Study Design and Sample
The adolescents included in this analysis were part of the Longitudinal Analysis of Biomarkers and Environmental Determinants of Physical activity (LabMed Physical Activity Study), which is a school-based prospective cohort study carried out in four Portuguese cities from the North. We selected 5 secondary schools, which had already collaboration agreements established with our research center and, therefore, they were selected primarily for pragmatic, budgetary and logistical reasons. Participants’ recruitment was conducted at the selected schools, and all pupils belonging to the 7th and 10th grades classes were invited to participate in the study (n=1678). The LabMed Physical Activity Study methodologies have been presented in elsewhere [20]. From that initial sample, 534 agreed to undergo blood sampling. However, five of them were later excluded from the analysis due to C-reactive protein values >10mg/L, which may be indicative of acute inflammation or illness, leaving 529 adolescents (267 girls, mean age 14.3±1.7 years) as the final sample for the present study. For the present study power analysis was calculated post hoc (for α =0.05) and it was higher than 0.8 for ROC curves and Logistic Regression Analysis sample size calculation in both sexes.
Ethical and legal requirements
The study was conducted in accordance with the World Medical Association’s Helsinki Declaration for Human Studies. The Portuguese Data Protection Authority (#1112434/2011), the Portuguese Ministry of Science and Education (0246200001/2011) and the Ethics Committee of the Faculty of Sport, University of Porto, approved the study. All participants in this study were informed of the study’s goals, and written informed consent was obtained from participating adolescents and their parents or guardians. Considering potential refusals to participate in the study due to blood analysis a “layered consent” was permitted, allowing the participants to consent some parts of the study protocol and not others. For example, an adolescent could perform physical fitness assessments and refuse to undergo blood sampling. Throughout the study no exclusion criteria were applied to avoid discriminations. However, for the present analysis, only apparently healthy adolescents were considered, i.e., without any medical diagnose of physical or mental impairment.
Measures
Anthropometrics
Body height was measured to the nearest 0.1 cm in bare or stocking feet with the adolescent standing upright against a portable stadiometer (Seca 213, Hamburg, Germany). Body weight was measured to the nearest 0.10 kg with the participant lightly dressed using a portable electronic weight scale (Tanita Inner Scan BC 532, Tokyo, Japan).To assess the body fat percentage a frequency current of 50 kHz (Tanita Inner Scan BC 532, Tokyo, Japan) was used to measure leg-to-leg Bio impedance. After manual introduction of the sex, age and height into the scale system, participants were asked to come up and remain still on the scale until measurement was completed, fulfilling the manufacturer's instructions.
Pubertal stage
Participants self-assessed their pubertal stage of secondary sex characteristics (breast and pubic hair development for girls, genital and pubic hair development for boys; ranging from stage I to V), according to the criteria of Tanner and Whitehouse[21] .
Blood Sampling
Blood samples were obtained from each subject early in the morning, following a 10-hour overnight fast by venipuncture from the antecubital vein. The samples were stored in sterile blood collection tubes in refrigerated conditions (4° to 8°C) for no longer than 4 hours during the morning of collection and then sent to an analytical laboratory for testing according to standardized procedures, as follow: (i) serum leptin and serum adiponectin by plate reader method (ELISA analyzer); (ii) glucose by hexokinase method (Siemens Advia 1600/1800 Erlangen, Germany); (iii) insulin by chemiluminescence immunoassay (Siemens ACS Centaur System, Erlangen, Germany. All assays were performed in duplicate according to the manufacturers’ instructions. Insulin resistance homeostatic model assessment (HOMA-IR) was calculated as the product of basal glucose (mmol/L) and insulin (µlU/mL) levels divided by 22.5, and was used as a proxy measure of insulin resistance (23). HOMA-IR were transformed to standardized values [Z-score = (participant’s value−mean value of the sample)/standard deviation)] by age and sex. High risk (at risk) was considered when the individual had ≥1SD (standard deviation) of this Z-score as previously suggested [22].
Adherence to the Mediterranean diet
To assess the degree of adherence to the Mediterranean diet the KIDMED index (Mediterranean Diet Quality Index for children and adolescents) was used [23](25). The index is based on a 16-questions self-administered, which sustain the principles of the Mediterranean dietary patterns, as well as, those that undermine it. The results of index varied between 0 and 12 points. The questions that have one negative connotation in relation to Mediterranean diet were equal to (-1), the questions that constitute positive aspect were equal to (+1). A continuum variable was computed to perform the statistical analyses.
Cardiorespiratory Fitness
Cardiorespiratory fitness was measured using the 20-metre shuttle run test. This test requires participants to run back and forth between two lines set 20 m apart. Running speed started at 8.5 km/h and increased by 0.5 km/h each minute, reaching 18.0 km/h at minute 20. Each level was announced on a tape player. The participants were instructed to keep up with the pace until exhausted. The test was finished when the participant failed to reach the end lines concurrent with the audio signals on two consecutive occasions. Otherwise, the test ended when the subject stopped because of fatigue. The participants received verbal encouragements from the investigators to achieve maximum performance, to keep running as long as possible. The number of laps performed by each participant was recorded [24].
Statistical Analysis
Measurement data mean, and standard deviations were evaluated for normality by the Kolmogorov-Smirnov test and compared using the t-test to examine sex differences.
A receiver operating characteristic curve (ROC) was drawn to calculate area under the curve (AUC) and the best cut-off value of the of leptin, adiponectin and L/A ratio to discriminate between low/high values of IR were determined using the maximum Youden’s index. Youden’s index is defined as: J = [sensitivity + specificity - 1]; the critical threshold value is the point at which the sensitivity and specificity is maximized. The positive likelihood ratio LR (+) and the negative likelihood ratio LR (−) were used to analyse the potential diagnostic accuracy. The prognostic accuracy of serum leptin, adiponectin and L/A ratio for predicting insulin resistance was performed by comparison of area under curve (AUC) of ROC curves. The AUC of ROC curves was derived by the method of Hanley and McNeil [25].
Based on the cut-off values suggested by the ROC curves, logistic regression analyses were performed to further study the relationships between adiponectin, leptin and L/A ratio and HOMA-IR, adjusted for the following potential confounders: age, pubertal stage, cardiorespiratory fitness, adherence to a Mediterranean diet and body fat percentage.
Data analysis was performed using the Statistical Package for the Social Sciences for Windows (Version 21.0 SPSS Inc., Chicago, IL) and MedCalc (Version 11.1.1.0 MedCalc, Mariakerke, Belgium). A p value < 0.05 denoted statistical significance.