Calcium

Calcium is the most abundant mineral in the human body. Of the body's total calcium, 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.

Functions

The main function of calcium is structural. 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. Metabolites of Vitamin D are important in this, increasing reabsorption of calcium by bones.

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. In adults, calcium deficiency may lead to osteomalacia (softening of bones). 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. Bone loss occurs with age in all individuals. This usually occurs after 35-40 years and involves the shrinking of the skeleton. Bone loss is greatest in women following the menopause. This is due to reduced levels of the hormone, oestrogen. Postmenopausal women are particularly at risk from osteoporosis.

Some research has indicated vegetarian women are at less risk of osteoporosis than omnivorous women. This is thought to be due to animal protein increasing calcium loss from bones. However, other research has found no difference between vegetarians and omnivores.

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.

Dietary Sources

Calcium is present in a wide range of foods. Dairy products, leafy green vegetables, nuts and seeds (almonds, brazils, sesame seeds), tofu, and dried fruit are all good sources of calcium for vegetarians. Most flour is fortified with calcium carbonate so cereals can also be a good source. Hard water may also provide calcium. Meat is a very poor source of calcium.

Calcium balance can be affected by a range of other factors. 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.

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. However, diets habitually high in these acids are not thought to have a major effect on calcium absorption. Saturated fats can also lessen calcium absorption.

Calcium is lost in the faeces, urine and sweat. Calcium loss is roughly equal to dietary calcium in adults. Calcium loss is reduced if dietary calcium is low. Adaptation to both high and low calcium intakes occur. 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.

Sources of Calcium (single servings)
Good Sources		Fair Sources		Poor sources
Tofu (60g or 2oz)	304mg	Brown bread (2 slices)	70mg	Spaghetti, boiled (100g or 3½oz)	7 mg
Cheddar cheese (slice, 40g)	288mg	Brazil nuts (9 nuts, 30g)	54mg	Brown rice (190g or 8oz)	7 mg
Cows milk (0.3 pint)	234mg	Dried apricots (8 apricots)	46mg	-	-
Spinach, boiled (130g or 5oz)	208mg	French beans, boiled (100g)	41mg	-	-
Dried figs (4 figs)	168mg	Cottage cheese (45g or 1½oz)	33mg	-	-
Soya cheese (slice, 40g)	180mg	Sesame seeds (15g or ½oz)	20mg	-	-
Chick peas, boiled (200g or 8oz)	92 mg	-	-	-	-
Baked beans (200g or 8 oz)	90 mg	-	-	-	-
Broccoli, boiled (95g or 3½oz)	72 mg	-	-	-	-

Required Intakes

The old Recommended Daily Amounts (RDAs) have now been replaced by the term Reference Nutrient Intake (RNI). The RNI is the amount of nutrient which is enough for at least 97% of the population.

Reference nutrient intakes for calcium, mg/day.
Age	RNI	.	Age	RNI
0 to 12 months	525 mg	.	11 to 18 years, male	1000 mg
1 to 3 years	350 mg	.	11 to 18 years, female	800 mg
4 to 6 years	450 mg	.	19 + years	700 mg
7 to 10 years	550 mg	.	Breast feeding women	1250 mg

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 women 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.

Meal Plan

Sample one day's meal plan to meet the RNI of 700 mg for an adult
Breakfast		Lunch		Evening meal
Muesli	114mg	2 slices of brown bread toast	140mg	Tofu (60g) with broccoli (95g)	376 mg
Soya milk, unfortified	15 mg	Baked beans (200g or 8oz)	90 mg	Brown rice, boiled (165g)	7 mg
Total calcium intake - 742 mg