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Iodised salt (also spelled iodized salt) is table salt mixed with a minute amount of various salts of the element iodine. The ingestion of iodine prevents iodine deficiency. Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of intellectual and developmental disabilities. Deficiency also causes thyroid gland problems, including "endemic goitre". In many countries, iodine deficiency is a major public health problem that can be cheaply addressed by purposely adding small amounts of iodine to the sodium chloride salt.
Iodine is a micronutrient and dietary mineral that is naturally present in the food supply in some regions, especially near sea coasts, but is generally quite rare in the Earth's crust, since iodine is a so-called "heavy" element (with the highest atomic mass of any element needed by mammals for life), and abundance of chemical elements generally declines with greater atomic mass. Where natural levels of iodine in the soil are low and the iodine is not taken up by vegetables, iodine added to salt provides the small but essential amount of iodide needed by humans.
Four inorganic compounds are used as iodide sources, depending on the producer: potassium iodate, potassium iodide, sodium iodate, and sodium iodide. Any of these compounds supplies the body with its iodine required for the biosynthesis of thyroxine (T4) and triiodothyronine (T3) hormones by the thyroid gland. Animals also benefit from iodine supplements, and the hydrogen iodide derivative of ethylenediamine is the main supplement to livestock feed.
Salt is an effective vehicle for distributing iodine to the public because it does not spoil and is consumed in more predictable amounts than most other commodities. For example, the concentration of iodine in salt has gradually increased in Switzerland: 3.75 mg/kg in 1952, 7.5 mg/kg in 1962, 15 mg/kg in 1980, 20 mg/kg in 1998, and 25 mg/kg in 2014. These increases were found to improve iodine status in the general Swiss population.
Salt that is iodised with iodide may slowly lose its iodine content by exposure to excess air over long periods. The halogen iodide, over time and exposure to excess oxygen and carbon dioxide, slowly oxidises to metal carbonate and elemental iodine, which then evaporates.
Edible salt can be iodised by spraying it with a potassium iodate or potassium iodide solution. 60 ml of potassium iodate, costing about US$1.15 (in 2006), is required to iodise a ton of salt. Dextrose is added as a stabilizer to prevent potassium iodide from oxidizing and evaporating. Anti-caking agents such as calcium silicate are commonly added to table salt to prevent clumping.
Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of intellectual and developmental disabilities. According to public health experts, iodisation of salt may be the world's simplest and most cost-effective measure available to improve health, only costing US$0.05 per person per year. At the World Summit for Children in 1990, a goal was set to eliminate iodine deficiency by 2000. At that time, 25% of households consumed iodised salt, a proportion that increased to 66% by 2006.
Salt producers are often, although not always, supportive of government initiatives to iodise edible salt supplies. Opposition to iodisation comes from small salt producers who are concerned about the added expense, private makers of iodine pills, concerns about promoting salt intake, and unfounded rumours that iodisation causes AIDS or other illnesses. Iodisation programmes are more likely to be successful in areas where most edible salt is produced by a small number of large companies, as opposed to hundreds of smaller companies.
As the world's fourth largest country, Brazil spans a large territory, encompassing an area larger than the contiguous United States, and much of its population is located in areas remote to the coast where most food with natural dietary iodine content is produced, and even in modern days consumption of marine fish, algae and seafood in Brazil is comparatively low (consumption of fish, presumably riverine, is higher in northern Brazil, though). In Brazil, the Iodine Deficiency Disorders were detected as a major public health issue by government authorities in the 1950s, when about 20% of the population had goiter.
The second most recent resolution on the mandatory iodisation of table salt is National Agency for Sanitary Vigilance's (ANVISA) Resolution RDC no. 130 of May 26, 2003 that mandates the content of iodine to be of between 20 and 60 miligrams for each kilogram of table salt. In order for the iodine content of salt to be within the limits established by law, the salt industry should adopt the requirements for Best Practices specific to the salt beneficiary industries regulated by Resolution RDC no. 28, of March 28, 2000.
Iodisation above legal limits is explicitly avoided according to ANVISA's site due to the risk of clinical and sub-clinical hyperthyroidism in elders and of the apparition of autoimmune Hashimoto's thyroiditis in those with a genetic predisposition to it. Brazilians consume on average 12g of table salt per day (more than twice the maximum recommended adult human consumption by international standards, which is strongly linked to the most common cause of death in the country, cardiovascular disorders, see health in Brazil). This level of salt consumption in conjunction with the maximum allowable fortification level of 60 mg/kg would lead to a daily intake of 0.72 mg of iodine, many times above the recommended 0.13 mg adult and 0.20 mg pregnant/breastfeeding consumption. However, a more moderate 6g salt consumption fortified at 30 mg/kg would still provide 0.18 mg of iodine.
All adjustments of salt iodisation, conducted by the Ministry of Health, are made in accordance with the recommendation of the World Health Organization and national experts on the subject. Any regulations on the salt is previously discussed in the Interagency Commission for the Prevention and Control of Iodine Deficiency Disorders.
In light of Brazil's high consumption of table salt at 12g per day (versus a recommendation of a maximum 5g a day), and given the tendency of excess consumption of iodine giving collateral health results that are worrisome in a minority of the population, the iodizing of Brazilian table salt was reduced to 15–45 mg/kg as of July 2013. Specialists criticized it, saying that the government would better invest in reducing the consumption of salt among the population, that contributes to the leading cause of death in the country, rather than diminishing the rate of iodizing.
Kazakhstan, a country in Central Eurasia in which local food supplies seldom contain sufficient iodine, has drastically reduced iodine deficiency through salt iodisation programmes. Campaigns by the government and non-profit organisations to educate the public about the benefits of iodised salt began in the mid 1990s, with iodisation of edible salt becoming legally mandatory in 2002.
In the U.S. in the early 20th century, goitre was especially prevalent in the region around the Great Lakes and the Pacific Northwest. David Murray Cowie, a professor of paediatrics at the University of Michigan, led the U.S. to adopt the Swiss practice of adding sodium iodide or potassium iodide to table and cooking salt. On May 1, 1924, iodised salt was sold commercially in Michigan. By the fall of 1924, Morton Salt Company began distributing iodised salt nationally. There was a gradual increase in average intelligence of 1 standard deviation, 15 points, in iodine-deficient areas, 3.5 points nationally, but also an increase in deaths of older people in iodine-deficient areas due to hyperthyroidism.
In contrast to table salt, which often has iodide as well as anticaking ingredients, special canning and pickling salt is made for producing the brine to be used in pickling vegetables and other food-stuffs. This salt has no iodine added because the iodide can be oxidised by the foods and darken them—a harmless but aesthetically undesirable effect.
Salt can also be double-fortified with iron and iodine. The iron is microencapsulated with stearine to prevent it from reacting with the iodine in the salt. By providing iron in addition to iodine in the convenient delivery vehicle of salt, it could serve as a sustainable approach to combating both iodine and iron deficiency disorders in areas where both deficiencies are prevalent.
Adding iron to iodised salt is complicated by a number of chemical, technical, and organoleptic issues. Since a viable DFS premix became available for scale-up in 2001, a body of scientific literature has been emerging to support the DFS initiative including studies conducted in Ghana, India, Côte d'Ivoire, Kenya and Morocco.