Capsaicinoid and Capsinoids as an Ergogenic Aid: A Systematic Review and the Potential Mechanisms Involved

Context : Capsaicinoids and capsinoids (CAP) are natural substances found primarily in chili peppers and other spicy foods that agonize the transient receptor potential vanilloid-1 in the mouth, stomach, and small intestine. Several studies have shown CAP to be a potential antiobesity agent and to exhibit an analgesic effect in both rodents and humans. However, there is no scienti ﬁ c consensus about the effects of CAP on physical exercise performance and its physiological mechanisms of action. Purpose : This systematic review aimed to better elucidate the effects of CAP compounds as ergogenic aids and to discuss underlying mechanisms of action by which this supplement may potentially enhance endurance performance and muscular strength. Conclusions : Among 22 studies included in the review, 14 examined the effects of capsaicinoid or capsinoid compounds on endurance and resistance exercise performance in animals, with 9 studies showing bene ﬁ ts on performance. In humans, 8 studies were included: 3 demonstrated signi ﬁ cant acute endurance bene ﬁ ts and 2 showed acute resistance exercise performance bene ﬁ ts compared with a placebo condition. Therefore, while more mechanistic studies are necessary to con ﬁ rm these outcomes in humans, the available scienti ﬁ c literature appears to suggest that these compounds could be considered an effective nutritional strategy to improve exercise performance.

Physical activities including endurance and resistance training programs are well-known strategies to improve cardiorespiratory fitness and muscular health according to the American College of Sports Medicine. 1 Fitness enthusiasts and recreational and trained athletes seek to optimize the benefits induced by exercise through targeted nutrition including nutritional supplements that are widely consumed in order to enhance training outcomes. 2 Capsaicinoids are the pungent compounds found primarily in chili peppers and other spicy foods. Capsaicin (trans-8-methyl-Nvanillyl-6-nonenamide) and dihydrocapsaicin (8-methyl-N-vanillylnonanamide) are among the most abundant capsaicinoids and are responsible for approximately 90% of total pungency. 3,4 Orally administered capsaicin is rapidly absorbed in the various tissues, such as blood, liver, kidney, and intestine with highest concentration in blood and intestinal tissue at 1 hour, liver at 3 hours, and kidney at 6 hours. Absorption is a passive process that does not require an ATP investment, with variable amounts ranging from 50% to 90% being absorbed, depending on the tissue distribution and analytical method employed (thin-layer chromatography or high-performance liquid chromatography). 5,6 Capsaicin concentration can be detected in plasma as soon as 10 minutes following administration, peaking at ∼47 minutes, with a half-life in blood of ∼25 minutes, and maximum plasma concentrations of 2.5 ± 0.13 ng·mL following a 26.6-mg dose of capsaicin in gel capsules. [7][8][9] The mode of oral delivery (gel capsules vs chewable delivery methods, and capsaicin vs a mix of capsinoids) may influence plasma availability and ergogenic effects. For example, Cross et al 10 found significant acute benefits of a chewable capsaicin supplement on isokinetic knee extensor contractile performance, whereas Opheim and Rankin 11 administered a commercially available cayenne pepper supplement and did not find an improvement in repeated-sprint tests. While a considerable amount of research has been done examining differences in bioavailability of various delivery methods of CAP when administered transdermally, 12 there are no studies to our knowledge that have investigated bioavailability or pharmacokinetics of oral ingestion.
Capsinoids, which include capsiate (4-hydroxy-3-methoxybenzyl [E]-8-methyl-6-nonenoate), dihydrocapsiate, and nordihydrocapsiate, on the other hand, are nonspicy compounds. Capsinoids are chemical analogs of capsaicin, a component of a sweet pepper called "CH-19 Sweet" that is highly concentrated, with an ester linkage replacing the amide bond between the vanillyl moiety. The fatty acid chain is very similar to capsaicin. 8 Ten studies investigating the ergogenic effects of capsaicinoids and capsinoids (CAP) supplementation have been published in the past 5 years. Our group recently investigated the effects of 12 mg of capsiate ingestion during high-intensity interval exercise in physically active men, and observed an increase in time to exhaustion without a concomitant change in blood lactate concentration or oxygen consumption during exercise and postexercise. 13 We also found that 12-mg capsiate ingestion increased the total volume performed during lower-body resistance exercise and decreased rating of perceived exertion (RPE). 14 Taken together, this data indicates that capsiate could serve as a potential strategy to improve both endurance and strength performance. On the other hand, Opheim and Rankin 11 reported that 25.8 mg of CAP per day for 7 days in the form of a commercially available cayenne supplement did not enhance repeated-sprint performance in experienced athletes. Best et al 15 recently reviewed a range of tastes including capsaicin/capsiate upon exercise performance and suggested that ingesting capsaicin as a capsule improved performance. To the best of our knowledge, there is no consensus regarding the effects of CAP on exercise performance. Therefore, the objective of this systematic review was to investigate the effects of CAP compounds as an ergogenic aid and to discuss the underlying mechanisms of action by which these supplements may potentially enhance endurance and resistance exercise performance.

Methods
The present systematic review was registered in an international database of prospectively registered systematic reviews in health and social care (PROSPERO, https://www.crd.york.ac.uk/prospero/) under registration number: CRD42018079825.
Two independent reviewers analyzed titles and abstracts and relevant full-text articles following specific inclusion criteria. Inclusion criteria for the present systematic review were (1) to use capsaicinoid (ie, capsaicin, dihydrocapsaicin) or capsinoids (ie, capsiate, dihydrocapsiate) compounds, (2) to include exercise, and (3) to investigate endurance or strength performances. The databases used in the search were as follows: PubMed, Science Direct, Scopus, Web of Science, SciELO, and LILACS.
The search strategy used consisted of a combination of databasespecific MeSH terms and Boolean operators ("AND" and "OR") following descriptors: Capsaicin OR capsaicinoid OR capsinoids OR capsiate AND Aerobic OR Endurance Exercise OR Endurance Training AND Performance AND Animal OR Human; Capsaicin OR capsaicinoid OR capsinoids OR capsiate AND Resistance Exercise OR Strength Exercise OR Resistance OR Strength Training AND Performance AND Animal OR Human. There was a restriction for English language but restrictions for publication period and systematic reviews were not included.

Data Extraction
Search results were exported to Excel software (Microsoft Corp, Redmond, WA), where duplicate articles were removed manually. First, 2 independent reviewers screened the exported studies to determine their relevance. Potentially relevant articles were retrieved as full-text articles and were evaluated against the eligibility criteria. If the 2 reviewers were uncertain or could not agree on the eligibility of studies, a third reviewer was invited to act as arbiter. The last examination of the literature was conducted in June 2019 and an update was performed in June 2020. After this process, 22 studies were included, as shown in Figure 1.

Risk of Bias (Quality) Assessment
Two independent reviewers assessed the quality of the studies and a third independent reviewer resolved any disagreement.
Methodological quality was not an inclusion criterion. The risk of bias tool covers 6 domains of bias: selection bias, performance bias, detection bias, attrition bias, reporting bias, and other bias, according to that proposed by the Cochrane Collaboration's tool for assessing the risk of bias in randomized trials. 16 The second part of the tool involved assigning a judgment of high, low, or unclear risk of material bias for each item, and the quality of all studies analyzed is presented in Figure 2.

Results
There were 14 studies conducted with animals, of which 6 studies were acute and all demonstrated significant increases in swimming endurance capacity. There were only 2 studies that investigated the acute effect of capsiate on force. 18,19 Faraut et al 20 analyzed the acute effect of 100 mg/kg body weight on force production in gastrocnemius muscle in female rats and observed that capsiate administration did not affect the overall time course of isometric force both in the control (39.5%) and capsiate groups (35.1%). On the other hand, Kazuya et al 19 compared 2 different doses of capsiate (10 mg/kg vs 100 mg/kg) on 6 minutes of maximal repeated isometric contractions and reported significant increases in total peak force (22%) with the highest dose.
There were 6 studies comparing the chronic effect of CAP in animals, of which 3 studies investigated the effect on running distance but only Luo et al 21 demonstrated increases in performance, where mice ran about 300 m more before than control after 3-month capsaicin treatment. In contrast, Dousset et al 22 verified that neonatally capsaicin-treated rats had impaired performance (ie, -35 min or -12% of run), and Ohyama et al 23 did not find benefits on running distance in mice treated with capsaicin analogs.
There were 3 chronic studies investigating the effect of resistance exercise in rats, but only Hsu et al 24 showed a dose-dependent effect in the forelimb grip strength (control = 118 g, 205 mg/kg CAP = 117 g, 410 mg/kg CAP = 125 g, and 1025 mg/kg CAP = 131 g) and time to exhaustion (control = 9.3 min, 205 mg/kg CAP = 11.06 min, 410 mg/kg CAP = 11.28 min, and 1025 mg/kg CAP = 46.99 min), while Faraut et al 18 verified isometric force production decreased throughout the stimulation period in control (69.1% [4.0%]) and capsiate groups (71.6% [4.6%]), but without significant difference between groups. Finally, Yashiro et al 25 did not find significant differences between 10 and 100 mg/kg body weight of capsiate on force-generating capacity and fatigability after 2 weeks. Table 1 summarizes the effects of CAP ingestion on endurance and strength performances in animals.
There were 5 studies that investigated the acute effect of CAP on running performance in humans. de Freitas et al 14 reported significant increases in time to exhaustion during high-intensity intermittent exercise (15s:15s at 120% peak oxygen uptake [VO 2 peak]) with 12-mg capsiate (102 efforts vs 89 efforts). The same authors also reported improvements in 1500-m running time trials (CAP = 371.6 [40.8]  Regarding the chronic effect in humans, there was only one study in the literature. Opheim and Rankin 11 investigated the effect of 6 treatment capsules with food per day for a total of 25.8 mg of capsaicin (cayenne pepper) per day or placebo for 7 days on a repeated-sprint test consisting of 15 × 30-m sprints with 35-second intervals. In this study, the authors did not find any significant improvements on performance. Table 2 summarizes the effects of CAP ingestion on endurance and strength performances in humans.

Acute CAP Supplementation and Performance
A total of 14 studies have investigated the effects of acute CAP supplementation on endurance performance in animals. For example, mice acutely supplemented with a nonpungent capsaicin analog (0.02 and 0.033 mmol/kg) were able to swim longer before exhaustion than the control mice (62.9 vs 49.6 min, P < .05). 27 The same authors later confirmed that the same dose of capsaicin increased the swimming endurance capacity, and also reported that co-supplementation of capsazepine (a capsaicin antagonist) with CAP reduced endurance capacity by suppressing the release of catecholamines. 26,28 In accordance with these findings, Oh and Ohta 29 demonstrated that supplementation of 15 mg/kg body weight of capsaicin 2 hours before exercise increased the swim time to exhaustion in rats, which was concomitant to increases in plasma concentrations of catecholamines, free fatty acids, and glucose. Interestingly, Oh et al 30 also reported the effects of various doses of capsaicin on swim endurance capacity in rats. The highest dose (15 vs 6 and 10 mg/kg) increased endurance performance time, plasma concentration of epinephrine, norepinephrine, free fatty acid, and spared tissue glycogen. These results tend to demonstrate that the more capsaicin ingested (at least up to 15 mg/kg), the greater the effects on performance in animals.
We were able to find only 5 studies in humans. de Freitas et al 31 analyzed the effects of capsiate supplementation on performance, RPE, and blood lactate concentration during middle-distance running in physically active adults. Capsiate supplementation significantly improved 1500-m run performance and reduced RPE more than a placebo supplement. Similarly, de Freitas et al 13 investigated the acute effect of capsiate supplementation on performance and physiological responses during high-intensity intermittent exercise. Thirteen physically active men performed 2 randomized, double-blind high-intensity intermittent exercise (15s:15s at 120% speed at VO 2 peak) trials 45 minutes after consuming 12 mg of capsiate or a placebo. The results showed that capsiate increased the time to exhaustion by 188 seconds (13 efforts more than in placebo, which represents a 13% improvement in performance), without modifying RPE and metabolic responses such as blood lactate and oxygen consumption during exercise and postexercise (fast phase of oxygen consumption and 20-min postexercise). Recently, Costa et al 32 investigated the effect of capsiate supplementation on short-(400 m) and middle-distance (3000 m) running timetrial performance, RPE, and HR in physically active men, and observed a decrease in both time trials but no change in RPE and HR. ↑ swimming endurance capacity Kim et al 27 6-wk-old mice (n = 21) C18-VA (Capsiate) 0.033 mmol/kg and placebo. Measurement of swimming time was evaluated 2 h after.
↑ swimming endurance capacity Oh and Ohta 29 4-wk-old male mice (n = 49) Vehicle or a dose of capsaicin, 6, 10, or 15 mg/kg of body weight, 2 h before exercise.
↑ swimming time to exhaustion in the highest dose Oh et al 30 4-wk-old male mice (n = 49) Rats were given orally either control (0) or 6, 10, or 15 mg capsaicin/kg body weight 2 h before exercise by stomach intubations using a round-ended needle.
↑ swimming time to exhaustion in the highest dose Haramizu et al 16 Male mice (n = 7) Mice orally given capsiate (10 mg/kg) and swam 30 min at a flow rate of 7 L/min. Padilha et al 33 investigated the effect of acute capsiate supplementation (12 mg) on time to exhaustion, physiological responses, and energetic systems contribution during continuous high-intensity exercise session in runners. The subjects completed 2 randomized, double-blind continuous high-intensity exercises at 90% sVO 2 peak, 45 minutes after consuming capsiate or an isocaloric placebo. Time to exhaustion, blood lactate concentration, oxygen consumption during exercise and 20-minute postexercise, energy systems contribution, time to reach VO 2 peak, HR, and RPE were evaluated. Acute capsiate supplementation did not increase the time to exhaustion during high-intensity continuous exercise nor alter physiological responses in runners. Langan and Grosicki 34 assessed the acute effect of chewable capsaicin supplement (1.2 mg) 45 minutes before a test until cycling exhaustion at ∼ 90% individual VO 2 max in active men and women. Capsaicin did not increase performance compared with ingested placebo, with no differences in HR, pedaling cadence, and RPE. Taken together, these studies suggest that acute capsaicin supplementation may improve endurance performance in humans, but this ergogenic effect depends upon the protocol and maybe the type of CAP used.
Studies investigating the acute effects of CAP on performances during resistance exercise are scarce. Kazuya et al 19 analyzed the effects of a single intake of a low (10 mg/kg) and high (100 mg/kg) dose of capsiate on gastrocnemius muscle performance and energetics in mice. The results showed that a higher dose increased force-generating capabilities in skeletal muscle and reduced the ATP cost for 6 minutes of repeated fatiguing isometric contractions.
To our best knowledge, only 2 studies investigated the effects of capsiate on resistance exercise performance in humans. de Freitas et al 14 investigated the effects of 12-mg capsiate supplementation on resistance exercise performance (4 sets to muscular failure with 70% of 1RM in squat exercise) in trained men. Capsiate significantly increased total weight lifted (capsaicin: 3919.4 vs placebo: 3179.6 kg) while reducing RPE, but did not significantly alter the blood lactate concentration. Cross 10 also verified the acute effect of chewable capsaicin supplementation (1.2 mg) on the isokinetic knee extensors in physically active men and women. Peak torque production was higher after capsaicin supplementation with no difference in the sum of torque or fatigue index. Although these preliminary data are very relevant to fitness conditioning, future research is certainly warranted to determine the efficacy of chronic capsaicin supplementation in various populations.

Chronic CAP Supplementation and Performance
Despite several studies investigating the acute effects of CAP on performance with relatively beneficial results, there is a lack of chronic studies in the research literature, particularly in humans. In mice, Luo et al 21 demonstrated that running distance on a treadmill increased over the course of 12 months when a high-fat diet was supplemented with 0.01% of capsaicin compared with animals maintaining a normal diet. On the contrary, Ohyama et al 23 showed that capsinoid supplementation associated with a high-fat diet did not increase running performance in rats; however, the authors reported increased energy expenditure with capsinoid supplementation via the activation of fat oxidation in skeletal muscle and lipolysis in brown adipose tissue. There is more data available in the literature about the combination of CAP with resistance training. For example, Hsu et al 24 reported that 4 weeks of capsaicin supplementation increased relative forelimb grip strength in a dose-dependent manner, with the greatest values observed in the group of mice receiving 1025 mg capsaicin/kg/ d (approximately 5-fold the usual human equivalent dose). On the other hand, with a much lower dose (100 mg/kg), Faraut et al 18 did not show any change in isometric force in the gastrocnemius muscle of rats after 14 days of capsiate supplementation. In humans, Opheim and Rankin 12 demonstrated that supplementation of 25.8 mg of capsaicin (cayenne pepper) per day over 7 days did not improve repeated-sprint performance (15 × 30-m sprints with 35-s intervals) in experienced athletes. However, with such a large dose in the form of cayenne pepper, the authors reported gastrointestinal discomfort in 25% of the participants. Thus, it is possible that such level of discomfort affected the physical capacity of the participants and, thereby, limited the interpretations derived from Opheim and Rankin 11 study.
On the other hand, Cross et al 10 found an improvement in isokinetic knee extensor torque, but not muscular endurance or fatigue, with a chewable, low-dose (1.2 mg) capsaicin supplement. It is unclear whether buccal absorption of capsaicin may have occurred or whether the pungent sensation of oral heat induced a stimulation of targeted receptors in the oral cavity. 15 Future studies are necessary to investigate the Pharmacokinetics and ergogenic response to different oral delivery methods.

Potential Mechanisms Underlying CAP Ergogenicity
The potential mechanism that may explain the ergogenic effects of CAP on performance has been through modulation of the transient receptor potential vanilloid 1 (TRPV1) channel, as illustrated in Figure 3. 35 Mechanisms related to TRPV1 agonists include the following: (1) increase in calcium release in the sarcoplasmic reticulum of muscle cells resulting in reduced muscular fatigue, 36 (2) increase in fatty acid oxidation thereby providing more available energy and improved body composition, 18,29,37 (3) having an analgesic effect and increasing pain tolerance during exercise, 35,38,39 (4) glycogen sparing properties attenuating declines in force production, 19,24,29 and (5) increase in the release of acetylcholine resulting in increased explosive power and increased endurance performance. 21,28 Skeletal muscle peripheral fatigue is a limiting factor in forcegenerating capacity, 40 and lower calcium release by the sarcoplasmic reticulum results in an impairment in contractile efficiency and myofiber force production. 41,42 In the skeletal muscle, TRPV1 are located in close proximity to the sarcoplasmic reticulum whereby activation by CAP increases the release of calcium, 36,43,44 which may improve the interaction between the actin-myosin filaments. 19,45 The TRPV1 are also present at the neuromuscular junction, whereby stimulation by CAP can induce presynaptic modulation at the neuromuscular junction leading to an increase in evoked acetylcholine release. 35,46 Luo et al 21 examined the influence of capsaicin on TRPV1 activation and calcium release in skeletal muscle of rats. Acute supplementation of capsaicin elevated the amounts of calcium in myotubes; however, when TRPV1 was blocked by 5′-iodo-resiniferatoxin (antagonist of TRPV1), the release of calcium was inhibited. Lotteau et al 36 further showed that capsaicin and resiniferatoxin, both agonists of TRPV1, elicit an increase in calcium in mouse muscle fibers. In addition, the inhibition of TRPV1 by capsazepine resulted in a strong inhibition of the calcium release by the sarcoplasmic reticulum. Based on Luo et al 21 and Lotteau et al 36 studies, it is likely that capsaicin increases performance by activating the TRPV1 receptor in skeletal muscle, improving sarcoplasmic reticulum function and muscular force output. That being said, future studies will need to test this hypothesis in humans.
The reduction of skeletal muscle glycogen content has also been proposed as one important potential mechanism of peripheral locomotor muscle fatigue. 47 There is a close association between low glycogen stores and impaired calcium release by the sarcoplasmic reticulum, suggesting that when muscle glycogen is lower, the efficiency of skeletal muscle to produce force is decreased. 47 In this perspective, delaying the reduction of muscle glycogen via carbohydrate ingestion has been shown to increase performance during endurance exercise. 48 Animal studies have suggested that CAP supplementation increases swimming time to exhaustion (see Table 2), and performance gains were explained in part by an increase in plasma free fatty acid availability as a result of higher epinephrine release, leading a glycogen sparing effect. 27,29 The hypothesis is that capsaicin may increase lipolysis in adipose tissue, raise free fatty acid levels, and increase fatty acid uptake by skeletal muscle, and glucose release from hepatic glycogenolysis and gluconeogenesis. Yashiro et al 25 demonstrated that 2 weeks of capsiate resulted in a reduced oxidative cost of contraction in exercising mouse skeletal muscle. The CAP ingestion also upregulates genes involved in fatty acid oxidation and mitochondrial respiration. 21 Theoretically, this would lead to a higher contribution of beta-oxidation toward the resynthesis of ATP, thereby decreasing the dependence on muscle glycogen to generate energy during exercise. These results provide a mechanistic rationale whereby CAP may improve muscular and aerobic endurance providing shifting substrate utilization. 37,49 The analgesic effect induced by CAP is another potential mechanism that may explain its ergogenic effects. Topical CAP has been used as a pain reliever in neuropathic conditions, 38,39 and sufficient doses of CAP that activate the TRPV1 receptor have been shown to possess analgesic effects by inactivating or desensitizing primary afferent nerve endings as a result of calcium overload. Studies have demonstrated that capsaicin may affect the RPE during exercise. For example, de Freitas et al 14 reported a lower RPE after consuming capsiate during 4 sets of squat exercise performed until muscular failure at 70% 1RM in trained men and during short-duration running in physically active adults. 31 It is possible that CAP supplementation may have increased the pain threshold and reduced discomfort induced by fatiguing exercise, thereby resulting in performance gains.

Practical Applications and Future Perspectives
Although CAP, a natural substance found in chili peppers and other spicy foods, presents promising ergogenic potential, the ergogenic effect of CAP on human exercise performance is currently equivocal, and significant scientific work is needed to demonstrate a consistent mechanism of action. Therefore, future research should investigate the effects of CAP in association with chronic resistance training, aerobic exercise, and concurrent exercise in humans. Research should also ascertain the mechanisms proposed in the present review, in particular, the hypothesis that CAP may increase the lipolysis and fatty acid oxidation, thereby decreasing the dependence on intramuscular glycogen stores to generate energy during exercise. The greater calcium release by the sarcoplasmic reticulum and the analgesic effect, which would increase the pain threshold and reduce discomfort induced by fatiguing exercise, should also be examined in humans. And since these physiological mechanisms (especially, substrate partitioning and pain threshold) may be different in men and women, 50,51 more research should include female participants in their trials to ascertain and understand the efficacy of CAP in women. In addition, since most studies to date have been focused on fat loss and energy intake, 52,53 CAP could be a potential tool for athletes competing in weight classes or in sports where reduced body weight is beneficial; however, further studies are necessary to verify this hypothesis and to measure the muscular hypertrophy outcomes. It is necessary to highlight that difference between exercise protocols used in the literature, different doses and types of supplementation strategy (capsaicinoid [capsaicin] and capsinoids [capsiate]), and levels of physical fitness (untrained vs trained) should be considered when interpreting the results between studies.

Conclusion
In summary, among 22 studies included in the present systematic review, 14 examined the effects of capsaicinoid or capsinoid compounds on endurance and resistance exercise performance in animals with 9 studies showing benefits on performance. In humans, 8 studies were included, of which 3 demonstrated significant acute endurance benefits and 2 acute resistance exercise performance benefits. Therefore, while more mechanistic studies are necessary to confirm these outcomes in humans, the available scientific literature appears to suggest that these compounds could be considered as potential new nutritional strategy to improve exercise performance.