Food and nutrition.pdf

108 Function of Foodmuch of it in the thyroid gland. Iodine is essential for the formation ofthyroid hormones secreted by this gland.Properties and FunctionsIn humans iodine functions as an essential component of thehormones of the thyroid gland, an endocrine gland situated in the lowerneck. Thyroid hormones, of which the most important is thyroxine (T4),are important for regvlating metabolism.In children they support normal growth and development, includingmental development.Iodine is absorbed from the gut as iodide, and excess is excreted inthe urine. The adult thyroid gland, in a person consuming adequate iodine,traps about 60 f.lg of iodine per day to make normal amounts of thyroidhormones. If there is insufficient iodine, the thyroid works harder to trapmore; the gland enlarges in size (a condition known as goitre), and itsiodine content might become markedly reduced.Thyroid stimulating hormone (TSH) from the pituitary glandinfluences thyroxine secretion and iodine trapping. In severe iodinedeficiency, TSH levels are raised and thyroxine levels are low.Dietary SourcesIodine is widely present in rocks and soils. The quantity in differentplants varies according to the soil in which they are grown. It is notmeaningful to list the iodine content of foodstuffs because of the largevariations in iodine content from place to place, depending on the iodinecontent of the soil. Iodine tends to get washed out of the soil, andthroughout the ages a considerable quantity has flowed into the sea. Seafish, seaweed and most vegetables grown near the sea are useful sourcesof iodine. Drinking-water provides some iodine but very seldom enoughto satisfy human requirements.In many countries where goitre is prevalent the authorities haveadded iodine to salt, a strategy which has successfully controlled iodinedeficiency disorders (IDD). Iodine has usually been added to salt in theform of potassium iodide, but another form, potassium iodate, is morestable and is better in hot, humid climates. Iodated salt is an important. dietary source of iodine.Deficiency StatesA lack of iodine in the diet results in several health problems, one ofwhich is goitre, or enlargement of the thyroid gland. Goitre is extremelyprevalent in many countries. There are other contributing causes of goitre,but iodine deficiency is by far the most common. -Iodine deficiency during