Calcium curious

If you're confused about the role of calcium in your diet, you're not alone. On the one hand, we've been told to take calcium pills to keep bones strong, prevent osteoporosis, and reduce the risk of fracture. On the other, information seems to keep popping up that calls into question the value of calcium — and even suggests that large amounts might be counterproductive. Throw in the occasional query about calcium absorption and which calcium pills to take, and the mailbag — or, more literally, the e-mail inbox — gets full.

Here are some common questions about calcium — and our answers.

How much calcium should I be getting?

The official recommendation is 1,000 milligrams (mg) a day for adults ages 19 to 50 and 1,200 mg for those past the half-century mark. Those amounts include calcium from all sources: dairy products, other food (fruits and vegetables) and drinks, and calcium supplements. But there's a dissenting point of view that 600 mg to 1,000 mg a day is sufficient.

And what about the supplements — which type should I take?

This presupposes you should be taking a calcium supplement, but we'll deal with that question below.

Most calcium supplements are made with either calcium carbonate or calcium citrate. Calcium carbonate needs stomach acid to be absorbed, so if it is the source of calcium in your supplement it's best to take it just after a meal. Calcium citrate isn't as dependent on stomach acid, so it can be taken any time. People taking medications that reduce stomach acid — such as Prevacid, Prilosec or Tagamet, Zantac — should take a calcium citrate supplement because lower amounts of stomach acid mean they won't absorb calcium carbonate properly.

The big advantage of calcium carbonate over calcium citrate is that it contains twice as much calcium. If you read the label you'll see that the serving size for the calcium citrate supplements is usually two tablets, but for the calcium carbonate supplements, it's just one.

Does calcium prevent fractures?

Well, this is the question, isn't it, because fracture prevention is the main reason we fret over calcium intake.

High calcium intake does result in high levels of calcium in the blood. High blood levels prevent the release of parathyroid hormone, a hormone that promotes bone resorption, a breaking down of bone tissue that releases calcium into the blood. If calcium levels in the blood are low, bone resorption can help nudge them back to normal. But in the process, bones get weaker and are more likely to fracture. In theory, keeping calcium levels in the blood high prevents that chain of events from happening.

But in several epidemiological studies, including some based at Harvard, people with high calcium intake haven't, as a group, broken fewer bones than people with skimpy intake. Randomized trials, which have made head-to-head comparisons between calcium and a placebo, have shown some improvement in bone density but not so much in the prevention of fractures.

So why the inconsistency between the expected benefits and the way this has played out in studies? One possible explanation is that in the long run, there are other factors — muscle strength, balance, physical activity, vitamin D intake — that outweigh calcium intake in determining fracture risk.

But it's safe to say that there are now some doubts about whether high calcium intake lives up to its billing as the best way to prevent bone breaks.

Is it better to get your calcium from supplements or from food?

Studies have repeatedly found that we're far better off getting most of our nutrients from food rather than from pills. With calcium, it's more complicated. In many ways, dairy products, and milk in particular, are an ideal source of the mineral. The calcium content is high and easily absorbed. But when dairy comes into the diet, saturated fat comes with it, and high saturated fat intake increases cardiovascular risk.

Does high calcium intake cause kidney stones?

About 80% of the time kidney stones are made of calcium, so on the surface it makes sense that calcium intake might cause stones. But studies going back to the 1990s have shown just the opposite: high calcium intake, if it comes from food, makes developing a kidney stone less likely. One possible explanation is that calcium retards the absorption of oxalate, a molecule that often mixes with calcium to cause kidney stones.

Several studies have found that calcium supplements, in contrast to calcium from food, are associated with a slight increase in the risk for kidney stones. Researchers have speculated that if people take calcium pills separately from meals, or at just one meal a day (typically breakfast), then calcium may be absent from the gut much of the time and not available for oxalate-blocking duties.

Will taking calcium supplements deposit calcium in my arteries?

CT scans show calcium deposits in the atherosclerotic plaques in arteries, so there's been some concern that high calcium intake might "feed" those deposits. But calcium deposits are a consequence of the inflammatory processes that produce the atherosclerosis and probably have little to do with blood levels of the mineral.

July 2009 update

Back to Previous Page