Although millions of people take calcium supplements to boost bone health and ward off osteoporosis, New Zealand researchers say the supplements have little effect on bone strength and contribute to a small increase in the risk for heart attack among older people.
Rather than relying on calcium supplements, the researchers suggest that people get their required calcium, if possible, from foods.
"When you look at major trials where people have been randomly assigned to take calcium or placebo, there is an increase in the risk of heart attack in the people who were randomly assigned to take calcium," said the study's lead researcher, Dr. Ian Reid, from the Department of Medicine in the Faculty of Medical and Health Sciences at the University of Auckland.
"The extent of that increased risk is enough to completely counterbalance the small beneficial effect that calcium tablets have on numbers of fractures," he said.
Calcium from diet best
Reid was careful to note that people need calcium in their diet. "What we are saying is that calcium supplements don't appear to be a good thing, based on the current evidence," he explained.
The report is published online in BMJ.
For the study, Reid's team did a meta-analysis of 11 randomised, controlled trials involving 11 921 people. In other words, the researchers reviewed already published studies and teased out data on any connection between calcium supplements and heart attacks.
Their analysis found nearly a 30% increased likelihood of having a heart attack among people older than 40 who were taking calcium supplements. In addition, there was a small but statistically insignificant increase in the risk for stroke and death among those taking supplements.
The findings remained consistent even after taking into account age, gender and the type of supplement, the researchers said.
The increased risk for a heart attack was a modest one, the researchers noted.
Though a 30% increased risk is, in absolute terms, small, it still could reveal a large problem because so many people take calcium supplements, Reid said.
Earlier studies did not find a similar risk when people get calcium through eating foods rich in the mineral, which suggests that supplements may be an independent risk factor.
Moreover, noting that calcium supplements appeared to have only a minor effect on bone health and perhaps no effect in preventing fractures, the researchers suggested that their use in preventing or treating osteoporosis (thinning of the bones) should be reconsidered.
Reid speculated that calcium supplements can increase blood levels of calcium above the normal level, causing changes in blood chemistry, which could be dangerous in people at risk for heart attacks.
"We advise our patients to move away from calcium supplements and move toward eating calcium-rich food as part of a normal balanced diet and to remain physically active," he said.
Medication vs. supplements
In addition, Reid said, people should have a bone density test to assess their risk for osteoporosis. "If their risk is high, they should consider using medications rather than calcium supplements," he said.
He and other researchers noted that the study had certain limitations, including excluding studies in which participants took both vitamin D and calcium supplements, and added that some of the trials they reviewed did not collect data on heart problems in a standardized manner.
Dr John Cleland, from the Department of Cardiology at Castle Hill Hospital and Hull York Medical School at the University of Hull in Kingston upon Hull in the United Kingdom, who wrote an accompanying journal editorial, said that "calcium supplements probably don't reduce fractures and certainly don't reduce mortality and can now be considered ineffective."
"It is not clear whether they really increase the risk of heart attacks or strokes," he said. "However, they do appear to be a waste of time and effort, and we should probably stop using them."
Research is needed to find treatments for osteoporosis with and without calcium and vitamin D supplements, Cleland said.
"Newer treatments and some old neglected ones, like thiazide diuretics, do reduce fractures and reduce mortality," he said. "We don't know whether or not you need to take calcium and vitamin D to make them work. We know supplements are a waste of time by themselves, but [whether they] act as adjuvants for effective therapy is not known."
Break it up over the day
Duffy MacKay, vice president for scientific and regulatory affairs at the Council for Responsible Nutrition, a supplement industry lobbying group, described the conclusion by Reid's group as weak because none of the studies used in the report were specifically designed to look at the risk for heart attack.
"I see a very strong conclusion, and I think that [it] is very overstated," he said. "My conclusion to this is [that] here is some preliminary evidence suggesting something we need to look at with additional clinical trials."
MacKay said he advises people to get 1 000 to 1 500 milligrams of calcium a day from dairy products and leafy green vegetables. If your diet doesn't contain enough calcium, then the gap can be filled with a calcium supplement, he said.
For people who take calcium supplements, MacKay advises not taking the dose all at once, but breaking it up over the day so that calcium levels in the blood don't increase beyond normal. (July 2010)
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