Title: Minerals - The Major Elements

Key words: enzyme systems, muscle contraction, nerve impulses, blood clotting, calcium, iron, magnesium, phosphorus, potassium, dietary sources, optimal intake, functions, deficiency, excess,

Date: June 2000

Category: Micronutrients

Type: Article

Author: DJE Candlish

Minerals - The Major Elements

Inorganic minerals are nutrients needed for the development of structural components of both hard and soft body tissues. They are also required for the normal functioning of enzyme systems, for muscle contraction, nerve impulse transmission and blood clotting. As the body cannot produce mineral nutrients, they must be supplied by the diet or by supplementation. There are two main classes of dietary mineral:

• The major elements - including calcium, iron, magnesium, phosphorus and potassium
• The trace elements - including chromium, copper, iodine, manganese, selenium and zinc

The best dietary sources of calcium include milk, cheese, sardines and green vegetables. Although the body can not produce calcium itself, it can be stored in bones and reabsorbed when needed.

Women who are pregnant or lactating need a higher intake, as their growing child needs calcium to form teeth and bones.

Post-menopausal women may benefit from calcium supplementation, as this may help to retain their bone density and strength.

A vegan diet is unlikely to supply the need for calcium and supplementation may be needed.

Antacids can interfere with calcium absorption, so people who take them regularly are at risk of calcium deficiency.

Calcium is needed for healthy bones, teeth and nails (a 70kg adult contains around 1200g of calcium, mostly in the skeleton).

Calcium is involved in a wide range of body processes, including blood clotting, transmission of nerve impulses and muscle contraction.

Calcium deficiency causes rickets and stunted growth in children, osteoporosis in menopausal women and the elderly and an increased risk of fractured bones, particularly in the elderly.

A high calcium intake is not associated with harmful effects, as the body excretes any that is excess to its needs.

Good dietary sources of iron include liver and red meat, poultry, wholegrain cereals and green vegetables.

Dietary iron exists in two forms: nonhaem (inorganic) and haem.

The two forms are absorbed by different mechanisms.

Iron absorption is influenced by the dietary iron content, its bioavailability, the amount of iron already stored in the body and the rate of erythrocyte (red blood cell) production. It also depends upon the types of food eaten and the interaction between the food and regulatory mechanisms in the intestinal mucosa which reflect the body's physiological need for iron ( See Table 2 ).

Loss of blood either through haemorrhage or menstruation can put the body into a negative iron balance. This eventually causes anaemia (pale skin, tiredness, depression). A simple blood test to measure haemoglobin levels can confirm the diagnosis of anaemia. Other effects include stunted growth and impaired mental function in children and pruritus (itching all over the body) in both adults and children.

Although iron supplements are generally safe, care should be taken as some people can not tolerate large amounts of the mineral. Vitamin C helps iron to be absorbed better.

Iron can irritate the stomach, causing nausea, vomiting, diarrhoea and constipation. Overdoses with extremely large amounts of iron can cause hypotension, coma and even death.

Iron absorption is influenced by the dietary content, bioavailability , amount of storage iron and the rate of erythrocyte production. The entry of iron into the body is regulated by the mucosal cells of the gastrointestinal tract. The percentage of non-haem iron absorbed is increased if the iron stores are low, after haemorrhage or pregnancy, for example. Men have higher iron stores than women and childen, who are able to absorb a higher percentage of iron from the diet.

Iron is transported around the body in the form of a complex formed from iron and transferrin, a plasma transport protein. Cell membrane receptors bind the transferrin-iron complex and carry it into the cell. Tissue affinity in different tissues varies. Erythroid precursors, the placenta and liver have a high iron uptake. In an iron-rich environment the number of receptors decreases. The regulator genes that control this are located on chromosome 3.

Sources of Iron losss are summarised in Table 2. Haemoglobin represents the largest iron containing protein in the body and loss of blood either through haemorrhage or menstruation can put the body into a negative balance and cause anaemia. Myoglobin, present in muscle, has a high affinity for oxygen. Training increases myoglobin levels and reduces lactic acid build up during exercise. The cytochrome system is critical to respiration and to energy metabolism in the mitochondria. Non haem iron-containing enzymes, such as NADH and Succinate dehydrogenase, are also involved in energy metabolism.

2. WORLD HEALTH ORGANISATION 1996; Trace Elements in Human Nutrition and Health .- WHO Geneva. ISBN 92-4-156173-4

3. ZIEGLER, E. E., FILER,L.J. 1996; Present Knowledge in Nutrition

p.277-289- International Life Sciences Institute. ISBN 0-944398-72-3

Table 2. Iron uptake and distribution.

Good dietary sources of magnesium include nuts, wholemeal bread, hard water, dairy products, fish and vegetables

Magnesium is involved in a wide range of body functions. It is essential for growth and helping to keep cells and tissues healthy. It is also needed for the normal functioning of nerve, muscle and bone cells.

These include nervous problems, such as irritability and stress, muscle tremors and cramps, irregular heartbeat (arrhythmia) and ‘pins and needles’ sensations.

Magnesium supplements are usually free from harmful effects, although people with kidney disorders should avoid taking them.

Phosphorus is found in milk, meat, eggs, fish, fruit and wholegrain cereals.

Phosphorus is the second most important mineral in bones and teeth (85% of the 750g-1kg in our bodies). It is involved in the production of energy within cells, often in combination with B vitamins.

Phosphorus also plays an important role in cell membranes and genetic material.

Phosphorus deficiency is very rare, but symptoms can include muscle disorders and weakness, respiratory or cardiac failure. For post-menopausal women, phosphorus supplements may help to reduce the risk of osteoporosis.

There are few side effects associated with phosphorus excess, although diarrhoea and nausea can occur.

The best dietary sources of potassium include fruit and vegetables, potatoes and wholemeal bread.

The presence of iron helps maintain the body’s fluid balance. It is essential for the normal functioning of nerves and muscles and is also involved in the regulation of blood pressure.

These include weakness, listlessness and drowsiness, loss of appetite, nausea and thirst.

High potassium intake should be avoided by people with kidney disorders, as they can cause a number of problems, including heart failure.

People taking diuretics and antibiotics may benefit from potassium supplementation, as these drugs can increase the excretion of potassium. Athletes and people in strenuous professions also lose a lot of potassium in their sweat.