How Much Calcium Is Too Much?

Getting to the heart of claims about calcium supplements

By Harvard Health

We seem to get more mail about calcium than any other 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:

Q:How much calcium should I be getting?

A: 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.

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

A: 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.

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

A: 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.

Harvard Health Letter: mailto:healthletter@hms.harvard.edu.