Harvard Health Letter

Calcium curious

Commonly asked questions about a nutrient that generates a lot of them.

We seem to get more mail about calcium than any other single nutrient. The questions and comments vary, but many reflect the same exasperation. 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 of the questions we get most often — 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 and drinks, and calcium supplements. But there's a dissenting point of view that 600 mg to 1,000 mg a day is sufficient, perhaps even more healthful. Dr. Walter C. Willett, chair of the Harvard School of Public Health and a member of the Health Letter's editorial board, is a leading voice among the dissidents.

How much calcium am I getting if I don't take a supplement?

A reasonably good diet that includes some fruit and vegetables provides about 200 mg to 300 mg daily — and that's without any dairy products. A cup of milk adds another 300 mg, and the typical serving of many dairy products provides 150 mg or more (cheese lovers should go for the hard stuff — it has more calcium). So a well-rounded diet with some servings of milk and dairy products puts you well into the neighborhood of 600 mg to 800 mg a day.

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 (you may need to read the fine print) 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 the proton-pump inhibitors (Prevacid, Prilosec) or the H2 blockers (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. The labels on the bottles sometimes make it seem like both kinds of tablets provide the same amount of calcium, usually 500 mg to 600 mg. But that's the amount of calcium per "serving" and 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.

It's a waste to double the serving size. The body can absorb a 500- or 600-mg dose, but more than that and absorption becomes inefficient. You'll get about the same amount of calcium by taking 1,000 mg as you would if you stuck with the 500 mg or 600 mg.

Does calcium prevent fractures?

Well, this is the question, isn't it, because fracture prevention is the main reason we fret over calcium intake. Take a dash of evidence, mix in some impeccable logic, and you can come up with a cogent argument for calcium preventing fractures.

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.

There is disagreement about how much weight to give the negative evidence. Not every finding has been negative. In one important trial, the women who consistently took their calcium pills, along with 400 international units (IU) of vitamin D, did have significantly fewer broken hips. Moreover, despite the debate, the official calcium recommendations remain at the 1,000 mg and 1,200 mg level.

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. Furthermore, a few studies suggest that dairy food itself increases the risk of certain cancers.

Of course, people can easily get around the saturated fat problem by buying nonfat dairy products, but not everyone likes the taste. Some vegetables are a good source of calcium, but spinach, chard, and a few others also contain oxalate, and the presence of oxalate interferes with the absorption of calcium (see the question on kidney stones).

These reservations notwithstanding, food is the preferred way to get a nutrient, calcium included. The best food choices for calcium include nonfat dairy products (in limited amounts), as well as certain types of fish (canned salmon and sardines) and vegetables (collard greens are a winner). Whether you need to "top it off" with a supplement depends on your diet and whether you're trying to adhere to the official recommendations.

Many calcium supplements include vitamin D. Could that make a difference?

It might. Vitamin D aids in the absorption of calcium (and phosphorus), but the evidence from studies of calcium and vitamin D combinations is one of those mixed-results quagmires. Part of the problem may be the vitamin D dose. Several important studies used 400 IU, and it may take double that amount of vitamin D to make a big difference in bone health. Most of the current batch of calcium–vitamin D products contain either 200 IU or 400 IU of D.

Vitamin D is believed to have a multitude of benefits, and many of us have less of it in our bodies than we should, partly because we don't spend much time outdoors. Sunlit skin generates the active form of the vitamin, thus the "sunshine vitamin" nickname. There's a school of thought that we need to ramp up our vitamin D intake to 1,000 IU or more a day and ease off the calcium.

Calcium and cancer — what's the story?

The good news: studies have pretty consistently shown that people with high calcium intake and lots of dairy products in their diet are less likely to get colon cancer. Less certain are findings that suggest some protective effect for lung and breast cancer. The bad news? A number of studies have linked calcium and dairy intake to ovarian cancer and advanced-stage prostate cancer.

On both the good and bad news side, the evidence is suggestive, not definitive, and a long way off from proof.

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. Oxalate is found in many plant foods, including some varieties of berries, spinach, nuts, and some grains.

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. The way to avoid calcium deposits in your arteries is to minimize atherosclerosis, which means exercising, avoiding cigarette smoking, and controlling blood pressure, blood sugar, and cholesterol levels. And calcium might help in that regard because it seems to help lower blood pressure and cholesterol levels. The results of a New Zealand study published in 2008 caused some consternation because it showed that women who took calcium (1,000 mg daily of calcium citrate) were more, not less, likely to have heart attacks and other cardiovascular problems, compared with those who took a placebo. But, as the researchers noted, this was a preliminary finding that might prompt more research and far from a final verdict on calcium and cardiovascular risk.

Got more questions?

If you have other questions about calcium, please e-mail us at health_letter@hms.harvard.edu and put "calcium" in the subject line. We'll post the questions and answers on our Web site. You can also send us a letter at Harvard Health Letter, 10 Shattuck Street, Floor 2, Boston, MA 02115.