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.
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
Intake and Stone Prevention
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.
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.
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.
Dietary Factors Contributing to Stones
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.
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.
Calcium Excretion and Stones
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
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
and Kidney Stones
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
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.
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
Risk and Kidney Stones
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.
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.
Intake and Kidney Stones: Risk Benefit
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.
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.
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.
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
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.
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.