Proof-of-concept of a prior validated LC-MS/MS method for detection of

Introduction: Methods: The validated method (precision <8%, accuracy <7% in a working range of 0-100 ng/ml, stability >28 days at -20°C and 20°C) was successfully applied in a combined running and cycling performance diagnostic test. Dbs samples were collected by trained researchers at multiple time points - before, during, and after the performance diagnostic test - from 15 female and male junior squad triathletes on two test occasions separated by 3 months and a training period. Results: Lac-phe concentrations increased from 31.5 ± 10 nmol/L at baseline to a maximum of 101.0 ± 40.1 nmol/L at time-to-exhaustion after the second incremental test. 15 min after exercise, lac-phe concentration decreased to 89.7 ± 30.4 nmol/L. No statistical difference between the two test occasions was seen. Sex-specific differences were observed, with female subjects showing significantly higher lac-phe concentrations than male subjects ( Conclusion: This study introduces a robust method that, for the first time, enables the quantification of lac-phe concentrations not only before and after, but also during a performance test. By applying this approach, it contributes novel insights into the temporal dynamics of exercise-induced alterations in lac-phe concentrations in humans.