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The trace mineral iodine is well known for its crucial role in enabling the body's manufacture of vital thyroid hormones, but it is also important for the health of the immune system and for optimal brain function. It is widely believed by many authorities that iodine deficiency should never be seen in the affluent West, although this problem affects millions throughout the developed world.

Some nutritionists argue, however, that this conventional view is too optimistic, because the content of all minerals in foods is heavily dependent on the mineral content of the soil from which those foods are derived. The assumption must therefore be that the continuing de-mineralisation of farm soils has led to a reduction in the amount of dietary iodine commonly consumed.

Fish and other seafood, however, remain a relatively rich source because these ocean creatures concentrate the sea's iodine in their flesh. Though not commonly eaten in the West, seaweed, or kelp, is also an excellent source of iodine for this reason, and is readily available in the form of a dietary supplements. Dairy products and certain meats may also be a good source, particularly where iodine is routinely added to farm animal feed. But in countries, including most of Western Europe, where animals are grazing fields growing on iodine depleted soils, levels are likely to be much lower.

So even in the West, those not including fish or seafood in their diets, and not using iodised or sea salt, may be at real risk of deficiency. In an effort to compensate for low levels of dietary iodine, the mineral has been routinely added to ordinary table salt in the US for many years. But the practice is not as common in the UK and other European countries, where specially iodised or natural "sea-salt" has been marketed more as a luxury alternative. The problem of insufficient dietary iodine has been compounded on both sides of the Atlantic, however, by increasing concern about the possible adverse health consequences, particularly high blood pressure, of excessive salt intake. Many nutritionists, however, regard these fears as exaggerated, and believe that any such potential problems are far less serious than the consequences of an insufficiency of iodine, and may be easily resolved by the use of the low sodium salt alternatives available.

Iodine, however, cannot in any case be regarded as a luxury. Its essential function lies in the production of the vital thyroid hormones; thyroxine, sometimes known as T4, and tri-iodothyronine, or T3. And as is well known, these hormones are crucially important in ensuring a healthy metabolic rate and the release of energy from food; so an underactive thyroid gland is commonly the villain in cases of excessive weight gain, particularly where this of sudden onset, and in cases of difficulty in losing weight even when following a sensible reducing programme. A healthy thyroid gland is also crucial for the optimal functioning of the immune system.

But perhaps even more importantly, iodine deficiency is also known as a major cause of avoidable brain damage; a problem which the World Health Organisation has estimated to affect an astonishing 50 million people worldwide. Sadly, many of these cases occur in children whose mothers were iodine deficient in pregnancy, resulting in a condition of severely retarded brain development known as congenital hypothyroidism, or "cretinism". Even where such catastrophic consequences are avoided, iodine deficiency in childhood may also have serious effects on the developing brain, leading to low energy and motivation for learning, and measurable impairment of IQ scores.

Since 2001 the Food and Nutrition Board of the US Institute of Medicine (FNB) has prescribed a Recommended Dietary Allowance for iodine of 150 mcg for all individuals over 14, rising to 220 mcg for pregnant women and 290 mcg for those breastfeeding. Somewhat confusingly, however, an excessive consumption of iodine is also associated with a malfunctioning or enlargement of the thyroid gland, as well as mouth ulcers, headaches and gastric upsets, and the FNB therefore advises an upper safe limit for daily iodine consumption of 1,100 mcg for adults. Most people eating a conventional Western diet are unlikely to exceed this level.

With the possible exception of pregnant and breastfeeding women, people in the West who use liberal quantities of iodised salt as a regular seasoning are unlikely to need further supplements. But many commercial multi-mineral preparations contain iodine in reasonable quantities, usually in the form of potassium iodide, and whilst not perhaps strictly necessary, such supplementary doses will do no harm and may be regarded as a useful insurance policy given that, like all minerals needed by the body, iodine functions best in the presence of adequate supplies of all the others. And it should be particularly noted in this context that the effects of any deficiency of iodine may be intensified by any deficiency of selenium, iron or vitamin A.