Nutritional status, biological maturation and cardiorespiratory fitness in Azorean youth aged 11–15 years

The study emerged from a protocol between the Government of the Azores and the University of Coimbra. The Azorean Archipelago consists of nine islands divided into three distinct groups (West, Central and East) located in the North Atlantic between 36° and 43° N and 25° and 31° W. According to 2011 census, the archipelago comprises 240,024 inhabitants [19]. In the 2007–2008 academic year, the educational system included 36 schools and 36,196 students distributed as follows: primary (grades 1–4), 11,928; early elementary (grades 5–6), 6,609; advanced elementary (grades 7–9), 10,231; and secondary (grades 10–12), 7,428. The present study was conducted in 2008–2009, and was part of the third edition of the Azorean Growth Study of secular change in body size, maturation and physical fitness in the school population 10–16 years of age (1988, 1998, and 2008–2009).

The project was approved by the Committee of the University of Coimbra and registered in the Portuguese Commission for Protection of Personal Data [Process #3132006]. Subsequently, the Azorean Government issued a written statement to all schools informing them of the objectives of the study. The Pedagogic Boards of each school approved the study as a part of the Physical Education program.

After obtaining all institutional permissions, participants were selected with a proportionate stratified random sampling strategy taking into account the group of islands (West: Flores, Corvo; Central: Terceira, Faial, Pico, São Jorge, Graciosa; East: São Miguel, Santa Maria). In the West, Central and East groups, 50% of the schools were visited. In Portugal, for each school classes are numbered within each grade and students are numbered within each class. In each chosen school, researchers only contacted PE teachers of even classes and from the selected classes consent terms were uniquely distributed among students having odd numbers. About 9% of the terms were not affirmative. From the same classes, equal portion of students having even numbers were contacted for taking part of the measurements. A total of 1,969 students were examined. The sample for this study was limited to the age range approximating adolescence in girls and boys aged 11–15 years (n = 1,206). Sixteen potential participants did not participate due to health limitations (asthma and flu).

Measurements were obtained by 14 local qualified teachers of Physical Education between November 2008 and March 2009. These observers were master students at the University of Coimbra and received one-year training in the protocol and procedures of the study: All signed a contract with the Government of the Azores that permitted their participation in research project as part of the master program. Before data collection, observers were trained in anthropometry (27 hours), fitness measurements (27 hours), data management and analysis (27 yours) taught by the first and last authors of the current manuscript.

Stature was measured to the nearest 0.1 cm using a stadiometer (Holtain Ltd., Crymmych, Pembrokeshire, UK). Body mass was measured to the nearest 0.1 kg with a portable electronic scale (Tanita Inner Scan BC 532). Subjects wore light indoor clothing with shoes and accessories removed. Subjects were classified by BMI as non-overweight, overweight or obese using the age and sex-specific cut-off values of the International Obesity Task Force (IOTF) [20].

Skinfold thickness was measured to the nearest 0.1 mm on the right side of the body in duplicate with a Lange caliper (Cambridge Scientific Industries, Inc., Cambridge, MD, USA), at the triceps and subscapular. In our laboratory, the intra-observer technical errors of measurement for skinfold thickness in this study were < 5%, measured in duplicate. Percentage body fat was estimated from the two skinfolds [21] and classification of normal-fat and over-fat was done using the cut-off points adopted by the FITNESSGRAM battery [17, 22, 23]. The prediction equation to estimate percentage of fat for males requires stage of puberty [21]. Consequently stages of pubic hair [24] were self-assessed.

The Khamis-Roche method [9] was used to estimate mature stature from decimal age, stature and body mass of the participant and midparent stature (average stature of biological parents). The protocol was developed on children from the Fels Longitudinal Study conducted in South-central Ohio in the United States. The median error bound (median absolute deviation) between actual and predicted mature stature at 18 years of age is 2.2 cm in males and 1.7 cm in females [9]. Parent statures were extracted from national identification cards which included stature measured to the nearest centimeter. Measurements were taken by experienced, but not necessarily trained observers. A similar protocol was used in surveys of conscripts in Portugal [25].

Current stature of the youngster was expressed as a percentage of predicted mature stature attained at the time of measurement. This was used as the indicator of biological maturity status. The percentage of predicted mature stature was then expressed as a z-score using half-yearly age- and sex-specific means and standard deviations for the longitudinal sample of the Berkeley Guidance Study[26]. The rationale for using the Berkeley data has been previously discussed [27, 28]. Participants were divided into three maturity groups based on z-scores: Late: z-score less than −1.0; On-time (average): z-score between −1.0 and +1.0; Early: z-score greater than +1.0.

The CRF was analyzed from the 20-m shuttle run test. This measurement of aerobic performance demonstrated a high correlation with direct assessment of maximal oxygen uptake [29‐31]. The 20-m shuttle run test was conducted as described by Léger et al. (29) on an indoor court with slip resistant floor in a 20-m space bounded by two parallel lines. Participants were instructed to run in a straight line, to pivot and turn on completing a shuttle, and to pace themselves in accordance with the audio signals. The initial speed was set at 8.5 km/h (2.4 m/s), which was increased by 0.5 km/h (0.1 m/s) each minute (one minute equals one stage). The CD used was calibrated for duration of one minute. Subjects were advised on proper pacing strategy and motivated to give their best effort. During the test, the subjects were encouraged and verbally communicated at each change of stage. The test ended when the participant stopped due to fatigue, or when they failed to reach the end lines concurrent with the audio signals on two consecutive occasions. The participants were familiar with the test in their physical education classes. Furthermore, the subjects were instructed to refrain from strenuous exercise in the 48 h before testing. All tests were conducted by the same investigators. Based on the 20-m shuttle run scores, participants were classified as fit or unfit relative to age- and sex-specific cut-offs of the FITNESSGRAM [23].

Decimal age was calculated as the difference between date of birth and date of data collection. Descriptive statistics for percentage of predicted mature height, stature, body mass, BMI, percentage of body fat and CRF were calculated and tested for sex differences using analysis of variance (ANOVA) for single year age groups from 11 [11.0-11.9] to 15 [15.0-15.9] years. Preliminary associations between all variables were verified with a matrix of correlation. The associations of weight status and fat status with the independent variables (five age groups, estimated maturity status [late, on time, early], CRF [fit, unfit]) were assessed using chi-square with a Yates correction in 2×2 contingency tables. All independent variables with p ≤ 0.20 in the chi-square test were then inserted simultaneously in multivariate models stratified by sex. The multivariate models were elaborated using the Poisson regression with robust variance (expressed as prevalence ratio [PR] and 95% confidence intervals [95% CI] adjusted for independent variables). In cross-sectional studies with outcomes of higher frequency (>10%), Poisson regression with robust variance is an alternative to logistic regression, which tended to overestimate the magnitude of associations [32]. Poisson regression corresponds to a more conservative multivariate model. The combined effect of weight status (normal and over BMI) and adiposity status (normal and over fat) was tested by comparing the means of the derived four groups using analysis of co-variance (controlling for chronological age). SPSS for Windows (version 17.0) and Stata 8.0 version were used for the analysis. Significance was set at 0.05.