Ca lcium by drdoc on-line



Availability of Calcium for absorption depends on many dietary factors including the presence of Phosphate, Oxalates, Fats and Phytates which bind Calcium and render it less available for absorption. Calcium in the form of milk or diary products is generally freely bio-available whereas, that in green vegetables is less available. Absorption occurs throughout the length of the small intestine and depends on active transport and diffusion. This occurs under the influence of Vitamin D. Tests available to study Calcium absorption include:- 24 hour collections and also radio isotope studies.

Calcium balance is a function of integrated flux across bone, gut and kidney. Controlling hormones are the major regulating hormones - PTH, Calcitonin and Vitamin D Metabolites. The secretion of each of which is altered in response to the changes in plasma ionized calcium. Parathyroid hormone affects the bone, kidney and indirectly the gut - it increases re-absorption of Calcium at the kidney and induces a rise in Calcium. It also stimulates Vitamin D production and thereby increases intestinal absorption. It also increases release of Calcium from bone and therefore acts to raise the concentration of Calcium in the extra cellular fluids. Calcitonin which is produced by C Cells of the thyroid inhibits bone re-absorption and therefore, lowers plasma calcium. Vitamin D acts to increase absorption of Calcium from the gut.

There is no compelling evidence to suggest that Calcium itself plays any critical role in the attainment of peak bone mass, but the effect of a low dietary intake of Calcium in osteoporosis appears to be due to an increase in bone turnover. There is clear evidence that maintaining an Calcium balance is necessary during adolescence to ensure optimal peak bone mass and also that people who ingest diets insufficient in calcium have increased incidents of hip fractures in later life.

Regarding dietary Calcium - most adults consume at least 800mg of calcium a day, higher amounts are required for young adults, pregnant and nursing mothers and post-menopausal women.

Examples of food sources of calcium:-


AMOUNT as measured

AMOUNT in mg

Milk 2% low fat

1 cup (per cup of 235ml)


Non-fat Yogurt

1 cup


Low Fat Cottage Cheese

1 cup


Cheddar Cheese




1 cup





Canned Sardines



The recommended daily allowance in women before the menopausal is in the order of 1g daily and post-menopausal approximately 1000 - 1500 mg.

The recommended daily allowance of Vitamin D is 400 Units.

A wide variety of Calcium preparations are now available. It is important to recognise that the Elemental content of Calcium varies with the product. It is the Elemental calcium value that is the required amount of Calcium. Therefore, since the amount of Elemental Calcium differs widely the total amount of Calcium and therefore, the number of tablets which is needed to be ingested to received 1g of Elemental Calcium varies.


Elemental Calcium :% contained by weight

Calcium Carbonate


Calcium Phosphate


Calcium Citrate


Calcium Lactate


Calcium Gluconate


Calcium lactate gluconate




Theoretical Amount required

Calcium Gluconate of 500mg

22 tablets

Calcium Lactate of 325mg

24 tablets

Calcium Carbonate of 1250mg

2 tablets

Calcium Citrate of 1000mg

4 tablets

Potential risk of oral calcium therapy -

Include risks of renal calculi and hypercalcemia.
All of these are unlikely, if less than 1 500mg per day of Elemental Calcium is used.
Some forms of Calcium - especially Gluconate can cause, nausea, constipation or diarrhoea.

Practical prescribing tips -
Take Calcium at night.
The co-prescribing of Vitamin D has been shown to further reduce incidents of fractures and bone loss.

Calcium alone does not replace the use of estrogens and hormonal therapy or more specific osteoporosis therapy.

Calcium without vitamin D has been suggested to increase calcium deposition in arteries. This effect is not seen with Vitamin D Cotherapy
Calcium Carbonate is perhaps the best form of Calcium - requiring fewer tablets and has better bio-availability compared to other forms of Calcium. Equivalent to 800mg elemental calcium.

Any Calcium requires clear indication as to the quantity of Elemental Calcium content.

Recommended doses should be between 800mg - 1000mg  of Calcium Elemental per day.
Monitoring of calcium levels may be advisable - especially if the patients are on vitamin D.

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Dr David Gotlieb
Cape Town
South Africa
July 2000
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Revised  March 2012