1. Introduction
As interest in weight management and metabolic health continues to grow, many individuals rely on dietary supplements to enhance caloric expenditure and fat loss [1,2]. Amongst the broad array of available supplements claiming to aid in this process, thermogenic supplements or “fat burners” have garnered significant attention in recent years [1,3,4]. It has been previously reported that these multi-ingredient supplements [5,6] may increase resting metabolic rate (RMR) [7,8,9,10] which may contribute to enhanced energy expenditure and create the potential for fat loss when sustained over time. Despite these findings, direct evidence documenting reductions in fat mass specifically attributed to the use of thermogenic supplements is limited. Most studies have focused on acute effects on metabolism or surrogate markers of fat loss, leaving a gap in understanding their long-term efficacy in altering body composition [4,8,9,10,11,12].
The effectiveness of thermogenic supplements may be related to the composition of their novel ingredients. Caffeine, a primary ingredient of most thermogenic supplements, has demonstrated a thermogenic response via increased RMR [13,14,15,16] and free fatty acid (FFA) utilization [13,17,18,19] as well as potentially providing benefits to sports performance [19,20,21]. Prior studies have indicated the role of caffeine as a central nervous system (CNS) stimulant, providing physiological benefits in physique and weight management mediated by catecholaminergic upregulation and appetite suppression [16,22]. When combined with other compounds, caffeine is believed to enhance metabolic and lipolytic effects by synergistically increasing energy expenditure and promoting fat oxidation [5,7,8,9,10,11,12]. This combination may amplify caffeine’s stimulation of the CNS, leading to a more significant overall impact on metabolism.
It is hypothesized that continued usage of caffeine-containing thermogenic supplements may subsequently support weight management and improve body composition by enhancing the body’s calorie-burning capacity. In addition to caffeine, thermogenic supplements are formulated with compounds such as green tea extract. Green tea is valued for its high content of catechins, especially epigallocatechin gallate (EGCG), which works synergistically with caffeine to enhance its lipolytic properties [23,24,25,26,27]. These compounds are believed to stimulate the sympathetic nervous system, increasing energy expenditure and promoting the breakdown of stored fat [28].
L-carnitine is another ingredient in many thermogenic supplements due to its role in transporting long-chain fatty acids into the mitochondria [29,30], where they are oxidized for energy [6,29,31]. This process is purported to support fat metabolism and energy production, particularly during exercise, which may potentially enhance endurance and facilitate fat loss. While some preliminary research suggests the use of maca and gymnema sylvestre leaf extract, common ingredients in thermogenic supplements, potentially benefitting endurance and weight management, more research is needed to clarify their specific roles and effectiveness in enhancing physical performance and body composition [32,33,34].
While existing data may support the use of thermogenic supplements for fat loss and potential performance benefits, there is limited evidence on the efficacy and safety of these aids with chronic usage in conjunction with a resistance training program. The unique combination of ingredients commonly found in thermogenic supplements presents a promising approach to enhancing body composition and strength adaptations in trained and untrained populations. Therefore, this study aims to investigate the effects of pre-exercise ingestion of a thermogenic supplement over an 8-week periodized resistance training program on maximal strength and body composition, potentially offering insights into the supplement’s role in supporting training outcomes through thermogenic and performance-enhancing mechanisms.
2. Materials and Methods
2.1. Subjects
Subjects were informed about the purpose of the research and the experimental procedures in addition to signing an institutionally approved informed consent before entering the research procedure. Thirty-four resistance-trained male subjects (21 ± 3.8 years; 177 ± 6.2 cm; 79 ± 11.2 kg) participated in this investigation, with resistance training defined as consistent engagement for at least one year and the capability to bench press their body weight and squat 1.25 times their body weight. Participants were randomized into either the Thermogenic Supplement (TS) or placebo group.
2.2. Procedures
The present study utilized a double-blind, placebo-controlled, randomized design. Participants were asked to visit the laboratory on three occasions. The first two visits were completed before the 8-week training program. During the first visit, subjects completed the informed consent document, health history form, and personal data form. On visit two, subjects completed baseline testing, which consisted of height and weight, body composition via a BodyMetrix hand-held ultrasound wand (IntelaMetrix, Livermore, CA, USA) with the American College of Sports Medicine 7-site formula prediction equation, bench press 1-RM, and back squat 1-RM. Each squat attempt had to reach at least parallel; a sound-activated lever (BFS Safety Squat®, Salt Lake City, UT, USA) was attached to the thighs of the subjects during each 1-RM squat attempt to confirm that a parallel depth was reached. The third and final visit occurred the week after subjects had finished the 8-week training program, at least 48 h after the final training session, with measurements taken in the same order before and after the training intervention.
2.3. Supplementation Protocol
Participants were ranked by Bench Press 1-RM and randomized in counter-balance fashion to either the TS or placebo group. For the first 4 weeks, subjects took one serving (three capsules containing 200 mg caffeine) at breakfast or lunch and during the last 4 weeks of the study took two servings of the thermogenic supplement (six capsules containing 400 mg caffeine) at breakfast and lunch. The TS and placebo capsules were identical in appearance and taste. The ingredients in the thermogenic fat loss supplement treatment are presented in Figure 1, while the placebo contained only inert ingredients (650 mg of maltodextrin and 88.8 mg of hemp protein). In addition to the pre-workout supplementation, all participants, regardless of TS or placebo treatment, were asked to ingest approximately 30 g of protein (Iron WheyTM) following each workout. Additionally, participants were instructed not to change their dietary habits during the investigation.
2.4. Resistance Training Protocol
Participants in the TS and placebo groups adhered to an identical resistance training program based on a daily undulating periodization model. The protocol involved three weekly training sessions designated as strength (S), hypertrophy (H), and power (P) days, respectively. Each session was structured as a full-body workout with similar basic lifts, differing primarily in repetition ranges and tempo.
On S days, exercises were performed at a 3–6 RM intensity with an explosive concentric phase and a controlled eccentric phase, including four sets with three minutes of rest between sets and exercises. Exercises included back squat, bench press, shoulder press, lat pulldown, chin-ups, and dips. H days involved exercises performed at a 10–12 RM intensity, emphasizing an explosive concentric and a 2 s eccentric phase, with four sets and 2.5 min of rest between sets. Exercises included back squat, bench press, barbell row, deadlift, barbell curl, and triceps pressdowns. On P days, exercises were completed explosively at approximately 60% 1-RM for 6 repetitions, with four sets and three minutes of rest. Exercises included back squat, bench press, lat pulldown, deadlift, barbell curl, and dips. Each workout session lasted about one hour. Table 1 details the resistance training protocol used over the 8 weeks.
2.5. Materials and Testing
At baseline and post-testing (after 8 weeks of resistance training), participants were assessed on 3 different variables: body composition (fat mass and lean body mass), bench press 1-RM, and back squat 1-RM. A description of participants’ characteristics at baseline are available in Table 2.
Body Composition: The Body-Metrix BX-2000 A-mode ultrasound (IntelaMetrix, Livermore, CA, USA) equipped with a 2.5 MHz probe was used to evaluate body composition. This device has been recognized as a reliable method for estimating lean body mass in male athletes, demonstrating comparable accuracy to dual-energy X-ray absorptiometry (DEXA) [35]. Measurements were taken at seven standardized anatomical locations, including the chest, midaxillary, triceps, subscapular, abdomen, suprailiac, and thigh. These values were then used to estimate fat mass and lean mass percentages utilizing the American College of Sports Medicine (ACSM) 7-site skinfold equation.
Bench Press and Back Squat 1-RM: The 1-RM for both the bench press and back squat was assessed using standardized procedures recommended by the National Strength and Conditioning Association (NSCA) [36]. Participants began with a light warm-up set (~10 repetitions) at a low resistance, followed by progressively heavier warm-up sets consisting of 3–5 repetitions with brief rest periods in between. Once adequately warmed up, participants proceeded to 1-RM testing, during which the weight was gradually increased. For the bench press, increments ranged between 4.5 and 9 kg, while for the back squat, the weight was adjusted in 13.6 to 18.2 kg increments. The heaviest successfully completed lift was recorded as the individual’s 1-RM for each exercise.
2.6. Statistical Analysis
Descriptive statistics are presented as means ± standard deviations (SD) while inferential statistics are expressed as means (95% confidence interval). To evaluate the effect of condition on the variables of interest, ANCOVAs were performed, with group (experimental vs. control) as the fixed factor, pre-measures as covariates, and post measures as the dependent variables. A significance level of alpha = 0.05 was adopted for all analyses. Statistical analyses were conducted using SPSS (version 29, IBM Corporation, Chicago, IL, USA).
3. Results
A total of 34 individuals completed the study, with 18 in the TS group and 16 in the placebo group. The mean age and height for the TS group were 21.5 ± 5.0 years and 177.8 ± 6.3 cm, while the placebo group had a mean age of 21.0 ± 2.0 years and a height of 176.2 ± 6.4 cm. Post-intervention descriptive statistics and raw change scores for all measured variables are summarized in Table 3.
There were no statistically significant differences between groups for changes in body mass (TS: 0.50 kg, 95% CI: −0.24, 1.24; placebo: 1.23 kg, 95% CI: 0.42, 2.05; p = 0.147) or lean body mass (TS: 1.15 kg, 95% CI: 0.53, 1.77; placebo: 1.12 kg, 95% CI: 0.29, 1.95; p = 0.965). However, significant differences were observed for fat mass and body fat percentage. Figure 2 illustrates the change in fat mass over the intervention for both groups. Fat mass decreased more in the TS group compared to the placebo group (TS: −0.65 kg, 95% CI: −1.15, −0.14; placebo: 0.12 kg, 95% CI: −0.40, 0.64; p = 0.029), and the TS group also experienced a larger reduction in body fat percentage (TS: −0.89%, 95% CI: −1.47, −0.32; placebo: −0.08%, 95% CI: −0.67, 0.52; p = 0.039).
Both groups demonstrated increases in squat and bench press strength following the resistance training intervention, but there were no statistically significant differences between groups for changes in squat strength (TS: 16.87 kg, 95% CI: 12.48, 21.26; placebo: 18.47 kg, 95% CI: 12.46, 24.48; p = 0.312) or bench press strength (TS: 7.80 kg, 95% CI: 6.09, 9.50; placebo: 7.13 kg, 95% CI: 3.33, 10.94; p = 0.883). Similarly, no group differences were observed for relative squat strength (TS: 0.21, 95% CI: 0.15, 0.27; placebo: 0.21, 95% CI: 0.14, 0.28; p = 0.719) or relative bench strength (TS: 0.09, 95% CI: 0.06, 0.12; placebo: 0.07, 95% CI: 0.03, 0.12; p = 0.643).
4. Discussion
The purpose of the present investigation was to address the gap in the current literature on chronic usage of a thermogenic supplement over an 8-week resistance training program on body composition and maximal strength. Our primary finding included significant reductions in fat mass and body fat percentage in the TS group as compared to the placebo group. Prior research has demonstrated acute thermogenic supplement usage leading to increased RMR and enhanced fat oxidation. Our observations extend these findings and further elucidate that continued usage may lead to significant reductions in body fat [5,7,8,9,10,11,12]. As these acute increases in resting metabolic rate are sustained over time, individuals utilizing thermogenics may experience chronic reductions in body fat beyond what may be expressed in a traditional RT protocol.
We hypothesize that caffeine is the primary ingredient responsible for the observed reductions in body fat, as it is well documented to significantly increase RMR [16,22]. Sustained elevations in RMR over several weeks may lead to meaningful reductions in fat mass, as supported by the findings of the current study. However, the influence of each ingredient in the supplement with caffeine is presently unknown; thus, we cannot evaluate the potential synergistic effects of the combination of ingredients in thermogenic supplements to contribute to these fat loss effects. As caffeine is a known CNS stimulant [16,22] and lipolytic agent [13,17,18,19], its impact in addition to commonly included thermogenic ingredient compounds such as green tea, L-carnitine, maca and gymnema sylvestre leaf extract may enhance its fat-burning capabilities, providing sustained increases in RMR and fatty acid oxidation. As these mechanisms remain elevated, the body may increase its overall energy expenditure and shift its reliance toward stored fat as a primary fuel source. Elevated RMR ensures that more calories are burned throughout the day, even during periods of rest, while enhanced fatty acid oxidation facilitates the mobilization and breakdown of fat stores to be used for energy. This synergy creates a metabolic environment that promotes greater fat loss over time, beyond what may be achieved with RT alone.
In the TS group, the synergistic actions of these ingredients may have amplified these effects. For example, caffeine accelerates the breakdown of triglycerides into free fatty acids (FFAs), providing readily available substrates for oxidation [13]. Green tea catechins and L-carnitine have been shown to further enhance fatty acid transport into mitochondria, optimizing their use as an energy source [24,28,31]. Additionally, maca and gymnema sylvestre leaf extract may contribute metabolic or hormonal regulatory effects that support these processes [32,33,34]. Together, these mechanisms sustain elevated energy expenditure and fat utilization, which could explain the greater reductions in fat mass and body fat percentage observed in the TS group compared to the placebo group. In contrast, the placebo group relied solely on the effects of resistance training to improve body composition, lacking the metabolic enhancements provided by thermogenic supplement ingredients. As a result, their fat mass reduction and body fat percentage improvements were less pronounced, highlighting the efficacy of the thermogenic supplement formulation in augmenting fat loss when combined with a RT protocol.
The significant decrease in fat mass in the TS group contributed directly to the larger reduction in body fat percentage observed, as fat mass accounts for a proportional component of total body weight. However, this decrease in fat mass did not translate to significant differences in overall body weight or lean body mass between groups. In both TS and placebo, increases in LBM may have offset reductions in fat mass, resulting in no overall changes in body mass. These findings suggest that while the thermogenic supplement enhanced fat loss, it did not hinder the preservation of lean tissue or impact body weight significantly beyond what is typically observed with RT alone. It was observed that both groups experienced insignificant increases in overall body mass and LBM, aligning with prior reports highlighting the importance of standardized protein intake in conjunction with RT for muscle hypertrophy [37]. It is important to reiterate that all subjects were instructed to maintain their current dietary habits throughout the duration of the investigation and were provided with a dietary protein supplement following the cessation of each supervised RT bout.
Additionally, it was found that both groups experienced increases in absolute and relative expressed strength for the back squat and bench press after the 8-week protocol. All participants in this study had been resistance-trained individuals and had met the minimum strength requirements in order to partake in this study; these requirements may have limited the magnitude of strength gains experienced [38]. All participants had performed the same RT protocol for the duration of the study, and the identical RT program across groups may have overshadowed any potential additive effects of the TS on strength. The absence of a change does not necessarily negate the efficacy of the TS, but rather highlights the rigor of controlling for external variables such as diet and outside activity.
The lack of change in strength between groups may also be due to the mechanisms by which TS operate. As previously mentioned, thermogenic supplements and the combination of ingredients predominately influence fat oxidation rather than muscle anabolism. Prior studies into the relationship between thermogenic ingredients and exercise performance support this finding [17,39]. While thermogenic supplements may aid in reducing fat mass and sparing lean body mass, its influence on muscle hypertrophy and strength adaptations appears minimal, particularly in trained individuals. Furthermore, because strength gains are multifactorial and influenced by neural adaptations, muscle hypertrophy, and mechanical efficiency, it is possible that these supplements would be more impactful in untrained individuals or under protocols with greater nutritional variability. This aligns with prior research suggesting that thermogenic supplement effects are more pronounced in areas related to metabolism rather than direct performance enhancements.
5. Conclusions
The findings of the present study indicate that chronic usage of a thermogenic supplement can result in more significant reductions in fat mass and body fat percentage, without hindering strength gains in trained populations as compared to a placebo group which experienced less pronounced changes in body composition and similar increases in strength. This study is among the first of its kind demonstrating the chronic effect of thermogenic supplement ingestion and its impact on body composition and strength measures. It can be elucidated that the enhanced impact on body fat and body fat percentage is a result of the combined nature of the thermogenic ingredients, though we cannot delineate if these enhancements are due to individual ingredients in the supplement or the combination of ingredients; many of these fat burners utilize proprietary blends when formulating their products, leaving ambiguity in the content, dosing, and possible efficacy of individual ingredients. Additionally, subjects had been provided an absolute dosage of active ingredients within the supplement. Further research may be necessary to investigate the implementation of thermogenic supplements over longer intervention durations, exploring if there is a dose–response of active ingredients, or different populations such as novice lifters, untrained individuals, or clinical populations.
Limitations for this study include the reliance on self-reported dietary intake that was not monitored by the research team which could introduce variability in caloric and macronutrient intake across participants. While efforts were made to control external variables such as outside activity and supplement adherence, compliance was not objectively verified, which may have influenced the results. The trained nature of the participants may limit the generalizability of the findings to untrained populations or those with differing baseline characteristics. These factors highlight the need for further studies employing more robust controls and examining the effects of thermogenic supplements under varying conditions.
Conceptualization, R.J.C. and B.I.C.; Data curation, G.S. and S.J.D.; Formal analysis, G.S. and S.J.D.; Funding acquisition, B.I.C.; Investigation, R.J.C., D.B., C.G., N.M., S.B. and B.I.C.; Methodology, R.J.C. and B.I.C.; Project administration, R.J.C. and B.I.C.; Supervision, R.J.C. and B.I.C.; Validation, G.S. and S.J.D.; Visualization, G.S. and B.I.C.; Writing—original draft, G.S.; Writing—review and editing, R.J.C., G.S., D.B., C.G., N.M., S.B., S.J.D. and B.I.C. All authors have read and agreed to the published version of the manuscript.
The study was conducted according to the guidelines of the Declaration of Helsinki and approved (IRB#: Pro00017704) by the Institutional Review Board of the University of South Florida, Tampa, Florida on 21 October 2014.
Informed consent was obtained from all subjects involved in the study.
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.
B.I.C. has received grants and contracts to conduct research on dietary supplements; has served as a paid consultant for industry; has received honoraria for speaking at conferences and writing lay articles about sports nutrition ingredients and topics; is a member of the International Protein Board that disseminates knowledge on protein and protein products; has served as an expert witness on behalf of the plaintiff and defense in cases involving dietary supplements; and receives compensation for writing and providing educational services related to exercise and nutrition-related topics. No other authors have conflicts of interest to report. All researchers involved in this study independently collected, analyzed, and interpreted the results from this study, and have no financial interests concerning the outcome of the study. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Footnotes
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Resistance training workouts over 8-week study period.
| Monday (S) | Wednesday (H) | Friday (P) | |||
|---|---|---|---|---|---|
| Exercise | No. of Sets × RM | Exercise | No. of Sets × RM | Exercise | No. of Sets × RM * |
| Back squat | 4 × 3–6 | Back squat | 4 × 10–12 | Back squat | 4 × 6 |
| Bench press | 4 × 3–6 | Bench press | 4 × 10–12 | Bench press | 4 × 6 |
| Shoulder press | 4 × 3–6 | Barbell row | 4 × 10–12 | Lat pull-down | 4 × 6 |
| Lat pull-down | 4 × 3–6 | Deadlift | 4 × 10–12 | Deadlift | 4 × 6 |
| Chin-ups | 4 × 3–6 | Barbell curl | 4 × 10–12 | Barbell curl | 4 × 6 |
| Dips | 4 × 3–6 | Triceps pressdowns | 4 × 10–12 | Dips | 4 × 6 |
* Repetitions at 60% 1-RM. RM: repetition maximum (maximum that could be lifted for indicated number of repetitions).
Descriptive characteristics of the participants at baseline (mean ± SD).
| Participant Characteristics | TS | Placebo | Total |
|---|---|---|---|
| Age | 21.5 ± 5.0 | 21 ± 2.0 | 21 ± 3.8 |
| Height (cm) | 177.8 ± 6.3 | 176.2 ± 6.4 | 177.1 ± 6.3 |
| Body-Mass (kg) | 78.2 ± 11.2 | 79.2 ± 11.6 | 78.6 ± 11.2 |
| Fat-Mass (kg) | 9.5 ± 4.5 | 9.6 ± 4.7 | 9.6 ± 4.5 |
| Body Fat % | 12 ± 4.8 | 11.8 ± 4.4 | 11.9 ± 4.5 |
| Lean Body Mass (kg) | 68.7 ± 9.2 | 69.5 ± 8.4 | 69.1 ± 8.7 |
| Squat Strength (kg) | 126.8 ± 22.8 | 130.4 ± 22.2 | 128.5 ± 22.3 |
| Relative Squat Strength (kg/kg) | 1.6 ± 0.2 | 1.7 ± 0.2 | 1.6 ± 0.2 |
| Bench Strength (kg) | 98.3 ± 20.2 | 100.5 ± 10.2 | 99.3 ± 16.1 |
| Relative Bench Strength (kg/kg) | 1.3 ± 0.2 | 1.3 ± 0.1 | 1.3 ± 0.1 |
Body composition and strength differences.
| Measure | TS | Placebo |
|---|---|---|
| Body-Mass (kg) | ||
| Pre | 78.2 ± 11.2 | 79.2 ± 11.6 |
| Post | 78.7 ± 10.5 | 80.4 ± 11.7 |
| ∆ | 0.50 (−0.24, 1.24) | 1.23 (0.42, 2.05) |
| Fat-Mass (kg) * | ||
| Pre | 9.5 ± 4.5 | 9.6 ± 4.7 |
| Post | 8.9 ± 4.0 | 9.7 ± 4.9 |
| ∆ | −0.65 (−1.15, −0.14) | 0.12 (−0.40. 0.64) |
| Body Fat % * | ||
| Pre | 12 ± 4.8 | 11.8 ± 4.4 |
| Post | 11.1 ± 4.2 | 11.7 ± 4.5 |
| ∆ | −0.89 (−1.47, −0.32) | −0.08 (−0.67, 0.52) |
| Lean Body Mass (kg) | ||
| Pre | 68.7 ± 9.2 | 69.5 ± 8.4 |
| Post | 69.8 ± 8.5 | 70.7 ± 8.3 |
| ∆ | 1.15 (0.53, 1.77) | 1.12 (0.29, 1.95) |
| Squat Strength (kg) | ||
| Pre | 123.6 ± 19 | 131.2 ± 23.7 |
| Post | 140.5 ± 16.5 | 149.7 ± 21.1 |
| ∆ | 16.87 (12.48, 21.26) | 18.47 (12.46, 24.48) |
| Relative Squat Strength (kg/kg) | ||
| Pre | 1.6 ± 0.2 | 1.7 ± 0.2 |
| Post | 1.8 ± 0.2 | 1.9 ± 0.2 |
| ∆ | 0.21 (0.15, 0.27) | 0.21 (0.14, 0.28) |
| Bench Strength (kg) | ||
| Pre | 95.0 ± 17.7 | 100.9 ± 10.7 |
| Post | 102.8 ± 16.9 | 108.1 ± 12.0 |
| ∆ | 7.80 (6.09, 9.50) | 7.13 (3.33, 10.94) |
| Relative Bench Strength (kg/kg) | ||
| Pre | 1.3 ± 0.2 | 1.3 ± 0.1 |
| Post | 1.3 ± 0.1 | 1.4 ± 0.2 |
| ∆ | 0.09 (0.06, 0.12) | 0.07 (0.03, 0.12) |
* p < 0.05, results are expressed as mean ± SD or mean (95% confidence interval).
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; Campbell, Bill I 2
1 Exercise & Nutrition Research Group, Department of Health, Kinesiology, and Sport, University of South Alabama, Mobile, AL 36608, USA;
2 Performance & Physique Enhancement Laboratory, Exercise Science Program, University of South Florida, Tampa, FL 33620, USA;
3 Health and Exercise Science Department, School of Nursing & Health Professions, Rowan University, Glassboro, NJ 08028, USA;