European Journal of Cardiovascular Medicine

Caffeine is the most widely consumed psychoactive substance worldwide, routinely ingested through coffee, tea, soft drinks, and energy beverages. Its popularity stems from its central nervous system stimulant effects, including enhanced alertness, reduced fatigue, and improved cognitive and physical performance. Given its ubiquitous use across age groups and cultures, understanding the cardiovascular and autonomic consequences of caffeine consumption remains an important public health and physiological concern.1,2

Pharmacologically, caffeine acts primarily as a non-selective antagonist of adenosine A₁ and A₂ receptors, resulting in increased neuronal excitability and modulation of autonomic nervous system activity. Through these mechanisms, caffeine has the potential to influence heart rate, blood pressure, and cardiac autonomic regulation. While caffeine is traditionally regarded as a sympathomimetic agent, emerging evidence suggests that its autonomic effects are complex, dose-dependent, and influenced by contextual factors such as habitual intake, posture, and physical activity.3,4

Heart rate variability (HRV) is a well-established, non-invasive tool for assessing cardiac autonomic regulation, reflecting the dynamic interplay between sympathetic and parasympathetic influences on the sinoatrial node. Reduced HRV has been associated with adverse cardiovascular outcomes, whereas preserved or enhanced HRV is considered a marker of autonomic flexibility and cardiovascular health.5,6 Time-domain and frequency-domain HRV parameters provide complementary information regarding autonomic modulation, enabling detailed physiological interpretation.

Studies examining the acute effects of caffeine on HRV have yielded inconsistent results. Several experimental investigations have reported increases in parasympathetic indices, such as high-frequency (HF) power and time-domain markers, along with reductions in the LF/HF ratio following moderate caffeine intake, suggesting enhanced vagal modulation under resting conditions.7,8 In contrast, other studies, particularly those conducted in post-exercise settings or using higher caffeine doses, have demonstrated sympathetic predominance or delayed autonomic recovery.9,10 Recent systematic reviews and meta-analyses have highlighted substantial heterogeneity across studies, attributable to differences in study design, dosing strategies, timing of HRV assessment, and participant characteristics.11–14

Young healthy adults represent a population with frequent caffeine consumption, yet systematic physiological data on acute autonomic responses in this group—particularly from South Asian settings—remain limited. Furthermore, potential sex-related differences in autonomic responses to caffeine have not been consistently evaluated, despite evidence of baseline sex differences in cardiac autonomic tone.15,16

Therefore, the present study aimed to evaluate the short-term effects of moderate caffeine intake on cardiac autonomic balance in healthy young adults using comprehensive time-domain and frequency-domain HRV analysis. By incorporating sex-specific comparisons, hemodynamic assessment, and standardized effect size estimation, this study seeks to clarify the autonomic and cardiovascular impact of acute caffeine ingestion and to place the findings within the context of existing national and international evidence.

The present cross-sectional interventional study was conducted on 100 healthy young adults aged 18–25 years. Participants were recruited on a voluntary basis from a tertiary care teaching institution after obtaining informed consent using a predesigned proforma. Individuals with a history of cardiovascular, respiratory, neurological, metabolic, or endocrine disorders, or those using medications known to affect autonomic function, were excluded. Participants were instructed to abstain from caffeine-containing products for at least 12 hours prior to assessment. Baseline anthropometric measurements were recorded, following which participants rested in the supine position in a quiet, temperature-controlled environment. Heart rate variability (HRV) was recorded at rest for 5 minutes using a digital electrocardiograph with HRV analysis software (Physio Pac, Medicaid Systems), with ECG acquisition in RR (beat-to-beat) mode. Hemodynamic parameters, including heart rate and blood pressure, were recorded concurrently. Following baseline recording, caffeine was administered orally at a dose of 3 mg/kg body weight. Repeat HRV and hemodynamic recordings were obtained 60 minutes post-ingestion under identical conditions. HRV analysis included time-domain and frequency-domain parameters, in accordance with standard guidelines.5 Heart rate variability (HRV) analysis was performed using both time-domain and frequency-domain parameters. Time-domain indices included the standard deviation of RR intervals (SDRR), root mean square of successive differences of NN intervals (RMSSD), number of successive NN interval differences greater than 50 ms (NN50), and the proportion of NN50 relative to the total number of NN intervals (pNN50). Frequency-domain analysis comprised very low frequency (VLF), low frequency (LF), and high frequency (HF) components, assessed in terms of peak frequency (Hz), absolute power (ms²), percentage power (%), and normalized units (n.u.), along with the LF/HF ratio. Statistical analysis involved paired comparisons for pre–post changes and independent comparisons for sex-based differences, with effect sizes calculated using Cohen’s d and Hedges’ g. A p-value <0.05 was considered statistically significant. Pre–post comparisons were performed using paired t-tests. Sex-wise comparisons of change (Δ) values were analysed using independent-sample Welch’s t-test. Sex-specific effect sizes are reported as Cohen’s d, calculated from Δ (post–pre) values (male vs female). Overall pre–post effect sizes are reported as Hedges’ g for the total cohort. Effect size interpretation: 0.2 = small, 0.5 = moderate, 0.8 = large. A p-value < 0.05 was considered statistically significant.

A total of 100 healthy young adults were included in the final analysis. Baseline demographic and anthropometric characteristics are summarised in Table 1. Males and females were comparable with respect to age and body mass index, while statistically significant sex-related differences were observed for height, weight, and body surface area (p < 0.001 for all). These differences reflect expected physiological sexual dimorphism rather than sampling bias.

Hemodynamic Parameters

Baseline and post-caffeine hemodynamic parameters are presented in Table 2. In the total cohort, systolic blood pressure, mean heart rate, and mean RR interval did not change significantly following caffeine intake. A small but statistically significant reduction in diastolic blood pressure was observed in the overall cohort (mean Δ −0.18 mmHg, p = 0.039), although the corresponding effect size was trivial (Hedges’ g = 0.14; 95% CI approximately −0.02 to 0.30), indicating limited physiological relevance.

Sex-wise change-score analysis demonstrated no significant differences between males and females for any hemodynamic parameter. Sex-specific effect sizes (Cohen’s d) for Δ values were uniformly small (d ≤ 0.35), with confidence intervals spanning zero, confirming the absence of meaningful sex-related differences. Overall, these findings indicate that acute moderate caffeine intake did not induce clinically significant hemodynamic alterations.

Time-Domain HRV Parameters                          

Time-domain HRV parameters before and after caffeine intake are shown in Table 3. In the total cohort, NN50 increased significantly following caffeine intake (mean Δ +1.23 counts, p = 0.015), with a small but consistent overall effect size (Hedges’ g = 0.21; 95% CI approximately 0.05–0.37). Other time-domain parameters, including SDRR, RMSSD, and pNN50, demonstrated upward trends post-caffeine but did not reach statistical significance, and their effect sizes were small (g ≤ 0.17).

Sex-stratified analysis revealed that the increase in NN50 was more pronounced in females compared to males. Change-score comparison confirmed a statistically significant sex difference for NN50 (p = 0.046), with a moderate sex-specific effect size (Cohen’s d = 0.39; 95% CI approximately 0.02–0.76). No significant sex differences were observed for SDRR, RMSSD, or pNN50, and effect sizes for these parameters were trivial to small.

Frequency-Domain HRV Parameters

Frequency-domain HRV findings are detailed in Table 4. In the total cohort, high-frequency (HF) power increased significantly after caffeine intake (p < 0.001), accompanied by significant reductions in HF power expressed as percentage and normalized units. The LF/HF ratio decreased significantly (p = 0.004), indicating a shift toward parasympathetic dominance. These changes were associated with small-to-moderate overall effect sizes (Hedges’ g ranging from 0.26 to 0.42), with confidence intervals excluding zero.

Very low frequency (VLF) power and its percentage contribution increased modestly but significantly in the total cohort, whereas peak frequencies for VLF, LF, and HF bands remained unchanged, suggesting that observed changes reflected autonomic modulation rather than respiratory artefact. Sex-wise comparisons of Δ values showed no statistically significant differences across frequency-domain parameters, with sex-specific effect sizes remaining small (Cohen’s d ≤ 0.39).

Collectively, the results demonstrate that acute moderate caffeine intake produces a measurable shift in cardiac autonomic balance toward parasympathetic predominance, evidenced by consistent changes in both time-domain and frequency-domain HRV parameters, without clinically meaningful hemodynamic effects.

Table 1. Baseline Demographic and Anthropometric Characteristics of Participants (N = 100)

Table 2. Comparison of Hemodynamic Parameters After Caffeine Intake (N = 100)

Table 3. Comparison of Time-Domain HRV Parameters After Caffeine Intake (N = 100)

Table 4. Comparison of Frequency-Domain HRV Parameters After Caffeine Intake (N = 100)

The present study evaluated the short-term effects of moderate caffeine intake on cardiac autonomic regulation in healthy young adults using detailed time-domain and frequency-domain heart rate variability (HRV) analysis. The principal finding is that acute caffeine ingestion resulted in a measurable shift in cardiac autonomic balance toward parasympathetic predominance, without inducing clinically significant hemodynamic changes. This conclusion is supported by consistent changes across multiple HRV indices, small-to-moderate effect sizes, and the absence of meaningful sex-related differences for most parameters.

Hemodynamic Responses to Caffeine

In the current study, systolic blood pressure, mean heart rate, and RR interval remained largely unchanged following caffeine intake, while a statistically significant but clinically negligible reduction in diastolic blood pressure was observed in the overall cohort. Importantly, the effect size for this change was trivial, and confidence intervals overlapped zero, indicating limited physiological relevance. These findings support the concept of hemodynamic neutrality of moderate acute caffeine intake in healthy young adults.

International literature demonstrates heterogeneity in hemodynamic responses to caffeine. Some studies report transient increases in blood pressure, particularly in caffeine-naïve or older individuals1,2, whereas others, especially in young healthy populations, have shown minimal or no change.7,8,17 The present findings align more closely with the latter, suggesting that in young adults, autonomic modulation rather than direct vascular effects predominates following moderate caffeine ingestion.

Time-Domain HRV Findings         

Among time-domain HRV parameters, NN50 emerged as the most sensitive marker of acute autonomic modulation. The significant increase in NN50 in the total cohort, along with a small but consistent overall effect size, indicates enhanced beat-to-beat vagal modulation following caffeine intake. Notably, females exhibited a significantly greater increase in NN50 compared to males, with a moderate sex-specific effect size. This finding suggests that sex-related differences in autonomic responsiveness to caffeine may be parameter-specific rather than global.

Other time-domain indices, including SDRR, RMSSD, and pNN50, demonstrated non-significant upward trends with small effect sizes. These results are consistent with previous reports indicating that short-term recordings may detect subtle parasympathetic changes more reliably through NN50 than through RMSSD or SDRR.3 From a methodological perspective, this highlights NN50 as a potentially more sensitive index for detecting acute dietary or pharmacological autonomic modulation.

Frequency-Domain HRV and Autonomic Balance

Frequency-domain analysis provided robust evidence of autonomic modulation following caffeine intake. The significant increase in high-frequency (HF) power, coupled with a reduction in LF/HF ratio, indicates a shift toward parasympathetic dominance. These changes were associated with small-to-moderate effect sizes, reinforcing their physiological relevance. Importantly, peak frequencies within the VLF, LF, and HF bands remained stable, suggesting that the observed changes reflected genuine autonomic modulation rather than alterations in respiratory frequency or recording artefacts.

These findings are in agreement with controlled experimental studies reporting enhanced vagal modulation following moderate caffeine intake at rest.7,18 Others have similarly observed increased parasympathetic indices following espresso consumption, particularly in non-habitual consumers.8 In contrast, studies conducted in post-exercise contexts often report sympathetic predominance or delayed autonomic recovery9,10, underscoring the importance of experimental context when interpreting caffeine’s autonomic effects.

Sex Differences in Autonomic Response

The present study found no consistent sex-related differences in frequency-domain HRV responses to caffeine, with sex-specific effect sizes for Δ values remaining small across most parameters. The only notable exception was NN50, which showed a greater increase in females. This limited sex specificity aligns with existing evidence suggesting that while baseline autonomic tone differs between males and females, acute autonomic responses to caffeine are largely comparable.15,16

From a clinical and physiological standpoint, these findings argue against broad sex-based stratification when evaluating acute autonomic responses to moderate caffeine intake in healthy young adults. Instead, they support the interpretation that caffeine exerts a cohort-wide autonomic effect, with only subtle parameter-specific sex modulation.

Effect Sizes and Clinical Interpretation

A key strength of this study is the systematic reporting of effect sizes alongside p-values. While several HRV parameters showed statistically significant changes, effect size analysis revealed that these effects were generally small to moderate. This distinction is crucial, as it prevents overinterpretation of statistically significant but physiologically modest findings. The magnitude of observed effects is comparable to those reported in international trials and meta-analyses,3,11 lending external validity to the present results.

Data on acute caffeine-induced autonomic modulation from Indian or South Asian populations remain limited. The present study contributes region-specific evidence, demonstrating that autonomic responses in young Indian adults are broadly consistent with findings from Western populations. This supports the generalisability of existing physiological models of caffeine–autonomic interaction while addressing a notable gap in the literature.

Limitations

This study has certain limitations. First, habitual caffeine intake was not quantitatively assessed, which may influence individual autonomic responsiveness. Second, the cross-sectional interventional design assessed only short-term effects, precluding conclusions regarding chronic caffeine consumption. Third, HRV was recorded under resting supine conditions, and findings may not be generalisable to stress or exercise settings. Finally, although effect sizes and confidence intervals were reported, direct measures of sympathetic activity such as muscle sympathetic nerve activity were not included.

Suggestions

Future studies should incorporate habitual caffeine consumption profiling and explore dose–response relationships to better characterise individual variability. Longitudinal designs assessing chronic caffeine intake and its interaction with lifestyle factors would enhance clinical relevance. Inclusion of additional autonomic markers, such as baroreflex sensitivity or direct sympathetic nerve recordings, may provide deeper mechanistic insight. Expanding research to diverse age groups and clinical populations would further clarify the cardiovascular implications of caffeine consumption.

This study demonstrates that acute moderate caffeine intake produces a measurable shift in cardiac autonomic balance toward parasympathetic predominance in healthy young adults, as evidenced by consistent changes in both time-domain and frequency-domain heart rate variability parameters. Importantly, these autonomic effects occur in the absence of clinically meaningful changes in heart rate or blood pressure, supporting the hemodynamic safety of moderate caffeine consumption under resting conditions. Sex-related differences were minimal and largely confined to parameter-specific responses. Overall, the findings indicate that caffeine acts primarily as an autonomic modulator rather than a cardiovascular stressor in healthy young individuals.

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