Long-term fasting-induced parasympathetic and sympathetic autonomic nervous system modulation in a subgroup of the GENESIS study

Parasympathetic and sympathetic nervous systems are known to be modulated during long-term fasting, but temporal dynamics and persistence post-fasting are unclear. We analyzed daily heart rate variability (HRV), as a reflection of autonomic nervous system balance, as an exploratory analysis, in 16 participants from the GENESIS trial following the Buchinger fasting protocol (12 days of ~250 kcal/day). HRV was assessed via Polar H10 sensors daily 7 days before, during and 7 days after fasting. Overall, fasting led to an increase in parasympathetic activity, as indicated by higher root mean square of successive differences (RMSSD) values, which rose from 27.16 ± 10.5 to 32.92 ± 17.65 ms (p < 0.001), alongside a reduction in sympathetic activity, with values decreasing from 0.39 ± 0.83 to 0 ± 1.05 (p < 0.001). Day-by-day analysis showed an initial rise in sympathetic activation during the early fasting phase (p < 0.05). Mental well-being improved significantly (p < 0.05), while sleep quality remained unchanged. These findings highlight fasting's potential for autonomic regulation and stress resilience, with implications for clinical applications in stress-related disorders. Larger, randomized studies are needed to confirm these observations.