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 |
(For more meal plans - see Dayplan)
|
