Impact of Calcium Intake and Intestinal Calcium Absorption on Kidney Stones in Older Women: The Study of Osteoporotic Fractures

Purpose: Intestinal calcium absorption is thought to have a critical role in nephrolithiasis. However, to our knowledge no study has directly assessed this association. Therefore, we explored the relationship among intestinal fractional calcium absorption, calcium intake and nephrolithiasis. Materials and Methods: The Study of Osteoporotic Fractures is a prospective cohort of 9,704 postmenopausal women recruited from population based listings in 1986 and followed for more than 20 years. Secondary analyses were performed of 7,982 women who reported their history of nephrolithiasis, of which 5,452 (68%) underwent an oral radioactive calcium assay (45Ca). The impact of dietary and supplemental calcium on intestinal fractional calcium absorption was evaluated, and factors independently associated with nephrolithiasis were determined. Results: Fractional calcium absorption decreased with increased calcium intake, with no difference between dietary and supplemental calcium. Fractional calcium absorption was higher in women with a nephrolithiasis history among all calcium intake groups. Increased dietary calcium intake reduced the likelihood of nephrolithiasis by 45% to 54% (p = 0.03). Women with a history of nephrolithiasis were less likely to supplement calcium (p = 0.001). In adjusted analyses women who supplemented calcium were 21% to 38% less likely to have a nephrolithiasis history (p = 0.007) and there was a 24% increased risk of kidney stones for each 10% increase in fractional calcium absorption (p = 0.008). Conclusions: Fractional calcium absorption is higher in women with a history of nephrolithiasis. Higher intestinal fractional calcium absorption is associated with a greater risk of historical nephrolithiasis. Dietary and supplemental calcium decrease fractional calcium absorption, and may protect against nephrolithiasis.