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.