The Influence of Diet on Isotope Ratio Mass Spectrometry Values

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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 δ13C 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 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).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.

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