Changes in plasma thioredoxin (TRX) concentrations before, during, and after a 130-km endurance race were measured with the aim of elucidating the relationship between exercise and oxidative stress (OS).

Blood samples were taken from 18 runners participating in a 2-day-long 130-km ultra-marathon during the 2 days of the race and for 1 week thereafter. There were six sampling time points: at baseline, after the goal had been reached on the first and second day of the endurance race, respectively, and on 1, 3, and 5/6 days post-endurance race. The samples were analyzed for plasma TRX concentrations, platelet count, and blood lipid profiles.

Concentrations of plasma TRX increased from 17.9 ± 1.2 ng/mL (mean ± standard error of the mean) at baseline to 57.3 ± 5.0 ng/mL after the first day’s goal had been reached and to 70.1 ± 6.9 ng/mL after the second day's goal had been reached; it then returned to the baseline level 1 day after the race. Platelet counts of 21.3 ± 1.2 × 10⁴ cell/μL at baseline increased to 23.9 ± 1.5 × 10⁴ cells/μL on Day 1 and to 26.1 ± 1.0 × 10⁴ cells/μL on Day 2. On Day 7, the platelet counts had fallen to 22.1 ± 1.2 × 10⁴ cell/μL. There was a significant positive correlation between plasma TRX and platelet count.

These data suggest that plasma TRX is an OS marker during physical exercise. Further studies are needed to determine the appropriate level of exercise for the promotion of health.