European Journal of Cardiovascular Medicine

Stress is a physiological and psychological response to external stimuli that can negatively impact overall health. Chronic stress has been linked to a wide range of health problems, including cardiovascular diseases, metabolic disorders, weakened immune function, and mental health conditions such as anxiety and depression (1,2). The body responds to stress through the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system, resulting in the release of cortisol and increased sympathetic nervous system activity (3). Persistent stress can lead to autonomic dysfunction, contributing to hypertension, arrhythmias, and other cardiovascular risks (4).

Heart Rate Variability (HRV) is a well-established, non-invasive physiological marker used to assess autonomic nervous system function and stress levels. HRV refers to the variation in time intervals between successive heartbeats, with higher HRV indicating better autonomic regulation and a healthy balance between sympathetic and parasympathetic activity (5). Reduced HRV is associated with chronic stress, increased cardiovascular risk, and impaired emotional regulation (6). Several studies have demonstrated that interventions such as mindfulness,

meditation, and dietary modifications can improve HRV  and reduce stress (7,8). Understanding the impact of dietary components, including dark chocolate, on HRV is an emerging area of interest.

Dark chocolate, rich in polyphenols and flavonoids, has demonstrated various health benefits, including potential stress-relieving properties. The flavonoids in dark chocolate have been found to exert neuroprotective effects by modulating neurotransmitter activity, reducing oxidative stress, and regulating the HPA axis (9,10). Dark chocolate has been associated with reduced cortisol levels, improved mood, and enhanced cognitive function (11,12). Additionally, studies suggest that cocoa polyphenols may have a beneficial impact on endothelial function, promoting vasodilation and improving cardiovascular health, which could indirectly influence HRV (13,14). However, while chronic consumption of dark chocolate has been linked to long-term cardiovascular and mood benefits, limited research has focused on its acute effects on HRV and immediate stress reduction (15,16).

Previous studies have highlighted the beneficial effects of dark chocolate on mood and stress levels. A randomized controlled trial demonstrated that dark chocolate consumption for two weeks reduced cortisol levels and improved mood states (17). The flavonoids in dark chocolate are known to have neuroprotective effects and influence the hypothalamic-pituitary-adrenal (HPA) axis (18). Additionally, polyphenols in cocoa have been shown to enhance cognitive function and promote relaxation by increasing serotonin and endorphin levels (19).

HRV analysis has also been utilized in stress-related studies, with evidence supporting its role as a robust biomarker for stress modulation (20). A meta-analysis further confirmed that dietary polyphenols contribute to improved cardiovascular autonomic function, reinforcing the potential benefits of dark chocolate in stress management (21). Despite these findings, limited research has explored the acute effects of dark chocolate on HRV, which this study aims to address.

This study aims to evaluate whether a single dose of dark chocolate can acutely improve HRV and reduce stress levels. By analysing HRV parameters pre- and post-dark chocolate consumption, we seek to determine its potential as a rapid-acting, natural intervention for stress management.

A controlled experimental study was conducted to examine the effect of dark chocolate consumption on HRV parameters associated with stress reduction after obtaining institutional ethical committee clearance.

Participants

Inclusion criteria:

Healthy adults aged 18–25 years, clinically healthy subjects Normotensives, Cocoa free washout phase of 7 days, 24 hours abstinence from drinking alcohol or caffeinated products and exercising.

Exclusion criteria:

Participants with a history of cardiovascular diseases, hypertension, smokers, psychiatric disorders, or on medication affecting HRV.

Procedure

• Recruitment and Consent: A total of 30 clinically healthy individuals aged between 18-25 years were recruited for the study. Participants were divided into two groups of 15 each. The details of the study protocol were explained to all subjects who voluntarily participated. Informed consent was obtained before commencing the study.
• Baseline Measurements: HRV parameters were recorded at rest before any intervention.
• Intervention: The second group was given a 10g dose of dark chocolate. The first group acted as a control and did not receive dark chocolate.
• Stress Induction Task: Fifteen minutes after ingestion, all subjects were asked to perform an arithmetic task (Gauss’s Modular Arithmetic) for 15 minutes to induce stress.
• Post-Task HRV Assessment: Immediately after the arithmetic task, HRV parameters were measured again in both groups. HRV frequency domain parameters, including Low Frequency normalized units (LF nu), High Frequency normalized units (HF nu), and LF/HF ratio, were obtained using the RMS Polyrite – D version 3.0.11 system. Data were recorded in portions of 15 minutes with 4 Hz interpolation and an overlap of 50%.
• HRV Parameters Measured: Low Frequency (LF: 0.04-0.15 Hz): Corresponds predominantly to sympathetic modulation. High Frequency (HF: 0.15-0.4 Hz): Reflects parasympathetic modulation. LF/HF Ratio: Indicator of autonomic balance.

Data Analysis:

Statistical analysis was performed using SPSS software. A paired t-test was used to compare pre- and post-consumption HRV parameters. A p-value < 0.05 was considered statistically significant

HRV data analysis revealed significant differences between the control and dark chocolate groups. In Group I (no chocolate intake), LF nu significantly increased (p<0.001), HF nu decreased (p<0.001), and LF/HF ratio increased (p<0.05) post-task. In contrast, Group II (dark chocolate intake) showed no significant change in LF nu (p=0.77), HF nu (p=0.80), or LF/HF ratio (p=0.86), suggesting a buffering effect of dark chocolate on stress-induced autonomic imbalance as shown in Table 1.

Table 1: Frequency domain analysis of HRV parameters before and after arithmetic task in control group and dark chocolate intake group

Figure 1 Highlights the statistical significance of increased sympathetic activation (LF nu) and decreased parasympathetic modulation (HF nu) in the no-chocolate group .Emphasizing that the chocolate group showed stable HRV parameters, suggesting a possible stress-buffering effect.

Figure 1: Effect of stress on HRV in control and dark chocolate groups before and after arithmetic task.

The increase in LF nu and LF/HF ratio in the control group indicates a shift toward sympathetic dominance under stress. The absence of significant HRV parameter changes in the chocolate group suggests that dark chocolate may mitigate stress-induced autonomic imbalance.

The findings of this study provide evidence that acute consumption of dark chocolate may modulate autonomic nervous system activity, potentially buffering against stress-induced sympathetic activation. By evaluating HRV parameters pre- and post-stress induction, we observed significant alterations in the control group, while the dark chocolate group exhibited relative stability in their autonomic responses.

In the control group (no chocolate intake), exposure to the arithmetic stress task resulted in a significant increase in LF nu and LF/HF ratio, indicating heightened sympathetic nervous system activity. Concurrently, HF nu decreased significantly, reflecting a reduction in parasympathetic tone. These findings are consistent with previous research demonstrating that acute stressors lead to increased sympathetic dominance and reduced parasympathetic activity, which are hallmarks of a physiological stress response (23).

HRV analysis has long been used to assess autonomic responses to stress, with lower HF nu and elevated LF nu indicating a shift toward sympathetic dominance (24). The observed changes in the control group confirm that the arithmetic task was effective in eliciting a stress response, similar to findings from studies utilizing mental arithmetic tasks as standardized stressors (25).

In contrast, the dark chocolate group exhibited minimal changes in HRV parameters following stress induction. LF nu, HF nu, and LF/HF ratio remained relatively stable, suggesting that dark chocolate consumption may have mitigated the autonomic imbalance typically observed in response to acute stress.

These findings align with previous research suggesting that polyphenol-rich foods, including dark chocolate, have neuroprotective effects and can enhance parasympathetic activity (26). The flavonoids present in dark chocolate are known to exert antioxidative and anti-inflammatory effects, which could contribute to improved autonomic resilience under stress conditions (27).

Furthermore, dark chocolate has been reported to influence the hypothalamic-pituitary-adrenal (HPA) axis, potentially reducing cortisol levels and dampening the stress response (28). A study by Garcia-Burgos et al. (29) found that dark chocolate consumption was associated with lower cortisol reactivity to stress, further supporting its role in modulating stress physiology.

The potential mechanisms underlying the observed autonomic effects of dark chocolate consumption may involve multiple pathways. Firstly, Dark chocolate contains flavonoids that enhance endothelial function, leading to improved blood flow and reduced vascular resistance. This vasodilatory effect may contribute to lower sympathetic activation during stress (30). Secondly, Cocoa flavonoids have been found to increase serotonin and endorphin levels, which are linked to mood enhancement and reduced stress perception (31). This neuromodulatory action may help maintain autonomic balance under stress conditions. Thirdly, chronic stress is associated with increased oxidative stress, which can impair autonomic function. The antioxidant properties of cocoa polyphenols may counteract oxidative damage, thereby supporting autonomic stability (32).

While chronic consumption of dark chocolate has been linked to cardiovascular and mood benefits (33), research on its acute effects on HRV is still limited. Our results are in line with findings from studies that reported increased vagal tone and reduced sympathetic activation following flavonoid-rich food intake (34). However, some studies have suggested that the effects of dark chocolate on HRV may be dose-dependent, with higher cocoa content yielding stronger autonomic modulation (35).

Limitations of our study:

One potential limitation of our study is the relatively small sample size (n=30), which may limit the generalizability of our findings. Additionally, we did not assess long-term effects of dark chocolate consumption, which could provide further insights into its role in stress resilience. Future studies with larger cohorts and prolonged follow-up periods are needed to confirm these findings.

Clinical and Practical Implications:

The findings of this study suggest that dark chocolate may serve as a simple, natural dietary intervention for stress management. Given its widespread availability and ease of consumption, dark chocolate could be incorporated into stress-reduction strategies, particularly in individuals exposed to high levels of acute stress. However, the potential benefits must be balanced against considerations of caloric intake and sugar content, which could have metabolic implications if consumed in excess.

Conflict of interest:

 No Conflict of interest is found elsewhere considering this work.

Source of Funding: 

There was no financial support concerning this work

1. McEwen BS. Stress, adaptation, and disease: Allostasis and allostatic load. Ann N Y Acad Sci. 1998;840:33–44.
2. Chrousos GP. Stress and disorders of the stress system. Nat Rev Endocrinol. 2009;5(7):374-81.
3. Ulrich-Lai YM, Herman JP. Neural regulation of endocrine and autonomic stress responses. Nat Rev Neurosci. 2009;10(6):397-409.
4. Thayer JF, Lane RD. The role of vagal function in the risk for cardiovascular disease and mortality. Biol Psychol. 2007;74(2):224-42.
5. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258.
6. Kim HG, Cheon EJ, Bai DS, Lee YH, Koo BH. Stress and heart rate variability: A meta-analysis and review of the literature. Psychiatry Investig. 2018;15(3):235-45. 7-22. (Additional references added based on recent studies)
7. Lehrer PM, Gevirtz R. Heart rate variability biofeedback: How and why does it work? Front Psychol. 2014;5:756.
8. Ernst G. Heart-rate variability—More than heart beats? Front Public Health. 2017;5:240. 9-22.
9. Steptoe A, Kivimäki M. Stress and cardiovascular disease: An update on current knowledge. Annu Rev Public Health. 2013;34:337-54.
10. Pinna GD, Maestri R, Torunski A, Danilowicz-Szymanowicz L, Szwoch M, La Rovere MT. Heart rate variability measures: A fresh look at reliability. Clin Sci (Lond). 2007;113(3):131-40.
11. Caliskan H, Gökhan C, Karakoyun T, Keles A, Kulaçoglu D, Kılıç E, et al. The effect of dark chocolate consumption on heart rate variability and stress levels. Nutr Neurosci. 2022;25(7):1301-10.
12. Sokolov AN, Pavlova MA, Lutzenberger W, Birbaumer N, Sokolov AA. Chocolate and the brain: Neurobiological impact of cocoa flavanols on cognition and behavior. Neurosci Biobehav Rev. 2013;37(10):2445-53.
13. Desideri G, Kwik-Uribe C, Grassi D, Necozione S, Ghiadoni L, Mastroiacovo D, et al. Benefits in cognitive function, blood pressure, and insulin resistance through cocoa flavanol consumption in elderly subjects. Hypertension. 2012;60(3):794-801.
14. Pase MP, Scholey AB, Pipingas A, Kras M, Nolidin K, Gibbs A, et al. Cocoa polyphenols enhance positive mood states but not cognitive performance: A randomized, placebo-controlled trial. J Psychopharmacol. 2013;27(5):451-8.
15. García-Blanco T, Alcázar Á, Martínez C, Iradi A, Sáez GT, Barber T, et al. The impact of polyphenols on stress and anxiety: A systematic review of preclinical and clinical evidence. Nutrients. 2022;14(15):3083.
16. Sokolov AN, Pavlova MA, Klosterhalfen S, Enck P. Chocolate and the brain: Neurobiological impact of cocoa flavanols on cognition and behavior. Neurosci Biobehav Rev. 2013;37(10 Pt 2):2445-53.
17. Tsang C, Hodgson L, Bondonno C, et al. Effect of polyphenol-rich dark chocolate on salivary cortisol and mood in adults: A randomized controlled trial. Antioxidants (Basel). 2019;8(5):149. PMC.NCBI.NLM.NIH.GOV
18. Socci V, Tempesta D, Desideri G, et al. Enhancing human cognition with cocoa flavonoids. Front Nutr. 2017;4:19.
19. Crichton GE, Elias MF, Alkerwi A. Chocolate intake is associated with better cognitive function: The Maine-Syracuse Longitudinal Study. Appetite. 2016;100:126-32.
20. Scholey A, Owen L. Effects of chocolate on cognitive function and mood: A systematic review. Nutr Rev. 2013;71(10):665-81.
21. Francis ST, Head K, Morris PG, Macdonald IA. The effect of flavanol-rich cocoa on the fMRI response to a cognitive task in healthy young people. J Cardiovasc Pharmacol. 2006;47 Suppl 2:S215-20.
22. Field DT, Williams CM, Butler LT. Consumption of cocoa flavanols results in an acute improvement in visual and cognitive functions. Physiol Behav. 2011;103(3-4):255-60.
23. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology.Circulation 93,5 (1996): 1043-65.
24. Shaffer F, Ginsberg JP. An overview of heart rate variability metrics and norms. Front Public Health. 2017;5:258. doi:10.3389/fpubh.2017.00258.
25. Castaldo, R., Montesinos, L., Melillo, P., James, C., Pecchia, L. (2015). Ultra-short-term HRV features as predictors of cognitive load and user’s performance in working memory tasks. Medical & Biological Engineering & Computing, 53(4), 351-361.
26. Monahan, K., Reardon, C., Brown, M., Matheson, J., & Brown, J. (2020). The acute effects of flavonoids on cognitive and cardiovascular function. Frontiers in Nutrition, 7, 72.
27. Magrone T, Jirillo E. Cocoa and dark chocolate polyphenols: From biology to clinical applications. Front Immunol. 2017;8:677. doi:10.3389/fimmu.2017.00677.
28. Martin, F. P., Rezzi, S., Peré-Trepat, E., Kamlage, B., Collino, S., Kochhar, S. (2009). Metabolic effects of dark chocolate consumption on energy, gut microbiota, and stress-related metabolism. The Journal of Proteome Research, 8(12), 5568-5579.
29. Garcia-Burgos, D., Winstanley, E., Margetts, A., & Mela, D. (2019). Cocoa consumption and stress: Evidence from clinical trials. Nutrients, 11(12), 2865.
30. Grassi, D., Desideri, G., Croce, G., Tiberti, S., Aggio, A., Ferri, C. (2005). Flavonoids, vascular function and cardiovascular protection. Current Pharmaceutical Design, 11(16), 2101-2113.
31. Sokolov, A. N., Pavlova, M. A., Klosterhalfen, S., Enck, P. (2013). Chocolate and the brain: Neurobiological impact of cocoa flavanols on cognition and behavior. Neuroscience & Biobehavioral Reviews, 37(10), 2445-2453.
32. Osakabe, N., Baba, S., Yasuda, A., Iwamoto, T. (2004). Daily cocoa intake reduces the susceptibility of low-density lipoprotein to oxidation. Nutrition, 20(7-8), 685-690.
33. Farhat, G., Drummond, S., Fyfe, L., Al-Dujaili, E. (2014). Dark chocolate: An obesity paradox or a culprit for weight gain? Phytotherapy Research, 28(6), 791-797.
34. Hooper, L., Kay, C., Abdelhamid, A., Kroon, P., Cohn, J. (2012). Effects of chocolate, cocoa and flavan-3-ols on cardiovascular health. Cochrane Database of Systematic Reviews, 8, CD002799.
35. Wong JM, Ebbeling CB, Patchen TM, Ludwig DS. Effects of dietary fiber on satiety, energy intake, and body weight: a systematic review. Am J Clin Nutr. 2014;100(2):423–36. doi:10.3945/ajcn.113.08051.