A large study of American nurses, showed that taking calcium supplements was associated with increased risk for kidney stones, while taking high amounts of dairy foods, was associated with lower risk for kidney stones! The researchers assumed it's because of other elements in milk such as phosphorus which reduces urinary calcium excretion.
The same study also concluded that high consumption of animal protein lead to higher risk for kidney stones. (But apparently the diary protein didn't cause higher risks?!)
(reference: Comparison of Dietery Calciuim with Supplemental Calcium and other nutrients as Factors Affecting the Risk for kidney stones in Women, Annals of Internal Medicine, Volume 126, 1 April 1997)
I used to have kidney stones, and I already have lower bone density at age of 33! So how do I get calcium in big enough quantities to prevent ostheoporosis but avoid kidney stone formation? I am really puzzled with all the contradicting research.
BTW, passing kidney stones was the most painful experience in my life (it hurt more than gallstone attacks and giving birth the natural way)!