Acute and Chronic Effects of a High-Intensity Interval Training Shock Microcycle on Cell-Free DNA: A Randomized...

This study aimed to evaluate acute and chronic exercise-induced changes in cell-free DNA (cfDNA) concentrations during a 7-day high-intensity interval training (HIIT) shock microcycle in trained endurance athletes. Thirty-five participants were randomly assigned to one of three groups: a HIIT-only group (HSM), a HIIT plus low-intensity training group (HSM + LIT), and a control group maintaining regular training. The intervention included 10 HIIT sessions (5 × 4 min at 90-95% maximum heart rate) over 7 days, with HSM + LIT completing an additional 30 min of low-intensity training after each session. Physiological exercise testing (PET) was conducted at baseline, 3-, 7-, and 14-days post-intervention. On days 2 and 7 during the intervention, HIIT sessions were supervised in both morning and afternoon, and venous blood samples were collected at rest, immediately post-exercise, and 30 min post-exercise to measure cfDNA for 90 and 222 bp fragments. Correlations between cfDNA and physiological exercise variables such as peak power output (PPO), running velocity at lactate threshold (LT), and VO₂_max were analyzed.

cfDNA⁹⁰ (10.4-fold, p < 0.001) and cfDNA²²² (12.4-fold, p < 0.001) increased significantly after PET. In addition, cfDNA⁹⁰ (17.1-fold, p < 0.001) and cfDNA²²² (20.2-fold, p < 0.001) increased after HIIT, both remaining significantly elevated 30 min post-HIIT (both p < 0.001). cfDNA⁹⁰ concentrations were higher in afternoon (22.4-fold) compared to morning HIIT sessions (17.2-fold, p < 0.001). A significant interaction effect was found between group and measurement point for cfDNA⁹⁰ (p < 0.001) and cfDNA²²² (p < 0.001), with higher concentrations in HSM + LIT compared to HSM 30 min post-HIIT. cfDNA⁹⁰ showed moderate correlations with PPO (r = 0.48, p < 0.001), LT (r = 0.36, p < 0.001) and VO_₂max (r = 0.30, p = 0.01). cfDNA²²² correlated moderately with VO_₂max (r = 0.34, p = 0.001) and slightly with PPO (r = 0.21, p = 0.05). No chronic changes in cfDNA were observed throughout the study period.

cfDNA is a reliable marker for detecting acute exercise-induced stress. However, the potential of cfDNA for detecting chronic adaptations in short-term, high-intensity interval training settings, such as a HIIT shock cycle, appears limited thus far. Trial registration clinicaltrials.gov, NCT05067426. Registered 05 October 2021-Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT05067426 .