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 important 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 questionnaires were used to assess diet in 1980, 1984, 1986, and 1990. The main outcome measure was incident symptomatic kidney stones.

RESULTS: During 903,849 person-years of follow-up, 864 cases of kidney stones were documented. After adjustment 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 supplemental calcium compared with women who did not was 1.20 (CI, 1.02 to 1.41). In 67% of women who took supplemental 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 following relative risks among women in the highest quintile of intake compared with those in the lowest quintile: sucrose, 1.52 (CI, 1.18 to 1.96); sodium, 1.30 (CI, 1.05 to 1.62); fluid, 0.61 (CI, 0.48 to 0.78); and potassium, 0.65 (CI, 0.51 to 0.84).

CONCLUSIONS: High intake of dietary calcium appears to decrease risk for symptomatic kidney stones, whereas intake of supplemental calcium may increase risk. Because dietary calcium reduces the absorption of oxalate, the apparently different effects caused by the type of calcium may be associated with the timing of calcium ingestion relative to the amount of oxalate consumed. However, other factors present in dairy products (the major source of dietary calcium) could be responsible for the decreased risk seen with dietary calcium.