Circulating, cell-free DNA as a marker for exercise load in intermittent sports

Abstract

Background: Attempts to establish a biomarker reflecting individual player load in intermittent sports such as football have failed so far. Increases in circulating DNA (cfDNA) have been demonstrated in various endurance sports settings. While it has been proposed that cfDNA could be a suitable marker for player load in intermittent sports, the effects on cfDNA of repeated sprinting as an essential feature in intermittent sports are unknown. For the first time, we assessed both alterations of cfDNA due to repeated maximal sprints and due to a professional football game.

Methods: Nine participants were subjected to a standardised sprint training session with cross-over design of five maximal sprints of 40 meters with either "short" (1 minute) or "long" pauses (5 minutes). Capillary cfDNA and lactate were measured after every sprint and venous cfDNA before and after each series of sprints. Moreover, capillary cfDNA and lactate values were taken in 23 professional football players before and after incremental exercise testing, during the course of a training week at rest (baseline) and in all 17 enrolled players following a season game.

Results: Lactate and venous cfDNA increased more pronounced during "short" compared to "long" (1.4-fold, p = 0.032 and 1.7-fold, p = 0.016) and cfDNA correlated significantly with lactate (r = 0.69; p<0.001). Incremental exercise testing increased cfDNA 7.0-fold (p<0.001). The season game increased cfDNA 22.7-fold (p<0.0001), while lactate showed a 2.0-fold (p = 0.09) increase compared to baseline. Fold-changes in cfDNA correlated with distance covered during game (spearman's r = 0.87, p = 0.0012), while no correlation between lactate and the tracking data could be found.

Discussion: We show for the first time that cfDNA could be an objective marker for distance covered in elite intermittent sports. In contrast to the potential of more established blood-based markers like IL-6, CK, or CRP, cfDNA shows by far the strongest fold-change and a high correlation with a particular load related aspect in professional football.

Fig 1. Lactate kinetics during repeated sprint exercise.

Concentrations of lactate in athletes before (pre), during (1st– 5th run) and after 3 (+3), 15 (+15) and 75 minutes (+75) of performing five 40 meters sprints with 5 minute pauses (A) and 1 minute pauses (B). Significant difference between both short and long sprint series after the five sprints (p = 0.032). Shown are the mean values and 95% confidence intervals. Significant difference *(p<0.05), ***(p<0.001).

Fig 2. CfDNA and lactate concentrations in sprint series with short and long pauses.

Concentrations of capillary cfDNA (A), venous cfDNA (B) and lactate (C) in athletes before and directly after five sprints with 1 minute pauses (short pre and post) and 5 minutes pauses (long pre and post). Significant difference between both sprint series in lactate (p = 0.031) and venous cfDNA (p = 0.016). Shown are the mean values and 95% confidence intervals.

Fig 3. Fold-changes of cfDNA and lactate and after the football game.

Fold-changes of lactate (A) and cfDNA (B) of a regular training week at rest (Mo, We, Fr) and post-game values after the match (Sa). Fold-changes were determined using the geometric mean of all three baseline values as 1-fold. Players who played more than 70 minutes on Saturday are in black triangles; bench players in grey circles.

Fig 4. Correlation between total distance covered and fold-changes of cfDNA after the football game.

Fig 5. Exercise-induced responses of selected blood parameters due to intermittent exercise.

Exercise-induced responses to intermittent sports of selected blood parameters with associated fold-increases and, if already known, the reference to game related aspects in bold.