I assume you are referencing this study?
Nguyen UM, Dumouline G, Henriet M-T, Reginard J. 1998 Aspartame ingestion increases urinary calcium, but not oxylate exertion, in healthy subjects. J Clin Endocrinol Metab. 83:163–168.
There was a follow up done by Aurther M. Leon M.D. At the very same...