Subjects

Subjects were 20 healthy young female volunteers (age: 22.4 ± 2.8 yrs; height: 163.4 ± 4.7 cms; weight: 62.2 ± 6.5 kgs) recruited from a university population. This sample size was justified by a priori power analysis using a target effect size of 0.6, alpha of 0.05 and power of 0.80 with percent body fat as an outcome measure based on previously established results [17]. All participants reported performing aerobic exercise several days a week on a regular basis and several were off-season collegiate track and field athletes. Initial screening required that subjects were between the ages of 18 35, not classified as obese based on a body mass index (BMI; calculated as kg/m2) of ≥30, and not involved in a resistance training program at the time of the study. Individuals meeting these criteria were invited to attend a familiarization session where a complete explanation of the study was provided, and a medical history and informed consent were obtained. Participants were excluded from the study if they are found to have any of the following: existing lower body injury; current participation in a resistance training program; metabolic or cardiovascular disorders (including coronary artery disease, cardiac arrhythmias, diabetes, thyroid disease, or hypertension); history of pregnancy within the past 6 months; any condition that would result in stratification as high risk based on criteria set forth by the American College of Sports Medicine [18], and/or taking a prescription or non-prescription weight-loss aid. Those meeting eligibility criteria and willing to participate in the study were scheduled for baseline testing. Approval for the study was obtained from the Institutional Review Board at Lehman College.

Testing sessions

Testing was carried out in the 24 48 hours prior to beginning the intervention and after the fourth week at completion of the study. For each testing session, subjects reported to the lab in the morning following an overnight fast having refrained from vigorous physical activity, alcohol intake, or consumption of over-the-counter medications for at least 12 hours. Baseline assessments included body mass, height, body composition, and waist circumference, After completion of testing, subjects were pair-matched based on initial body mass measurements and randomly assigned to 1 of 2 experimental groups: a fasted training (FASTED) group that performed exercise after an overnight fast (n = 10) or a post-prandial training (FED) group that consumed a meal prior to exercise (n = 10). The number of athletes and non-athletes were evenly distributed between groups.

Exercise training intervention

Training consisted of 1 hour of steady-state aerobic exercise performed 3 days per week on a LifeFitness, treadmill (model CLST-0100R-01, Brunswick Corporation, Rosemont, IL) at a 0% grade. Subjects performed a warm-up for the first 5 minutes at an intensity equating to 50% of maximal heart rate (MHR), determined by the formula 220 age, then increased intensity to 70% MHR for the next 50 minutes, and finished with a 5 minute cool down at 50% MHR. Heart rate monitors (model F7U, Polar Electro Inc, Lake Success, NY) were used to ensure that exercise remained at the appropriate intensity. A low-to-moderate training intensity was used because it has been shown to maximize lipid oxidation during fasted aerobic exercise as compared to higher-training intensities [19]. All training sessions were supervised by research assistants who were upper level undergraduate students in exercise science. Subjects were instructed to refrain from performing any additional structured exercise for the duration of the study.

Dietary intervention

Subjects were provided with customized dietary plans prepared by one of the researchers (A.A.A.) for the length of the study. In order to facilitate weight loss, energy consumption was set so that subjects remained in a caloric deficit. The determination of energy intake was based on the Mifflin-St. Jeor Equation, which is considered an accurate formula for estimating resting metabolic rate [20]. The formula is as follows:

10 × weight kg + 6.25 × height cm – 5 age × y – 161

The formula was multiplied by a moderate activity factor (1.5) to estimate energy balance, and the total was then reduced by 500 calories to impose a caloric deficit. Dietary protein intake was set at 1.8 g/kg of body mass, as higher protein consumption has been shown to help offset losses in lean tissue mass and promote greater adherence to the nutritional regimen [21],[22]. After accounting for protein intake, dietary fat was 25-30% of total calories and the remaining calories were obtained from carbohydrate. Sample meal plans were provided to guide the participants in acceptable food choices.

Dietary plans included provision of a meal replacement shake (Pursuit Recovery, Dymatize Nutrition, Dallas, TX). The shake contained 250 calories consisting of 40 g carbohydrate, 20 g protein, and 0.5 g fat. On exercise days, FED consumed the shake immediately prior to the exercise bout and FASTED consumed the shake immediately after finishing the bout. Shakes were consumed under the supervision of a research assistant to ensure adherence within the context of the subject s respective participation in the fed or fasted protocol.

Dietary adherence was assessed by self-reported food records using MyFitnessPal.com (http://www.myfitnesspal.com), which were collected and analyzed during on a daily basis to ensure that intake was not based on recall. Subjects were instructed on how to properly record all food items and their respective portion sizes that were consumed for the designated period of interest. Each item of food was individually entered into the program, and the program provided relevant information as to total energy consumption, as well as amount of energy derived from proteins, fats, and carbohydrates over the length of the study. Continued nutritional guidance was provided to the subjects at the time of each training session by the research team to encourage dietary adherence.

Anthropometrics and body composition measurements

Height and body mass measurements were made using a double beam scale. Circumference measurements of the waist was made using an Intelametrix tape measure (Intelametrix Inc., Livermore, CA) according to established criteria [18]. Body mass index (BMI) was calculated as body mass in kg divided by height in meters squared. Percent fat mass and lean body mass was obtained via air displacement plethysmography (ADP) using the BodPod body composition analyzer (model 2000a, Life Measurement, Concord, CA) as per the user manual and described previously in the literature [23]. ADP has been shown to have good validity in measuring body fat percentage when compared to dual x-ray absorptiometry in the sampled population [24],[25]. Subjects were tested in tight clothing (either compression shorts and a sports bra or a swimsuit) and Lycra swim cap. Based on body mass and volume as well as through body density, total fat mass, total fat free mass and body fat percentage were calculated by the BodPod system software.

Statistical analyses

Normality assumptions were checked using a one-sample Kolmogorov-Smirnov test; all data was found to meet normality assumptions. Independent t-tests were used to assess differences in baseline measurements between groups as well as energy and macronutrient intake over the length of the study period. Cohen s D effect sizes were calculated for all pre- to post-study outcome measures using the following formula:

M 1 – M 2 / S D

where M 1 represents the pre-study mean, M 2 represents the post study mean, and; SD represents the pooled standard deviation.