This is a very worthwhile review of the importance of iodine in our diet. Zimmermann et al 2008 Iodine-deficiency disorders, Lancet vol 272 pp 1251-62
Iodine is an essential component of the thyroid hormones., and therefore iodine, is essential for mammalian life. Iodine (as iodide) is widely but unevenly distributed in the earth’s environment. Most iodide is found in the oceans (about 50 ug/L), and iodide ions in seawater oxidise to form elemental iodine, which is volatile and evaporates into the atmosphere and returns to the soil by rain, completing the cycle. However, the cycle of iodine in many regions is slow and incomplete, and soils and groundwater become deficient in iodine. Crops grown in these soils will be low in iodine concentration, and man and animals consuming food grown in these soils become deficient in iodine.
In plants grown in deficient soils, iodine concentration might be as low as 10 ug/kg of dry weight, compared with about 1 mg/kg in plants from iodine-sufficient soils. Iodine-deficient soils are common in inland regions, mountainous areas, and places with frequent flooding, but can also occur in coastal regions. Iodine deficiency in populations residing in these areas will persist until iodine enters the food chain through addition of iodine to foods (eg, iodisation of salt) or dietary diversification introduces foods produced in iodine-sufficient regions.
The native iodine content of most foods and beverages is low, and most commonly consumed foods provide 3-80 ug per serving. Major dietary sources of iodine in the USA and Europe are bread and milk.” On the basis of direct food analysis, an adequate mean intake of dietary iodine is about 100-180 ug per day Boiling, baking, and canning of foods containing iodated salt cause only small losses (2 billion individuals worldwide have insufficient iodine intake, with those in south Asia and sub-Saharan Africa particularly affected. Iodine deficiency has many adverse effects on growth and development due to inadequate production of thyroid hormone. Iodine intake is very important during pregnancy and lactation. Assessment methods include urinary iodine concentration, goitre, newborn thyroid-stimulating hormone, and blood thyroglobulin. In nearly all countries, the best strategy to control iodine deficiency is iodisation of salt, which is one of the most cost-effective ways to contribute to economic and social development. When iodisation of salt is not possible, iodine supplements can be given to susceptible groups. Introduction of iodised salt to regions of chronic iodine-deficiency disorders might transiently increase the proportion of thyroid disorders, but overall the small risks of iodine excess are far outweighed by the substantial risks of iodine deficiency.
- Martin Eastwood