Introduction
Capsaicin (8-methyl-N-vanillyl-trans-6-nonenamide) is a natural substance found primarily in chili peppers and other spicy foods that agonizes transient receptor potential vanilloid-1 (TRPV1) in the mouth, stomach, and small intestine. TRPV1 activation leads to the sensation of heat and signals a chemical cascade that activates the sympathetic nervous system (29) and potentially increases energy expenditure, lipolysis, and fatty acid oxidation (9,10,18). Capsaicin has been studied as an antiobesity agent in both rodents and humans. Activation of TRVP1 receptor by capsaicin has been shown to increase mitochondrial heat production through regulation of uncouple protein expression and induce mitochondrial biogenesis (6,14,17). Snitker et al. (28) reported 12 weeks of 6 mg per day of mixed capsinoid supplementation that resulted in enhanced abdominal fat loss in overweight men and women.
Fatiguing exercise decreases the rate of calcium release in sarcoplasmic reticulum vesicles (5), which has been shown to contribute to reductions in myofiber force generation (30). Activation of the TRPV1 receptor by capsaicin in skeletal muscle increases the release of calcium by the sarcoplasmic reticulum (16), thus resulting in an enhanced interaction between actin-myosin fillaments and resulting in greater force output. A single dose of both 10 mg·kg−1 and 100 mg·kg−1 capsaicin was recently shown to reduce the adenosine tri-phosphate cost during 6 minutes of repeated fatiguing isometric contractions with the higher dose also increasing force-generating capabilities in mouse skeletal muscle (11).
Several studies performed in rodents have demonstrated ergogenic effects of capsaicin supplementation. Kim et al. (12) demonstrated that acutely supplemented with approximately 10–15 mg·kg−1 capsaicin per day increased swimming time to exhaustion in mice. Oh and Ohta (21) observed that capsaicin increased endurance capacity and spares tissue glycogen in swimming rats too, and the effect of capsaicin on endurance capacity seems to be dose dependent (20). The benefits in endurance performance was hypothesized to be partially due to an increase in plasma-free fatty acids and glycogen sparing effect as a result of an increase in adrenal hormone secretions. In support of this hypothesis, capsaicin administration to adrenalectomized mice did not improve endurance performance (12) and cosupplementation of capsazepine (a capsaicin antagonist) with capsaicin reduced endurance capacity by suppressing the release of catecholamines (13).
Based on a US National Library of Medicine National Institutes of Health database search performed in January of 2017, the effects of capsaicin on exercise performance in humans are scarce. In one study conducted in humans, Opheim and Rankin (22) reported no improvements in repeat sprint ability (15 × 30-m sprints with 35-second intervals) after 7 days of 28.5 mg of capsaicin ingested through 3 g of powdered Capsicum frutescens (cayenne pepper) capsules. To our knowledge, only 1 study has evaluated the effects of capsaicin supplementation on strength, and this study was conducted in mice. Hsu et al. (8) 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 receiving 1,025 mg·kg−1·d−1 capsaicin (approximately 5-fold the human equivalent dose). Whether capsaicin influences strength performance in humans is currently unknown.
Thus, the purpose of this study is to investigate the acute effect of capsaicin supplementation on strength performance, rate of perceived exertion (RPE), and blood lactate concentrations during resistance exercise in healthy trained young men. We hypothesized that capsaicin consumption would enhance performance during resistance exercise.
Methods
Experimental Approach to the Problem
This study used a randomized, double-blind, crossover design. Subjects completed 3 experimental trials at the laboratory which were separated by 1 week. All trials were performed at the same time of day (in the morning) to ensure chronobiological control. The first visit measured anthropometrics, body composition, and back squat 1 repetition maximum (1RM). On the following 2 visits, each participant consumed randomly either the placebo or capsaicin and then completed 4 sets of back squats until momentary muscular failure with a load corresponding to 70% of the 1RM and 90 seconds of rest between sets. The total number of repetitions performed was recorded for each set and was used to analyze performance. Blood lactate was analyzed to measure glycolytic parameters during and after resistance exercise, and the RPE was collected immediately after all sets of exercise (Figure 1).
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Figure 1.: Experimental design. CAP = capsaicin; 1RM = 1 repetition maximum.
Subjects
Ten young men (mean ± SD age, 22.7 ± 4.0 years; height, 175 ± 0.07 cm; and mass, 82.3 ± 9.6 kg) with at least 1 year of resistance training experience at a frequency of 3 days per week (experience, 3.5 ± 1.5 years; weekly frequency training, 5.3 ± 0.6 days) were recruited for this study (Table 1). The project was approved by the Ethics Research Group of the University of São Judas, São Paulo-SP, Brazil (Protocol number: 66523717.2.0000.0089) and the research was conducted according to the 2008 Revision of the Declaration of Helsinki. All participants signed a consent form and were informed about the purpose of the study and the possible risks. During the study, all participants were instructed not to use any other supplement or ergogenic substance, as well as make changes to their regular diet and exercise. Additional exclusions criteria included smoking, injuries, or medical conditions that might interfere with the exercise protocol.
Table 1.: General characteristics of the sample, dietary intake, and macronutrient distribution.*
Procedures
Anthropometric Measurements, Body Composition, and Dietary Intake Assessment
Height was measured on a fixed stadiometer of the Sanny brand, with an accuracy of 0.1 cm and a length of 2.20 m. Body mass was measured using an electronic scale (Filizola PL 50; Filizola Ltda., Brazil), with a precision of 0.1 kg. Body composition was assessed via skinfold thicknesses of the triceps, suprailiac, and abdominal according to the procedures described by Lohman et al. (1991), and body density was estimated through the Guedes's formula (1986). To convert body density to %BF, Siri's (27) 2-compartment model was used. The fat-free mass was calculated as the difference between each subject's body mass and body fat mass.
The volunteers were instructed not to consume chili peppers or other spicy foods as well as coffee, tea, alcohol, or stimulant drinks for a period of 12 hours before the assessment. Food questionnaires were distributed to all participants to record food and fluid intake for 24 hours before each trial in addition to their pre-exercise meal (breakfast). Participants were instructed to replicate the first trial's dietary intake for the subsequent trial. All food intakes were analyzed for total kilocalorie and macronutrient intakes (Software—Dietpro version 5.8) to ensure that dietary intake was similar between experimental trials. The software used the database of Brazilian food composition table (TACO) to calculate dietary intake.
Supplementation Protocol
A previous pilot study was performed with 4 subjects to identify sensations associated with capsaicin ingestion. The form and dose of capsaicin in this study was well tolerated, and none of the subjects reported any “hot” sensations or gastrointestinal distress.
On the exercise testing sessions, each participant randomly consumed either the placebo (50 mg of starch) or 12 mg of purified capsaicin (Pharma Nostra—Campinas, Brazil). This dosage was selected because other studies have reported that supplementation of more than 33 mg per day of capsaicinoids increases gastric motility (29). The capsules were identical to ensure a double-blind design. Capsaicin or placebo was ingested 45 minutes before the first resistance exercise test. This timing was selected because capsaicin reaches peak concentrations 45 minutes after supplementation (2); the half-life of capsaicin is approximately 25 minutes; and full clearance from the plasma occurs approximately 105 minutes after supplementation (23).
Blood Lactate Concentration and Rate of Perceived Exertion
Blood samples were collected from the ear lobe of each participant to analyze the concentration of lactate. This measurement was obtained immediately after each set as well as 3, 5, and 30 minutes postresistance exercise. The blood lactate analysis was performed using the Yellow Spring 1,500 Sport lactimeter (Yellow Springs, United States). The RPE was measured at the end of each session of exercise using the 6–20 point Borg scale, according Borg et al. (1).
Resistance Exercise Protocol
Subjects completed 2 familiarization sessions to become acquainted with the 1RM test procedures and exercise equipment. Before 1RM testing, subjects completed a warm-up protocol, which consisted 5 minutes of walking and a subsequent set of 10 repetitions at approximately 50% of the 1RM. The load was increased gradually (10–15%) during the test until the participants were no longer able to perform the entire movement, and 3–5 attempts were allowed to meet the corresponding 1RM load (24).
Before each testing session, participants completed a 5-minute walk on a treadmill and a subsequent set of 15 repetitions at 30% of 1RM for a warm-up. Five minutes later, each participant completed 4 sets of back squats until movement failure at 70% of 1RM with normal speed (1-second concentric, 1-second eccentric) and 90 seconds of rest interval between sets. For better control of the 1RM testing procedures and repetitions to failure tests, a wooden seat with adjustable heights was placed behind the participant to keep the bar displacement and knee angle constant on each repetition. Two fitness professionals supervised all testing sessions.
Statistical Analyses
The data normality was verified using the Shapiro-Wilk test. The comparison of the total mass lifted and RPE under the different conditions was analyzed through a repeated measured t test. A 2 × 4 repeated measures analysis of variance (RMANOVA) with the Bonferroni adjustment for multiple comparisons was used to compare the maximum number of repetitions performed in each set across conditions and time, respectively. A 2 × 8 and 2 × 5 RMANOVA was used to compare lactate and heart, respectively, across condition and time. For all measured variables, the estimated sphericity was verified according to Mauchly's W test, and the Greenhouse-Geisser correction was used when necessary. Statistical significance was set at p ≤ 0.05. The effect size for total repetitions performed and workload was calculated using Cohen's d ([treatment mean−placebo mean]/pooled SD), whereby a value of >0.20 was considered small, >0.50 moderate, and >0.80 large. The data were analyzed using the Statistical Package for Social Sciences 17.0 (SPSS Inc., Chicago, IL, USA).
Results
Table 1 presents the mean and SD values for age, body mass, height, experience, resistance training frequency, body composition, and dietary intake before the 24 hours of experimental trials.
The total mass lifted was significantly (t = 3.88, p = 0.002) greater in the capsaicin (3,919.4 ± 1,227.4 kg) compared with the placebo (3,179.6 ± 942.4 kg) condition (Figure 2A). A moderate (d = 0.68) effect size was found for total mass lifted. The RPE was significantly (t = 2.28, p = 0.048) less in the capsaicin (17.2 ± 1.0) compared with the placebo (18.3 ± 1.7) condition. A large effect size (d = 0.80) was found for RPE.
Figure 2.: Comparison between placebo and capsaicin groups on the performance. A) total mass lifted (kg); (B) maximum number of repetitions in each series; a = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-1. b = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-2.
Figure 2B shows the differences in repetitions performed across time and between conditions. There was a main effect of time (F = 57.588, p < 0.001). Post hoc analysis revealed that volume decreased significantly after all sets compared with set-1 and after set-3 and set-4 in relation to set-2. There was a significant (F = 18.855, p = 0.002) main effect for condition, but no interaction (F = 0.278, p = 0.841). Total repetitions performed were greater in the capsaicin condition (42.6 ± 12.6) compared with the placebo condition (35.1 ± 12.3). A moderate (d = 0.60) effect size was found for total repetitions.
Figure 3 presents the differences in lactate concentrations across time and between conditions. There was a main effect of time (F = 108.5, p < 0.001), but no significant differences between conditions or interactions (p > 0.05). Post hoc analysis showed that lactate was greater at all periods compared with baseline and lactate remained elevated 30 minutes after exercise.
Figure 3.: Comparison between placebo and capsaicin groups on the lactate concentration. a = main effect of time with Bonferroni's test and p-value < 0.05 compared with rest. b = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-1; c = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-2; d = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-3; e = main effect of time with Bonferroni's test and p-value < 0.05 compared with set-4; f = main effect of time with Bonferroni's test and p-value < 0.05 compared with post-3 minutes (P3); g = main effect of time with Bonferroni's test and p-value < 0.05 compared with post-5 minutes (P5); P30 = post-30 minutes.
Discussion
To our knowledge, this was the first study to investigate the effects of 12 mg of capsaicin supplementation on performance, RPE, and blood lactate concentrations during resistance exercise in healthy trained young men. We hypothesized that acute capsaicin supplementation would improve resistance training performance. Results from this study confirm this hypothesis. Capsaicin supplementation resulted in improved performance, specifically in more repetitions and a greater total workload performed during 4 sets of back squats to muscle failure. These improvements in total repetition and total workload were associated with moderate effect sizes. In addition, capsaicin supplementation resulted in a lower postexercise RPE compared with placebo.
Currently, there is a lack of studies investigating the effects of capsaicin on exercise performance in humans. The single previous study conducted in humans contrasts with the results of this study. Opheim and Rankin (22) reported that capsaicin supplementation (25.8 mg·d−1) for 7 days did not enhance repeat sprint performance (15 × 30-m sprints with 35-second intervals) in experienced athletes. Possible explanations for the discrepancy between the study by Opheim and Rankin and this study could be due to the exercise test used (resistance exercise vs. intermittent sprint exercise) or the form of the capsaicin supplement (pure capsaicin vs. cayenne pepper), respectively. In addition, the supplement administration protocol (acute vs. chronic) may be partially responsible for the discrepancies in the results. In this study, capsaicin was administered acutely, whereas in the study by Opheim and Rankin, it was administered chronically. It is possible that chronic capsaicin ingestion may result in a “desensitization effect” that blunts the performance benefits. Several studies in mice have demonstrated that chronic capsaicin administration through intraperitoneal injection reduces spontaneous ambulation (4,7) administered a 50 mg dose of capsaicin to mice the day after birth to desensitize the TRVP1 channel and then measured exercise performance 4 months later. TRVP1-desensitized mice demonstrated reduced exercise capacity compared with the control group. By contrast, Lou et al. (17) reported increased time to exhaustion after 3 months of capsaicin ingestion in rats. Whether a desensitization or tolerance effect happens in adult humans, ingesting small doses of capsaicin, and if so, to what extent it might affect exercise outcomes is currently unknown.
In contrast to Opheim and Rankin (22), many studies in animals support an ergogenic effect of capsaicin (15 mg·kg−1) on time until exhaustion in endurance exercise (10,20,21), and one study in mice suggests that capsaicin may improve strength performance (8). The researchers reported that the increase in endurance performance by capsaicin supplementation may be attributed to a muscle glycogen sparing effect as a result of increased lipolysis during exercise. Given the anaerobic nature of the exercise task and lack of difference in lactate concentrations, alterations in lactate flux or substrate oxidation are not likely to explain the ergogenic effects observed in this study. One potential mechanism that may explain the ergogenic effects is the analgesic effect of capsaicin activating the TRPV1 receptor. Topical capsaicin has been used as a pain reliever in neuropathic conditions (3), and sufficient doses of capsaicin that activate the TRPV1 receptor have been shown to possess analgesic effects by inactivating or desensitizing primary affect nerve endings as a result of calcium overload (15). Ratings of perceived exertion were lower in the capsaicin condition compared with the placebo condition, which suggests that capsaicin supplementation may have increased the pain threshold, thus leading to the greater repetitions performed.
A second potential mechanism that may explain the ergogenic effects of capsaicin on resistance exercise performance observed in this may have been through the modulation of the TRPV1 channel. The activation of this receptor in skeletal muscle increases calcium release by sarcoplasmic reticulum (16), leading to greater interaction of actin-myosin filaments and greater tension generation (11). In addition, an increase in the CNS activity and epinephrine secretion through activation of TRPV1 receptor by capsaicin (19,26) may also have contributed toward the increase in total volume performed. Given that greater resistance training volumes are highly associated with muscular hypertrophy (25), it is therefore possible that chronic pretraining capsaicin supplementation could lead to greater strength and hypertrophic adaptations. However, future research is required to test this hypothesis.
A previous capsaicin study using cayenne pepper in humans reported gastrointestinal distress (22). The form and dose of capsaicin in this study was well tolerated, and none of the subjects reported gastrointestinal distress. Despite the importance of our data, some limitations need to be considered: such as a small sample size and infrequent collection of blood lactate time points. Therefore, we suggest future research to analyze the blood lactate concentration at more frequent time points, the catecholamine response to capsaicin supplementation during resistance training, and the differences between acute and chronic capsaicin ingestion protocols.
In summary, acute capsaicin supplementation improved resistance training performance and reduced ratings of perceived exertion during resistance training in healthy trained young men.
Practical Applications
The volume, intensity, and recovery intervals used in this study typically reflect those used by the general weight-training population or athletes in a hypertrophy phase. In addition, the dose and form of capsaicin was well tolerated. Thus, this study suggests that acute capsaicin supplementation can be used as a nutritional strategy to improve total mass lifted with lowered rates of perceived exertion.
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