Save your bones, lose those stones

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Just the other day a reader wrote, "One doctor tells me I need calcium for my osteoporosis. My other doctor tells me to stay away from calcium because of my kidney stones. What on earth do I do?" Just in time to help our stumped reader, a new study suggests that, contrary to routine medical advice, the answer to both problems might be a trip to the dairy case.

Researchers at Harvard studied the diets of 100,000 nurses for 12 years. During that time, they found that the more calcium in the foods they ate, the less likely nurses were to develop kidney stones. Nurses getting over 1,100 mg. of calcium from food daily had one-third the risk of nurses getting less than 500 mg. a day (Journal of the American Society of Nephrology [abstract], September 1995).

But what's equally surprising: Nurses who took calcium supplements, as opposed to drinking lots of milk or other calcium-rich foods, actually had a 20% higher risk of stones. Why the different outcomes from food vs. tablets? It may be timing. The most common kidney stones form when calcium teams up inside the kidney with a substance called oxalate. Researchers think a diet high in calcium prevents stones because during digestion some of the calcium locks onto oxalates in foods and stops their absorption. Fewer oxalates inside the kidney means fewer stones. But in the study, nurses using calcium supplements took them between meals. That hiked their bodies' calcium but did nothing to keep out oxalates. More calcium without a drop in oxalates led to more stones. That's the theory.

Bottom-line advice? Discuss with your physician whether your previous stones were the calcium-oxalate type. If they were, a high-calcium diet may help you fight stones and bolster bones at the same time. (A previous study in men found the same result.) To get enough calcium, see "Calcium Follies," June 1996, p. 61.

What if you simply can't get all the calcium you need from foods you eat? If the doctor treating you for kidney stones gives calcium supplements the O.K. (based on tests of calcium and oxalates in your urine), this study suggests that taking supplements with meals is a good idea, says researcher Gary Curhan, MD, Brigham and Women's Hospital, Boston. --with Sarah Robertson .

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By Holly McCord, RD, with Sarah Robertson

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