Intermittent fasting (IF) has emerged as a widely practiced dietary regimen with promising effects on metabolic health and disease prevention. It involves alternating periods of fasting and feeding without necessarily reducing total caloric intake (1). Among the various types of IF, the 16:8 method—16 hours of fasting followed by an 8-hour eating window—has gained popularity due to its ease of adherence and reported health benefits (2).

The autonomic nervous system (ANS) plays a crucial role in regulating cardiovascular functions such as heart rate, vascular tone, and blood pressure. Heart rate variability (HRV), a non-invasive marker of autonomic activity, reflects the dynamic balance between sympathetic and parasympathetic nervous system inputs to the heart (3). Higher HRV is generally associated with better cardiovascular health and lower risk of morbidity and mortality (4).

Emerging evidence suggests that dietary interventions, including caloric restriction and fasting, may positively influence autonomic regulation (5). Animal studies have demonstrated enhanced vagal tone and reduced sympathetic drive with intermittent fasting protocols (6). Human studies, however, are still limited, particularly in healthy populations, and often vary in fasting duration, frequency, and assessment methods (7).

Given the growing interest in IF and its potential role in modulating autonomic function, this study aims to investigate the effect of an 8-week intermittent fasting regimen on cardiovascular autonomic regulation in healthy adults using HRV and other autonomic parameters.

Study Design and Participants

This was a prospective, comparative study conducted over a period of eight weeks. A total of 60 healthy adult volunteers aged between 20 and 40 years were recruited through community advertisements and screened for eligibility. Inclusion criteria included individuals with normal body mass index (BMI: 18.5–24.9 kg/m²), non-smokers, not on any long-term medications, and free from any known cardiovascular, metabolic, or endocrine disorders. Participants with a history of recent weight loss programs, irregular sleep cycles, or those involved in shift work were excluded.

Group Allocation

Participants were randomly assigned into two groups:

• Intermittent Fasting (IF) Group (n = 30): Followed a 16:8 fasting protocol, which involved fasting for 16 consecutive hours daily (typically from 8:00 PM to 12:00 PM the next day), with an 8-hour feeding window.
• Control Group (n = 30): Continued with their habitual dietary routine without any imposed restrictions.

All participants were instructed to maintain their usual level of physical activity and sleep pattern throughout the study period.

Assessment of Autonomic Function

Cardiovascular autonomic regulation was assessed using heart rate variability (HRV) parameters derived from a 5-minute electrocardiogram (ECG) recording in a seated resting position. Measurements were taken at baseline and at the end of the 8-week intervention period. The key HRV metrics analysed included:

• SDNN (Standard deviation of all NN intervals)
• RMSSD (Root mean square of successive differences between adjacent NN intervals)

Resting heart rate and blood pressure (systolic and diastolic) were also recorded using a digital sphygmomanometer following 10 minutes of rest.

Data Analysis

Statistical analysis was performed using SPSS version 25.0. Data were expressed as mean ± standard deviation (SD). Paired t-tests were used to compare pre- and post-intervention values within groups, while independent t-tests were used to compare changes between groups. A p-value < 0.05 was considered statistically significant.

A total of 60 participants completed the 8-week study, with no dropouts reported. The mean age of participants in the IF group was 28.4 ± 4.5 years, and 27.9 ± 5.1 years in the control group, with no significant difference between groups at baseline (p > 0.05).

Changes in Heart Rate Variability

After 8 weeks, the intermittent fasting group showed a statistically significant improvement in HRV parameters. The mean SDNN increased from 48.5 ± 10.2 ms to 61.3 ± 9.8 ms (p < 0.01), and RMSSD increased from 35.2 ± 7.5 ms to 44.6 ± 6.9 ms (p < 0.05). In contrast, the control group exhibited no significant changes in HRV metrics (Table 1).

Changes in Resting Heart Rate and Blood Pressure

The IF group demonstrated a notable reduction in resting heart rate (RHR) from 74.8 ± 6.3 bpm to 68.2 ± 5.7 bpm (p < 0.01), whereas the control group showed minimal change. A slight but non-significant decrease in systolic and diastolic blood pressure was also observed in the IF group compared to the control (Table 2).

Comparisons between Groups

Comparison of changes between the IF and control groups revealed significant differences in SDNN, RMSSD, and resting heart rate (p < 0.05), favouring the IF group. No significant difference was found in blood pressure parameters between the groups (Table 3).

Table 1. Pre- and post-intervention heart rate variability (HRV) parameters within each group

Table 2. Changes in resting heart rate and blood pressure

Table 3. Between-group comparisons of parameter changes

This study evaluated the effect of an 8-week intermittent fasting (IF) regimen on cardiovascular autonomic regulation in healthy adults. The key findings indicate that IF significantly improved heart rate variability (HRV) metrics—SDNN and RMSSD—and led to a reduction in resting heart rate. These changes suggest enhanced parasympathetic activity and better autonomic balance, potentially contributing to improved cardiovascular health.

Heart rate variability is an established non-invasive marker of autonomic nervous system function, with higher values indicative of robust vagal tone and cardiovascular adaptability (1,2). In our study, the IF group exhibited a significant increase in both SDNN and RMSSD, while the control group showed no meaningful changes. These findings are in line with earlier research suggesting that dietary interventions like fasting can positively modulate autonomic responses (3,4).

The reduction in resting heart rate observed in the IF group also aligns with previous studies indicating improved cardiac efficiency and reduced sympathetic drive during fasting periods (5,6). This bradycardic response may reflect an adaptive physiological mechanism to conserve energy during caloric deprivation (7). Moreover, although changes in blood pressure were not statistically significant, the downward trend observed may still have clinical relevance over longer durations (8).

The mechanisms underlying these autonomic shifts are thought to involve improvements in metabolic flexibility, reduction in systemic inflammation, and enhanced mitochondrial function (9,10). IF has been shown to increase levels of brain-derived neurotrophic factor (BDNF) and ketone bodies, both of which may influence central autonomic regulation (11,12). Additionally, fasting reduces insulin levels and improves insulin sensitivity, which are closely linked to sympathetic activity and baroreflex sensitivity (13).

Our findings are supported by prior clinical studies. For example, Mager et al. demonstrated that intermittent fasting improved HRV parameters in animal models, showing reduced sympathetic dominance (14). Human studies, although fewer, have also reported improvements in autonomic regulation following various IF protocols (4,6). However, inconsistencies in fasting duration, timing, and participant health status make direct comparisons challenging.

This study has several strengths, including a well-defined fasting protocol, objective HRV measurement, and inclusion of healthy participants. However, limitations include a relatively short duration, a modest sample size, and lack of long-term follow-up. Moreover, factors such as stress, sleep quality, and physical activity—though controlled to some extent—could still have influenced autonomic parameters.

In summary, the results suggest that intermittent fasting significantly improves cardiovascular autonomic regulation in healthy adults, as evidenced by enhanced HRV and reduced resting heart rate. These findings support the potential role of IF as a non-pharmacological strategy for promoting autonomic and cardiovascular health. Future studies with larger samples, longer follow-up, and mechanistic exploration are warranted.

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