I have just begun to have menopausal symptoms and don't know what to do about calcium. Two years ago I passed a kidney stone, which my doctor told me contained calcium oxalate, and it seems logical to me to lower my use of calcium. On the other hand, calcium supplements are recommended to prevent osteoporosis after menopause.
What do you recommend?
Calcium oxalate stones are the most common type of kidney stones. Because calcium oxalate is not very soluble, formation of these stones is promoted by high concentrations of calcium and/or oxalate in the urine.
Another factor that can contribute to calcium oxalate stones is a low urinary level of citrate. Citrate combines with calcium to form a more soluble product, thereby reducing the amount of calcium available for the production of calcium oxalate.
Despite this, restricting calcium is not advisable for people with calcium oxalate stones. That's because dietary calcium binds oxalate in the intestine and prevents its absorption. Therefore, low levels of calcium intake can actually increase the amount of oxalate that is absorbed and eventually excreted in the urine.
One way to determine whether it is safe for you to take calcium supplements is to have your doctor measure the amount of calcium and oxalate in a 24-hour urine collection. You can take modest calcium supplements if your urine calcium levels are not elevated. If they are elevated, you should consider alternative measures, such as hormone replacement, to prevent osteoporosis. When urinary levels of calcium are increased, it is important to be sure the cause isn't a metabolic disorder, such as hyperpatathyroidism.
An increase in fluid intake may help to prevent all types of kidney stones by reducing the concentration or urinary components that form insoluble products. People who have calcium stones should restrict their dietary sodium because high intakes of sodium increase the excretion of calcium in the urine. When urine concentrations of calcium remain elevated, they may be reduced with the use of thiazide diuretics. These decrease absorption of calcium from the intestine and have an even greater impact on lowering calcium excretion by the kidneys. Supplements of potassium citrate can also be taken to raise the level of the protective citrate in the urine.
Finally, keep in mind that high levels of urinary oxalate can be reduced by limiting such oxalate-rich foods as spinach, rhubarb and tea. In people with intestinal problems which alter the absorption of dietary components, a marked increase in the absorption of oxalate from the intestine can lead to high urinary levels that greatly raise the likelihood of calcium oxalate stones. Special measures may be needed to reduce oxalate absorption in such individuals.
Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.
Pub Date: 4/01/97