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Is caffeine a thermogenic?

In addition to an increased thermogenic effect during rest [16], caffeine has been shown to potentially increase the rate of fat oxidation, while separately enhancing exercise performance [17, 18]; although, less is known about the metabolic response to a TFD during exercise [19].

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Experimental protocol

Three testing visits separated by a minimum of 48 h were completed within a two-week period. The timeline for each testing visit is outlined in Fig. 1. Participants were asked to maintain a consistent diet and track their food and beverage intake for the entire day prior to each of the three testing visits. Each testing visit was scheduled in the morning between 8 am, and 9 am, following an 8-h fast with no caffeine consumption and a 24-h period of no exercise or alcohol consumption. Baseline hydration status was evaluated upon arrival to the laboratory. After assessing height, body mass, and body composition, participants were led to a calm and quiet environment for baseline measurements consisting of a baseline blood draw for determining serum glycerol concentration followed by analysis of resting metabolic rate. A randomized, double-blind, crossover design was employed where participants were assigned to complete three trials, each of which required consumption of one of the following beverages: (a) 140 mg formula (10 kcal drink containing a total of 140 mg of caffeine from a proprietary blend of caffeine, guarana, ginger, and green tea extract containing EGCG), (b) 100 mg formula (10 kcal drink containing a total of 100 mg of caffeine from a proprietary blend of caffeine, guarana, ginger, and green tea extract containing EGCG), (c) Placebo (artificially sweetened non-caloric/non-caffeinated drink). Fig. 1 Experimental design of the study; = hydration test, = anthropometrics and body composition, = blood draw, RMR= resting metabolic rate, = thermogenic fitness drink formula, =graded exercise test Full size image Assessments were repeated at 30, 60, and 90 min following consumption of each beverage. Immediately following the last resting measurements, a graded exercise test was conducted to determine metabolic responses and performance outcomes.

Participants

Thirty-two recreationally active men (n = 15) and women (n = 17) between the ages of 18 and 35 years old who were regular caffeine consumers of no more than 250 mg per day were recruited to participate in this research investigation (Table 1). After participants signed the informed consent they completed the Physical Activity Readiness Questionnaire (PARQ+), medical and activity history questionnaire (MHQ), and a caffeine consumption questionnaire adapted from Landrum [26]. This study was approved by the university’s Institutional Review Board. Participants were excluded if they had any physical limitations, metabolic diseases, were caffeine naïve or consumed more than 250 mg of caffeine per day according to the caffeine consumption questionnaire, and/or did not meet the ACSM recommendation of at least 150 min of exercise per week for the past 6 months [27].

Table 1 Participant demographics Full size table

Nutrient intake and dietary recall

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Participants were required to complete a 24-h dietary recall. Dietary intake data for 24-h recalls were collected and analyzed using the Automated Self-Administered 24-h (ASA24) dietary assessment tool (version 2018, National Cancer Institute, Bethesda, MD) [28]. The ASA24 dietary recall assessment was utilized to estimate mean total energy intake (TEI) in kilocalories (Kcal) before each testing day. Participants were provided with a login and a password and detailed tutorial on how to access and complete the ASA24. The dietary recall was completed the night before each testing visit and after the last food item or drink consumed. During the recall, participants received automated prompts that would assist them in quantifying portion sizes, actual volume of food consumed at each meal or snack, and commonly forgotten items (condiments, supplements, sugar-sweetened beverages). A total of 27 participants complied with the dietary recall instructions and were included in the data analysis.

Hydration status, anthropometrics, and body composition

Participants were asked to refrain from food or drink consumption—except water—for 8 h prior to testing and to be euhydrated. Urine samples were analyzed for hydration status using the refractometry method (Human Urine Refractometer, MISCO Refractometer, Cleveland, OH, USA). Participants could not initiate testing until proper hydration was confirmed, and specific gravity of the urine was less than or equal to 1.020. Following hydration testing, height was assessed using a stadiometer (500KL Health O Meter, Alsip, IL, USA). Body fat percentage (%BF) was estimated using a multi-frequency bioelectrical impedance analysis device (InBody 770, InBody, Seoul, Korea) and body mass (BM) was measured with a built-in scale. Participants were tested wearing minimal clothing and barefoot without socks.

Resting metabolic rate testing

Resting metabolic rate (RMR) was measured using an automated metabolic gas analysis system (TrueOne 2400, Parvo Medics, Sandy, Utah, USA) to examine changes in whole-body metabolism after drink ingestion. After hydration status and body composition measurements were obtained, participants were led to a calm, quiet, mild-light, temperature (21–24 °C) controlled environment. Participants were instructed to lie in a supine position while enclosed in a clear hard plastic canopy, which was attached to the metabolic cart and dilution pump via a breathing tube. Oxygen uptake (V̇O 2 ) and carbon dioxide production (V̇CO 2 ) were measured for 30 min at baseline and for 20 min at the 30-, 60-, and 90-min time points post-ingestion. Respiratory gas values were averaged over one-minute intervals and posteriorly averaged for the last 10 min of each time point to estimate resting energy expenditure (REE). Total REE was also estimated by conducting area under the curve analyses over the 90-min procedure. As recommended by the manufacturer, a non-protein stoichiometric equation was used to estimate resting fat oxidation rate (RFO) (1.695 · V̇O 2 –1.701 · V̇CO 2 ) [29].

Blood venous sampling and glycerol analysis

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Venous blood was obtained during rest from the antecubital area of the arm using a Teflon cannula with a three-way stopcock with a male luer lock adapter. The cannula was maintained patent using a non-heparinized isotonic saline solution for the duration of the trial. A total of four blood draws occurred for each trial (baseline, 30, 60, and 90 min post-ingestion) collected in two 10 mL serum Vacutainer® tubes. Following a given blood draw, the tube was allowed to clot for 30 min followed by centrifugation at 4000 x g for 15 min. Serum samples were placed into separate 1.8-mL microcentrifuge tubes and stored at -80°C in the Exercise Biochemistry Lab for later analysis. Serum glycerol was determined via direct enzymatic analysis using a commercially available assay (Clinical Glycerol II Reagent Kit GMRD-177; Analox Instruments Ltd., Stourbridge, UK). All samples for each assay were thawed once and analyzed in duplicate by the same technician to reduce potential inter-assay variance (CV:7.3%). Due to technical issues, glycerol concentration analyses were not completed for three participants.

Graded exercise test, indirect calorimetry, and calculations

Participants performed a graded exercise test to exhaustion (GXT) on an electromagnetically-braked cycle ergometer (Corival, Lode B.V., Groningen, Netherlands). The GXT protocol consisted of a 10-min warm-up at 50 watts for male participants and 30 watts for female participants. Work rate was increased by 35 watts for males and 25 watts for females every 3 min until volitional fatigue. Breath-by-breath gas exchange data were collected using a metabolic gas analyzer (K-5 CPET, Cosmed, Rome, Italy) and used to determine maximal oxygen uptake (V̇O 2max ) and total energy expenditure during exercise (EE). The rating of perceived exertion from Borg’s 10-point scale was recorded during each stage of the GXT and immediately upon completion to confirm maximal exertion [30]. Average values for V̇O 2 and V̇CO 2 for the last minute of each stage were calculated using stoichiometric equations and used to determine fat oxidation, while assuming negligible protein oxidation [31]. Maximal fat oxidation (MFO) and the exercise intensity at which MFO occurred (Fat max ) were then determined using a third order polynomial function for each participant [32]. Two participants did not complete the GXT due to technical issues and Fat max could not be obtained for an additional two participants; therefore, a total of 28 participants were included in the final analysis.

Statistical analysis

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