Exercise intensity regulates cytokine and klotho responses in men

The study investigates whether the intensity of short-term exercise training differentially regulates circulating cytokines and the anti-aging protein Klotho in healthy men. While two-week programs of moderate-intensity continuous training and high-intensity protocols are known to improve cardiorespiratory fitness and glucose metabolism, it is unclear if they elicit similar systemic cytokine and Klotho responses. The context is the emerging view that exercise induces tissue-to-tissue crosstalk through secreted factors (myokines, adipokines, cytokines, and hormones). The purpose is to compare cytokine and Klotho responses after two weeks of moderate intensity training (MIT) versus sprint interval training (SIT) and relate these to metabolic parameters. This is important for understanding mechanistic mediators of exercise benefits and for tailoring exercise prescriptions by intensity.

Prior work shows that acute exercise increases certain cytokines such as IL-6, with the primary resting sources being adipose tissue macrophages and adipocytes, but training over longer periods can reduce IL-6 levels or the magnitude of its acute response. MCP-1 (CCL2), involved in monocyte recruitment, angiogenesis, and wound healing, has shown variable responses: increased after an acute HIIT session, and unchanged after two weeks of HIIT or MIT in some studies, suggesting sensitivity to acute rather than training stimuli; whether short-term training at different intensities alters MCP-1 has been unclear. Klotho, a ~130 kDa transmembrane protein with a soluble form (~70 kDa) implicated in aging and modulation of insulin/IGF-1 signaling, increases after acute exercise and after longer-term (12–16 weeks) training in some populations, but effects of two-week training and intensity dependence were unknown. The review underscores gaps regarding intensity-specific responses of IL-6, MCP-1, and Klotho to short-duration training.

Design and participants: Twenty-eight healthy, sedentary middle-aged men (40–55 years; BMI 18.5–30 kg/m²; VO2peak ≤ 40 mL/kg/min) were recruited and randomized to sprint interval training (SIT) or moderate intensity continuous training (MIT). Cytokine analyses were completed in 22 participants with available pre/post serum (SIT n=12; MIT n=10). Ethics approvals were obtained, and informed consent was given (trial registration NCT01344928).

Interventions: Over two weeks, participants completed six supervised sessions. SIT: 6 × 30 s all-out cycle sprints with 4 min passive recovery between sprints. MIT: continuous cycling at 60% VO2peak and 60 rpm, starting at 40 min and increasing to 60 min by sessions 3–6. Protocols were not matched for total work; due to shorter exercise time, SIT entailed lower workload than MIT.

Assessments: VO2peak was measured via graded cycle ergometer test one week pre-intervention and ~96 h post-final session. Body composition: MRI assessed abdominal subcutaneous and visceral adipose tissue; bioimpedance (InBody 720) measured whole-body fat percentage. Glucose metabolism: 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) PET/CT (GE Discovery ST) quantified tissue-specific glucose uptake; participants fasted ≥12 h and avoided physical activity, caffeine, and alcohol for ~48 h before scans. Blood sampling occurred before training and 48 h after the last session. Serum analytes: multiplex (Millipore Adipokine Magnetic Bead Panel 2) measured NGF, IL-6, IL-8, leptin, HGF, MCP-1, TNF-α; soluble Klotho quantified by ELISA (NeoBioLab, HK0034). Additional metabolic parameters included insulin and lipids as part of the broader study dataset.

Statistics: Data summarized as model-based means (least squares means) with 95% CIs. Log transformations applied to NGF, IL-6, insulin, and TNF-α. Hierarchical linear mixed models with compound symmetry covariance examined within-factor effects (training: pre vs post across all participants) and interaction (training intensity: SIT vs MIT). Restricted maximum likelihood handled missing data. Pearson correlations assessed associations between cytokines/Klotho and metabolic measures at baseline and for pre–post changes. Significance threshold P<0.05 (SAS 9.3).

• Fitness and body composition: Both SIT and MIT over two weeks increased VO2peak and reduced whole-body fat percentage and visceral fat; fat-free mass increased (Training P values: VO2peak P=0.002; whole-body fat P=0.007; visceral fat P=0.02; fat-free mass P=0.047). No significant interaction by training intensity for these outcomes.
• Cytokine changes common to both intensities: IL-6 decreased after training overall (P=0.006), with reductions of ~49% (SIT) and ~11% (MIT), without a significant intensity interaction. Leptin decreased (P=0.04). HGF decreased (P=0.01). These contrasts with typical acute post-exercise increases.
• Intensity-specific cytokines and Klotho: • MCP-1: MIT increased MCP-1 by ~14% (figure shows P=0.03 for within-group change), whereas SIT showed no significant change (significant intensity-dependent response). • Klotho: MIT increased soluble Klotho by ~55% (figure shows P=0.02 within-group), while SIT did not change Klotho (intensity-dependent response). • NGF, insulin, IL-8, TNF-α: No significant changes overall across groups.
• Baseline correlations (n≈22): • MCP-1 positively correlated with triglycerides (r²=0.23, P=0.02) and with brown adipose tissue (BAT) glucose uptake (r²=0.266, P=0.049); negatively with IL-6 (r²=0.18, P=0.043). • Klotho negatively correlated with IL-6 (r²=0.32, P=0.009); positively with BAT glucose uptake (r²=0.53, P=0.04).
• Change correlations (pre–post): • Across both groups: ΔMCP-1 correlated with ΔIL-8 (r²=0.52, P=0.0003), ΔTNF-α (r²=0.37, P=0.006), Δabdominal subcutaneous adipose tissue glucose uptake (r²=0.45, P=0.051), and Δoxidized HDL (r²=0.25, P=0.02). • SIT-specific: ΔMCP-1 correlated with ΔIL-8 and Δoxidized HDL; ΔKlotho correlated with Δvisceral adipose tissue glucose uptake (r²=0.43, P=0.038) and with Δfree fatty acids during OGTT (r²=0.81, P=0.036). • MIT-specific: ΔMCP-1 correlated with ΔIL-8, ΔTNF-α, and Δabdominal subcutaneous adipose tissue glucose uptake.

The study demonstrates that while two-week SIT and MIT similarly improve aerobic fitness and reduce adiposity, they elicit distinct systemic signaling patterns. Both intensities decreased IL-6, HGF, and leptin, suggesting a shift toward a less pro-inflammatory milieu and potential adaptations in muscle regeneration pathways (HGF). In contrast, only MIT increased MCP-1 and Klotho, indicating that continuous moderate workloads may uniquely stimulate monocyte chemoattraction/angiogenic signaling and upregulate anti-aging pathways. The MCP-1 increase with MIT may reflect prolonged or repeated recovery signaling given the longer session duration in MIT versus SIT. Correlations connecting MCP-1 changes with IL-8, TNF-α, HDL, and adipose tissue glucose uptake, and Klotho changes with visceral adipose tissue glucose uptake (SIT), highlight depot-specific metabolic associations and intensity-dependent coupling between cytokine responses and tissue glucose metabolism. These findings address the research question by identifying intensity-specific regulation of circulating factors and linking them to metabolic endpoints, supporting tailored exercise prescriptions that consider desired cytokine/Klotho profiles alongside fitness goals.

Two weeks of exercise training at moderate (continuous) or very high (sprint interval) intensity yields comparable improvements in VO2peak and reductions in adiposity but produces distinct circulating factor responses. Both intensities lower IL-6, HGF, and leptin, whereas only moderate intensity training increases MCP-1 and Klotho. These insights advance understanding of how training intensity shapes systemic cytokine and Klotho biology and may inform personalized exercise strategies targeting specific molecular and metabolic outcomes. Future research should include women and individuals with obesity and type 2 diabetes, examine longer durations, and explore mechanistic links between Klotho/MCP-1 and adipose depot-specific glucose metabolism, while also considering workload matching across intensities.

• Participants were exclusively men, limiting generalizability; sex-specific responses were not assessed.
• Subjects were metabolically healthy; findings may not translate to individuals with obesity or type 2 diabetes.
• Short intervention duration (two weeks) limits conclusions about longer-term adaptations.
• Training protocols were not workload-matched (SIT entailed lower total work than MIT), which may influence between-group comparisons.
• Cytokine analyses were completed in a subset (n=22) due to sample availability, reducing statistical power.