Participants
The municipality of Pamplona comprises a total population of 203 382 inhabitants (2018 census, Instituto de Estadística de Navarra), of which 9772 (4.8%) constituted the population of 10.1 to 14.9 years of age in the year 2018. The sample frame considered included these 9772 adolescents (5042 males and 4680 females). We applied the worst case estimate (0.50), a 95% confidence level and a precision of 0.04 in order to calculate the sample size, and the result was a minimum number of participants necessary of 600 individuals.
Anthropometric data were collected from a group of 940 healthy Caucasian adolescents (370 men and 570 women), aged 10.1 to 14.9 years. These were all students of the corresponding ages who were enrolled in four public schools located in the city of Pamplona (Navarra, Spain) in the period January-June 2018.
The normality of the nutrition status was the condition sine qua non to be included in this study; this means, BMI should range between +1 and -1 standard deviations (Z-score). In addition, non-Caucasian adolescents and those diagnosed with chronic pathologies that might affect growth, body composition, food ingestion or physical activity were excluded. The response rate after the exclusions was 85.5%.
Anthropometric measurements
The following anthropometric measurements were registered: weight, height, body mass index (BMI) and skinfold thickness (biceps, triceps, subscapular and suprailiac).
Weight and height measurements were taken with participants in underwear and barefoot. An Año-Sayol scale was used for weight measurement (reading interval 0 to 120 kg and a precision of 100 g), and a Holtain wall stadiometer for height measurement (reading interval 60 to 210 cm, precision 0.1 cm). BMI was calculated according to the following formula: weight (kg) / height2 (m).
Skinfold-thickness measurements were performed in triplicate at the biceps, triceps, subscapular, and suprailiac sites, and the mean of the 3 values was used, and measurements were performed by the same trained individual. Skinfold thicknesses values were measured to a precision of 0.1 mm on the left side of the body with Holtain skinfold calipers (CMS Weighing Equipment, Crymych, United Kingdom). The percentage of total body fat, fat mass (kg) and fat-free mass (kg) were calculated using the equations reported by Slaughter et al. [16], adjusted for sex and age. In the same way, the fat mass index (FMI) and the fat free mass index (FFMI) were estimated using the following formulas: fat mass (kg) / height2 (m), and free fat mass (kg) / height2 (m), respectively.
The z-score values for the BMI were computed using the program Aplicación Nutricional, from the Spanish Society of pediatric gastroenterology, hepatology and nutrition (Sociedad Española de Gastroenterología, Hepatología y Nutrición Pediátrica, available at http://www.gastroinf.es/nutritional/). The graphics from Ferrández et al. (Centro Andrea Prader, Zaragoza 2002) were used as reference charts [17].
Statistical analysis
Results are displayed as means (M) with corresponding standard deviations (SDs). The statistical analysis (descriptive statistics, percentiles calculation, Student’s t test, and analysis of variance) was executed using the program Statistical Packages for the Social Sciences version 20.0 (SPSS, Chicago, IL, USA). The condition for statistical significance was a P-value <0.05.
Parents and/or legal guardians were informed and provided written consent for the participation in this study in all cases. This study was approved by the Ethics Comm ittee for Human Investigation of Navarra Hospital Complex (in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and later amendments).