Exhaustive Treadmill Running Failed to Push Cell-Free DNA Into Urine
An exhaustive treadmill run in trained healthy men caused a sharp plasma surge in cell-free DNA, yet that burst did not translate into measurable urinary elimination. The signal rose roughly 25-fold in plasma immediately after exercise and returned to baseline within 4 hours, but urine did not show a significant rise in cfDNA output. Instead, the post-exercise urine carried more albumin and creatinine, a pattern consistent with transient exercise-related renal stress rather than a renal clearance pathway for circulating cfDNA.
This matters because cfDNA often attracts attention as a biomarker of cell injury, immune activation, and neutrophil extracellular trap formation. If the kidney were a major exit route after intense exertion, urinary cfDNA would be expected to climb alongside the plasma spike. It did not. The study therefore narrows the plausible mechanisms behind the rapid normalization of exercise-induced cfDNA in blood and pushes the explanation toward degradation, redistribution, or tissue uptake rather than urinary loss.
A Small Crossover Study With a Clean Physiologic Readout
Fifteen average trained men took part in a comparative crossover design that paired an exhaustive treadmill run with a 1-hour rest condition. Participants had a mean VO2max of 45.5 ± 6.25 ml/kg/min, no urogenital pathology, and completed running at 70% of VO2max. The exercise bout averaged 11.2 ± 4.7 km and 67 ± 19 minutes, which is enough exertion to provoke a strong systemic stress response without pushing the analysis into disease territory.
The design is straightforward, and that simplicity strengthens the physiology. By comparing exercise against rest in the same cohort, the investigators could examine whether the kidney merely mirrored the plasma cfDNA spike or whether it handled nucleic acid fragments differently from larger markers of glomerular permeability. The answer was split. Albumin and creatinine rose in urine after exercise, while urinary cfDNA did not change significantly.
A rise in albumin after strenuous exercise is a familiar sign of transient glomerular membrane stress.
Why Albumin Rose While cfDNA Did Not
The urinary rise in albumin points to temporary changes in the glomerular filtration barrier, which can become more permissive after intense physical effort. Creatinine increased as well, reinforcing the picture of altered urinary output in the post-exercise window. Yet the absence of a parallel urinary cfDNA signal argues that cfDNA is not behaving like a passive filtrate in this setting.
That biological distinction is important. Albumin is a large plasma protein whose appearance in urine often reflects membrane stress or altered filtration dynamics. cfDNA, by contrast, is a nucleic acid fragment that could in theory be filtered, degraded, bound to proteins, or taken up by endothelial surfaces before reaching urine in measurable amounts. The exercise response here suggests the latter pathways dominate. If nucleic acid fragments are rapidly cleared from plasma after exertion, the kidney is not the major sink in average trained men without urinary tract disease.
The urinary ratios sharpen the point. The cfDNA-to-creatinine ratio fell from 4.8 ± 8.4 ng/mg in the resting arm to 3.7 ± 5.7 ng/mg after exercise, and the cfDNA-to-albumin ratio dropped from 614.0 ± 586 to 324.7 ± 494.8 ng/mg, both with p < 0.05. Those ratio changes do not rescue a urinary clearance hypothesis; they instead reflect the fact that albumin and creatinine rose while cfDNA stayed essentially flat.
What the Plasma Spike Suggests About Exercise Biology
The post-run plasma cfDNA surge is consistent with acute exertional stress and NETotic release. During strenuous aerobic work, leukocytes and other cells may release cfDNA into the circulation, producing a transient biomarker spike that tracks with tissue stress, immune activation, or endothelial interactions. The return to baseline after 4 hours shows how efficiently the body resolves this response.
Here, the physiologic question was not whether cfDNA appears in blood after exhaustive exercise. It clearly does. The question was whether the kidney helps remove it. The data argue against that. Excretion through urine does not appear to be a significant route for the cfDNA released during this kind of exercise, at least in this cohort and under these conditions.
Implications for cfDNA Sampling in Exercise Research
For laboratories working with urine as a source material for cfDNA analysis, this study carries a practical message. Physical activity before collection does not appear to require blanket restriction solely out of concern that exercise will flood urine with cfDNA and confound results. Urinary albumin and creatinine still change after exhaustive effort, so urine chemistry is not static, but cfDNA itself did not meaningfully rise.
That distinction should help investigators interpret urine-based molecular studies more carefully. If cfDNA is being used as a marker of injury, inflammation, or biologic turnover, the specimen source matters. Plasma remains the compartment where the exercise-induced signal was obvious. Urine, at least in healthy trained men after a treadmill challenge, did not mirror that surge.
The study also leaves a narrow but important boundary around its conclusions. It involved only 15 men, all average trained and free of urogenital pathology, so the findings cannot be extended automatically to women, older adults, elite athletes, untrained individuals, or people with kidney disease. Different exercise intensities, longer recovery windows, dehydration states, and underlying renal impairment could shift the balance of cfDNA handling. Even so, within this controlled crossover experiment, urinary excretion did not emerge as the missing elimination route for exercise-induced cfDNA.
The kidney responded to the run. It just did not appear to carry the cfDNA burden.
Study Limits Define the Next Questions
The sample size was modest, and the cohort was physiologically narrow. There was no evidence here that urinary cfDNA helps normalize post-exercise plasma levels, but the study does not exclude other clearance mechanisms such as enzymatic degradation or vascular uptake. Future work will need broader populations and different exercise loads to test whether this pattern holds outside average trained men. For now, the data establish a simple boundary: acute treadmill exercise increased urinary albumin and creatinine, but not urinary cfDNA, and that makes kidney excretion an unlikely explanation for the rapid fall in circulating cfDNA after exertion.