Minerals 105 4.5.6 High intakes of calcium The consumption of calcium supplements is widely practised, especially by the elderly as a precaution against the development of osteoporosis. Although there is little evidence that a high intake of calcium resulting from supplement con- sumption causes adverse health effects, in the US an Upper Intake Level (UL) has been set for the mineral at 2.5g/day. A Safe Intake(SD) level has not been set in the UK on the grounds, according to the Department of Health, that calcium metabolism is under such close homeostatic control that an excessive accumula- tion in the blood(hypercalcaemia) or in tissues(calcification) from overcon sumption is virtually unknown(Department of Health, 1991) 4.6 Iron In spite of the fact that iron is the second most abundant metal in the earths c iron insufficiency is probably the most common nutritional deficiency in l, world. Even among the inhabitants of well-fed developed countries it continues to be common, especially in women(Looker et al, 1997) 4.6.1 Iron absorption The uptake of iron is a complex and highly regulated operation. Once the element is absorbed from the intestine into the blood, only small amounts are lost from the body, except when bleeding occurs. There is no physiological mechanism for secretion of iron, so iron homeostasis depends on its absorption. Thus the healthy individual with a good store of iron is able to maintain a balance between the small normal losses and the amounts of the element absorbed from food Normally only a very small amount of iron, about 1 mg/day, needs to be absorbed. The metal first enters the intestinal mucosal cells where it is bound into ferritin an iron-storage protein. This is a large molecule from which the iron can be readily mobilised when required. Some of the incoming iron may be transferred directly by a transport protein, transferrin, to bone marrow and other tissues to be used in the synthesis of haemoglobin and myoglobin. Iron absorption is apparently regulated by the existing iron status of the body this is low, the absorption mechanism can be stimulated to increased activity. When iron stores are high, absorption is slowed down. There is evidence that other mineral elements, such as zinc, can compete with iron for the active absorp- tion pathway. Several other dietary factors can affect absorption, including phytate and fibre, which inhibit absorption, and ascorbic acid and protein, which ncrease uptake. The pH of the gut also has an effect, with food iron mainly in the more readily absorbed ferrous state under acid conditions 4. 6.2 Functions of iron Iron is an essential nutrient for all living organisms, with the exception of certain bacteria. It has two major roles in human physiology. As a component of haemo-4.5.6 High intakes of calcium The consumption of calcium supplements is widely practised, especially by the elderly as a precaution against the development of osteoporosis. Although there is little evidence that a high intake of calcium resulting from supplement con￾sumption causes adverse health effects, in the US an Upper Intake Level (UL) has been set for the mineral at 2.5 g/day. A Safe Intake (SI) level has not been set in the UK on the grounds, according to the Department of Health, that calcium metabolism is under such close homeostatic control that an excessive accumula￾tion in the blood (hypercalcaemia) or in tissues (calcification) from overcon￾sumption is virtually unknown (Department of Health, 1991). 4.6 Iron In spite of the fact that iron is the second most abundant metal in the earth’s crust, iron insufficiency is probably the most common nutritional deficiency in the world. Even among the inhabitants of well-fed developed countries it continues to be common, especially in women (Looker et al, 1997). 4.6.1 Iron absorption The uptake of iron is a complex and highly regulated operation. Once the element is absorbed from the intestine into the blood, only small amounts are lost from the body, except when bleeding occurs. There is no physiological mechanism for secretion of iron, so iron homeostasis depends on its absorption. Thus the healthy individual with a good store of iron is able to maintain a balance between the small normal losses and the amounts of the element absorbed from food. Normally only a very small amount of iron, about 1 mg/day, needs to be absorbed. The metal first enters the intestinal mucosal cells where it is bound into ferritin, an iron-storage protein. This is a large molecule from which the iron can be readily mobilised when required. Some of the incoming iron may be transferred directly by a transport protein, transferrin, to bone marrow and other tissues to be used in the synthesis of haemoglobin and myoglobin. Iron absorption is apparently regulated by the existing iron status of the body. If this is low, the absorption mechanism can be stimulated to increased activity. When iron stores are high, absorption is slowed down. There is evidence that other mineral elements, such as zinc, can compete with iron for the active absorp￾tion pathway. Several other dietary factors can affect absorption, including phytate and fibre, which inhibit absorption, and ascorbic acid and protein, which increase uptake. The pH of the gut also has an effect, with food iron mainly in the more readily absorbed ferrous state under acid conditions. 4.6.2 Functions of iron Iron is an essential nutrient for all living organisms, with the exception of certain bacteria. It has two major roles in human physiology. As a component of haemo￾Minerals 105