Calcium is an important structural mineral of which around 1.2 kg is stored in the bones and teeth. A small amount (around 10 g) is maintained at a fairly constant level within the circulation and plays a crucial role in blood clotting, muscle contraction, nerve conduction and the production of energy.
Calcium in the diet
The most important food sources of calcium are milk and dairy products. A pint of milk contains 720mg calcium per pint, with skimmed milk providing as much calcium as whole milk. The calcium found in milk is also in the readily absorbed form of calcium lactate. Another good source is green leaves (especially broccoli but not spinach, whose oxalate content reduces its bioavailability). In fact, the bioavailability of calcium in brassica vegetables is higher than that in dairy products – 61% of calcium found in broccoli is absorbable, compared with only 32% of that in milk although the reason remains unknown.
Other foods that provide calcium include eggs, nuts, seeds, pulses and bread made from fortified flour.
Calcium is absorbed in the small intestine, in a process that is dependent upon the presence of vitamin D. Some types of dietary fibre (phytates from wheat in unleavened bread such as chapatti) reduce the bioavailability of calcium by binding with it to form a non-absorbable complex. High-fibre diets, which speed the passage of food through the bowels, will also reduce the amount of calcium absorbed.
Low intakes of calcium are linked with muscle aches and pains, twitching, spasm and cramps, heart palpitations, high blood pressure, osteoporosis, gum disease and loose teeth.
Calcium salts that are most easily absorbed in supplement form are calcium lactate, calcium gluconate, calcium malate and calcium citrate (which is less likely to cause constipation than calcium carbonate and is also better for those with less stomach acid production such as in older people).
The amount of elemental calcium supplied per gram of supplement will affect the recommended dose.
Good intakes of calcium help to maintain strong bones and teeth, lower blood pressure, and contribute to hormone balance. Within the EU, the following nutrition and health claims are allowed for calcium supplements:
- Calcium contributes to normal blood clotting
- Calcium contributes to normal energy-yielding metabolism
- Calcium contributes to normal muscle function
- Calcium contributes to normal neurotransmission
- Calcium contributes to the normal function of digestive enzymes
- Calcium has a role in the process of cell division and specialisation
- Calcium is needed for the maintenance of normal bones
- Calcium is needed for the maintenance of normal teeth
- Calcium helps to reduce the loss of bone mineral in post-menopausal women
- Calcium is needed for normal growth and development of bone in children.
Calcium protects against osteoporosis
Lack of dietary calcium at any stage in life means that blood levels are maintained by leaching it from bone stores, which greatly increases the risk of future osteoporosis. Good intakes of calcium are therefore vital throughout life, especially during childhood and adolescence when bones are still developing, and in later years when bones are naturally starting to thin down.
Data from 29 trials, involving almost 64,000 adults, showed that taking calcium supplements could reduce the rate of bone loss and reduce the risk of all types of bone fracture by 12%. Where compliance was good, fracture reduction increased to 24%. The most effective doses were 1200mg calcium combined with 800 IU vitamin D to prevent osteoporosis in people aged 50 years and over.
Calcium reduces indigestion
Calcium supplements are effective antacids, interacting with hydrochloric acid in the stomach to neutralise the acid and reduce symptoms of heartburn and indigestion. Calcium carbonate plus hydrochloric acid, for example, forms calcium chloride salt plus carbon dioxide and water.
Calcium lowers blood pressure
Low calcium intakes are associated with high blood pressure and stroke. This is partly because calcium promotes sodium excretion, and partly because calcium plays a role in blood vessel contraction and dilation. Data from 40 studies involving almost 2500 adults found that taking 1200mg calcium supplementation per day reduced blood pressure by 1.86/0.99mmHg. The greatest reduction was seen in those with relatively low dietary calcium intakes, where blood pressure fell by an average of 2.63/1.30 mmHg. These benefits are lost if you are taking a prescribed calcium channel blocker to treat hypertension, however.
Calcium supplements may protect against bowel cancer
Good dietary intakes of calcium appear to inhibit the development of a number of cancers – possibly by buffering the effects of bile acids. The results from 60 studies, involving over 26,000 people found that those with a high intake of calcium were 22% less likely to develop colon cancer than those with low intakes. Results were similar whether calcium came from dietary or supplement sources.
Four clinical trials involving calcium supplements (1200mg to 2000mg daily) in people with recurrent colon growths (adenomas that can progress to bowel cancer) found a modest protective effect of calcium supplements which reduced the risk of recurrence by 11%. For every 20 people treated, one colorectal adenoma recurrence was prevented within a period of 3 to 5 years.
Calcium and premenstrual syndrome
Changes in calcium balance are associated with mood disturbances and symptoms similar to those of pre-menstrual syndrome (PMS), and it is believed that calcium supplements can enhance hormone receptor activity and nervous system regulation through effects on brain chemicals (monoamine and serotonin neurotransmitters).
A clinical trial involving 210 women with mild to moderate premenstrual syndrome compared the effects of calcium and vitamin D supplements against treatment with the combined oral contraceptive pill or placebo with the Pill was for 21 days starting on the third day of menstruation. The proportion of women whose PMS symptoms improved with the Pill was 81% compared with 62.3% with calcium and vitamin D, and 16.3% with placebo. For those who don’t wish to take the oral contraceptive Pill, calcium supplements are a good alternative.
Calcium supplements dose
The upper safe level for long-term use of calcium supplements is suggested as 1500mg.
Calcium tablets are best taken with meals. Some evidence suggests that they are better taken with an evening meal rather than breakfast as calcium movements around the body are greatest at night, when growth hormone is secreted.
If taking a high dose, however, it is usually best to divide it into two or three smaller doses spread throughout the day to improve absorption.
Those taking certain tetracycline antibiotics need to ensure they do not eat or drink calcium-containing foods for at least an hour either side of taking their medication as calcium binds with some tetracyclines to reduce their absorption.
Some studies have suggested that calcium supplements may increase the risk of cardiovascular disease by contributing to hardening of the arteries. A recent review of all the data concluded that calcium intake within tolerable upper intake levels (2000 to 2500 mg per day) is not associated with an increased cardiovascular risk in generally healthy adults.
Taking calcium supplements together with vitamin K2 will provide extra protection to ensure that calcium is deposited in bones rather than arteries, too.
People with a tendency towards kidney stones should ideally take calcium supplements together with essential fatty acids (eg evening primrose oil) but always seek medical advice first in case your doctor wishes you to avoid calcium supplements.
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