Experimental design

In order to examine the primary hypotheses of the present study, a double-blinded, randomized, within-group crossover study design was used. After familiarization and preliminary baseline resistance exercise performance testing, subjects were randomly assigned (using a balanced design) to either a p-synephrine (100 mg; S), p-synephrine + caffeine (100 mg p-synephrine plus 100 mg caffeine; SCF), or placebo (P) treatment. The resistance exercise protocol consisted of performing 6 sets of back squats for up to 10 repetitions per set using 80 % of subjects’ maximal strength with 2 min of rest in between sets at a standard time early in the morning in a fasted state. The protocol was performed 4 times (baseline and following consumption of each supplement). Subjects consumed each supplement in the form of chews for 3 days prior to and upon arrival at the laboratory the day of each protocol and then washed-out for 3 days following each protocol day. The 3-day pre-protocol supplementation period was utilized in order to expose subjects to the supplements on multiple occasions (to examine how subjects tolerated the supplemental doses) prior to performance assessment. Performance (repetition number, force, velocity, and power), blood lactate, and ratings of perceived exertion data were collected during each protocol.

Subjects

The subjects were healthy, college-aged (e.g., 20–26 years) men (N = 12) who were former athletes with at least 2 years of resistance training experience from the student population at The College of New Jersey. Descriptive subject characteristics are shown in Table 1. None of the subjects were taking any medications, anabolic steroids, or nutritional supplements known to affect resistance exercise performance. All subjects had refrained from caffeine intake for at least 3 weeks prior to starting the study. Each subject was monitored during this time for symptoms such as headaches and fatigue in order to minimize potential confounding effects of caffeine withdrawal reversal. Three weeks was selected as withdrawal effects from low-to-moderate caffeine consumption typically subside within 10 days [28]. No withdrawal effects were reported upon initiation of pre-study familiarization and testing. Prior to this period, 5 subjects reported low daily caffeine intake (<10–20 mg/day), 3 subjects reported moderate caffeine intake (100–200 mg/day), and 4 subjects reported high daily caffeine intake (250–400 mg/day). Subjects were consistently questioned, completed diet records, and instructed not to consume known sources of caffeine during the experimental period. None of the subjects had any physiological or orthopedic limitations that could have affected lifting performance as determined by completion of a health history questionnaire prior to initiating the study. Subjects were instructed to refrain from exercise for 2 days prior to each protocol. This study was approved by The College of New Jersey’s Institutional Review Board and each subject subsequently signed an informed consent document prior to participation.

Table 1 Subject descriptive characteristics Full size table

Preliminary screening and familiarization

During the first visit to the Human Performance Laboratory, subjects completed a medical history questionnaire, informed consent document, were familiarized with the equipment and procedures, and were instructed on how to properly complete a 3-day dietary record. Height was measured using a wall-mounted stadiometer and body mass was measured using an electronic scale. VO 2 max was assessed using a progressive, multi-stage ramp protocol on a treadmill using a metabolic system (MedGraphics ULTIMA Metabolic System, MedGraphics Corporation, St. Paul, MN). Percent body fat was estimated via a three-site skinfold test. The sites measured were the pectoral, anterior thigh, and abdominal skinfolds [29]. The same research assistant performed all skinfold assessments. Body density was calculated using the equation of Jackson and Pollock [29] and percent body fat was calculated using the equation of Siri [30]. Dietary records were used to ensure subjects maintained normal kilocalorie and macronutrient intake throughout the study, and were completed for 3 days prior to the baseline protocol and for 3 days prior to each subsequent protocol. Each dietary record was analyzed using the Nutrition Calc Plus Version 3.4 software program (ESHA Research, Salem, OR). Total kilocalories, grams of fat, carbohydrates, and protein, and percent dietary intake of fats, carbohydrates, and protein were analyzed.

Strength testing

The one-repetition maximum (1RM) squat was used as a measure of strength using a standard protocol [31]. A warm-up set of 5–10 repetitions was performed using 40–60 % of the perceived 1RM. After a 1-min rest interval, a set of 2–3 repetitions was performed at 60–80 % of the perceived 1RM. Subsequently, 2–4 maximal trials were performed to determine the 1RM with 2–3 min rest intervals in between trials. A complete range of motion and proper technique was required for each successful 1RM trial. Subjects descended with the bar on the rear shoulders until their upper thighs were parallel to the ground. At that point a “lift” signal was given by a research assistant (to ensure proper depth) and the subject ascended to the starting position. Assessment of 1RM strength enabled calculation of the protocol loads (i.e. 80 % of 1RM).

Control protocol

A control (no supplement) resistance exercise protocol was performed first to establish baseline resistance exercise performance. Subjects reported to the laboratory early in the morning following a 10-h fast to minimize any confounding influence from prior meal consumption. Only water consumption was permitted. Upon arrival, each subject was encouraged to drink water ad libitum to pre-hydrate. Subjects had a cannula inserted into an antecubital vein for blood sampling. Subsequently, each subject was positioned on a reclining chair and sat quietly for 15 min prior to measurement of baseline blood lactate. Subjects then proceeded to sit quietly for an additional 45 min in a recumbent position. The quiet sitting period was used to mimic the time of supplement consumption during subsequent protocol sessions.

Subjects performed a standard warm-up consisting of 3 min of stationary cycling or walking, light stretching, and 2–3 light sets of squats with 30–65 % of 1RM. Water was provided ad libitum during this time. The resistance exercise protocol consisted of performing 6 sets of up to 10 repetitions (i.e. when momentary muscular failure was attained prior to the completion of 10 repetitions) of the free-weight back squat with ~80 % of 1RM using 2-min rest intervals in between sets. Standard exercise technique was used and only those repetitions that met the criteria were counted. Resistance remained constant while total numbers of repetitions were recorded. Subjects used a self-selected cadence (with no rest in between repetitions) in order to maximize resistance exercise performance. Following each set, ratings of perceived exertion (RPE) were obtained using a category ratio (CR) 10-point (0–10) scale. Volume load (kg) was calculated as the number of completed repetitions x resistance used.

Experimental protocol

Following the control session, subjects were randomly assigned (in double-blind manner) to a balanced treatment sequence. The treatments involved use of 3 supplements: p-synephrine (100 mg), p-synephrine + caffeine (100 mg of p-synephrine plus 100 mg of caffeine), or a placebo treatment. Subjects consumed random treatment 1 for 3 consecutive days prior to returning to the laboratory. Subjects then reported to the laboratory the following morning in a fasted state similar to the control protocol session. After baseline measures and blood sampling, subjects consumed a dose of treatment 1. They sat quietly for 45 min and subsequently initiated the resistance exercise protocol discussed previously. Each subject did not consume a supplement for the next 3 days (i.e. a 3-day “wash-out” period was used based on the half-lives and patterns of elimination of p-synephrine and caffeine) but then began supplementing with treatment 2 for 3 days prior to their next scheduled protocol session. Subjects arrived at the laboratory, consumed treatment 2, sat for 45 min, and repeated the protocol described above. Following a 3-day wash-out period, subjects consumed treatment 3 for 3 days and repeated the protocol in a similar manner. Each supplement protocol session was identical to the control session with the exception of supplement consumption prior to the 45 min quiet sitting period (following baseline assessments). This cross-over design allowed each subject to experience all supplemental conditions in random sequence.

Resistance exercise kinetics and kinematics

Each resistance exercise protocol was performed on a portable force plate (Advanced Medical Technology Inc., Watertown, MA) with data collected at a frequency of 200 Hz. Average and peak concentric ground reaction force per repetition were recorded and analyzed. Average bar velocity and power for the each repetition was measured with a Tendo™ Power Output Unit (Tendo Sports Machines, Trencin, Slovak Republic). The Tendo™ unit consists of a linear position transducer attached to the end of the barbell which measured linear displacement and time. Subsequently, average bar velocity and power were determined for each repetition. Power and velocity were averaged for each set (for all completed repetitions) and for each protocol. Test-retest reliability for the Tendo™ unit in our laboratory has consistently shown R > 0.90 [32].

Blood lactate measurements

Subjects arrived at the laboratory in the early morning (at a standard time of day for all sessions) following an overnight fast. Venous blood samples were collected from subjects in a seated, semi-recumbent position at rest (T1), following the 45 min quiet sitting protocol (T2), immediately post-exercise (T3), and 15 min (T4), and 30 min (T5) post exercise. All blood samples were obtained using a 20-gauge Teflon cannula placed in a superficial forearm vein. The cannula was maintained patent via infusion of a heparin solution and blood was removed via a plastic syringe connected to a 3-way stopcock with a male luer lock adapter. T1 blood samples were drawn following a 15 min equilibration period. T3 blood samples were taken within 30 s of exercise cessation. Blood samples were collected into a Vacutainer® tube containing SST® Gel and Clot Activator. An aliquot of each whole blood sample was removed and immediately used for determination of blood lactate. Whole blood lactate was analyzed in duplicate using an Analox GM7 enzymatic metabolite analyzer (Analox Instruments USA, Lunenburg, MA).

Supplement composition and procedures

The supplements used in the present study were in “chew” form (Advantra Z®, Nutratech, Inc., West Caldwell, NJ). Each chew was identical in appearance, chocolate flavored, and identical in taste. All three supplements contained isomalt, maltitol syrup, cocoa powder, natural flavors, palm oil, soy lecithin, glycerin, and stevia. One serving consisted of two chews. Each serving of chews contained 30 kcal from 8 g of carbohydrates. Thus, the placebo contained only these nutrients. The chews contained these nutrients with 100 mg of p-synephrine from Citrus aurantium extract in two chews. The p-synephrine + caffeine chews consisted of the same nutrients plus 100 mg of p-synephrine plus 100 mg of caffeine. The supplements were administered to subjects in absolute doses. The rationale was to examine the supplement in a manner consistent with manufacturer’s recommendations and to expand the p-synephrine literature base by providing a larger amount compared to previous studies which have mostly investigated absolute doses of 5–80 mg per day [12].

Subjects were instructed to consume each supplement for 3 days prior to arriving to the Human Performance Laboratory. Subjects were given the chews (6 in total) in bags labeled “A”, “B”, or “C”, specific instructions on consumption, forms to complete by recording the time of day each supplement was consumed, and were instructed to return the empty wrappers as documentation of use. Two chews were consumed each day generally during the late morning, early afternoon hours. Both chews were consumed simultaneously and subjects were instructed to chew them completely and hold remnants under the tongue for at least 2 min prior to swallowing. Subjects were also given two chews during each resistance exercise protocol, i.e. prior to the 45-min quiet sitting period resulting in consumption of 8 chews in total per treatment condition.

Statistical analyses

Descriptive statistics (means ± SD) were calculated for all dependent variables. A 2-way (treatment x time point) analysis of variance (ANOVA) with repeated measures was used to analyze all within-subject data. Subsequent Tukey’s post hoc tests were utilized to determine differences when significant main effects were obtained. A one-way ANOVA was used to analyze diet records and body weight changes. Pearson-product moment correlations were calculated for selected variables. For all statistical tests, a probability level of P ≤ 0.05 denoted statistical significance.