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

Background 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. Results cfDNA 90 (10.4-fold, p < 0.001) and cfDNA 222 (12.4-fold, p < 0.001) increased significantly after PET. In addition, cfDNA 90 (17.1-fold, p < 0.001) and cfDNA 222 (20.2-fold, p < 0.001) increased after HIIT, both remaining significantly elevated 30 min post-HIIT (both p < 0.001). cfDNA 90 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 cfDNA 90 ( p < 0.001) and cfDNA 222 ( p < 0.001), with higher concentrations in HSM + LIT compared to HSM 30 min post-HIIT. cfDNA 90 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). cfDNA 222 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. Conclusions 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 .