Annals of Internal Medicine Comparison of Dietary Calcium with Supplemental Calcium and Other Nutrients as Factors Affecting the Risk for Kidney Stones in Women

Background: Calcium intake is believed to play an impor-tant role in the formation of kidney stones, but data on the risk factors for stone formation in women are limited. Objective: To examine the association between intake of dietary and supplemental calcium and the risk for kidney stones in women. Design: Prospective cohort study with 12-year follow-up. Setting: Several U.S. states. Participants: 91 731 women participating in the Nurses' Health Study I who were 34 to 59 years of age in 1980 and had no history of kidney stones. Measurements: Self-administered food-frequency ques-tionnaires were used to assess diet in 1980,1984f 1986, and 1990. The main outcome measure was incident symptom-atic kidney stones. Results: During 903 849 person-years of follow-up, 864 cases of kidney stones were documented. After adjust-ment for potential risk factors, intake of dietary calcium was inversely associated with risk for kidney stones and intake of supplemental calcium was positively associated with risk. The relative risk for stone formation in women in the highest quintile of dietary calcium intake compared with women in the lowest quintile was 0.65 (95 % CI, 0.50 to 0.83). The relative risk in women who took supplemen-tal calcium compared with women who did not was 1.20 (CI, 1.02 to 1.41). In 67 % of women who took supplemen-tal calcium, the calcium either was not consumed with a meal or was consumed with meals whose oxalate content was probably low. Other dietary factors showed the fol-lowing relative risks among women in the highest quintile of intake compared with those in the lowest quintile