A transient augmentation in metabolic rate, fat oxidation (FO), and perhaps performance is the fundamental basis for supplementing the diet with thermogenic aids. Thermogenic supplements, or compounds proposed to acutely enhance overall energy expenditure (EE), exist primarily to support efforts to reduce adiposity and improve overall body composition. Most thermogenic supplements incorporate multiple ingredients rich in various bioactive compounds that are conducive to metabolic activity and thereby EE. Of these bioactive compounds, caffeine and polyphenolic catechins of various plant extracts remain arguably as the most predominant ingredients found in thermogenic supplements today. Commensurate with their thermogenic properties, these compounds have also shown potency in inducing ergogenic/performance enhancing effects (8,23), hence revealing their versatility as a dietary supplement especially in the athletic and fitness communities.
First, caffeine, perhaps the most researched thermogenic or ergogenic substance, has been reported to significantly increase total daily EE, facilitate body fat reduction, improve alertness, and enhance physical performance (1,15,28,33,35). On that basis, dietary weight-loss supplements often incorporate a variable but rather large dosage of caffeine, typically around 200–300 mg per serving. Green tea extract is another common constituent of thermogenic supplements because of their high polyphenolic catechin and moderate caffeine content. Among these polyphenols, epigallocatechin gallate (EGCG) has demonstrated efficacy in inducing a metabolic response supportive of lipid catabolism and oxidation (10,19). When these sympathomimetic compounds are combined in supplement form, they may function synergistically to enhance metabolism, FO, and ultimately facilitate weight loss (19). In fact, a combined treatment of a catechin polyphenol-caffeine mixture has shown to induce a significantly greater thermogenic response than an equivalent amount of caffeine (10,19).
Besides thermogenic supplementation, postexercise conditions have also been characterized by a transient rise in resting metabolic rate lasting up to 48 hours (7,24,34). Excess postexercise oxygen consumption (EPOC) reflects this temporary augmentation in metabolic rate, and thereby total daily EE. From a weight control perspective, efforts to identify specific exercise conditions most favorable for optimum EPOC responses would be of great value to weight-loss programing. Sprint or high-intensity interval exercise is now widely considered a time efficient, low volume, and possibly more effective alternative to traditional endurance exercise in terms of facilitating weight-loss efforts (25). In support of this contention, previous studies have reported significantly greater thermogenesis/EPOC after a single bout of sprint interval exercise (SIE) compared with a volume-matched bout of endurance exercise (17).
Thus, an investigation examining the potential synergistic effects of caffeine-polyphenol supplementation and SIE on overall metabolic responses is warranted. Moreover, the dual benefits of caffeine and green tea extract on metabolic rate and performance assert the need to conduct a comprehensive analysis on both metabolic and ergogenic responses. What remains relatively unknown is the acute effects of a caffeine and polyphenolic supplement compound on (a) SIE performance, (b) resting metabolism, and (c) postexercise metabolism. Therefore, the purpose of this study was twofold: to examine the efficacy by which a single dose of a caffeine-polyphenolic supplement acutely enhances (a) metabolic rate and FO at rest and after a bout of SIE and (b) performance during a bout of SIE. We hypothesized that a single dose of a caffeine-polyphenolic supplement would acutely enhance metabolic rate/EE at rest and after a bout of SIE compared to the placebo control condition. Second, we hypothesized that the supplement would improve performance during a bout of SIE compared to the placebo control condition.
Experimental Approach to the Problem
This study incorporated a randomized, double-blind, placebo-controlled, crossover experimental design. Subjects visited the Human Performance Research Laboratory at California State Polytechnic University, Pomona on three separate occasions each separated by 1 week: once for a familiarization visit followed by two experimental trials. During the familiarization visit, subjects first underwent assessments for exercise and health history, body composition, and anthropometric measures (i.e., height, weight, and body fat percentage). Afterward, subjects were familiarized with the physiological testing and SIE protocol to minimize potential learning effects during subsequent experimental trials.
For both experimental trials (Figure 1), subjects reported to the laboratory after an 8-hour overnight fast to ensure proper assessment for resting EE (EEREST). In addition, subjects were instructed to refrain from any exercise 24 hours before each experimental trial and to wear similar exercise apparel to each visit. The subjects initially rested in a supine position on a padded therapy table for 20 minutes. The subjects then consumed either a placebo or the experimental thermogenic supplement (EXP) and continued to rest for additional 10 minutes. During this time, subjects were fitted with an electronic heart rate (HR) monitor (Polar Electro Inc., Lake Success, NY, USA). Afterward, EEREST was assessed for 45 minutes using the dilution technique and an open-circuit indirect calorimeter (TrueOne 2400; ParvoMedics, Salt Lake City, UT, USA). Also, during this period, resting heart rate (HRREST) and blood pressure (BPREST) were assessed. After the pre-exercise rest period, the subjects were placed on a computer-integrated cycle ergometer (Velotron; RacerMate, Inc., Seattle, WA, USA) and began the 30-minute SIE protocol. During SIE, the subjects were measured for exercise HR (HRSIE) and muscular power output. After SIE and 5 minutes of active recovery, subjects returned to the padded table and underwent 45 minutes of rest during which time postexercise metabolic measures, heart rate (HRPOST-SIE), and blood pressure (BPPOST-SIE) were measured.
Twelve subjects (male: n = 11; female: n = 1) participated in this study (body mass = 76.1 ± 2.2 kg; height = 169.8 ± 1.6 cm; body mass index = 22.7 ± 3.0 kg·m−2; body fat % = 21.6 ± 2.0%). Each volunteer completed a preparticipation exercise and health history questionnaire and signed an informed consent document. Subjects met the following inclusion criteria: (a) age = 18–32 years and (b) recreationally active (i.e., within the past 6 months: 3–5 h·wk−1 of aerobic exercise, resistance exercise, or recreational sports, and not a competitive athlete). Subjects were excluded from participation if they reported (a) a history of medical or surgical events in which the study protocols would be contraindicated or confound the interpretation of results. These include, but are not restricted to, cardiovascular, metabolic, pulmonary, renal, or kidney diseases, hypertension, or musculoskeletal impediments; (b) use of any medication including those with cardiovascular, pulmonary, thyroid, hyperlipidemic, hypoglycemic, hypertensive, endocrinologic, psychotropic, neuromuscular, neurological, or androgenic implications; (c) pregnancy; (d) daily caffeine consumption >200 mg; and (e) daily use of ergogenic aids or dietary sports supplements within 6 weeks before the study. Subjects were asked to maintain normal physical activity/exercise levels during the 3-week time span. All subjects signed an informed consent form prior to participation, and the study was approved by California State Polytechnic University, Pomona.
Anthropometric and Body Composition Testing
During the initial familiarization visit, investigators first assessed the subject's height (m) and bodyweight (BW) (kg) using a standard stadiometer and a digital BW scale, respectively. Body composition was measured by dual-energy x-ray absorptiometry (DXA) (Hologic Discovery; QDR Series Densitometer, Bedford, MA, USA). The DXA machine was calibrated before each scan using a manufacturer-provided phantom. All DXA measurements and analyses were conducted by a single certified x-ray technologist.
The measurement of EEREST during pre-exercise and postexercise time points (i.e., EEREST and EEPOST-SIE, respectively) were acquired using the dilution technique and an open-circuit indirect calorimeter metabolic cart (TrueOne 2400; ParvoMedics) (3,26). The assessment was administered in a thermoneutral (∼24° C), dimly lit room with the participant lying in a supine position, without speaking or sleeping and with minimal movement. Before the pre-exercise resting assessment, subjects laid quietly in the supine position on a padded table for 20 minutes followed by consumption of the assigned supplement (EXP or placebo) and additional 10 minutes of rest. During these 10 minutes after supplementation, the participant's head and upper torso were enclosed and sealed under a ventilated hood apparatus (VacuMed; Ventura, CA, USA), which was interfaced to the indirect calorimeter through a corrugated plastic tube. The dilution pump pulled diluted air at a flow rate of 20 L·min−1 from the hood into the mixing chamber for O2 and CO2 analysis. Breath-by-breath oxygen uptake (V[Combining Dot Above]O2) and carbon dioxide output (V[Combining Dot Above]CO2) were measured for 55 minutes. The initial 10 minutes were used for subject habituation, and data were excluded from analysis. Data acquired during the subsequent 45 minutes were used to compute EE according to the Weir equation (36). Criteria for a valid measurement were a minimum of 45 minutes of steady state, determined as a <10% and <5% fluctuation in V[Combining Dot Above]O2 and respiratory exchange ratio (RER), respectively (16). Additional time was applied to the protocol in the event a 45-minute steady state was not observed. Based on recorded gas exchange rates (L·min−1), FO rate was derived from the following equation with the assumption that urinary nitrogen excretion was negligible: FO (g·min−1) = 1.689(V[Combining Dot Above]O2) − 1.689(V[Combining Dot Above]CO2) (22). For postexercise measurements, the subjects rested for 10 minutes after SIE and underwent 45 minutes of metabolic testing as described above. The test-retest correlation was r = 0.96.
From the beginning of the pre-exercise period to the end of postexercise recovery, HR was continuously measured by an electronic HR monitor (Polar Electro Inc.). In addition, arterial BP (i.e., systolic [SBP] and diastolic [DBP]) was measured in 10-minute intervals during the pre-exercise and postexercise rest periods using an automated digital BP system (Omron Healthcare, Inc., Vernon Hills, IL, USA).
Exercise Performance Assessment
Muscular performance was assessed during the execution of the SIE protocol (see Sprint Interval Exercise Protocol) by data acquisition software (Velotron Wingate Software Version 1; RacerMate, Inc.) interfaced to the Velotron cycle ergometer. For both SIE bouts (experimental trials), muscular power output (W) was measured only during each of the four 30-second sprint intervals and not during the low-intensity recovery separating each sprint. Performance variables included peak power (PWRPEAK), average power (PWRAVG), mean PWRPEAK and PWRAVG of sprint intervals 1–4 (PWRPEAK1-4 and PWRAVG1-4), and fatigue index (FI), which have shown significant reliability when derived from the Velotron system (4). Other than PWRPEAK1-4 or PWRAVG1-4, each performance variable will be denoted specific to the sprint interval number, e.g., PWRPEAK1, PWRPEAK2, etc.
Sprint Interval Exercise Protocol
The SIE protocol was performed on the Velotron DynaFit Pro cycle ergometer and comprised four 30-second maximal effort intervals, each separated by 5 minutes of low-intensity, constant workload cycling (Figure 2). First, the ergometer was properly adjusted for the subject. Adjustment specifications for each subject were recorded during their familiarization visit and repeated for all experimental trials. Subjects initiated the SIE protocol with a 5-minute interval of low-intensity cycling at a constant workload of 75W. Immediately after, subjects cycled with maximal effort for 30 seconds against an added resistance that is 7.5% of BW for males and 7.2% for females (5). These two intervals were repeated three additional times. After the last 30-second sprint interval, the subjects performed an additional low-intensity 75 W interval plus an extra 3 minutes of cool-down at a constant workload of 30 W. The total duration of the SIE protocol was 30 minutes.
Dietary Supplementation Protocol
For the two experimental trials, subjects received 1 single serving of either a caffeine-polyphenol supplement (EXP) or placebo in randomized order. The supplements were administered in a double-blind manner. Both supplements were in powder form and matched for taste, appearance, and calories. The caffeine-polyphenol supplement contained 250 mg of caffeine anhydrous and green tea extract (400 mg/serving; standardized for 50% EGCG; 5 mg of caffeine). The caffeine-polyphenol supplement was incorporated into a serving of a noncaloric beverage powder mix. The placebo supplement was a serving of the noncaloric powder mix but without the thermogenic compounds. The supplements were mixed with 0.5 L of water on administration in a solid-colored bottle.
With α = 0.05 and 1-β = 0.80, a sample size of 12 participants was required to detect changes in EE, which was our primary outcome measure. A dependent t-test was used to analyze data and compare mean values for all variables between treatments. All statistical analyses were performed using Statistica10 for Windows (StatSoft; Tulsa, OK, USA) with significance set at p ≤ 0.05. Data are reported at mean ± SEM.
Resting and Postexercise Metabolism
Resting (postsupplementation, pre-SIE) EE (EEREST; kcal·d−1), V[Combining Dot Above]O2 (V[Combining Dot Above]O2 REST; ml·kg−1·min−1), and fat oxidation (FOREST; g·d−1) were significantly (p < 0.02) greater during EXP compared with placebo (Figure 3). When compared to placebo, EXP resulted in 7.99, 9.64, and 10.60% greater EEREST, V[Combining Dot Above]O2 REST, FOREST, respectively. Resting respiratory exchange ratio (RERREST) was significantly lower during EXP (RERREST = 0.82 ± 0.01) compared with placebo (RERREST = 0.85 ± 0.01) (p = 0.01). After SIE, EEPOST-SIE, V[Combining Dot Above]O2 (V[Combining Dot Above]O2 POST-SIE), and fat oxidation (FOPOST-SIE) were significantly (p < 0.02) greater during EXP compared with placebo (Figure 4). When compared to placebo, EXP resulted in 10.16, 12.10, and 9.76% greater EEREST, V[Combining Dot Above]O2 REST, FOREST, respectively. Post-SIE respiratory exchange ratio (RERPOST-SIE) was 4.4% lower during EXP than placebo (p = 0.02).
Resting (postsupplementation, pre-SIE) and exercise heart rate (HRREST and HRSIE) did not differ between treatments; however, HRPOST-SIE was significantly (p = 0.001) greater during EXP (90.8 ± 3.5 b·min−1) compared with placebo (85.1 ± 3.6 b·min−1). No significant difference between treatments was detected for SBP or DBP at rest and post-SIE, although the analysis approached significance (p = 0.06).
Sprint Interval Exercise Performance Measures
There were no between-treatment differences for peak power output at intervals 1, 2, 3, or 4 (Figure 5). The mean peak power across all four intervals was not significantly different between treatments. Average power output at intervals 1, 2, 3, or 4 was not significantly different between treatments. The mean average power across all four intervals did not differ between treatments. When normalized to BW, all muscular power measures were not significantly different between treatments. Fatigue index during interval 1 was significantly (p = 0.04) greater during EXP (18.5 ± 1.2 W·s−1) compared with placebo (17.1 ± 1.0 W·s−1). Fatigue index during all other intervals and mean FI across all four intervals failed to differ between treatments.
In young, nonhabitual caffeine users, the current findings expose the overall efficacy of the experimental caffeine-polyphenol compound as a thermogenic dietary supplement. First, during resting state, a single dosing of the thermogenic supplement induced an approximate 8% transitory rise in EE with a concomitant augmentation in oxygen uptake. Accompanying this thermogenic response was a 4% decrease in RER, corroborating a 10% increase in FO rate. These results correspond with previous reports of enhanced EEREST and lipid oxidation with caffeine-polyphenol supplementation of various dosage combinations (10,29,32). In fact, Hursel et al. (19) conducted a meta-analysis that elucidated the importance of combining caffeine and polyphenol catechins to stimulate a thermogenic response that accompanied an increase in FO. When considering both the mutual and unique mechanisms through which these bioactive compounds promote thermogenesis, it is reasonable to suggest that each ingredient within the experimental compound contributed to the observed metabolic responses.
To expound further on possible mechanistic underpinnings for the observed thermogenic responses, caffeine acts on molecular pathways concomitant with sympathetic activity, such as those regulating fat metabolism (20). As a stimulant compound, caffeine antagonizes the suppressive actions of adenosine on the sympathetic nervous system, resulting in greater secretion of catecholamines, such as epinephrine (EPI) and norepinephrine (NE) (31). In turn, elevated circulating levels of EPI and NE may facilitate activation of β-adrenergic receptor signaling, which involves cyclic adenosine monophosphate (cAMP) and multiple downstream enzymatic targets such as those responsible for lipid metabolism, e.g., lipases (11). Moreover, caffeine seems to suppress phosphodiesterase, an enzyme that deactivates cAMP from further signaling induction (11). In so doing, caffeine promotes cAMP activation and ensuing sympathetic signaling mechanisms. Additionally, research suggests a multifaceted mechanism through which EGCG promotes thermogenesis, specifically fat metabolism. For instance, EGCG directly inhibits COMT (catechol-O-methyl-transferase), an enzyme that degrades EPI and NE, thereby promoting higher circulating concentration of sympathetic-induced catecholamines (30). In support, previous studies, such as Murase et al. (27), demonstrated enhanced beta-oxidation during exercise after catechin-rich green tea extract treatment.
As a more novel aspect of this study, postexercise (specifically SIE) metabolic responses were also examined. When compared to resting conditions, similar dynamics were evident after an acute bout of SIE with respect to EE and lipid metabolism. Specifically, post-SIE EE was approximately 10% greater with pre-exercise caffeine-polyphenol supplementation versus placebo. These outcomes were substantiated by differing EPOC responses as thermogenic supplementation resulted in an 11% V[Combining Dot Above]O2 augmentation after SIE. The results for RER and FOPOST-SIE also suggest that the acutely heightened lipid oxidation rate after SIE is further amplified with a single caffeine-polyphenol dosing. Similar postexercise metabolic responses were reported by Jitomir et al. (21) after subjects ingested a proprietary multi-ingredient thermogenic supplement. With these particular results, a question may be posed as to whether the enhanced metabolic activity after SIE was attributable to any changes in SIE performance associated with caffeine-polyphenol supplementation.
Considering the multifaceted benefits of caffeine and certain polyphenol catechins like EGCG, it is of significant importance to better understand the relationship among caffeine-polyphenol supplementation, exercise performance, and postexercise thermogenesis. In the context of performance (specifically during SIE), these findings revealed no significant ergogenic advantages with caffeine-polyphenol supplementation. For instance, peak and average power outputs for each sprint interval were similar between nutritional treatments. Although subjects exhibited improved fatigue resistance during the initial sprint interval with caffeine-polyphenol supplementation, the results from an inclusive perspective fail to offer any compelling evidence reflective of its ergogenic value. The absence of detectable performance benefits, especially during anaerobic sprint exercise and caffeine consumption, corroborates a number of previous reports (6,9,14,18,37,38) but disagrees with some others (2,12,13). Commensurate with the mixed outcomes across previous studies is the equivocal rationale for these differential results. One of the more common explanations relates to divergent dosing protocols. It can certainly be argued that the current absolute dosing of 255 mg of caffeine may have lacked potency to induce an ergogenic response and that a dosing protocol normalized to body mass would have been more effective. However, Glaister et al. (14) conducted a caffeine dose-response study on sprint cycling performance using a dosing spectrum normalized to individual body mass. Results indicated no ergogenic advantages across the entire dosing spectrum (caffeine of 2–10 mg·kg−1 body mass) when compared to placebo treatment. By average, the dosage treatment in this study equated to approximately 3.4 mg·kg·body mass−1, thus falling within the range tested by Glaister et al. and thereby agreeing with their results. On the other hand and by which the current dosing protocol was based on, Anselme et al. (2) found a significant improvement in sprint cycle performance with 250 mg of caffeine supplementation. As demonstrated, there remains quite a perplexity with respect to the optimum dosage protocol for acute performance benefits, thus precluding proper application. Overall, on the basis of these performance outcomes, it may be concluded that the observed thermogenic response after SIE was directly attributable to caffeine-polyphenol supplementation as opposed to an indirect manifestation of enhanced performance and work output.
Additionally, resting HR and BP remained unchanged between experimental and placebo control conditions. Although sympathomimetic substances such as caffeine and polyphenol catechins are known cardiovascular stimulants, especially in nonhabitual users, the dosage administered during this study may have precluded a detectable response. A recent randomized controlled trial examining physiological responses to a thermogenic supplement containing similar caffeine and green tea extract dosages indicated no significant hemodynamic or HR changes compared to a placebo treatment (29). The lack of a significant hemodynamic response to the administered dose may suggest that the experimental combination of caffeine and polyphenols accompanies minimal risk for individuals with cardiovascular contraindications to sympathomimetic use at least during rest. However, it must be noted that a more comprehensive and sophisticated investigation on cardiovascular and hemodynamic responses must be conducted to confirm this conjecture. No difference in HR during SIE between treatments was expected as subjects were working at near maximal HR because of the nature of the exercise protocol. After SIE, however, the thermogenic treatment resulted in an approximate 6.5% greater HR than placebo. As with the observed metabolic responses, the outcomes for HRPOST-SIE were not attributable to any increase in work during the SIE performance.
Collectively, these results corroborate the use of dietary caffeine and polyphenol catechins, e.g., green tea extract, to support efforts to reduce adiposity and improve overall body composition especially in conjunction with SIE. Although the practical application of our findings is limited because of the acute nature of the study design, it may be reasonably inferred that a long-to-moderate term application of caffeine-polyphenol supplementation with SIE would act favorably toward weight control. Moreover, a majority of marketed thermogenic supplements today comprise an abundant mixture of ingredients each proposed to enhance EE through various purported mechanisms. The complexity of these proprietary blends may accompany questions of safety and futility because some ingredients may not have empirical backing in terms of effectiveness and potency. This study indirectly reveals a fresh perspective on the use of thermogenic aids in that positive effects may be achieved through a relatively simple blend containing two bioactive and empirically supported ingredients of acceptable dosages. The complexity of a thermogenic supplement in terms of included ingredients does not always reflect efficacy and safety.
The study was conceived and designed by E. Jo; data were collected and analyzed by E. Jo, K. Lewis, D. Higuera, J. Hernandez, and A. Osmond; data interpretation and manuscript preparation were undertaken by E. Jo. There were no professional relationships with companies or manufacturers who may benefit from the results of this study. There was no specific grant support for the study. The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association.
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