Iodised salt (also spelled iodized salt) is table salt mixed with a minute amount of various iodine-containing salts. The ingestion of iodide prevents iodine deficiency. Worldwide, iodine deficiency affects about two billion people and is the leading preventable cause of mental retardation.[1] It 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 iodisation of salt.
Iodine is a micronutrient that is naturally present in the food supply in many regions. However, 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 needed by humans.
Iodide-treated table salt slowly loses its iodine content through the process of oxidation and iodine evaporation.
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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.[2]
Edible salt can be iodised by spraying it with a potassium iodate solution. 60 ml of potassium iodate, costing about US$1.15 (in 2006), are required to iodise a ton of salt.[1] Salt is an effective vehicle for distributing iodine to the public because it does not spoil and is consumed by everyone in the population in fairly predictable amounts. For example, the concentration of iodine in salt has gradually increased in Switzerland: 3.75 mg/kg in 1975, 7.5 mg/kg in 1962, 15 mg/kg in 1980, and 20 mg/kg in 1998. These increases were found to improve iodine status in the general Swiss population.[3]
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.[4]
Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of mental retardation.[1] 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.[1] 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.[1]
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.[1] 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.
The United States Food and Drug Administration recommends[5] 150 micrograms of iodine per day for both men and women.
Kazakhstan, a country in Central Asia 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.[1]
In the U.S. in the early 20th century, goitre was especially prevalent in the region around the Great Lakes and the Pacific Northwest.[6] 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.[7] By the fall of 1924, Morton Salt Company began distributing iodised salt nationally.
The South African government instructed that all salt for sale was to be iodised after December 1, 1995.[8][9]
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.[10] Also, typical anticaking agents are not soluble in water, and form an unwanted precipitate, which is also undesirable in pickling jars.
Salt can also be double-fortified with iron and iodine (see below).[11] 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.[12]
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.[13]
In some countries, table salt is treated with potassium fluoride to enhance dental health.[14]