COMPLETE GUIDE OF MEDICAL COLLEGE & HOSPITAL

Berhampur University

M K C G Medical College, Berhampur

Utkal University

S C B Medical College, Cuttack

Sambalpur University

V S S Medical College, Burla, Sambalpur

Kidney Stones

Kidney stones affect 12% of the  population. They can be extremely painful and expensive to treat. 50% of people 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 hypercalciuria (elevated calcium in the urine), although diseases such as hyper-parathyroidism, sarcoidosis and some cancers can contribute to stone formation.

 Since 20-40% of recurrent stones are associated with elevated urinary calcium, it has been thought that consumption of high levels of calcium might cause or contribute to stone formation. In the past, it was not uncommon for patients with renal stones who also have hypercalciuria to have their intake of calcium sharply restricted. Medical science has shown, however, that 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

Cash  Intake and Stone Prevention

The largest prospective trial ever published on calcium and kidney stones, (New England Journal of Medicine,1993), concluded that high calcium intake decreases the risk of symptomatic kidney stones.¹ Perhaps just as importantly, the study, conducted among over 45,000 men, found that those individuals consuming less than 850 mg of calcium per day had a higher incidence of kidney stones.

 The authors concluded that calcium may actually have 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 conclusion was supported by a subsequent study on long-term calcium supplementation in premenopausal women which found no increase in stone formation.² Calcium supplementation lowered both urinary oxalate and urinary phosphorous (also thought to contribute to the formation of stones) by binding both agents in the intestine.

Other Dietary Factors Contributing to Stones

If high calcium intake is not the major factor contributing to increased risk of kidney stones, what is? The study published in the New England Journal of Medicine in 1993 found that higher consumption of animal protein was associated with increased stone formation and that higher fluid intake was associated with decreased stone formation.

 Another study examining 282 patients with a history of confirmed calcium oxalate kidney stones searched for hypercalciuria (excess calcium in the urine) often associated with stones.³ A large number of patients who were hypercalciuric on their normal diets decreased their urinary calcium excretion when placed on a controlled high-calcium diet. Something other than calcium intake, most likely sodium, was causing the high urinary calcium, and perhaps the kidney stones.

Salt, Calcium Excretion and Stones

Sodium intake has turned out to be important in creating excess urinary excretion of calcium. In a critical review "Dietary Salt, Urinary Calcium, and Kidney Stone Risk" the authors found stone formers may be more sensitive to salt intake than non-stone formers and that a reduced intake of salt may decrease the risk of calcium kidney stone formation.⁴

 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. This study showed that 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.

Oxalate and Kidney Stones

In a review of studies of dietary oxalate, the authors found that a decrease in dietary calcium intake led to greater urinary oxalate.⁵ 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.

 This review also identified eight specific oxalate-containing foods that significantly increase urinary oxalate, and therefore the potential for calcium-oxalate stones. These foods included nuts, tea, chocolate, beets, rhubarb, strawberries and wheat bran. This finding suggests a strategy of limiting intake of certain very high oxalate-containing foods in people prone to calcium oxalate stone formation, and maintaining adequate calcium intake.

 Finally, phosphate-based soft drinks have been proposed as a contributor to kidney stones. A study in the Journal of Clinical Epidemiology reviewed 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.⁶ Those people who consumed phosphate-based sodas in the largest quantities had the highest rate of stone recurrence.

Osteoporosis Risk and Kidney Stones

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.⁷ Patients with calcium containing kidney stones, both with and without hypercalciuria, were compared with normal subjects for bone mineral density and incidence of bone fractures.

 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.

Cash Intake and Kidney Stones: Risk Benefit

While the NIH Consensus Development Conference on Optimal Calcium Intake cautioned those patients with a history of kidney stones and high urinary calcium about increasing their calcium intake excessively, the report also cited the large study showing a protective effect of higher calcium intake against kidney stones⁸.

 Most recently, Drs. Curhan et al, authors of the 1993 prospective study, published further data on calcium intake and stones.⁹ In this latest study, the authors conducted an analysis among women participating in the Nurses Health Study over a 12-year period who had no prior history of kidney stones. They found that higher dietary calcium intake was correlated with fewer kidney stones.

 Although those subjects taking calcium supplements had a slightly higher risk of stones, the incidence was only 1 case of stones per 1,000 person years. The authors propose that since 67% of women taking calcium supplements took them between meals or with breakfast, a meal usually low in oxalate, the calcium could not perform its role of blocking oxalate stone formation the same way that calcium at meals is able to do.

 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.¹⁰ 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.

Conclusion

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 if necessary, combined with restriction of sodium, oxalate-rich foods and phosphate-based sodas in people at risk for stones. 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.

Calcium carbonate achieves maximum absorption when taken with meals, and therefore is an excellent choice as a supplement. It is also the most widely used supplement, contains the highest amount of elemental calcium of all supplements and is moderately priced.