December 1, 2021

Flavanol's Effects on Vascular Function During Acute Stress

Results from a double-blind, randomized, controlled study
This study showed that dietary flavanols are effective in eliminating post-stress weakening of endothelial function.


Baynham R, Veldhuijzen van Zanten JJCS, Johns PW, Pham QS, Rendeiro C. Cocoa flavanols improve vascular responses to acute mental stress in young healthy adults. Nutrients. 2021;13(4):1103.

Study Objective

To explore the effect of cocoa flavanol consumption in response to stress-induced changes on vascular function in young, healthy adults following acute mental stress


A double-blind, randomized, controlled study at the College of Life and Environmental Sciences, University of Birmingham in the United Kingdom


The study recruited 30 male participants from the University of Birmingham, aged 19 to 36 years. All the participants were of white European ethnicity and had healthy values for heart rate (HR) in beats per min (bpm), body mass index (BMI), blood pressure (BP), and brachial flow-mediated dilatation (FMD).

Exclusion criteria included:

  • Age under 18 or over 45
  • Women
  • Consumption of more than 21 units of alcohol per week
  • Current smokers
  • Recent infection or acute illness
  • History of respiratory, cardiovascular, metabolic, inflammatory, or liver diseases
  • Use of a weight-reducing dietary regimen
  • Frequent use of dietary supplements
  • Blood-clotting disorder
  • Use of antibiotics 3 months prior to the study
  • Chronic use of long-term medication

Study Parameters Assessed

Participants completed high- and low-flavanol interventions. The dietary order of interventions was randomized and equally weighted for each participant. Food intake was assessed using a 24-hour diet recall. The interventions included high-flavanol cocoa powder containing 150 mg of (-)-epicatechin powder and 681.4 mg of total flavanols per serving, and the low-flavanol powder containing <4 mg of (-)-epicatechin and 4.1 mg of total flavanols per serving. Pre-intervention baseline measurements included FMD, brachial BP, and forearm blood flow (FBF).

Researchers measured FBF for an 8-minute period during which they took readings during the intervals of minutes 2, 4, 6, and 8. After completion of the pre-intervention baseline assessments, the participants received either the low- or high-flavanol drink to ingest for another measurement of FBF 1.5 hours later during an 8-minute period of rest and also during an 8-minute mental stress task. Researchers collected measurements for HR, beat-to-beat BP, heart rate variability (HRV, a measure of parasympathetic activity), and pre-ejection period (PEP, a measure of sympathetic activity). The cardiovascular variables and FBF were averaged during the 4 minutes of data collection, providing an overall pre-intervention baseline rest and stress value. They measured brachial FMD and BP 30 and 90 minutes post-stress interventions.

Researchers used the Paced-Auditory Serial Addition Test (PASAT) to provoke mental stress for 8 minutes by inducing a physiological stress response. Participants had to add single-digit numbers to the last number they were presented with. Time intervals between the numbers were reduced every 2 minutes starting from intervals of 2.8, 2.4, 2, and 1.6 seconds in the final 2 minutes. Two body-language assessors, who played a loud noise after every 10 answers following an incorrect response or at the end of the questions, recorded and evaluated the participants. The participants, therefore, were in competition with each other; researchers added elements of difficulty to the task to enhance the levels of stress to be rated by each participant at the end, scored on a 7-point scale ranging from 0 (no difficulty) to 6 (extremely difficult).

Researchers measured HRV, HR, and PEP with an electrocardiogram (ECG) and an impedance cardiogram (ICG), which was recorded using the Ambulatory Monitoring Systems, VU-AMS5s. A finometer was used to record beat-to-beat arterial BP during the time periods in which FBF was also collected. Both systolic blood pressure (SBP) and diastolic blood pressure (DPB) were obtained from the data. Brachial DBP and SBP were measured after a 10-minute rest period in the supine position for 3 consecutive readings taken in which only the last 2 readings were averaged for each time period.

Researchers used venous occlusion plethysmography to measure FBF. Three blood flow measurements were provided each minute at 15-second intervals. Limb circumference measurements were averaged to yield a mean value for blood flow per minute. Forearm vascular conductance (FVC) calculation was based on dividing FBF by BP per minute of each assessment. The differences between FBF/FVC resulted in this data being collected a few months apart.

Researchers evaluated brachial artery endothelial function using FMD for measurement of diameter and blood velocity. After 20 minutes of rest supine, they imaged the brachial artery for several different vascular responses. Peak diameter occurred after occlusion was released, indicating the largest diameter. FMD calculation was based on the change of diastolic diameter between baseline and peak diameter. Resting arterial diameter and anterograde and retrograde blood flow were additionally assessed based on an estimation of on-time average of the first recording.

Researchers analyzed the data collected during this study using the IBM SPSS version 25 software. All the cardiovascular variables (HRV, HR, PEP, DBP, and SBP) were analyzed by 2-way ANOVA (ANalysis Of VAriance) with dietary interventions of high- and low-flavanol cocoa and time periods consisting of the following pre-intervention baseline, rest, and stress. FVC and FBF were also analyzed using the 2-way ANOVA with the same factorial components as the cardiovascular variables with the exception of time (rest percent change, stress percent change) as factors. Arterial diameter, FMD, anterograde flow, retrograde flow, and resting BP were analyzed using the 2-way ANOVA system similarly to the above analysis. However, the time measurement was pre-intervention baseline, at 30 minutes, and at 90 minutes). The vascular and cardiovascular reactivity scores were calculated as the difference between stress and rest value average.

Primary Outcome Measures

The significance of this study had a value of P<0.05 for the analyses. Power analyses conducted measured a power of 85% based on the sample size of 30 participants and an alpha set at 0.05 allotted for detection of a medium-size interaction effect of 0.25 as the primary outcome measure for brachial FMD. During the study, there was an intervention effect (P=0.001) and intervention-per-time interaction effect (P<0.001); however, no time effect was on brachial FMD (P=0.146). FMD decreased after ingestion of low-flavanol cocoa at 30 minutes (P<0.001, 3.86±1.97% compared to 5.23±1.39%). However, no decline was noted at 90 minutes (P=0.213) post-stress. FMD was higher for high-flavanol cocoa in comparison to low-flavanol cocoa for both post-stress at 30 minutes (P<0.001, 5.75±1.74% compared to 3.86±1.97%) and at 90 minutes (P=0.026, 5.38±1.50% compared to 4.66±2.22%).

At 30 minutes, a greater time effect revealed arterial diameter (P<0.001) to be 0.07±0.20 mm and 0.30±1.02 mm (P=0.006) in low-flavanol cocoa and high-flavanol cocoa, respectively post-stress. A great time effect was also revealed for arterial diameter 90 minutes post stress as well (P<0.001) with results of 0.11±0.20 mm and 0.33±1.09 mm (P=0.001) in low-flavanol cocoa and high-flavanol cocoa, respectively. In comparison, there were no effects found during pre-intervention baseline (P=0.113) or intervention-per-time interaction (P=0.441). In retrograde blood flow, a time effect (P=0.046) was shown with an even greater flow being observed 30 minutes after the stress task compared to 90 minutes post-stress, for low-flavanol cocoa (P=0.032, 7.69±25.72 cm3/min) and high-flavanol cocoa (7.95±15.10 cm3/min), respectively. The anterograde flow had no time changes detected (P=0.764). Effects on intervention were not found in anterograde blood flow (P=0.756) nor retrograde blood flow (P=0.874), and in addition, similarly there was no effect of intervention per time in anterograde (P=0.762) or retrograde blood flow (P=0.518).

For the rating of the mental stress task, participants were characterized to the same extent for both high- and low-flavanol interventions (n=29). The value for each category is as follows: equally difficult (P>0.99), stressful (P=0.442), competitive (P=0.310), enjoyable (P=0.630), and tried to perform well (P=0.326). No significant difference was noted for the PASAT score following completion of that task performance between interventions (n=29, P=0.713).

The cardiovascular response for the intervention per time revealed for HR (n=13, P<0.001), HRV (n=10, P<0.001), and PEP (n=13, P<0.001). Post-analysis following mental stress HR was found to be elevated compared to rest (P<0.001, 19.88±10.78 bpm and 15.45±2.42 bpm in low-flavanol cocoa and high-flavanol cocoa, respectively) and also to pre-intervention baseline (P<0.001). Post-analysis of HRV revealed different results indicating lower values during mental stress compared to rest and pre-intervention baseline. The values for low-flavanol cocoa were (P=0.006, -37.19±38.92 ms), for high-flavanol cocoa (P=-28.20±22.12 ms), and for pre-intervention (P=0.004). Post-analysis of PEP also revealed lower values during mental stress compared to rest and pre-intervention baseline. The values were (P=0.002, –26.38±17.90 ms and –23.34 ± 19.72 ms and P=0.001 in low-flavanol cocoa, high-flavanol cocoa, and pre-intervention respectively). This study showed no significant intervention (HR: P=0.964, HRV: P=0.425, PEP: P=0.833) or intervention-per-time interaction effects (HR: P=0.483, HRV: P=0.360, PEP: P=0.883).

ANOVAs for BP revealed a substantial time effect between SBP (n=30, P<0.001) and DBP (n=30, P<0.001) for intervention per time. Post-analysis highlighted an increase in SBP during mental stress when compared to rest and pre-intervention baseline. The values were (P<0.001, 34.67±23.38 mmHg and 29.84±15.98 mmHg and P<0.001 in low-flavanol cocoa, high-flavanol cocoa, and pre-intervention respectively). Post-analysis also showed a significant increase in DBP during mental stress in comparison to rest and pre-intervention baseline. The values for low-flavanol cocoa were 19.10±18.34 mmHg (P<0.001) and for high-flavanol cocoa 17.27±9.60 mmHg (P<0.001) and were also significantly increased for pre-intervention (P<0.001). This study showed no significant intervention (SBP: P=0.527, DBP: P=0.876) or intervention-per-time interaction effects (SBP: P=0.287, DBP: P=0.716).

Mental stress affected post-stress SBP (n=30, P<0.001). However, DBP was not impacted (n=30, P=0.670). Brachial SBP was higher following post stress than pre-intervention baseline. The values were higher at 30 minutes (P<0.001, 2.93±5.48 mmHg and 4.56±6.90 mmHg in low-flavanol cocoa and high-flavanol cocoa, respectively) and 90 minutes (low-flavanol cocoa: P=0.002, 2.26±5.72 mmHg and high-flavanol cocoa: 2.80±4.93 mmHg). This study did not detect significant intervention (SBP: P=0.923, DBP: P=0.682) or intervention-per-time interaction effects (SBP: P=0.554, DBP: P=0.738).

For FBF (n=29, P=0.025), an effect for intervention-per-time interaction was found. However, FVC (n=29, P=0.089) did not have a significant find. FBF was noted to be higher after ingestion of high-flavanol cocoa when compared to low-flavanol cocoa. At rest, high-flavanol cocoa (P<0.001, 0.34±0.83%) in comparison to low-flavanol (–0.38±0.28%) and during stress (P=0.002, 1.46±1.81% compared to 0.26±0.66%) for high-flavanol and low-flavanol cocoa, respectively. FBF increased significantly after a time effect showed changes during mental stress. FBF (P<0.001, 0.63±0.51% and 1.12±1.16% in low-flavanol cocoa and high-flavanol cocoa, respectively). This effect was also noted for FVC in low-flavanol cocoa (P<0.001, 0.32±0.32%) and for high-flavanol cocoa (P=0.51±0.67%). This effect was indicated in comparison to FBF and FVC at rest. Greater values of FBF (P=0.001) and FVC (P=0.001) were found as a significant intervention effect following high-flavanol cocoa when compared to low-flavanol cocoa.

Key Findings

Flavonoids were shown to be effective at offsetting endothelial dysfunction induced by mental stress and improving vascular response during acute mental stress. Cocoa flavanols prohibited the reduction in brachial FMD 30 minutes after mental stress. High-flavanol cocoa in particular attenuated this reduction in FMD, allowing it to stay elevated in comparison to low-flavanol cocoa at 90 minutes post-stress. FBF was significantly increased by high-flavanol cocoa at rest and during stress, unlike with low-flavanol cocoa. There was a 1.4% decline in FMD in this study, which correlates with literature reporting a 1% to 3% decrease in endothelial function following stress in healthy adults.

There was consistency within the stress experience across the interventions. The consumption of flavanols had no effect on the changes mental stress induced on HR, HRV, PEP, and BP. Based on the physiological responses, the central nervous system played a role as sympathetic activity was activated and parasympathetic activity was more withdrawn. Although flavanol had no effect on HR, HRV, PEP, and BP during stress, flavanol increased peripheral vasodilation. This confirms the benefit of flavanol in FBF at rest and demonstrates the contribution of flavanol when the vasculature is inundated with stress.

There was no decrease in the rise of BP during stress despite increased vasodilation during mental stress after consumption of high-flavanol cocoa. This aligns with previous reports of discrepancies between BP and FBF response during stress. Vasodilation during stress, which results in increased BP, indicates a protective mechanism by the vasculature and possibly a more effective method of responding to stress. This study showed that dietary flavanols are effective in eliminating post-stress weakening of endothelial function.

Brachial SBP was consistently elevated after the 30- and 90-minute intervals following stress, despite flavanols having no effect except on brachial DBP. SBP and DBP have been found to be raised for up to 20 minutes after mental stress based on previous studies, with SBP slowly reducing to baseline rate. A substantial increase in resting arterial diameter occurred at both 30 and 90 minutes following mental stress, indicating that arteries are more likely to be dilated after a stressful event; therefore, increased BP is not driven by vasoconstriction.

No adverse effects were reported.

Practice Implications

In today’s society, stress is exceedingly prevalent. During the pandemic, there has been a significant rise in mental health conditions both nationally and globally.1 Stress contributes to the risk of cardiovascular events and an array of health problems.2 Mental stress has been shown to have a negative impact on vascular function.3 Stress can evoke brief myocardial ischemia, and peripheral vascular responses during mental stress have been associated with stress-induced ischemia.4 In healthy adults, mental stress can lead to increased BP and HR in which peripheral arterial vasodilation occurs as measured by FBF.4 Studies have shown that in addition to acute responses to stress, weakened vascular function can also occur post-stress.5 Cardiovascular disease (CVD) is the leading cause of mortality and a major contributor to morbidity in both genders.6

In the United States, an estimated 83 million adults suffer from CVD.7 High blood pressure contributes to 50% of cardiovascular events worldwide and is a significant risk factor for CVD.8 It is the leading cause of death in the United States, contributing to 37% of deaths.9,10

Flavanols are polyphenolic compounds derived from plants.4 They exhibit antioxidant properties, especially those found in grapes, berries, cocoa, apples, tea, and red wine.4,11 Cocoa flavanols exert a substantial effect on endothelium vasodilation, regulate inflammatory markers, and reduce platelet aggregation, insulin resistance, BP, and lipid oxidation.11 Cocoa rich in flavanols helps to promote cardiovascular health and is beneficial to reducing CVD.11 Flavanols help improve human endothelial function and BP within 1 to 3 hours of consumption, benefiting FMD and arteriolar and microvascular vasodilator capacity.4 The first human study using cocoa, in 1996, found that cocoa-flavanols ingestion could protect LDL particles from oxidation ex vivo.11 Recent meta-analysis results following an 18-week randomized trial with flavanol-containing cocoa produced evidence that the consumption of cocoa flavanols can facilitate positive changes in blood pressure.10 Due to the fact that blood pressure is a cardiovascular risk factor, the use of flavanol-containing cocoa can reduce blood pressure within the range of 3 to 5 mmHg, fully substantiating its use and making it a way to significantly impact public health.11

Catechins are a part of the flavonoid family, and cocoa-based products such as chocolate contain significant amounts of both catechin and epicatechin.12 Catechins contain potent antioxidant properties, despite being able to act as pro-oxidants inside the cell.12 Catechins are scavengers of reactive oxygen species (ROS), which contribute not only to cardiovascular disease, but also to cancer, neurodegenerative disease, and diabetes. Catechins may be beneficial in the prevention of disease triggered by oxidative stress due to their antioxidant properties.12 Flavanol metabolites circulating in the body, particularly (-)-epicatechin-derived flavanols, stimulate the beneficial effects of cocoa flavanols on the endothelium by reducing endothelin-1 (ET-1) and increasing bioavailability of nitric oxide (NO).4 Cocoa flavanols increase development of endothelial NO, which promotes vasodilation and allows reduction of BP.9 Flavanols increase bioavailability of NO by stimulating endothelial nitric oxide synthase (eNOS) activation through calcium-mediated initiation signaling pathways, such as phosphoinositide 3-kinases (PI3K), protein kinase B (Akt), and protein kinase A (PKA).4 Evidence shows that NO increases blood flow by contributing to the rise of FBF during stress by inhibiting NO synthase.4

Endothelial function can temporarily decrease for 15 to 90 minutes based on measurements of brachial FMD following periods of stress in young, healthy adults, postmenopausal diabetic women with depression, older adults, and in individuals with metabolic syndrome and high cholesterol.4 According to the authors, significant reductions in FMD that occur after mental stress (1%–3%) should be considered a risk for development of future cardiovascular events, with a 1% FMD reduction resulting in a 13% increase of a cardiovascular-related incident. They also propose that studies suggest that decreased bioavailability of NO, cortisol, and inflammatory markers may play a role in stress-induced decline in endothelial function as well as increased vasoconstrictors, such as ET-1.4 Flavanols decrease the bioavailability of ET-1, indicating relevancy in context of vascular responses to stress.4

Reduction in the steady state of NO concentration can result in failure of smooth muscle relaxation, leading to the development of hypertension.13 Furthermore, evidence shows that cocoa flavanols can lower interleukin 6 (IL-6) production and reduce ROS.4 ROS help to regulate vascular cell functions. However, excessive levels of ROS lead to vascular disease via oxidative damage and disrupts signaling processes, which can negatively affect myocardial function, generating arrhythmias and cardiac remodeling.14 Flavanols help to maintain oxidative balance by their antioxidative properties, which aid in ROS removal.14

For healthy, young adults, BP does not typically change after acute intake of cocoa flavanols, but is more prone to change in older adults or after chronic intake by individuals who are overweight and hypertensive.4 NO bioavailability helps to improve arterial blood pressure by increasing vascular function.15 Flavanol-induced vasodilation that occurs during stress may be clinically relevant as there have been reports of associations between responses of peripheral vasodilation and myocardial ischemia.4 However, flavanols are effective in improving peripheral blood flow and offsetting mental stress that induces endothelial impairment during stress.4 SBP elevation for a prolonged period of time after stress potentiates further implications for risks on the vascular system.4 Although this does not cause great concern for healthy individuals, patients having prolonged increased SBP due to stress, especially patients with hypertension or CVD, have increased risk for vascular health issues.4 A study found that in young males, the rate of rise in BP during mental stress determines whether or not muscle sympathetic nerve activity has a role in driving a response.16 In studies of mental stress, larger SBP responses have been reported in young males than in young females.16 Although this particular study does not focus on a comparison of the female population, it would be interesting to know the impact it would produce for women, who are more likely to report higher stress levels than men.17 Future studies should look at the impact of flavanols on women in stress responses as prior evidence shows gender differences in vascular responses to stress between the 2 groups.4 Additionally, individual studies have confirmed that cocoa consumption has a positive effect on several cognitive outcomes, particularly in young adults.18 After consumption of cocoa flavanols, young adults had increased cerebral blood flow, resulting in better cognitive performance and neuroplasticity, further indicating the benefits of cocoa flavanols during acute mental stress.18

More research is needed to explore the efficacy of approaches to protect the cardiovascular system from the damaging effects of psychological stress.4 In a 16-week intervention where a hypocaloric diet and exercise were combined for obese children, there was a reduction in BP and an increase in peripheral vasodilation during acute mental stress.19 These findings indicate the potential use of high-flavanol dietary strategies to protect vascular health against stress.4 This implies that important lifestyle factors, such as diet and exercise, can help to target and protect the vasculature system from acute mental stress.4 Diets with higher contents of fruits and vegetables are beneficial for human health and wellness. Similarly, diets rich in fruits and vegetables have been shown to lower the risk of CVD, myocardial infarction, cardiovascular mortality, and overall mortality.4 Most fruits and vegetables contain flavonoids, and epidemiological evidence has shown associations between increased intake of flavonoids and reduced risk of CVD-related morbidity and mortality.20 Cocoa-flavanol-rich products have significant benefits for lowering the risk of CVD.9 Foods containing higher amounts of flavonoids can be effective against the impact of stress on vascular function.4 Inclusion of these types of foods into a balanced diet can provide support for cardiovascular health.10 Findings from this study show the importance of using strategies pertaining to the diet, by consuming plant-derived flavanols to provide cardiovascular protection during stressful periods, especially in populations more susceptible to the effects of mental stress.4 Overall lifestyle modifications can be implemented to lower risk factors that contribute to cardiovascular disease such as hypertension. Making the necessary lifestyle modifications can help to manage cardiovascular function and improve long-term health.

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