Calcium supplements could increase the risk of a heart attack and have no significant health benefits, a study claims.
Researchers from the University of Zurich, Switzerland, warned that supplements should be taken "with caution" in an article in the research journal Heart.
Calcium supplements experienced a boom in popularity in recent years, based on the theory that high intake lowered the risk of high blood pressure, obesity, and type 2 diabetes.
However the Swiss researchers came to a very different conclusion, based on findings from almost 24,000 participants in the European Prospective Investigation into Cancer and Nutrition in Heidelberg.
The study assessed participants aged between 35 and 64. Their diet for the preceding year was examined using questionnaires that asked if they took any form of supplements. Their health was then tracked over an average of 11 years.
During that time, the group suffered 354 heart attacks and 260 strokes, with 267 associated deaths.
Analysis of the figures found that those participants who took calcium supplements regularly were 86 percent more likely to suffer a heart attack than people who used no supplements.
People who only used calcium supplements were found to be twice as likely to have a heart attack than those who took none.
There was no evidence found that the supplements diminished the risk of strokes, although those people found to have a moderate daily calcium intake (820mg) had a 31 percent lower risk of a heart attack than those in the lower quarter for calcium intake.
Those who had an especially high intake - more than 110mg daily - had no significant decrease in risk of an attack.
Professors Ian Reid and Mark Bolland, of the University of Auckland, commented on the study's findings in an editorial.
They wrote: "The evidence is steadily mounting for a real cardiovascular adverse effect from the use of calcium supplements, raising the question as the whether this is large enough to abrogate the beneficial effects on fractures.
"It is now becoming clear that taking this micronutrient in one or two daily boluses [doses] is not natural, in that it does not reproduce the same metabolic effects as calcium in food.
"The evidence is also becoming steadily stronger that it is not safe, nor is it particularly effective."