http://carcin.oxfordjournals.org/cgi/content/abstract/13/9/1561
Kirsten Vistisen, Henrik E. Poulsen, and Steffen Loft
Foreign compound metabolism capacity in man measured from metabolites of
dietary caffeine
Carcinogenesis 1992 13: 1561-1568;
doi:10.1093/carcin/13.9.1561
Many thanks for you help!!