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Thursday, April 14, 2011

Minerals (2)

The daily RNI of iodine is 140 μg (1.1 μmol) for men and women, and it is found in milk, meat and seafoods.

It exists in foodstuffs as inorganic iodides which are efficiently absorbed. Iodine is a constituent of the thyroid hormones

Many mountainous areas throughout the world lack iodine in the soil, and so iodine deficiency, which impairs brain development, is a WHO priority. Endemic goitre occurs in remote areas where the daily intake is below 70 μg, and in those parts 1-5% of babies are born with cretinism. In these areas, iodized oil should be given intramuscularly to all reproductive women every 3-5 years. In developed countries, salt is iodized and endemic goitre has disappeared.

In areas where the level of fluoride in drinking water is less than 1 p.p.m. (0.7-1.2 mg/L), dental caries is relatively more prevalent. Fluoridation of the water provides 1-2 mg daily, resulting in a reduction of about 50% of tooth decay in children. There is little fluoride in food except for seafish and tea, the latter providing 70% of the daily intake. Fluoride-containing toothpaste may add up to 2 mg a day.

Excessive fluoride intake in areas where the water fluoride level is above 3 mg/L can result in fluorosis, in which there is infiltration into the enamel of the teeth, producing pitting and discoloration.

Clinical deficiency of selenium is rare except in areas of China where Keshan disease, a selenium-responsive cardiomyopathy, occurs. Selenium deficiency may also cause a myopathy. Toxicity has been described with very high intakes.

In the UK, the daily RNI of calcium is 700 mg (17.5 mmol), but substantially higher values are now recommended in the USA. It is found in many foodstuffs, with two-thirds of the intake coming from milk and milk products, only 5% from vegetables. In the UK most flour is fortified. Calcium absorption from the gastrointestinal tract is vitamin D-dependent. Ninety-nine per cent of body calcium is in the skeleton.

Increased calcium is required in pregnancy and lactation, when dietary intake must be increased. Calcium deficiency is usually due to vitamin D deficiency.

The daily RNI of phosphate is the same as that of calcium, i.e. 17.5 mmol. Phosphates are present in all natural foods, and dietary deficiency has not been described. Patients taking large amounts of aluminium hydroxide can, however, develop phosphate deficiency owing to binding in the gut lumen. It can also be seen in total parenteral nutrition. Symptoms include anorexia, weakness and osteoporosis.

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