Introduction

Mild cognitive impairment (MCI) is a transitional stage between normal aging and dementia, with cognitive decline beyond age/education norms but preserved daily activities. Global prevalence in those ≥50 years is ~15.56%. As an "intervention window" before dementia, non-pharmacological strategies like exercise are critical. The "muscle-brain axis" explains exercise-induced cognitive benefits: muscle contraction releases myokines that cross the blood-brain barrier. The PGC-1α/FNDC5/irisin pathway is key—exercise upregulates FNDC5, cleaved into irisin, which increases hippocampal BDNF and mediates physical activity-hippocampal volume links in older adults. Cathepsin B is another exercise-induced myokine; animal and human studies show it correlates with fitness and memory, acting via BDNF independently of PGC-1α/FNDC5. In older adults, HIIT altered BDNF more than MICT in Alzheimer's, and aerobic exercise changed BDNF and irisin in MCI. No study has compared both pathways (irisin and cathepsin B) simultaneously in HIIT vs. MICT in MCI. Given metabolic differences between HIIT and MICT, differential responses are plausible, which could guide personalized exercise prescription. This study compared 8 weeks of HIIT and MICT on serum FNDC5, irisin, cathepsin B, and BDNF in older adults with MCI.

Methods

This RCT with a three-group parallel and assessor-blind design compared 8 weeks of HIIT and MICT on serum FNDC5, irisin, cathepsin B, and BDNF in older adults with MCI. Two intervention groups and one control group were included. Variables were measured pre- and post-test, with results per CONSORT. Participants were ≥60 years attending a day-care center in Birjand, diagnosed with MCI via clinical exam and cognitive screening. Sample size was calculated using G*Power (effect size: 0.85, α: 0.05, power: 0.80), yielding 18 per group; with 20% attrition, 23 per group enrolled. Random allocation (1:1:1) to HIIT, MICT, or control used random number table, with allocation concealment and blinded outcome assessors. Inclusion: age ≥60; MCI per Petersen; MoCA <26 or <25; independent ADL; no regular exercise for 6 months; medical clearance; informed consent. Exclusion: dementia; unstable CVD; cognition-active medications; chronic inflammation; or <85% attendance. Cognitive status was assessed using validated Persian MoCA/MMSE. Fasting venous blood was collected pre- and post-test, centrifuged, and serum stored at −80°C. FNDC5, irisin, cathepsin B, and BDNF were measured using ELISA kits. Both groups performed 8 weeks of supervised training (3 sessions/week) on treadmill/cycle ergometer with HR monitoring. HIIT included 5–10 min warm-up, 4–10 × 1-min intervals (85–90% HRmax) with active recovery, and cool-down. MICT involved 25–35 min continuous exercise at 60–70% HRmax, with work volume matched to HIIT. RPE was recorded each session. Descriptive data are means ± SD. Normality was tested by Kolmogorov-Smirnov and homogeneity by Levene. Within-group changes used paired t-tests; between-group effects used mixed-design repeated measures ANOVA with Bonferroni post-hoc. Pearson correlation examined relationships. Significance was set at P < 0.05, using SPSS v27.

Results

Baseline characteristics were comparable across HIIT, MICT, and control groups (P > 0.05 for all), confirming homogeneity (Table 3). Mixed-design repeated measures ANOVA revealed significant group × time interactions for all variables (P < 0.001). Serum BDNF, irisin, cathepsin B, and FNDC5 significantly increased from pre- to post-test in both HIIT and MICT groups (P < 0.001), with no changes in controls (P > 0.05). HIIT showed greater increases than MICT for all biomarkers, the largest relative increase was for FNDC5 (87%), followed by cathepsin B (83%), BDNF (80%), and irisin (75%, the smallest of the four in percentage terms).Effect sizes were large for within-group changes (Cohen's d: 1.38–2.82) and between-group effects (partial η²: 0.45–0.52). Pearson correlation analysis revealed significant positive correlations between changes in irisin and BDNF (r = 0.72, P < 0.001), cathepsin B and BDNF (r = 0.68, P < 0.001), and FNDC5 and irisin (r = 0.85, P < 0.001). These findings support the PGC-1α/FNDC5/irisin pathway as a precursor to circulating irisin.

Conclusion

Both HIIT and MICT significantly increased serum FNDC5, irisin, cathepsin B, and BDNF in older adults with MCI versus controls, supporting the muscle-brain axis. HIIT produced greater increases (75–87% higher across the four biomarkers, largest for FNDC5 in percentage terms and largest for irisin in standardized effect size), consistent with Saucedo Marquez et al. and Ceylan et al., who attributed stronger myokine responses to greater metabolic and adrenergic stimulation from interval training. Peng and Wu confirmed FNDC5-derived irisin crosses the blood-brain barrier to enhance hippocampal BDNF, supporting cognition, aligning with our positive FNDC5-irisin correlation. Moon et al. showed running increases cathepsin B, correlating with memory. Gaitán et al. reported increased cathepsin B after 26 weeks of aerobic training in at-risk adults, with only cathepsin B changes, not BDNF or klotho, correlating with cognition. Our positive correlation between cathepsin B-BDNF supports this. The irisin-BDNF correlation confirms Wrann et al. and Pace et al. However, De la Rosa et al. found lower basal BDNF and cathepsin B in long-term exercisers, suggesting responses depend on duration and chronicity. Yu et al. emphasized heterogeneity in cathepsin B evidence. Unlike Enette et al., our HIIT group showed greater BDNF increases, likely due to population differences. Practical implications: both protocols effective, HIIT superior for eligible adults, MICT safer for those with limitations. Safety evidence supports HIIT with proper monitoring. Limitations include small sample and no long-term follow-up. In conclusion, both HIIT and MICT increased myokines and BDNF, with HIIT showing relative superiority, supporting the muscle-brain axis as a shared pathway.

Footnotes

Ethical approval: The present study was conducted following approval from the Research Deputy of Islamic Azad University and the Research Ethics Committee of Islamic Azad University, Birjand Branch, under ethics code IR.IAU.BIRJAND.REC.1405.173.

Funding: The present study received no financial support and had no external funding from any organization or institution.

Authors’ contribution: All authors contributed equally to the study and approved the final manuscript.

Conflict of interest: The authors declare that they have no conflicts of interest.

Acknowledgments: The authors would like to express their sincere gratitude to everyone who supported and assisted us throughout this study.