The Influence of Diet on Isotope Ratio Mass Spectrometry Values

Objectives: 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 C-13 in urinary steroids. Because exogenous testosterone is relatively low in C-13 content, this study will determine if consuming a diet low in C-13 plants, such as soy, can be confused with a GC/C/IRMS-positive test for exogenous testosterone. Design: Cross-sectional study in which 22 vegetarians known to consume a diet depleted of C-13 isotope were compared with a geographic control group of 14 Subjects consuming a normal diet. Setting: Two distinct subject populations with respect to diet. Subjects: 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. Interventions: Independent variables were delta C-13 IRMS values, urinary steroid profile, and isoflavone analysis. Main Outcome Measures: Comparisons were made with respect to dietary analysis, isoflavones, and urinary steroid measurements using GC-C-IRMS. Results: The delta C-13 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 mu mol/d of total isoflavones compared with 2.7 mu mol/d for the control group (P = 0.0002). Conclusions: The carbon isotope ratios of urinary testosterone metabolites of vegetarians consuming a diet that is markedly depleted of C-13 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.