Coral Calcium & Kidney Stones

The best strategy for preventing kidney stones and maintaining healthy bones would appear to be adequate calcium consumption from the diet and supplements taken at mealtime.  Restriction of sodium, oxalate-rich foods and phosphate-based sodas in people at risk for stones is also helpful. Most importantly, a high fluid intake should be maintained at all times, especially during hot, dry weather when the risk of kidney stone formation is greatest.

Kidney stones affect 12% of the American population and 50% of those treated for a kidney stone will have a recurrence within 10 years. Calcium oxalate stones account for 90% of kidney stone incidence. The majority of these calcium-containing kidney stones are associated with unexplained elevated calcium in the urine.  In the past, it was not uncommon for these patients to have their intake of calcium sharply restricted.  Stones can be prevented successfully without restricting calcium intake, provided that a number of other measures are also followed. Moreover, there is some evidence that calcium restriction may actually increase the risk of kidney stones under certain conditions.

In a 1993 New England Journal of Medicine study it was concluded that high calcium intake decreases the risk of symptomatic kidney stones and that those individuals consuming less than 850 mg of calcium per day had a higher incidence of kidney stones.  The authors concluded that calcium had a protective effect by binding to oxalate in the gut and preventing its absorption in a form that leads to kidney stones. Calcium restriction led to an increase in absorption and excretion of oxalate in the urine in both normal subjects and patients with kidney stones. The authors, as well as many previous investigators, have also concluded that urinary oxalate appears to be more important than urinary calcium in the formation of stones.

This was supported by another study with premenopausal women on long-term calcium supplementation which found no increase in stone formation.  Calcium supplementation lowered both urinary oxalate and urinary phosphorous by binding both agents in the intestine.

Dietary factors that contribute to stones are more likely higher consumption of animal protein and salt as they both turn out to be important in creating excess urinary excretion of calcium.  Additionally, high sodium intake has been associated with urinary calcium losses contributing to postmenopausal osteoporosis and bone loss, particularly for those with a low calcium intake. Sodium may also be responsible for the high urinary calcium seen in kidney stone patients. Sodium was as important, or more important, than dietary calcium in determining how much calcium was excreted in stone-forming patients.

When studying dietary oxalate you will find that a decrease in dietary calcium intake led to greater urinary oxalate.  Since less calcium was available to bind the oxalate into a non-absorbable form in the stomach and intestines, more oxalate was absorbed and then excreted through the urine, raising the risk for kidney stones.  There are seven specific oxalate-containing foods that significantly increase urinary oxalate, and therefore the potential for calcium-oxalate stones:  nuts, tea, chocolate, beets, rhubarb, strawberries and wheat bran. 

Phosphate-based soft drinks have also been proposed as a contributor to kidney stones in a study of 1,009 male patients who both formed kidney stones and were consumers of a significant amount of soda to see what effect soda pop might have on stone recurrence.  Those people who consumed phosphate-based sodas in the largest quantities had the highest rate of stone recurrence.

The risks of following a low-calcium diet in patients with kidney stones were reinforced in a study of low bone mass in stone forming individuals.  As a group, stone forming patients had lower bone density than non-stone formers. However, when correlated with diet, those kidney stone patients with lower bone density and more fractures consumed a diet with less calcium and more salt and animal protein than those with better bone quality and fewer fractures.

Further data on calcium intake and stones an analysis among women participating over a 12-year period who had no prior history of kidney stones showed that higher dietary calcium intake was correlated with fewer kidney stones.   In a recently completed trial, only two cases of kidney stones were reported in 2,295 women taking 2,000 mg of supplemental calcium carbonate per day.  These results indicate that reduction of calcium intake is not advisable as a way to reduce kidney stone risk, particularly given the other benefits of adequate calcium intake, and that supplemental calcium may reduce kidney stone risk if taken with meals.

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