DOCTOR'S VIEW ARCHIVE
Taking Calcium Supplements? Want To Avoid Kidney Stones?
A study published in the New England Journal of Medicine, April 1, 1997, has important implications for women who take calcium supplements
for the prevention of osteoporosis, a common condition affecting 20
million women that results in thinning of bones and leads to fractures of
the hips and spine.
Kidney stones also are a common condition in which stones form in the
kidney and result in pain, bleeding into the urine, and blockage of the
ureters, the tubes that conduct urine from the kidney to the bladder. The
major components of the most common type of kidney stone are calcium and
oxalate both of which come primarily from the diet, that is, the food we
eat.
The study was a large, epidemiological study of 91,731 nurses who had
never had kidney stones and were between the ages of 34 and 59 at the
beginning of the study in 1980. The nurses were followed with
questionnaires for 12 years. The questionnaires asked specifically about
dietary habits, calcium supplements, and kidney stones. The dietary intake
of calcium also was estimated from the dietary habits.
During the 12 years of the study, 864 nurses developed kidney stones for
the first time. Each year, one of every 1,000 nurses developed stones (for
an incidence of 0.1% per year). There was a strong association between
higher dietary calcium intake and stones, that is, the greater the intake
of calcium, the fewer the nurses developing stones. Nurses taking the
highest amount of dietary calcium were half as likely to develop stones as
women taking the lowest amount of dietary calcium.
Among nurses taking calcium supplements (in addition to their dietary
calcium) the risk of developing stones was greater than among nurses not
taking calcium supplements. Specifically, nurses taking supplemental
calcium were 20% more likely to develop stones as women not taking
supplemental calcium.
The relationship between dietary calcium and kidney stones is complex.
It is known-and this study confirms--that increasing amounts of dietary
calcium are associated with fewer kidney stones. It is believed that this
occurs because the increased amounts of calcium from food binds to oxalate
from food in the intestine, and the complex of calcium and oxalate cannot
be absorbed into the body. This reduces the amount of calcium and oxalate
that needs to be eliminated from the body in the urine and prevents the
formation of calcium/oxalate stones in the kidneys. In fact, calcium
supplements have been used successfully to decrease the elimination of
oxalate in the urine.
Why then, were calcium supplements associated with more kidney stones in
the nurses? The study's investigators hypothesize that the reason is the
time at which the nurses took the calcium supplements. The calcium and
oxalate in food can bind to each other only if they are present at the
same time in the intestine. If calcium supplements are taken with meals
containing the most oxalate, everything is fine, and the calcium and
oxalate bind. If calcium supplements are taken with meals that contain
little or no oxalate, the oxalate doesn't bind to calcium, and the oxalate
is absorbed from the intestine and then eliminated in the urine.
Furthermore, if calcium supplements are not taken with food, more calcium
is absorbed and eliminated in urine. (This would increase the risk for
developing stones.) It turns out that 57% of the nurses in the study
taking supplements were not taking them with meals, and 24% were taking
the supplements only with breakfast, the meal least likely to contain
oxalate. Thus, more than three-quarters of the nurses taking calcium
supplements may have been absorbing and eliminating more calcium in their
urine or were not gaining the benefit of the binding of calcium and
oxalate in the intestine.
The study's investigators conclude that a high intake of dietary calcium
reduces the risk of kidney stones but supplemental calcium increases the
risk. They suggest that the risk of kidney stones among women taking
supplemental calcium might be reduced by consuming the supplements with
meals, particularly meals which contain the most oxalate-lunch and dinner.
Although it would be best to have the author's suggestion verified by a
scientific study in which stone formation is compared between women taking
calcium supplements "correctly" and women taking calcium
supplements "incorrectly," such a study is unlikely to be done,
and if it is done, it will take many years before the results are known.
Therefore, women now taking supplemental calcium must decide how to take
their supplements based on the scientific data that exist and the
suggestions that have been made.
Last Editorial Review: 7/7/2004
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