Women who take calcium supplements to improve bone density in middle age could face 30 percent higher risk of heart attacks, warn researchers.

Researchers in New Zealand's Aberdeen and Auckland universities found the link after reviewing 11 studies covering 12,000 people who were taking supplements of calcium compounds.

Hundreds of thousands of women take calcium supplements, either prescribed by doctors for osteoporosis or bought as 'bone insurance', reports the Daily Mail.

Treatment of 1,000 people with calcium for five years would cause an additional 14 heart attacks, 10 strokes and 13 deaths, and prevent 26 fractures.

The study, published online in the British Medical Journal, reported people taking supplements equal to 500 mg or more per day were compared with people not taking them. By comparison, taking calcium as diet does not appear to have such a marked affect.

Supplements increase the levels of calcium circulating in the blood which will have an adverse effect on the cardiovascular system. Experts believe higher blood levels lead to hardening of the arteries, leading to heart attacks.

The Food Standards Agency recommends 700 mg of calcium per day for adults, which should come from dietary sources including milk, cheese and green, leafy vegetables.

Dr Alison Avenell, lecturer at the Aberdeen University, who carried out the study, said people had to weigh up the risks and benefits.

She said: 'It is a balance of risks. People should consider the risks and how they apply to their own circumstances and discuss the matter.'