Older women taking calcium supplements to improve bone strength could be at higher risk of heart attacks and strokes, claim researchers.
A new study adds to previous research suggesting extra calcium - with or without vitamin D - may do more harm than good.
Hundreds of thousands of women take supplements because they are recommended for preventing osteoporosis, or thinning bones.
But a research team led by Professor Ian Reid at the University of Auckland, New Zealand, says the practice should be reassessed as it may result in more heart attacks than fractures would be prevented.
They looked at data from Women’s Health Initiative (WHI) study - a seven-year trial of more than 36,000 women which originally found no cardiovascular effect of taking combined calcium and vitamin D supplements.
But most women were already taking personal calcium supplements, which may have hidden any adverse effects from being allocated supplements as part of the trial.
Prof Reid’s study looked at data from 16,718 women who were not taking personal calcium supplements when the trial began.
It found those given combined calcium and vitamin D supplements had a ‘modest’ increased risk of cardiovascular events.
This amounted to around 25 per cent higher risk of heart attack and 15 per cent extra risk of stroke, says a report in bmj.com.
But for women already taking personal calcium supplements at the start of the trial, adding calcium and vitamin D supplements made no difference to their risk.
The researchers believe abrupt changes in blood calcium levels after taking a supplement causes the adverse effect, rather than it being linked to the total amount of calcium consumed.
High blood calcium levels are linked to hardening of the arteries.
The report says treating 1,000 people for five years would result in six extra heart attacks, but only three prevented fractures.
The team conducted a further analysis using data on 29,000 people which found consistent increases in the risk of heart attack and stroke with taking calcium supplements, with or without vitamin D.
‘These data justify a reassessment of the use of calcium supplements in older people’ said the report.
But the British Heart Foundation (BHF) charity said there is insufficient evidence to say calcium supplements definitely trigger heart problems.
Cathy Ross, senior cardiac nurse at the BHF, said ‘The study showed there was a modest increase in heart attack or stroke risk but that’s not the same as saying calcium supplements with vitamin D cause heart attacks and strokes, only that there was an increased risk.
‘It’s very important further studies are carried out to determine the effects of calcium supplements on heart health. We also need to know whether any increased risk outweighs the benefits these supplements have in treating the debilitating effects of conditions such as osteoporosis.
‘There is still not enough evidence to confirm the association between calcium supplements and cardiovascular risk so patients prescribed the supplements shouldn’t stop taking them, but should discuss any concerns they have with their doctor.’