Iodine is a trace element that is essential for the synthesis of thyroid hormones by the thyroid gland. These hormones are involved in growth, development and control of metabolic processes in the body. The iodine content of foods depends on the iodine content in the soil. Thus, low iodine concentrations in soil and water result in iodine-deficient plants and animals (4). As populations depend on food sources grown in areas where iodine may be deficient, iodine can only enter the food supply through the addition of this nutrient to nutrient-delivery interventions, and the most common has been a common staple: edible food-grade salt, a crystalline product consisting predominantly of sodium chloride (5).
The most susceptible group for iodine deficiency disorder is women of reproductive age, whose neonates, if iodine deficient in utero, are at high risk of irreversible mental impairment. Moreover, the other susceptible group is women providing breast milk to their children, as this may be the only source of iodine for an infant during the first 6 months of life (4). It is estimated that about 1.88 billion people worldwide are at risk of insufficient iodine intake if iodine-delivered interventions are not kept in place (4). Approximately one third of the world's population lives in areas where natural sources of iodine are low, and therefore they require the permanent presence of iodine-supplying interventions. Approximately 29.8% (241 million) school-age children globally are estimated to have insufficient intake of iodine. Many of these 241 million children live in the World Health Organization (WHO) South-East Asia Region (76 million) and African Region (58 million) (6).
Salt iodization is the preferred strategy for control of iodine deficiency disorders and is implemented in more than 120 countries around the world (7). Many countries worldwide have successfully eliminated iodine deficiency disorders or made substantial progress in their control, largely as a result of salt iodization and dietary diversification. Dairy products are also a good source of iodine if cattle are fed with iodine-enriched feed, and therefore including milk and other dairy products as part of promotion of dietary diversification is important to prevent iodine deficiency disorders.
The most visible sign of iodine deficiency is goitre, an enlargement of the thyroid gland. Individuals living in areas affected by severe iodine deficiency may have an intelligence quotient (IQ) of up to 13.5 points below that of those from comparable communities in areas where there is no iodine deficiency (8–10).
When iodine intake is inadequate, the thyroid gland may no longer be able to synthesize sufficient amounts of thyroid hormones. Low levels of thyroid hormones in the blood, referred to as hypothyroidism, are responsible for damage to the developing brain and for the whole spectrum of iodine deficiency disorders. The spectrum of iodine deficiency disorders includes goitre; hypothyroidism; increased susceptibility to nuclear radiation; spontaneous abortion; stillbirths; congenital anomalies; perinatal mortality; endemic cretinism, including mental deficiency with a mixture of mutism, spastic dysplegia, squint, hypothyroidism and short stature; infant mortality; impaired mental function; delayed physical development; and iodine-induced hyperthyroidism (11, 12). These can be prevented by ensuring that the population has an adequate intake of iodine.
Both insufficient and excess iodine can have negative effects on thyroid functioning. Excessive intake of iodine may be associated with complications such as iodine-induced hyperthyroidism in some cases (13) or hypothyroidism in others (14). Data indicate a small increase in the risk for iodine-induced hyperthyroidism with increasing iodine intakes in older adults, mainly those with pre-existing nodular goitre (14, 15).
On a population basis, iodine-induced hyperthyroidism represents a transient increase in the incidence of hyperthyroidism that occurs when there is an increase in iodine intake in severely iodine-deficient populations. The number of people at risk of iodine-induced hyperthyroidism is directly proportional to the number of individuals with nodular goitre. This condition will disappear with the correction of iodine deficiency (16, 17).
Based on studies of balance and excretion, an adult iodine intake is in the range of 100–300 μg/day. The recommended daily intake of iodine is 90 μg/day for infants and children aged less than 6 years, 120 μg/day for children aged 6–12 years, and 150 μg/day for adolescents and adults from 13 years of age through to adulthood. It is recommended that pregnant and lactating women consume 250 μg of iodine per day (11). Sea fish and some seaweeds are good sources of dietary iodine. Iodine can also be found in vegetables and cereals if they are produced in iodine-sufficient soils, poultry or meat and milk, if the animals consume foods grown in iodine-sufficient soils or are given iodine-containing feed supplements.
At all intake levels, a proportionate amount of iodine is excreted in the urine, so the urinary iodine excretion (UIE) using 24-h urine collections is the ideal biochemical indicator for assessing iodine status. However, owing to the difficulties associated with using this indicator for assessing populations and monitoring programmes, the simple urinary iodine concentration (UIC) has been used (18), although this may not reflect the intake distribution of individuals and populations as well as measurement of 24-h UIE.
Both UIC and UIE correlate with recent iodine intakes and, therefore, they are useful for estimating iodine intake. Currently, the median value of ≥100 μg/L for UIC is being used to identify when an adult population has sufficient intake of iodine (19), but it has mainly been applied to school-age children, from whom is easier to collect urine samples (4). However, because of the lower urinary volume in children, the UIC may be higher in that age group than in adults. As of 2013, there were 57 million school-age children who were not attending school (20), which may affect the interpretation of the results. For pregnant women, however, a median UIC of <150 μg/L has been proposed to indicate insufficient iodine intake (21).
In the past, the prevalence of goitre was also a commonly used indicator for the assessment of iodine deficiency in a population, although it has little practical usefulness now because goitre has nearly disappeared in many countries. Goitre is also not recommended for assessing the response to an intervention, as it can take months or years for goitre to regress after the normalization of iodine intake (11, 22). However, it remains useful in countries or regions that have not yet introduced iodised salt programmes.
The concentrations of thyroid-stimulating hormone (TSH) and thyroid hormone (T3 and T4) in serum/plasma are additional indicators of iodine status. However, these indicators are usually not recommended for monitoring iodine nutrition, because they are more expensive and less sensitive (11). Measurement of serum or dried blood spot thyroglobulin (Tg) in school-age children appears to be a sensitive indicator of iodine status in a population and it may be used to monitor improving thyroid function after iodine repletion (11). However, further validation is required for standardization of collection and measurement techniques for Tg, and for establishment and interpretation of cut-off values (21).
Recognizing the importance of preventing iodine deficiency disorders, in 1991 the World Health Assembly adopted the goal of eliminating iodine deficiency as a public health problem. In 1990, world leaders endorsed this goal when they met at the World Summit for Children at the United Nations, and it was reaffirmed by the International Conference on Nutrition in 1992 (23). In 1993, WHO and the United Nations Children's Fund (UNICEF) recommended universal salt iodization as the main strategy to achieve elimination of iodine deficiency disorders (24). In 2005, the importance of eliminating iodine deficiency disorders was again recognized when the World Health Assembly adopted a resolution committing to reporting on the global situation of iodine deficiency disorders every 3 years (25). In 2013, the World Health Assembly further recognized that the fight against iodine deficiency disorders contributes directly to many of the MDGs, including anti-poverty, reduction of infant mortality, maternal health, education for all, gender equity and private–public partnership (26).
Universal salt iodization
Since 1993, when WHO and UNICEF recommended universal salt iodization, many countries worldwide have made substantial progress in the control and prevention of iodine deficiency disorders, largely as a result of salt iodization. Salt iodization has been implemented in more than 120 countries around the world (27) and 71% of households worldwide are estimated to have access to adequately iodized salt (28). However, iodine deficiency disorders still constitute a public health problem in many countries where the salt iodization programmes are weakly implemented. On the other hand, it is important to avoid excess iodine fortification levels. Eleven countries have populations with apparent excessive iodine intakes based on UIC (6); however, the true iodine intake has not been assessed in most of them. Such excess intakes, if they exist, may be mainly due to excess fortification levels rather than excess salt intake.
Salt is considered an appropriate vehicle for fortification with iodine, for the following reasons: (i) it is widely consumed by virtually all population groups in all countries, with little seasonal variation in consumption patterns, and salt intake is proportional to energy intake/ requirements; (ii) in many countries, salt production is limited to a few centres, facilitating quality control; (iii) the technology needed for salt iodization is well established, inexpensive and relatively easy to transfer to countries around the world; (iv) addition of iodate or iodide to salt does not affect the taste or smell of the salt or foods containing iodized salt, and therefore consumer acceptability is high; (v) iodine (mainly from iodate) remains in processed foods that contain salt as a main ingredient, such as bouillon cubes, condiments and powder soups, and hence these products become sources of iodine; and (vi) iodization is inexpensive (the cost of salt iodization per year is estimated at US$ 0.02–0.05 per individual covered, and even less for established salt-iodization programmes) (12). Additionally, the concentration of iodine in salt can easily be adjusted to meet policies aimed at reducing the consumption of salt in order to prevent cardiovascular disease (29, 30).
The amount of iodine added to salt should be based on the estimated daily consumption of salt. Salt is the main dietary source of sodium and a high intake of sodium has been associated with hypertension, cardiovascular disease and stroke; and decreasing sodium intake may reduce blood pressure and the risk of associated noncommunicable diseases. Salt consumption varies widely in different countries and sometimes within a country. For instance, some indigenous populations of Brazil have salt intakes of less than 1 g/day, while some countries like Korea or Japan have reported consumptions close to 20 g/day (31, 32).
Since decreased sodium intake in the population is a cost-effective public health intervention that could potentially reduce morbidity and mortality associated with noncommunicable diseases, and because of the increasing importance of noncommunicable diseases for health-care costs and burden of disease, WHO recently updated guidance on sodium intake for adults and children (33). WHO currently recommends a reduction to <2 g/day sodium (5 g/day salt) in adults, and in children the recommended maximum level of intake should be adjusted downward based on the energy requirements of children relative to those of adults. This applies to all individuals, with or without hypertension, and should be complemented with the WHO guideline on potassium intake and other nutrient guidelines and recommendations, to guide the development of public health nutrition programmes and policies (34–36).
It has been recognized that polices on salt reduction and salt iodization are compatible (29, 30). Monitoring of salt intake and salt iodization at country level is needed to adjust salt iodization over time as necessary, depending on observed salt intake in the population, to ensure that individuals consuming the recommended amount of sodium continue to consume sufficient iodine. Resolution WHA57.17 on the Global strategy on diet, physical activity and health (37) confirmed that the policy on salt iodization for preventing iodine deficiency should be compatible with the recommendation to limit salt (sodium) consumption from all sources.
Although WHO recommends a reduction in the intake of salt to less than 5 g/day, on average, it is estimated that individuals still consume around 10 g of salt per day in countries where most of the salt in the diet comes from household salt, used for home cooking and at the table (36). Previous estimations assumed an average salt intake of 10 g per capita per day, and recommended that salt be iodized at a level of 20 mg iodine per kg of salt. The minimum and maximum levels used for the iodization of food-grade salt (expressed as mg iodine/kg salt) needed to be established by the national health authorities in the light of the local iodine deficiency situation, local consumption of salt and quality systems in place.
Although universal salt iodization calls for fortification with iodine of all food-grade salt for human and animal consumption, including salt for food processing, in practice, focus on implementation had only been on table salt and not on all salt destined for the food industry or animal consumption. Food-grade salt can obtained from the sea, from underground rock salt deposits or from natural brine (5). Washed and refined salt is the most widely used form. The main bulk of salt is sold for industrial use, where it has great commercial value as a necessary ingredient in many manufacturing processes.
Recent trends, particularly in high-income and middle-income countries, but also urban populations in low and middle income countries, show that individuals are consuming the majority of their salt through processed foods, in which iodized salt is generally not used, rather than through iodized salt. Nevertheless, in many low- and middle-income countries, discretionary salt (cooking and table) continues to be the main source of salt (29). Thus, countries that focus on iodization of table salt alone may not achieve optimal iodine nutrition of their population (38) and it is necessary to include iodized salt in processed foods.
There have been some concerns about technical difficulties, retention, losses and changes in organoleptic properties of final products, with the inclusion of iodized salt in industrialized processed foods (39, 40). However, studies in more than 20 food products containing salt fortified with potassium iodide or potassium iodate found no adverse effects on food quality (39, 41). Concerns still remain regarding the stability of iodine during food processing (42) and supply of iodine through these sources remain uncertain, although products with high content of salt, such as bouillon cubes, condiments and powder soups made with iodized salt, retain the iodine after production. It has been reported that considerable losses, up to 100%, are possible during food processing, depending on the product and the procedure, moisture, heating and storage (43–48).
Successful and sustainable implementation of the use of iodized salt in processed foods requires awareness of the risks of iodine deficiency, adequate legislation in line with a country's dietary customs and “legislation culture”, regular monitoring of iodine nutrition status and dietary intake, and a cooperative role of the salt industry and food processors (39).
