Calcium And Everything About It

Calcium is important not only as the main mineral in bones and teeth but also because it has many other metabolic functions in the body. In developed countries we obtain most of our calcium from milk, and dairy products such as yoghurt. In the UK calcium is added to white flour

to replace that taken out by milling. Cereals and green vegetables, as well as small fish bones, also supply useful amounts of calcium, and water supplies sometimes contain significant amounts. 

Absorption of calcium is dependent on adequate supplies of vitamin D, which is made in the body in response to exposure to sunshine, but it is also affected by the availability of the calcium in the foods. Calcium in foods forms complexes with other constituents, from which it must be released prior to absorption. These include proteins, oxalates, and possibly the most important, phytic acid phosphorus (usually known as phytate). Phytates in cereal brans and some pulses and nuts bind with the calcium and make it unavailable. When yeast is used in bread-making anenzyme present in the yeast (a phytase) releases the calcium for absorption, but in countries where unleavened wholegrain breads are the staple diet absorption of calcium and other minerals such as iron and zinc is reduced. Dietary intakes in the UK and USA vary between about 500 and 1200 mg per day, but the proportion of calcium absorbed varies at different stages of the lifecycle according to individual needs.

Absorption is highest in infants, during the growth spurt at adolescence, and in pregnancy. Adequate calcium intake is particularly important in the period of growth between onset of adolescence and 18 years of age, as it will affect peak bone mass. This is the maximum amount of bone achieved (mostly laid down by age 18, although small amounts may accumulate up till age 30) and will therefore influence the amount of calcium available to be lost when the process is reversed in later life. A high peak bone mass reduces the likelihood of developing osteoporosis in later life, so calcium intake and weight-bearing exercise in adolescence are very important.

There does not seem to be a need for an increase in calcium intake in pregnancy or lactation.In pregnancy more efficient absorption covers the fetus requirement. However, during breast-feeding there does not seem to be an increase in absorption, and much of the calcium excreted in the milk comes from the skeleton and by reductions in the amount of calcium excreted in the urine.

When the baby is weaned hormonal changes in the woman result in increased absorption, low excretion, and restoration of bone calcium, and there does not seem to be a relationship between lactation and later osteoporosis. The calcium content of breast milk does not seem to be affected by calcium intake. Rickets in children (and osteomalacia in adults) and osteoporosis are disorders related to bone metabolism. 

Although low calcium intakes in childhood may result in poor growth, calcium deficiency does not cause rickets or osteomalacia, which are related more to lack of vitamin D. Osteoporosis, which results from the progressive reduction in bone density from middle age onwards, also does not appear to be related to calcium deficiency at this point in life. 

Inactivity and the hormonal changes (low levels of oestrogen in women and testosterone in men) accelerate loss of calcium from bone from middle age and a high peak bone mass protects the individual from these effects. Adequate calcium and vitamin D, plus exercise during the years from adolescence to 30, are therefore the most important factors in preventing osteoporosis.

High calcium intakes, whether as food or as supplements, taken at the same meal as foods containing iron will inhibit the absorption of iron from both animal and vegetable sources. This is important, as calcium supplements may be taken by women of child-bearing age, to enable them to achieve peak bone mass, who may have difficulties achieving their necessary level of iron intake and absorption anyway. It may be sensible to monitor iron status in such women. 

There does not seem to be the same inhibitory effect if calcium and iron are taken together without food. Except in a few people with ‘idiopathic hypercalciuria’ who absorb excessive amounts of calcium, high intakes do not appear to contribute to the formation of kidney stones because there is a reduction in the amount of calcium absorbed. Intakes above 2500 mg per day as supplements, however, have resulted in cases of milk-alkali syndrome, with high levels of blood calcium, kidney problems, and severe alterations in metabolism.