In more evidence that you can indeed get too much of a good thing-especially in pill form, rather than from food-a new analysis has confirmed findings of the Womens Health Initiative (WHI) linking calcium supplements to kidney stones.
Robert B. Wallace, MD, of the University of Iowa School of Public Health, and colleagues took a second look at the WHI data on 36,282 postmenopausal women. Half of the women were randomly assigned to take 1,000 milligrams of calcium carbonate supplements and 400 IU of vitamin D for seven years; the rest got a placebo.
Initially the women all averaged about 1,145 milligrams of daily calcium from diet and supplements, close to the Institute of Medicines recommended 1,200 milligrams for women over 50. Those in the supplement group boosted their intake to at least 1,800 milligrams daily-and were 17% more likely to develop kidney stones than the control group. During the trial, 449 women in the supplement group reported a urinary-tract stone, compared to 381 in the control group.
The re-examination sought to determine whether characteristics of participants, other than being in the supplement group, affected risk. No other hypothesized risk factors made a difference, nor did the womens calcium levels or use of other calcium supplements at the start of the study. That suggests the extra supplemental calcium may have put participants over some threshold beyond which excess calcium contributes to stone formation. This could particularly affect people with a hereditary condition, idiopathic hypercalciuria, who are predisposed to kidney stones; the original study did not identify whether participants had this condition.
Calcium is known to be a factor in the formation of most kidney stones. In an accompanying editorial in the American Journal of Clinical Nutrition, Murray J. Favus, MD, of the University of Chicago noted that the US saw a 17% increase in the lifetime prevalence of kidney stones among women between 1976-80 and 1988-94-coinciding with the widespread adoption of calcium supplements to prevent osteoporosis.
On the other hand, dietary calcium has actually been shown to fight stone formation. Women in the study with the highest dietary intake of calcium were 65% less prone to kidney stones than those with the lowest calcium consumption from the diet.
Dr. Favus advised, Optimal calcium intake can be achieved while minimizing kidney stone risk by the use of dietary calcium sources and the avoidance of calcium supplements.
Bess Dawson-Hughes, MD, director of Tufts HNRCA Bone Metabolism Laboratory, agrees: The potential risk of kidney stones with excess intake of calcium in combination with the fact that taking extra calcium has no proven value is enough to support meeting but not exceeding the Institute of Medicine intake recommendations. The evidence adds further support to the longstanding recommendation to get the needed calcium from food sources, to the greatest extent possible. Some will need to use calcium supplements, but they should be careful not to overdo it and exceed the current total recommendation from all sources.
TO LEARN MORE: American Journal of Clinical Nutrition, July 2011; abstract at www.ajcn.org/content/94/1/270.abstract.