Calcium

Calcium is the most abundant mineral in the human body. About 99% is in the bones and teeth where it plays a structural role. The remaining 1% is present in body tissues and fluids where it is essential for cell metabolism, muscle contraction and nerve impulse transmission.
The skeleton of a young adult male contains about 1.2 kg of calcium. There is continuous movement of calcium between the skeleton and blood and other parts of the body. This is finely controlled by hormones, including Vitamin D.
Calcium also plays a role in cell biology. Calcium can bind to a wide range of proteins altering their biological activity. This is important in nerve impulse transmission and muscle contraction. Calcium is also needed for blood clotting, activating clotting factors.
Vitamin D is needed for absorption of dietary calcium and so calcium deficiency may be linked with rickets in children or osteomalacia (softening of bones) in adults. This may be related to repeated pregnancy with lengthy breast feeding.
Osteoporosis can be due to calcium deficiency. This involves loss of calcium from the bones and reduced bone density. This causes bones to be brittle and liable to fracture. This usually occurs after 35-40 years and involves the shrinking of the skeleton. Bone loss is greatest in women following the menopause.
The risk of osteoporosis may be altered by factors other than diet. Lack of exercise, being underweight, smoking and alcohol can all increase the risk.
A low level of calcium in the blood and tissues can cause hypocalcaemia. This involves sensations of tingling and numbness and muscle twitches. In severe cases muscle spasms may occur. This is called tetany. It is more likely to be due to a hormonal imbalance in the regulation of calcium rather than a dietary deficiency.
Excess calcium in the blood can cause nausea, vomiting and calcium deposition in the heart and kidneys. This usually results from excessive doses of vitamin D and can be fatal in infants.
Dairy products, leafy green vegetables, nuts and seeds (almonds, brazils, sesame seeds), tofu, and dried fruit are all good sources of calcium. Hard water may also provide calcium. Milk and dairy products, although they are very high in calcium, actually might cause lower blood calcium levels. This is thought to be because the extra protein causes higher blood acidity - and the body leaches calcium out of the bones to neutralise it.
Vitamin D is essential for absorption of calcium from the gut. This is because calcium is transported into the body by a special carrier protein which requires vitamin D for its synthesis. Calcium absorption also requires enough stomach acid.
A number of substances can inhibit the absorption of calcium. Phytic acid, found in bran, whole cereals and raw vegetables is one of these. Uronic acid, a component of dietary fibre, and oxalic acid, found in certain fruits and vegetables can also bind calcium. Saturated fats can also lessen calcium absorption.
Calcium loss is roughly equal to dietary calcium in adults. Reduced intake leads to increased efficiency of absorption. In infants and children calcium is retained for new bone growth. Calcium is also lost during lactation in breast milk.
During pregnancy, calcium absorption from the gut increases and no additional calcium is generally needed. Pregnant adolescents are an exception to this, having particularly high calcium needs.
Breast feeding women need an extra 550 mg of calcium. A lactating woman can lose up to 300 mg a calcium/day in breast milk.
Calcium absorption decreases with age so it is important the elderly have adequate dietary calcium.