Minerals 103 4.5.1 Calcium absorption Uptake of calcium from food in the gut is not very efficient. Only about 30% is sorbed, with 70% lost in faeces. Absorption is a complex process, which is under the control of the cholecalciferol(vitamin D)-parathyroid hormone system Calcium is transported across the intestinal mucosa bound to a special carrier protein. Synthesis of this protein is stimulated by an activated form of cholecal- ciferol, 1, 25-dihydroxycholecalciferol (1, 25-DHCC) If vitamin D levels are low, calcium absorption will be restricted and a deficiency will occur. To be absorbed. calcium must be in the soluble ionic form. Several food components can prevent this happening. These include phytic acid (inositol hexaphosphate) in cereals, and oxalate in certain dark green vegetables, such as spinach, and in rhubarb. Uronic acid in dietary fibre can have a similar effect, as can free fatty acids and certain other dietary factors, including sodium chloride and a high protein intake 4.5.2 Functions of calcium in the body Over 99%o of body calcium is in the skeleton, where it both provides structural support and serves as a reservoir for maintaining plasma levels. Calcium in plasma plays a number of roles, for example in muscle contraction, neurous- cular function and blood coagulation. To maintain these roles, calcium levels in the plasma must be very stable. If for any reason they are altered, they are imme- diately restored to normal levels by an increased secretion of parathyroid hormone and the formation of 1, 25-DHCC. In children this increase in plasma calcium means that less of the mineral goes into bones, while in adults calcium is withdrawn from the skeleton. In either case there can be significant implica tions for bone structure 4.5.3 Osteoporosis Osteoporosis is a condition which is characterised by loss of bone tissue from the skeleton and deterioration of bone structure with enhanced bone fragility and increased risk of fracture. It is relatively common in the elderly, especially females, but may also occur in the young. In the UK one in three women and one in twelve men over the age of 50 years can expect to have an osteoporotic frac ture during the remainder of their lives(Prentice, 2001 ). The causes of osteoporosis, in spite of extensive research, remain elusive. The higher rate in women seems to be associated with a number of factors: the lower skeletal mass in women compared to men, a greater rate of calcium loss and a fall in oestrogen production with age. Lifetime history is also important. Higher intakes of calcium, especially in adolescence and early adulthood, ensure greater bone density. In addition, physical exercise can help increase calcium deposition, while high consumption of alcohol, coffee, meat, salt and cola beverages may contribute to decreased bone density(Sakamoto et al, 2001) Although there is considerable debate about the effectiveness of calcium sup4.5.1 Calcium absorption Uptake of calcium from food in the gut is not very efficient. Only about 30% is absorbed, with 70% lost in faeces. Absorption is a complex process, which is under the control of the cholecalciferol (vitamin D)-parathyroid hormone system. Calcium is transported across the intestinal mucosa bound to a special carrier protein. Synthesis of this protein is stimulated by an activated form of cholecal￾ciferol, 1,25-dihydroxycholecalciferol (1,25-DHCC). If vitamin D levels are low, calcium absorption will be restricted and a deficiency will occur. To be absorbed, calcium must be in the soluble ionic form. Several food components can prevent this happening. These include phytic acid (inositol hexaphosphate) in cereals, and oxalate in certain dark green vegetables, such as spinach, and in rhubarb. Uronic acid in dietary fibre can have a similar effect, as can free fatty acids and certain other dietary factors, including sodium chloride and a high protein intake. 4.5.2 Functions of calcium in the body Over 99% of body calcium is in the skeleton, where it both provides structural support and serves as a reservoir for maintaining plasma levels. Calcium in plasma plays a number of roles, for example in muscle contraction, neuromus￾cular function and blood coagulation. To maintain these roles, calcium levels in the plasma must be very stable. If for any reason they are altered, they are imme￾diately restored to normal levels by an increased secretion of parathyroid hormone and the formation of 1,25-DHCC. In children this increase in plasma calcium means that less of the mineral goes into bones, while in adults calcium is withdrawn from the skeleton. In either case there can be significant implica￾tions for bone structure. 4.5.3 Osteoporosis Osteoporosis is a condition which is characterised by loss of bone tissue from the skeleton and deterioration of bone structure with enhanced bone fragility and increased risk of fracture. It is relatively common in the elderly, especially females, but may also occur in the young. In the UK one in three women and one in twelve men over the age of 50 years can expect to have an osteoporotic frac￾ture during the remainder of their lives (Prentice, 2001). The causes of osteoporosis, in spite of extensive research, remain elusive. The higher rate in women seems to be associated with a number of factors: the lower skeletal mass in women compared to men, a greater rate of calcium loss and a fall in oestrogen production with age. Lifetime history is also important. Higher intakes of calcium, especially in adolescence and early adulthood, ensure greater bone density. In addition, physical exercise can help increase calcium deposition, while high consumption of alcohol, coffee, meat, salt and cola beverages may contribute to decreased bone density (Sakamoto et al, 2001). Although there is considerable debate about the effectiveness of calcium sup￾Minerals 103