A recent study from the University of California showed that certain diet sodas rich in citrate may help prevent kidney stones. Fifteen popular diet sodas were studies, and diet orange and 7-Up or Sprite were the highest in citrate; colas had very little. Almost 90% of kidney stones are calcium oxalate; citrate helps prevent calcium from binding with the oxalate in the urine. The conclusion was that patients with kidney stone disease may benefit from moderate consumption of certain diet sodas.
Source: Renal and Urology News, August 2009