Qualitative determination of iodate and iodide species
Qualitative experiments exhibited positive results for iodate, which are blue colorations with the addition of 1 % starch solution. That indicates the presence of iodate (IO
3−) species in all of the salt products [
25]. However, in the case of iodide determination, none of the products gave any color change with the 1 % starch solution. That indicated the absence of iodide species in iodized salt products which has been tested [
25]. The iodide (I
−) ions convert iodate (IO−
3) to elemental iodine (I
2). This elemental iodine reacts with the iodide ion (I
−) to form a tri-iodide anion (I
3−) and this further reacts with it to give a penta-iodide anion (I
5−). This penta-iodide anion (I
5−) forms a visible blue–black complex with starch molecules [
20].
Total iodine content of iodized salts
The results showed that all of the products have exceeded the recommended iodine fortification level of 15–30 mg kg
−1 of Sri Lanka (Table
3). Although the qualitative methods exhibited no iodide present in the samples, the quantitative experiments indicated high concentrations, which express the less importance of qualitative examinations during standardization tests. The iodide concentrations of saturated salt solutions decreased with the time when exposed to air (Table
4). The loss of iodine as elemental iodine after air oxidation is the reason behind that [
14,
26]. The iodine loss percentages of salts were 13.0, 10.7, and 11.2 % for C, F and A products respectively (Table
4). Saturated solutions were not kept beyond 24 hours since the water also can evaporate thus it can cause errors in the results. And in household, the salt solutions are replaced and the new salts crystals are added regularly.
Table 3
Total iodine contents of different commercial iodized salts in Sri Lanka, 2015
A | 17.2 ± 1.1 | 39.4 ± 2.2 | 56.6 ± 2.5 |
B | 14.3 ± 0.5 | 38.6 ± 0.4 | 52.9 ± 0.7 |
C | 31.9 ± 2.4 | 40.1 ± 2.0 | 72.0 ± 3.2 |
D | 19.0 ± 0.4 | 35.4 ± 1.2 | 54.5 ± 1.3 |
E | 4.2 ± 0.5 | 36.5 ± 0.9 | 40.7 ± 1.0 |
F | 18.5 ± 0.0 | 40.9 ± 0.7 | 59.4 ± 0.7 |
Table 4
The loss of iodine as iodide and iodate in saturated salt solutions in different conditions
Product | Iodine content ± SD (mg L−1) | Final iodine content ± SD (mg L−1) | Open Environment ± SD (%) | 50 °C ± SD (%) | 100 °C ± SD (%) |
| Initial | 24 h | | | |
C | 27.5 ± 2.9 | 23.9 ± 3.3 | 13.0 ± 3.5 | 4.6 ± 1.7 | 11.1 ± 2.7 |
F | 26.1 ± 0.1 | 23.3 ± 0.1 | 10.7 ± 0.2 | 8.6 ± 0.8 | 15.9 ± 0.3 |
A | 24.0 ± 1.4 | 21.3 ± 1.4 | 11.2 ± 3.6 | 7.8 ± 2.7 | 11.4 ± 0.6 |
The loss of iodine as iodate in saturated salt solutions at different conditions |
C | 9.6 ± 0.2 | 9.6 ± 0.0 | 0 ± 0.0 | 1.6 ± 2.8 | 0 ± 0.0 |
F | 5.7 ± 0.2 | 5.4 ± 0.2 | 5.6 ± 0.2 | 5.5 ± 4.8 | 0 ± 0.0 |
A | 6.6 ± 0.0 | 6.6 ± 0.0 | 0 ± 0.0 | 0 ± 0.0 | 0 ± 0.0 |
According to the results obtained, salt solutions from C and A have not lost their iodine content in the form of iodate after 48 hours at the open environmental conditions (Table
4). Iodate is a stable anion and resistant to get reduced when exposed to the air [
26,
27]. However, F product has shown a 5.6 % of loss after 48 hours. That may be due to the presence of hygroscopic impurities like magnesium chloride (MgCl
2), reducing agents like ferrous ions or lower pH that enhance the reduction of iodate [
11,
26,
27]. Factors like impurities, reducing agents, metal ions and pH value vary from one salt product to another. So that, further experiments are required to analyze the activity of those factors that affect the stability of iodine in iodized salts under any environmental condition [
26].
Risk assessment of iodine exposure
The possible exposure of iodine from each product was calculated under assumptions of 10 g/day as the average salt intake per capita and iodine loss during cooking is 20 % [
12,
23] (Table
2). According to the data in the Table
5 with and without the addition by loss during cooking, only 16.6 % of the units at low salt consumptions can cause optimal nutrition (150–299 μg/day) and 83.3 % belong to iodine exposure above requirements (300–449 μg/day). Among medium salt consumption from all salt products, 50 % can cause iodine exposure above requirements while the other 50 % belong to excessive iodine exposure (>449 μg/day). Among moderate high salt consumptions, only 16.6 % of the units show iodine exposure above requirements while the rest of 83.3 % belong to the excessive iodine exposure. High salt consumption from all salt products (100 %) with cooking can cause excessive intake of iodine. Out of those 24 cases including low, medium, moderate high and high salt consumptions, only one of them (4.1 %) can lead to optimal iodine nutrition. And the rest of 95.8 % can possibly cause IIH at a population [
4].
Table 5
Iodine exposure assessment based on iodine contents of salt, with and without adjustments for cooking losses
with the adjustments for cooking losses |
A | 56.7 ± 2.5 | 340.1 ± 13.0 | 453.5 ± 17.3 | 566.9 ± 21.6 | 680.2 ± 26.0 |
B | 52.9 ± 0.7 | 317.5 ± 1.1 | 423.4 ± 1.5 | 529.3 ± 1.9 | 635.1 ± 2.3 |
C | 72.0 ± 3.2 | 432.2 ± 25.8 | 576.3 ± 34.5 | 720.4 ± 43.1 | 864.4 ± 51.7 |
D | 54.5 ± 1.3 | 327.0 ± 6.5 | 436.0 ± 8.6 | 545.1 ± 10.8 | 654.1 ± 12.9 |
E | 40.7 ± 1.0 | 244.4 ± 2.3 | 325.9 ± 3.1 | 407.4 ± 3.9 | 488.8 ± 4.6 |
F | 59.4 ± 0.7 | 356.8 ± 4.1 | 475.8 ± 5.5 | 594.8 ± 6.9 | 713.7 ± 8.2 |
without the adjustments for cooking losses |
A | 56.7 ± SD | 425.1 ± 16.2 | 566.9 ± 21.6 | 708.6 ± 27.0 | 850.3 ± 32.5 |
B | 52.9 ± SD | 396.9 ± 1.4 | 529.3 ± 1.9 | 661.6 ± 2.4 | 793.9 ± 2.9 |
C | 72.0 ± SD | 540.3 ± 32.3 | 720.4 ± 43.1 | 900.5 ± 53.9 | 1080.6 ± 64.7 |
D | 54.5 ± SD | 408.8 ± 8.1 | 545.1 ± 10.8 | 681.3 ± 13.5 | 817.6 ± 16.2 |
E | 40.7 ± SD | 305.5 ± 2.9 | 407.4 ± 3.9 | 509.2 ± 4.8 | 611.1 ± 5.8 |
F | 59.4 ± SD | 446.1 ± 5.1 | 594.8 ± 6.9 | 743.5 ± 8.6 | 892.2 ± 10.3 |
WHO has estimated the average salt intake as 10 g per capita per day, based on the data throughout the world [
4,
12] (Table
1). It was revealed that the salt consumption is higher in Asia than the rest of the world [
28]. Central Asia ranks highest in salt consumption and many countries in central Asia consume salt more than 12 g per day [
28]. Therefore, the average salt intake per capita in Sri Lanka also can be higher than the estimated value due to same factors stated at above. People in Sri Lanka regularly consume seafood, marine fish, cereals, grains, vegetables, milk, dairy products which are the main food sources of dietary iodine, apart from iodized salts [
2]. Processed food made by grains may also contain higher amounts of iodine due to the addition of iodized salt or additives that contain iodine [
2].
Geographically, Sri Lanka is an island nearer to the equator. Therefore, the climate is tropical with warm and humid weather conditions year around. Due to warmer and humid climate, the excretion of urine is also high. Therefore, daily average water intake per capita of a population could also be high. Due to those factors, the average salt intake per capita in Sri Lanka also can be higher than the estimated value. But the true value has not been estimated by surveys yet. Therefore, the moderate high (12.5 g/day) and high salt consumption (15 g/day) were considered to mimic the possible average salt intake per capita in Sri Lanka.
In our simulation (Table
5) taking into account the level of salt consumption in the population and the iodine content of the salt brands, it is only in the case of low salt consumption of the brand with the lowest iodine content that excessive iodine intake would not be attained in the population. The susceptible groups are the patients who have autoimmune thyroid disease, thyroiditis or a history of thyroid surgeries [
16,
20]. And the rest of 75 % also can cause adverse health effects of IIH and autoimmune thyroid diseases due to the iodine exposure in excessive amounts.
Therefore, almost all of the above cases of salt intake, with or without cooking, at low, medium, and high salt consumptions can cause risks of adverse health effects such as IIH [
2]. However, most of people in a population are tolerable to the excessive iodine intake from food, water and other supplements like drugs [
2]. Only the susceptible individuals are the ones at the risk of IIH and the actual risks for each individual are influenced by many variables including age, gender, genetic predisposition, environmental factors, personal history of thyroid diseases, concurring diseases, and some medications [
9]. Tolerable upper level (UL) for iodine is 1100 μg/day [
2].
The potential adverse effects of iodine intake above the tolerable UL include dysfunction of thyroid gland, thyroiditis, goiter, hyperthyroidism, sensitivity reactions, thyroid papillary cancers and acute responses [
2]. However, those effects are related with the chronic exposure of excessive iodine [
4]. Acute exposure of iodine can be occurred after sudden intake of doses of many grams at once [
2]. Effects of acute excessive iodine exposure are mouth and stomach burnings, abdominal pain, nausea, vomiting, diarrhea, weak pulse, cardiac irritability and coma [
2]. However, studies on time durations for the acute exposure have not been performed yet. Hence, further studies are required in future to assess the effects of acute iodine exposure on humans most probably using lab rats [
16]. There is a need for proper monitoring of the salt iodization programs to achieve an acceptable and optimal iodine status in the population. To achieve it, rigorous measures should be implemented to ensure that manufacturers of iodized salt are committed to the recommended specifications.