Impact on health absorption and recommended intakes

Minerals function mainly in three ways in the body:

  1. As structural components, e.g. calcium, phosphate and magnesium in bones and teeth.
  2. In organic combinations as physiologically important compounds, e.g. phosphorus in nucelotides, zinc in enzymes such as carbonic anhydrase, iodine in thyroid hormone.
  3. In solution in body fluids to maintain pH, help conduct nerve impulses, control muscle contraction, e.g. sodium and potassium in blood and intra-cellular fluids.

The macrominerals are mainly involved in functions 1 and 3, and the microminerals in function 2.

A normal diet, composed of a mixture of both plant and animal foodstuffs, should supply all the minerals required by the body. When such a diet is not available, or in some other situations, it may be necessary to provide the missing elements in the form of supplements or by fortifying the diet with additional minerals. The minerals ingested in food are absorbed after digestion from the gut into the blood stream, which transports them to the sites where they function or are stored. Not all minerals are absorbed to the same extent. Some, including sodium and potassium, are readily absorbed as ions or as simple compounds. Others, such as calcium, magnesium and phosphorus may be combined as indigestible or insoluble compounds in food and are less easily taken up from the gut. A few others, especially some of the trace elements such as iron, are poorly absorbed.

Uptake of certain minerals from food can be affected by other components of the diet. Thus phytic acid and phytates in cereals can inhibit absorption of iron and zinc. The same effect can be caused by oxalate in certain vegetables. Iodine absorption can be limited by sulphur-containing compounds known as goitrogens, which occur in certain plants, such as some brassicae and cassava. Consumption of these vegetables can acerbate iodine deficiency and increase the likelihood of goitre.

If an essential element is at a low level in the diet, a nutritional deficiency may occur, with specific symptoms. Thus an inadequate intake of iron can cause anaemia when there is insufficient haemoglobin to meet the needs of the body for oxygen transport. A deficiency of iodine can lead to goitre when the body tries to compensate for a low production of the iodine-containing thyroid hormone by increasing the size of the thyroid gland. Inadequate zinc may result in growth failure in children. Usually these conditions are corrected when intake of the missing element is increased by improving the diet or by providing supplements.

An excessive intake of a mineral may also have serious consequences for health. Too much sodium in the diet may be associated with high blood pressure and increased risk of a stroke. A condition known as siderosis, in which an excess of iron is deposited in the body, can result when too much iron is absorbed. Selenosis, a sometimes fatal effect of an excessive intake of selenium is known to occur in parts of China where high levels of the element enter locally grown foods from selenium-rich soil. Less serious effects, such as nausea, can be caused by a high intake of zinc.

Table 4.2 Reference nutrient intakes and safe intakes for minerals

Mineral

male (19-50 years)

female (19-50 years)

Calcium (RNI) mg/day

700

700

Phosphorus (RNI) mg/day

550

550

Magnesium (RNI) mg/day

300

270

Sodium (RNI) mg/day

1600

1600

Potassium (RNI) mg/day

3500

3500

Chloride (RNI) mg/day

2500

2500

Iron (RNI) mg/day

8.7

14.8+

Zinc (RNI) mg/day

9.5

7.0

Copper (RNI) mg/day

1.2

1.2

Selenium (RNI) mg/day

75

60

Iodine (RNI) mg/day

140

140

Manganese (SI) mg/day

above 1.4

above 1.4

Molybdenum (SI) mg/day

50-400

50-400

Chromium (SI) mg/day

above 25

above 25

Fluoride (SI) mg/kg body weight/day

0.5

0.5

  • insufficient for women with high menstrual losses where the most practical way of meeting iron requirements is to take iron supplements. adapted from Department of Health (1991) Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. HMSO: London.
  • insufficient for women with high menstrual losses where the most practical way of meeting iron requirements is to take iron supplements. adapted from Department of Health (1991) Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. HMSO: London.

Health authorities in most countries have established recommendation for intake levels of essential minerals which both meet the nutritional requirements of consumers and at the same time prevent excessive intakes. In the UK, Reference Nutrient Intakes (RNI) for 11 minerals have been published by the Department of Health to meet the requirements for the different age groups and sexes in the Community (Department of Health, 1991). The RNI is defined as 'an amount of the nutrient that is enough, or more than enough, for about 97 per cent of people in a group'. In addition Safe Intakes (SI) have been established for another four minerals. The SI is 'a term used to indicate intake or range of intakes of a nutrient for which there is not enough information to estimate RNI... it is an amount that is enough for almost everyone but not so large as to cause undesirable effects'. The RNI for minerals for adult men and women are given in Table 4.2.

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