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 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.

Design: 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.

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 delta13C 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 delta13C 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 micromol/d of total isoflavones compared with 2.7 micromol/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 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.