Study sample

This cross-sectional study is part of a research project entitled “Bone mineral density in adolescents: What is its relationship with body fat, physical activity, and sedentary behavior?”, conducted in Sao José, SC, Brazil, from April to December 2016. The project was approved by the Human Research Ethics Committee at Santa Catarina State University (protocol No. 1,468,045/2016). The target population was male and female adolescents aged between 10.0 and 14 years enrolled in a primary school in Sao José, SC, Brazil. Participants were selected from a list, provided by the school, containing body mass index (BMI) values collected in the previous year. The exclusion criteria were fracture history, malignant neoplasms, chromosomal abnormalities, paralysis, renal and hepatic insufficiency, hyper- and hypothyroidism, chronic viral infections, such as human papillomavirus (HPV), human immunodeficiency virus (HIV), human T-lymphotropic virus (HTLV), and any other disease that could interfere with body composition. All participants and their parents/guardians provided informed written consent to participate in the study.

The total sample consisted of 118 adolescents (60 girls and 58 boys) who were grouped according to confirmed BMI: normal weight and (34 girls and 32 boys) overweight (26 girls and 26 boys), according to sex and age (Cole et al., 2000; Cole et al., 2007) (Fig. 1).

This sample was deemed adequate for the study according to the sample size power calculation (G*Power, version 3.1.9.4). R² values of the multiple linear regression model for total body less head (TBLH) BMD (normal weight = 0.571, overweight = 0.502) were used for calculations, affording statistical power values of 1.00 and 0.99 for the normal-weight and overweight groups, respectively.

Data collection was performed in two stages. The first stage was carried out at school to confirm participant BMI. The second stage was conducted at the Laboratory of Anthropometry, Health Sciences Center, Santa Catarina State University. Prior to data collection, students were instructed to (i) observe an overnight fast or fast for ∼10 h before their scheduled appointment, (ii) wear lightweight and comfortable clothing, (iii) not wear any metal objects, such as earrings, necklace, or watch, (iv) not exercise up to 8 h before measurements, (v) bladder voided, and (vi) not drink alcohol up to 48 h prior (Nana et al., 2015).