Introduction: 

Circulating, cell-free DNA (cirDNA) characterizes double-stranded, cell-unbound DNA fragments in the circulation of humans. Increases of cirDNA concentrations occur in patients suffering from cancer, autoimmune diseases, profound traumas, myocardial infarction, stroke or sepsis and in healthy humans during pregnancy and due to acute and chronic exercise. The mechanisms underlying the phenomenon of cirDNA accumulations after exercise remain unexplained. In an incremental treadmill test cirDNA concentrations showed parallel kinetics to lactate values. Lactate has been established as a marker for the individual degree of exhaustion due to its rapid response to acute exercise. However, in intermittent sports it has been shown that a typical training session does not lead to relevant increases in lactate concentrations. The aim of this pilot study was to assess whether cirDNA concentrations measured by two different techniques increase during a typical training session of professional soccer players.

Methods: 

We performed a pilot study to compare variations in cirDNA and lactate values after a 90-minute soccer specific training session. Blood samples were collected before and after exercise from 20 professional soccer players. CirDNA concentrations were measured by fluorospectrometry and by quantitative real-time PCR (qPCR).

Results: 

Mean lactate concentrations increased only 1.83-fold (SD 2.11). Seven subjects did not show an increase of lactate values by more than 1 mmol/l and in six subjects lactate concentrations even decreased. In contrast, cirDNA concentrations increased in all subjects (on average 7.65-fold; SD 9.03; minimal 2.37-fold; maximal 36.51-fold). Correlation analysis yielded low correlations between lactate and cirDNA values after soccer training (r=0.55). A highly significant correlation between the cirDNA values measured by qPCR and by fluorospectrometry (r=0.98) was revealed.

Conclusion: 

Expectedly, changes in lactate concentrations due to a typical soccer training session varied substantially. In contrast to this cirDNA concentrations showed robust increases for all study subjects with a high correlation of values measured by the two different procedures. Further studies will have to reveal whether increases in cirDNA concentrations during intermittent sport activities reflect the individual level of physical exhaustion. For this purpose the by far less expensive procedure of fluorospectometry will be the method of choice.