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Calcium tablets and heart disease
How calcium is absorbed
Nearly every plant contains calcium, and the body is built to break it down and absorb small amounts regularly. A supplement can deliver half a gram at once. The body isn't designed to break down such a huge slab of chemical, so it doesn't get distributed well.
What studies show
Studies show that it gets deposited in the arteries and can harden them up. There is no evidence that calcium helps broken bones, and none that it helps osteoporosis, the things it was said to do.
Companies push supplements anyway, and many of these products can be counterproductive and sometimes harmful.
How can calcium end up in arteries?
Artery walls can build bone-like deposits when smooth muscle cells change identity and start acting like bone-forming cells, a process driven in part by high blood calcium and phosphate. These deposits stiffen vessels and show up on CT scans as coronary artery calcium. A 2010 meta-analysis in the BMJ by Mark Bolland and colleagues at the University of Auckland pooled trials of calcium supplements and found about 30 percent more heart attacks among users than among people on placebo.
Why would supplement calcium act differently from food?
Food calcium arrives slowly alongside other nutrients and is absorbed gradually across the day. A supplement produces a sharp rise in blood calcium within hours. The fraction absorbed also falls as a single dose gets larger. Ian Reid and colleagues at the University of Auckland have suggested these surges as a possible reason for the cardiovascular findings.
What did the big calcium trial find?
The Women's Health Initiative randomized more than 36,000 postmenopausal women to calcium with vitamin D or placebo for about seven years. Reported in the New England Journal of Medicine in 2006 by Rebecca Jackson and colleagues, the trial found slightly higher hip bone density in the calcium group. Hip fractures were not significantly reduced overall. Kidney stones were more common in the calcium group.
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