The median UI level of all the family members was 235 µg/L. Median UI levels were >200 µg/L in girls, boys, women and men (Table 3). The differences in the mean UI between boys and women, boys and men and boys and girls were statistically significant. These differences might be attributable at least in part to the variation in UI in early morning urine collection.

TABLE 3. Urinary iodine (UI) µg/L in family members

	Mean (SD)		n	Median	Minimum	Maximum
Boys		268.4 (90.16)	6 8	277.5	50	450
Girls		238.1 (88.00)	7 7	242.0	70	470
Men		223.5 (88.71)	132	223.0	40	425
Women		223.2 (83.86)	156	225.0	49	430
Total		233.0 (88.30)	433	235.0	40	470

Numbers in parenthesis indicate number of persons in whom UI excretion was done Student t test two-tailed: boys vs women 0.0004; boys vs men 0.001; boys vs girls 0.04

Data on UI estimated from urine samples collected in 2017 and samples collected in 2019 were pooled to assess the differences, if any, in the frequency distribution of UI in school-age boys, girls, men and women. Frequency distribution of the UI in girls, boys, men and women is shown in Fig. 1. Less than 2% of the girls and boys and <10% of men and women had UI below 100 μg/L. In none of the families, all persons in the family had UI levels below 100 μg/L, suggesting that inadequate iodine content in salt was rare. These data suggest that these families were using IS adequately.

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FIG 1.

Frequency distribution of urinary iodine levels in family members (µg/L)

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Intra-family differences in UI were assessed by cross-tabulation of UI values in WRA and other members of the family (Fig. 2). Data show that when UI in the WRA was <100 µg/L, UI was <100 µg/L in only 4.5% of the other members of the family. These data suggest that the low UI in WRA might have been due to the known variations in UI in spot urine samples and not the lack of iodine in the IS being used by the family.

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FIG 2.

Intra-family differences in urinary iodine between women of reproductive age and other family members (µg/L)

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When UI in WRA was >300 µg/L, over one-third of the other persons in the family had UI of >300 µg/L and another third had UI between 200 µg/L and 299 µg/L, suggesting the possibility that IS consumed by the family had high iodine content.

The mean, median and minimum and maximum levels of UI in the urine samples obtained from persons who had been provided with IS or DFS (study A) and IS or one of the two formulations of DFS (study B) is given in Table 4.

TABLE 4. Urinary iodine (UI) µg/L in iodised salt (IS) and iron fortified iodised salt (DFS) users

		n Mean (SD)	Median	Minimum	Maximum
Study A	IS	125 202.1 (98.18)	183	30	450
	DFS	172 194.3 (102.17)	182	30	480
Study B	IS	100 159.7 (92.88)	139	38	430
	DFS	208 145 (79.19)	125	40	480

Student t test two-tailed Study A and study B; IS vs DFS not significant Study A IS vs study B IS p value <0.001 Study A DFS vs study B DFS p value <0.0001

There were substantial differences in the median UI both in the IS and DFS groups between Study A, completed in 2019 and Study B, completed in 2021. These differences were statistically significant. Both in Study A and Study B, the differences between IS and DFS were not significant.

The frequency distribution of UI in IS and DFS users in Study A and Study B is given in Fig. 3a and b. There were substantial differences in the frequency distribution of UI between Study A and Study B both for IS and DFS. In study A, in about 1/6th of the samples, UI was <100 µg/L; in study B, nearly a third of the samples had UI <100 µg/L. UI was over 200 µg/L in 40%–50% of IS/DFS users in 2019, but only in 20%–30% of the IS/DFS users in 2021. However, there were no significant differences in the distribution of UI levels between DFS and IS users in either Study A or Study B.

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FIG 3.

Urinary iodine (UI) in iodised salt (IS) and iron-fortified IS (DFS) users (µg/L) (a) study A, (b) study B

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DISCUSSION

India had been monitoring the progress towards universal salt iodisation by surveying household access to IS, testing the quality of IS at household levels, goitre prevalence and UIE, mostly in school children. UI is an objective, relatively inexpensive and non-invasive method for estimating the current iodine intake of the population groups and is globally used for monitoring the universal salt iodisation programme.^8–11 Initially, the focus was on assessing the prevalence of iodine deficiency as assessed by low UI, but currently, the detection of both low and high median UI levels is important. In the past three decades, there have been numerous publications reporting high levels of UI and an increase in hypothyroidism and thyrotoxicosis in populations using IS.^14–20 Epidemiological studies have suggested that the increase in hyperthyroidism might be due to excess iodine intake.²⁰ Taking cognisance of these reports, the WHO had provided cut-off values for UI that is indicative of a more than adequate or excessive iodine intake.^8–11 The WHO recommended that UI above 300 µg/L should be considered excessive and might be associated with adverse health consequences such as iodine-induced hyperthyroidism and autoimmune thyroid diseases. Furthermore, in populations who had earlier suffered from iodine deficiency, median UI above 200 µg/L in adults may be associated with the risk of iodine-induced hyperthyroidism.^8–11

In India during the 1980s and 90s, most of the IS was packed in jute bags and transported in open rail rakes. Salt bags were often stored in the open in retail shops. Under these conditions, iodine in the salt may leach/evaporate. To ensure iodine content at the household level of 15 ppm, Prevention of Food Adulteration and later FSSAI standards mandated that iodine content should be at least 30 ppm at the manufacturing level but no upper limit for iodine fortification was set.²⁶ UI surveys conducted in India a decade ago had shown that median UI levels were above 100 µg/L in over 80% of the districts, suggesting that the iodisation levels were adequate. However, in about 10% of the districts, UI was over 200 µg/L and some districts had median UI levels above 300 µg/L.¹³ Delhi was one of the states with districts in which high median UI had been reported. In the past decade, most of the IS is sold in polythene packets and the loss of iodine in transport and storage has been minimised. As no upper limit was defined at the manufacturing level, the amount of added iodine might be higher than 30 ppm. In some older manufacturing units, the mixing units might not have been upgraded to ensure thorough mixing. The high median UI reported in some districts in recent years might be due to a combination of these factors. FSSAI had organized meetings to build awareness about the changing scenario of packaging and transport of IS, the potential adverse health consequences of salt fortification with iodine content above 30 ppm and the need for the manufacturers to ensure that the iodine content at the manufacturing level does not exceed 30 ppm.²⁶

Data from our study provides information on UI between 2017 and 2021 in a well-defined urban low-middle-income family residing in South Delhi. There was no difference in the UI between 2017 and 2019 when families purchased and used IS from the market. UI in members of the families who were participating in the randomized study on the impact of DFS showed that there was a significant reduction in UI between initial (when they bought the IS from the market) and follow-up (when they were provided with IS or DFS by the research team for 1 year or longer). Samples for UI estimation were collected after the use of DFS or IS for 1 year or more, both in Study A (2019) and Study B (2021). There was a significant reduction in the median UI levels both in IS and DFS groups between 2019 and 2021. However, there were no differences in median UI levels between IS users and DFS users at any time point. The reduction in the median UI levels over time might partly be due to the efforts of FSSAI and other stakeholders to sensitise salt manufacturers that iodine content should not exceed 30 ppm at the manufacturing level. These data indicate that the UI is a valuable tool for monitoring the ongoing changes in the universal salt iodisation programme and ensuring that iodine fortification is kept at optimal levels. FSSAI had organized expert panel meetings to discuss the reported high median UI and associated risk of thyroid dysfunction. The Expert Panel recommended that iodine content at the manufacturing level should be between 20 and 30 ppm; FSSAI had notified this modification.²⁶ Monitoring UIE in the future will help in assessing compliance with the notified standards. Ensuring that the UI levels remain within the normal range will reduce the risk of thyroid dysfunction associated with low as well as high iodine consumption.

Most of the global and Indian data on UI pertain to school children. This might be mainly due to the fact that goitre surveys were done in school-age children, and at that time, spot urine samples were collected. Salt is provided to the families and not individuals. No studies report the UI status taking the family as the unit. Analysis of data from the present study indicates that, in both Study A and Study B, at the initial and at the time of follow-up, in none of the families was the UI level below 100 µg/L in all persons in the family. These data suggest that even when median iodine levels are lower in 2019 and 2021, none of the families used salt with inadequate iodine. It is important to assess the UI in WRA because adverse health consequences of IDDs affect the mother-child dyad.^23,25 In our study, urine samples were collected from all available willing members of the family; therefore, it was possible to estimate UI levels in boys, girls, men and WRA. It was reassuring to note that median UI levels in WRA were within the WHO norms. Most national health and nutrition surveys use the family as the unit of analysis. It might be relatively easy for the survey team to collect spot urine samples from available and consenting members of the family. If this approach is used, it might be possible to get a good estimate of the families using non-IS or inadequately iodised salt and also UI levels in different age, sex and physiological groups.

Data from the study show that when UI in the WRA was <100, UI was <100 µg/L in only 4.5% of the other members of the family. These data suggest that the low UI in WRA might have been due to the known variations in UI in spot urine samples and not the lack of iodine in the IS being used by the family. When UI in WRA was >300 µg/L, over one-third of the other persons in the family had UI of >300 and another third had UI between 200 and 299 µg/L, suggesting the possibility that the IS consumed by the family had high iodine content. These findings suggest that estimating UI in spot urine samples collected from all available members of the family may be the most appropriate method to monitor iodine deficiency or excess in salt consumed by the families and enable initiation of appropriate remedial methods and also monitor the impact of the remedial interventions.

India has the highest prevalence of anaemia in the world and is home to the largest number of anaemic persons in the world. The Sustainable Development Goal has set the target of a 50% reduction in the prevalence of anaemia in WRA to be achieved by 2030. India has initiated the Anaemia Mukht Bharat programme to accelerate the pace of anaemia reduction. One of the interventions under this programme is the use of iron-fortified foodstuffs. At present, iron-fortified rice/wheat flour and iron-fortified salt are being used in the preparation of the hot cooked meal in ICDS and the MDM programme. There are few studies on UI in Indians who have been using iron-fortified salt.²⁷ The data from our two studies, completed in 2019 and 2021 showed that there were no differences in the median UIE between persons who had used IS and those who had used DFS for 12 months. This is a reassuring finding and can provide impetus to improve the use of DFS in supplementary feeding programmes.