1. Introduction

Among the many commercially available supplements are cocoa flavanols, which are extracted from beans of the cacao tree. [1] Cocoa flavanols have previously been studied in the literature. [2–4] There are many proposed theories regarding their physiological effects on endothelial function that may tie into cardiovascular health and exercise performance. However, not all of the exact mechanisms and reasons for this are known. In addition, there is some variability in terms of the correct dose needed to elicit any of these responses. There are also inconsistencies regarding the proper intake duration to yield significant results. Overall, it is important to address these discrepancies in order to fully understand the current state of cocoa flavanols in the literature. A clear understanding is necessary to observe the effects that a moderate dose may have on endothelial function and exercise performance.

Many physiological mechanisms are thought to contribute to the positive effects of cocoa flavanols on endothelial function. These include decreases in known vasoconstrictors and nitric oxide (NO) scavengers, as well as an increase in the enzyme endothelial nitric oxide synthase (eNOS), which is essential for NO production. [2] Together, these changes lead to a robust increase in NO availability. [2] Cocoa flavanols appear to improve endothelial function through a combination of decreased blood pressure and enhanced flow-mediated dilation (FMD). [3,5–7]

It is also necessary to observe the effects that cocoa flavanol supplementation may have on exercise performance. There is little research on this topic, but there may be positive effects on both aerobic and anaerobic exercise performance. [8] The extent to which cocoa flavanols may aid performance still warrants more research. [8,9] It is important to determine whether or not this lesser-known nutritional supplement can impact various aspects of exercise performance, as well as the populations that may benefit from it. If cocoa flavanols are found to enhance endothelial function, they could be utilized in clinical settings to help lower the risk or severity of hypertension and cardiometabolic disease. [6] If cocoa flavanol supplementation is found to positively influence exercise performance, coaches and athletes could utilize this lesser-known nutritional supplement as a method of enhancing performance in both practice and competition.

2. Methods

Both PubMed and Google Scholar databases were utilized to search for appropriate studies published between 2003 and 2018. Using the following keywords including ‘cocoa flavanols,’ ‘endothelial function,’ and ‘exercise,’ relevant studies were searched and evaluated. The article screening process was in accordance with the guidelines set forth by the preferred reporting items for systematic reviews and meta-analyses (PRISMA) statement. [10] Studies were evaluated based on a variety of inclusion and exclusion criteria. Accepted studies needed to utilize cocoa-based flavanols, as opposed to other known sources of dietary flavanols (fruits, tea, etc.). In addition, studies needed to include components of endothelial function and/or exercise performance. Finally, studies using both human and animal models were included. Studies were excluded if they did not include relevant information regarding cocoa flavanol supplementation with exercise or with chronic disease related to poor endothelial function. Finally, studies were excluded if supplementation methods were unclear.

3. Results

At the completion of the PRISMA screening process, 27 out of 59 qualifying studies were included for the review. As shown in Figure. 1, a total of 32 studies were excluded for not meeting the abovementioned criteria, such as not including cocoa flavanols (N=17), endothelial function (N=7), or cocoa-based sources of cocoa flavanols (N=8). [6]

Of the 27 studies included in the review, the majority (N=24) exhibited significant results demonstrating how cocoa flavanols enhance endothelial function and exercise performance. A smaller number of studies reported no change following cocoa flavanol supplementation (N=3).

Figure 1. Screening process for eligible studies

4. Discussion

4.1. Cocoa Flavanols and Nitric Oxide

Cocoa flavanols may impact a variety of physiological mechanisms related to endothelial function through an increase in NO availability. [2] Specifically, cocoa flavanols are thought to increase eNOS activity and inhibit a variety of factors, such as superoxide anion, angiotensin-converting enzyme (ACE), and endothelin-1 (ET-1). [2,7,11,12] NO is a well-known vasodilator, causing blood vessel diameter to increase and allowing for a subsequent enhancement in blood flow. [2] Endothelial cells use an amino acid known as L-arginine to form NO. [13,14] eNOS, known as one of the limiting factors of NO availability, catalyzes this reaction. Indeed, there is a direct relationship between eNOS and NO availability. Once NO has been synthesized, it moves into the tunica media of the vasculature via diffusion. [14] NO then binds to the heme group of guanylyl cyclase, an important enzyme that catalyzes the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP). [13,14] cGMP acts as a second messenger in the process of vasodilation, signaling vascular muscle tissue to stop contracting. [13] This is the pathway by which NO-mediated vasodilation occurs, and it is essential to understand this pathway when studying the effects of cocoa flavanols on endothelial function. The basic relationship between cocoa flavanols and vasodilation is depicted below in Figure. 2.

Figure 2. Effects of cocoa flavanol-derived epicatechin on NO availability and vasodilation. ACE = angiotensin-converting enzyme, cGMP = cyclic guanosine monophosphate, eNOS = endothelial nitric oxide synthase, ET-1 = endothelin-1, GTP = guanosine triphosphate, NO = nitric oxide

Cocoa flavanols contain two very important polyphenols that are thought to influence endothelial function: catechin and epicatechin. [2] Epicatechin is thought to play the greater role in endothelial function as it is the most plentiful polyphenol in cocoa powder and other cocoa products. [2,15,16] It is theorized that epicatechin causes a variety of physiological changes that lead to increased availability of NO. [16,17] One such change is that cocoa flavanols (especially epicatechin) cause an increase in eNOS activity. The exact mechanisms behind this enhancement are unclear. However, previous research has proposed that cocoa flavanols may inhibit arginase activity. [17] Arginase plays a role in the breakdown of L-arginine. As mentioned above, L-arginine is used to form NO with the help of eNOS. Therefore, upregulated L-arginine availability allows for eNOS to be much more active in converting L-arginine to NO. [4,17] Other studies have confirmed that cocoa flavanol supplementation does indeed lead to increased eNOS activity, resulting in greater NO availability. [4,13,17] Indeed, it is clear that cocoa flavanols have a significant effect on eNOS.

4.2. Cocoa Flavanols and Oxidants

Cocoa flavanols also appear to impact oxidants contained in endothelial cells, specifically superoxide anion. Superoxide anion’s main function is to break down leftover NO within endothelial cells. [2,13] Cocoa-derived epicatechin has been shown to suppress cellular superoxide anion concentration, likely due to its ability to clear free radicals. [13] In addition, further research has speculated that cocoa flavanols may further limit superoxide anion concentration via inhibition of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, an enzyme involved with superoxide anion-mediated breakdown of NO. [2] Suppression of superoxide anion is yet another mechanism brought about by cocoa flavanols that appears to enhance endothelial function.

4.3. Cocoa Flavanols and ACE

Cocoa flavanols have also been reported to inhibit angiotensin-converting enzyme (ACE), an enzyme that is crucial in the conversion of angiotensin I to angiotensin II. Angiotensin II is a potent vasoconstrictor, which causes smooth muscle to contract and its diameter to decrease. [2,12,18] There is evidence that cocoa flavanols may inhibit ACE activity; thus, there is less vasoconstriction in the blood vessel. [2,5,11,12,19] This may lead to improved endothelial function through an increase in flow-mediated dilation (FMD). [17] There is no definitive evidence of the exact mechanism behind this inhibition. However, Actis-Goretta and colleagues proposed that, while monomers of epicatechin and catechin can inhibit ACE activity, polymers contained in high amounts in flavanol-rich foods (specifically cocoa products) are the most effective. [12] The polymers include more hydroxyl groups than the monomers, meaning that the polymers of epicatechin and catechin can create more hydrogen bonds with ACE. This causes ACE inhibition and a subsequent decrease in vasoconstriction. [7] This potential mechanism is quite relevant, as it brings forth yet another pathway by which cocoa flavanols stimulate enhanced endothelial function.

4.4. Cocoa Flavanols and ET-1

The final proposed physiological mechanism is the inhibition of endothelin-1 (ET-1). ET-1 is a potent vasoconstrictor activated by the onset of tissue inflammation. [11] Located within the endothelium, ET-1 brings about vasoconstriction by causing an increase in superoxide anion, resulting in decreased NO availability within the endothelial cells. [11] There is evidence that cocoa flavanol supplementation (epicatechin in particular) can result in reduced expression of the ET-1 gene by inhibiting its transcription. [11] This in turn reduces the amount of ET-1 within endothelial cells, thereby decreasing the concentration of superoxide anion even further and adding to the strength of aforementioned mechanisms.

4.5. Clinical Significance of Previous Studies

In addition to the mechanistic physiological effects brought about by cocoa flavanols, it is important to observe how previous research has measured possible enhancements in endothelial function and the methodology used to produce significant results. Cocoa flavanols significantly impact both blood pressure and FMD%. [20,21] Both are measurements of endothelial function frequently used in clinical settings. [20] Even though cocoa flavanol supplementation has shown promising results, there are discrepancies within the literature regarding the proper dose and intake period necessary to elicit significant positive effects.

Blood pressure is commonly used in clinical settings as a measurement of endothelial function and the presence of hypertension. [19] Many previous studies have measured the potential effects of cocoa flavanol supplementation on resting blood pressure with interesting results. A previous study conducted by Fraga and colleagues observed that a dose of only 168 mg of cocoa flavanols for 14 days led to a significant decrease (5 mmHg) in diastolic blood pressure (DBP) and a significant decrease (5 mmHg) in mean blood pressure in 18-20 year old male soccer players. [7] This shows that cocoa flavanols can have a positive impact on blood pressure, even in normotensive young adults. In contrast, there is evidence that doses of 33 mg, 372 mg, and 712 mg, respectively, do not have any significant effect on blood pressure in older adults with mild hypertension. [5] The authors of the latter dose response study reported that a dose of only 1052 mg for 6 weeks significantly reduced blood pressure, causing 5.3 mmHg and 3.0 mmHg decreases in systolic blood pressure (SBP) and DBP, respectively. [5] Another similar study supplementing 213 mg of cocoa flavanols for 14 days found no significant change in blood pressure for moderately trained males. [8] However, a study involving healthy males and females aged 35-60 using a 450 mg dose measured after 14 days observed significant decreases in both SBP and DBP of 4.4 and 3.9 mmHg, respectively. [21] These are peculiar results, as one study found significant effects with a low dose for only 14 days, while the other only found significant results with a much higher dose for 6 weeks. Thus, more research must be done in order to develop a clearer dose-dependent relationship between cocoa flavanols and significant reductions in blood pressure. [8]

There is even further discrepancy within the literature between different meta-analyses on the subject. According to Ried and colleagues, a dose range of 49-3,675 mg of cocoa flavanols resulted in an average decrease of 2 mmHg in both SBP and DBP over a span of at least two weeks. [22] In addition, another meta-analysis reported that an even smaller dose range (5-174 mg) resulted in a significant reduction in SBP by an average of 4.5 mmHg and DBP by 2.5 mmHg. [23] There are certainly discrepancies between proper doses and intake durations for cocoa flavanols to have a meaningful effect on blood pressure. However, it appears that even lower doses may have an effect if administered for at least two weeks. [7,22]

There is also evidence that cocoa flavanol supplementation can positively affect FMD%, a popular method of measuring endothelial function of the brachial artery using ultrasound-imaging technology. [17,24,4] FMD involves measurement and analysis of brachial artery diameter before and after a short period of blood vessel occlusion using a rapidly deflating cuff on the forearm. [4] According to Davison, Coates, Buckley, and Howe, a high cocoa flavanol dose of 902 mg for 12 weeks enhanced FMD% by 2.4% in sedentary adults aged 18-65. [6] Other studies showed improvements in FMD% between 1.40 and 3.99% with dose ranges of 371 to 918 mg over durations of 1 day to 2 weeks. [3,4,17,25,26] The considerable variability between doses and duration intakes certainly does not result in any specific insights. However, similar to blood pressure, lower doses administered for at least 2 weeks can still result in significant effects in FMD. [17]

4.6. Cocoa Flavanols and Exercise

In addition to having physiological and clinical relevance, cocoa flavanols may have a positive impact on exercise performance as well through enhancements in endothelial function. [8] Cocoa flavanol supplementation has been used in studies measuring both aerobic and anaerobic exercise performance. Aspects of performance within these areas have been analyzed in the literature with conflicting results.

The combination of cocoa flavanol supplementation and aerobic exercise performance has been given the most amount of attention in the literature. This likely due to the effects of cocoa flavanols on NO availability. [2,8] According to Patel, Brouner, and Spendiff, 14 days of 213 mg of cocoa flavanol supplementation resulted in a 6% increase in maximal oxygen uptake (VO_2max). [8] The authors also observed a decrease in oxygen consumption during moderate intensity exercise among moderately-trained male participants. [8] In contrast, another study reported that 900 mg of cocoa flavanol supplementation did not have a significant effect on a 30-minute cycling time trial in well-trained male participants with an average age of 30. [27] These are interesting results, as one study showed significant results with a smaller dose, while another found nothing using a much larger dose. [8,27] However, there is one notable difference between the two studies. Patel, Brouner, and Spendiff tested moderately trained individuals with an average VO_2max of 41 ml/kg/min, while Decroix and colleagues tested well-trained cyclists with an average VO_2max of 63 ml/kg/min. [8,27] Therefore, cocoa flavanol supplementation may only enhance aerobic performance in participants that are recreationally active or moderately trained.

Very little research has been performed on the effects of cocoa flavanol supplementation on anaerobic exercise performance. One relevant study reported that 213 mg taken for 14 days led to a significant (17%) increase in distance covered during an all-out time trial on a cycle ergometer. [8] Even though this is an interesting finding, further research needs to be performed using more recognized methods of testing anaerobic performance. Studies have, however, proposed ways of enhancing exercise performance through indirect mechanisms. [7,28,29] Exercise-related oxidative stress has been analyzed before and after cocoa flavanol supplementation with promising results. Doses of both 168 mg and approximately 2800 mg of cocoa flavanols had positive effects on oxidative stress caused by exercise. [7,28] In contrast, a study using downhill treadmill running as a stimulus for muscle damage found no significant enhancement in recovery from 350 mg of cocoa flavanols. [29] It is important to note, however, that the latter study utilized endurance-trained participants with an average VO_2max of 64.4 ml/kg/min. [29] These results point to training status as a potential limitation for the effects of cocoa flavanol supplementation. Overall, there are interesting findings that point to a possible indirect benefit of cocoa flavanols on exercise performance. There is a great need for more research on the subject.

There may be some interesting connections between endothelial function, clinical health, and exercise performance. In-depth studies on pathways related to NO availability have uncovered significant mechanistic effects that bring about an increase in vasodilation together with a decrease in vasoconstriction. [2,11,13,18,19,30] Nitric oxide availability is increased by cocoa flavanols through increased eNOS availability, as well as inhibition of superoxide anion, ACE, and ET-1. The comprehensive effects of these mechanisms allows for enhanced endothelial function. [2,11,13,18,19,30]

The physiological effects of cocoa flavanols can easily be measured in clinical settings using both blood pressure and FMD. Previous research on both methods has shown promising results for having implications on assessing clinical populations [17,19,23] This means that there is potential for cocoa flavanols to be used in conjunction with other treatments to help those who suffer from chronic diseases related to vascular dysfunction. Previous evidence shows that cocoa flavanols can benefit clinical measurements of endothelial function, such as blood pressure and FMD%. [3,4,7,17,25,26]

After a thorough review, it appears that doses between 168 and 918 mg per day for at least 14 days yield positive results in both blood pressure and FMD%. [3,4,13,17,25,26] This shows that even lower doses can have a significant impact on clinical measurements of endothelial function. It also presents definitive evidence that a daily intake of cocoa flavanols for 14 days can lead to significant beneficial effects. The clinical impact of this information remains somewhat unclear. However, there is potential for cocoa flavanols to attenuate risk factors for certain chronic diseases. [6,11,20,31] Further research is needed to determine how this may affect other populations. In addition, lower and upper dose guidelines need to be established for various populations.

Cocoa flavanols could certainly be supplemented in conjunction with exercise performance as well. Even though there is more conflicting evidence in this area, the mechanisms behind the supplement’s functional use are quite clear. Aerobic performance stands to gain the most from cocoa flavanols. More research is needed in this area. However, after a review of the literature, it appears that the training status of individuals may play a role. This could mean that highly trained individuals are simply too fit to benefit from cocoa flavanols. Future research should compare and contrast different training statuses with an equal dose of cocoa flavanols. Even though anaerobic performance evidence is lacking, there are some potential tie-ins to recovery from oxidative stress that could lead to enhanced anaerobic exercise performance. [7,28] This may allow athletes to recover from high intensity activity much faster and allow them to subsequently increase their performance. Specific studies involving anaerobic exercise performance and markers of recovery would be quite helpful in this area. A summary of each article reviewed is included below in Table 1.

Table 1. Summary of Studies Reviewed

5. Conclusions

The present review explored the current status of cocoa flavanols in the literature, specifically relating to endothelial function and exercise performance. Their positive effects on endothelial function are well documented, and cocoa flavanols could be utilized in clinical settings in the ongoing battle against vascular dysfunction. In addition, exercise performance may stand to benefit as well. Cocoa flavanols hold a large amount of un-tapped potential in various areas of research. Further work is certainly necessary to fully understand their benefits. Overall, it is important to realize that cocoa flavanols may be beneficial to human health, and that their effects on endothelial function can be applied in many directions to benefit different populations. With future research in more specific directions, cocoa flavanols could likely be seen as a more effective and useful nutritional supplements.