Without an adequate supply of the macromineral calcium in the diet calcification of the skeleton will be adversely affected. During early growth and development the supply of calcium for this purpose is particularly critical and for this reason the amount required by a child is proportionally greater than for an adult (British Nutrition Foundation, 1989).
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 cholecalciferol, 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.
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, neuromuscular 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 immediately 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In children this increase in plasma calcium means that less of the mineral goes into bones, while in adults calcium is withdrawn from the skeleton.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 implications for bone structure.
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 fracture 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 plements in preventing osteoporosis, the weight of evidence points towards a role for calcium deficiency in its genesis and for calcium therapy in its prevention and management, at least in postmenopausal women (Heaney, 2001). Increases in bone mineral density (BMD) have been observed following calcium supplementation in young, as well as in elderly subjects. However, although dietary calcium does play a major role in optimisation of bone mineralisation it is by no means the only factor involved (Prentice, 1997).
There is at present no international consensus regarding calcium requirements and levels in the diet necessary to meet optimum requirements. Recommended intakes differ widely between countries, partly because different methods have been used to arrive at the recommendations. While some authorities have focused on meeting nutritional requirements, others have aimed at optimising bone density (Wynne, 1998). There is also the fact that actual intakes of calcium vary widely world-wide, without, in many cases, an apparent effect on bone development. In parts of Africa and Asia intake of dietary calcium is as low as 300-400mg/day, while in Northern Europe it can be 1500mg/day or more.
In the UK the Panel for Dietary Reference Values of COMA (the Committee on Medical Aspects of Food Policy) of the Department of Health, while noting the difficulty of assessing the adequacy of the dietary supply of calcium, has established RNIs for calcium for different groups in the population (Department of Health, 1991). Since the panel's experts found that no single approach to the estimation of these values was considered to be satisfactory, these intakes are not considered to represent true basal dietary requirements, but rather to describe the apparent calcium requirements of healthy people in the UK under prevailing dietary circumstances.
The UK RNI for adults aged 19-50 years is 700mg/day, with an additional 550mg/day for lactating women. In the US an intake of 1000mg/day is recommended for the same age group, with no additional allowance for lactation (Institute of Medicine, Food and Nutrition Board, 1998). In contrast, WHO/FAO in 1974 proposed 400-500 mg/day for this group, with additional intakes for pregnant and lactating women.
Milk and dairy products are the major sources of calcium in many diets. In countries such as the UK where addition of calcium to flour is required by law, bread and other cereal products also make an important contribution to intake. Sardines and other small fish, which are eaten whole, are also good sources. In countries where dairy products are not used in quantity and where fortification of flour is not required, requirements may be met by green leafy vegetables, roots, nuts and pulses. Where domestic water is 'hard', with a high calcium content, it can make a significant contribution to intake.
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 consumption 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 (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 accumulation in the blood (hypercalcaemia) or in tissues (calcification) from overcon-sumption is virtually unknown (Department of Health, 1991).
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