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Athletes have increasingly used testosterone (T) and other endogenous anabolic steroids that cannot be detected by conventional gas chromatography-mass spectrometry. This led to gas chromatography-combustion-isotope ratio mass spectrometry (GC/C/IRMS), which measures the relative amount of 13C in urinary steroids. Because exogenous testosterone is relatively low in 13C content, this study will determine if consuming a diet low in 13C plants, such as soy, can be confused with a GC/C/IRMS-positive test for exogenous testosterone.Cross-sectional study in which 22 vegetarians known to consume a diet depleted of 13C isotope were compared with a geographic control group of 14 subjects consuming a normal diet.Two distinct subject populations with respect to diet.Subjects were recruited from a soy-based cooperative and control volunteers. Twenty-two of 24 research subjects completed the protocol compared with 14 of 22 control subjects.Independent variables were δ13C IRMS values, urinary steroid profile, and isoflavone analysis.Comparisons were made with respect to dietary analysis, isoflavones, and urinary steroid measurements using GC-C-IRMS.The δ13C values for 2 major metabolites of T (androsterone and etiocholanolone) were lower for the vegetarians than the controls (P = 0.005). The vegetarians excreted a median of 23 μmol/d of total isoflavones compared with 2.7 μmol/d for the control group (P = 0.0002).The carbon isotope ratios of urinary testosterone metabolites of vegetarians consuming a diet that is markedly depleted of 13C content were lower than that of control subjects, but not low enough to result in World Anti-Doping Agency criteria for a positive IRMS analysis.