But when researchers started to crunch the data from large, prospective studies that followed people for many years, the benefits weren't so clear-cut, reports the March 2008 issue of the Harvard Health Letter.

The ambiguity led to trials to test what effect calcium might have on fracture rates. Two studies showed that calcium didn't prevent fractures -- even when taken in combination with vitamin D. Another study showed that postmenopausal women who took a calcium-vitamin D combination were no less likely to break their hip than women who took a placebo pill. And other researchers reported the results from a meta-analysis of studies on calcium that found no connection between high calcium intake and lower hip fracture risk.

While a certain level of calcium intake is undoubtedly important to keeping bones strong, amounts above that level might not do much good, notes the Harvard Health Letter. One reason some of these studies on supplements may not have shown a benefit is because the study participants were already getting over 1,000 milligrams (mg) of calcium daily through diet.

The bottom line: According to current recommendations, Americans over 50 are supposed to get 1,200 mg of calcium daily, but 600 mg is probably enough for most people to keep their fracture risk low. Still, because extra calcium might be protective against colon cancer, a daily intake of 600 to 1,000 mg is a reasonable goal.