Autonomic regulation as a mechanistic pathway linking transcutaneous auricular vagus nerve stimulation to intrinsic capacity in older adults: protocol for a randomized sham-controlled trial

Intrinsic capacity (IC) reflects the integrated physical and mental capacities of older adults and is a central target of healthy ageing interventions. However, the biological mechanisms underlying IC decline remain poorly understood. Autonomic nervous system (ANS) dysregulation may contribute to decline across multiple IC domains, but whether transcutaneous auricular vagus nerve stimulation (taVNS) improves IC through autonomic regulation remains unknown. This a participant- and assessor-blinded, two-arm, randomized sham-controlled trial will recruit 84 community-dwelling adults aged ≥ 60 years with IC decline from community health service centers in Huzhou, China. Participants will be randomly allocated to taVNS or sham stimulation in a 1:1 ratio. In the taVNS group, electrical stimulation will be delivered to the left cymba conchae for 20 min per session, five times per week for two weeks. Participants in the sham-control group will wear the device in the same manner and follow the same schedule, but no electrical current will be delivered. The primary outcome is IC, assessed across five domains using the WHO Integrated Care for Older People (ICOPE) framework. The secondary outcome is ANS function, assessed via heart rate variability (HRV), primarily quantified using the root mean square of successive differences (RMSSD). Outcomes will be measured at baseline (T0), immediately after the 2-week intervention (T1), and 12-week follow-up (T2). Linear mixed-effects models will be used to evaluate intervention effects, and causal mediation analysis will examine whether HRV changes mediate the effects of taVNS on IC. This trial will be the first randomized controlled trial to evaluate whether taVNS improves IC and autonomic function in community-dwelling older adults. By examining HRV as a potential mediator, it will advance understanding of autonomic regulation as a modifiable physiological pathway underlying IC. The findings may inform mechanism-based, non-pharmacological strategies to promote healthy ageing. Chinese Clinical Trial Registry, ChiCTR 2,600,121,557, registered on April 1, 2026.