Association between sedentary behaviour and socioeconomic factors, diet and lifestyle among the Balearic Islands adolescents

Study design

The study is a population-based cross-sectional nutritional survey carried out (2007–2008) in the Balearic Islands (Spain), a Mediterranean region.

Selection of participants, recruitment and approval

A multicenter study was performed on Balearic Islands’ adolescents aged 12–17 years. The population was selected by means of a multiple-step, simple random sampling, taking into account first the location (Palma de Mallorca, Calvià, Inca, Manacor, Maó, Eivissa, Llucmajor, Santa Margalida, S’Arenal, Sant Jordi de Ses Salines) and then by random assignment of the schools within each city. Sample size was stratified by age and sex. The socio-economic variable was considered to be associated to geographical location and type of school. As the selection of schools was done by random selection and fulfilling quota, this variable was also considered to be randomly assigned.

To calculate the number of adolescents to be included in the study in order to guarantee a representative sample of the whole Balearic Islands, we selected the variable with the greatest variance for this age group from the data published in the literature at the time the study was planned; that was BMI [16]. The sampling was determined for the distribution of this variable; the CI was established at 95% with an error ± 0.25. The established number of subjects was 2400. The total number of subjects was uniformly distributed in the cities and proportionally distributed by sex and age group. Exclusion criteria were: type 2 diabetes, pregnancy, alcohol or drug abuse, and non-directly related nutritional medical conditions.

The sample was oversized to prevent loss of information and as necessary to do the fieldwork in complete classrooms. In each school, classrooms were randomly selected among those of the same grade or level, and all the adolescents of one classroom were proposed to participate in the survey. A letter about the nature and purpose of the study informed parents or legal tutors. After receiving their written consent, the adolescents were considered for inclusion in the study. All responses of questionnaires were filled in by adolescents. After finishing the field study, the adolescents who did not fulfil the inclusion criteria were excluded. Finally, the sample was adjusted by a weight factor in order to balance the sample in accordance to the distribution of the Balearic Islands’ population and to guarantee the representativeness of each of the groups, already defined by the previously mentioned factors (age and sex). The final number of subjects included in the study was 1961 adolescents (82% participation). The reasons to not participate were (a) the subject declined to be interviewed, and (b) the parents did not authorize the interview.

This study was conducted according to the guidelines laid down in the Declaration of Helsinki, and all procedures involving human subjects were approved by the Balearic Islands’ Ethics Committee (Palma de Mallorca, Spain).

Anthropometry measurements

Height was determined using a mobile anthropometer (Kawe 44444, Asperg, Germany) to the nearest millimetre, with the subject’s head in the Frankfurt plane. Body weight was determined to the nearest 100 g using a digital scale (Tefal, sc9210, Rumilly, France). The subjects were weighed in bare feet and light underwear. Waist circumference (WC) and hip circumference (HC) were measured using a non-stretchable measuring tape (Kawe, 43972, Asperg, Germany). The subjects were asked to stand erect in a relaxed position with both feet together on a flat surface. WC was measured as the smallest horizontal girth between the costal margins and the iliac crests at minimal respiration. Measurements were made to the nearest 0.1 cm. HC was taken as the greatest circumference at the level of greater trochanters (the widest portion of the hip) on both sides. Measurements were made to the nearest 0.1 cm. Triceps and subscapular skinfold thickness (ST) were measured at the right side of the using a Holtain skinfold caliper (Tanner/Whitehouse, Crosswell, Crymych, UK), and the mean of three measurements was used. Body fat percentage (%BF) was measured from triceps and subscapular ST according to Slaughter et al. [17]. This equation has been proposed as the most accurate for estimation of %BF from ST in this particular population of adolescents [18]. Height and weight measures were used to calculate body mass index (BMI, kg/m²) and WC and height were used to calculate waist-to-height ratio (WHtR). %BF and height were used to calculate fat mass index (FMI; kg/m²).

Defining overweight and obesity

Adolescents were classified into three groups as follows: (i) not at risk (BMI for age and sex < P97, and FMI < 4.58 kg/m² in boys, FMI < 7.76 kg/m² in girls); (ii) overweight (BMI for age and sex < P97, and FMI ≥ 4.58 kg/m² in boys, FMI ≥ 7.76 kg/m² in girls); and (iii) obesity (BMI for age and sex ≥ P97, and FMI ≥ 4.58 kg/m² in boys, FMI ≥ 7.76 kg/m² in girls). The variable was labeled as ‘body composition’ [19]. Age- and sex-specific BMI cut-offs were used according to the International Obesity Task Force and Cole et al. definitions [20], and FMI cut-offs according to Alvero-Cruz et al. [21] criteria for adolescents: 4.58 kg/m² in boys and 7.76 kg/m² in girls, as the limit between normal-fat and overfat.

Body image

Perceived body image was measured using the Stunkard scale [22], which consists of silhouette drawings ranging from 1 to 9 with monotonic increments in overweight percentage where 1 is the leanest and 9 is the heaviest. Separate figures for boys and girls were used. Participants were asked to identify of the 9 body figures: (a) ‘Which silhouette looks most like yourself?’ and (b) ‘Which silhouette would you like to look like?’ The difference between perceived body image and desired body image was used to determine the level of dissatisfaction with current body image. Values other than zero represent dissatisfaction with perceived body image. A positive value was indicative of the participant’s desire to be thinner than his/her perceived current size, while a negative value reflected the participant’s desire to be thicker than his/her current perceived size [23, 24].

Assessment of physical activity

Physical activity was evaluated according to the guidelines for data processing and analysis of the international physical activity questionnaire (IPAQ) [25] in the short form, and its specific modification for adolescents (IPAQ-A) [26]. The specific types of activity assessed were walking, and moderate (i.e. physical activity at school), and vigorous (i.e. sport practice) activity. According to the AVENA (Food and Assessment of Nutritional Status of Adolescents) study [27], the questionnaire also included information on television (TV) viewing, computer use and video games and homework in h/d, and usual sleep duration to the nearest 10 min. According to recent reports of physical activity for adolescents [7, 28], sedentary behaviour was established with a cut-off level of 300 min of moderate/vigorous physical activity (MVPA) per week.

Socioeconomic factors

Socio-demographic factors were recorded using a questionnaire that included age group; parental education level (according to years and type of education: low, <6 years; medium, 6–12 years; high, >12 years); and parental profession level (based on the occupation of parents and classified as low, medium and high, according to the Spanish Society of Epidemiology [29]. The number of daily meals and snacks was calculated from the total eating occasions that participants declared among the following: breakfast; mid-morning snack; lunch; mid-afternoon snack; dinner; before going to sleep; others. According to previous studies that found an inverse relationship between eating frequency and BMI [11], three groups of eating frequency were considered: ≤3, 4 and ≥5 times/d.

Dietary assessment

A semi-quantitative food-frequency questionnaire (FFQ) previously validated [30] was used. The FFQ, which asked the subject to recall average use over the past year, consisted of 145 items (118 of the original validated FFQ plus the most characteristic Balearic Islands foods in order to make easy the interviewee answer), and arranged by food type and meal pattern. Edible fractions of foods were recorded in the database. Frequency of food consumption was based on times that food items were consumed (per day, week or month). Consumption <1/month was considered no consumption. The period of consumption of seasonal items was also considered. The 145 foods items from the FFQ were collapsed to twenty-nine food groups, which may have practical importance in daily diet and clinical practice among Mediterranean younger [31, 32].

Adherence to the Mediterranean diet

The degree of adherence to the Mediterranean diet was evaluated using the KIDMED index (Mediterranean diet quality index for children and adolescents), described elsewhere [33]. Based on the given answers, the test classified individuals according to the quality of the Mediterranean diet categorized as: high, medium or poor.

Statistics

Analyses were performed with Statistical Package for the Social Sciences version 19.0 (SPSS, Inc., Chicago, IL, USA). All tests were stratified by sex. Significant differences in prevalence were calculated by means of χ ². Differences between means were tested using ANOVA. Logistic regression models with the calculations of corresponding adjusted odds ratios and 95% confidence intervals were used to examine possible differences between those adolescents who were sedentary or active. Univariate analysis was first carried out for all the socio-demographic and lifestyle variables that could be associated with sedentary behaviour. Any factor that was significantly associated was considered as a candidate for the multivariate model. Multiple logistic regression analyses were used to simultaneously examine the effect of different socio-demographic and lifestyle variables on the prevalence of sedentary behaviour. Multiple logistic regression analyses were also used to simultaneously examine the association between sedentary behaviour and dietary patterns adjusted by potential confounders (e.g. age, parental education level, and parental profession level, body composition, number of daily meals and snacks, desire for weight change, and energy intake). Level of significance for acceptance was P < 0.05.

Limitations of the study

In the literature there are important methodological differences in instruments and cut-off points which frequently prevent comparisons among studies [52]. Questionnaires have inherent limitations, mainly because they are subjective in nature. One bias could be that all responses of questionnaires were filled in by adolescents; however, doubts were solved immediately in the classroom by interviewers. Self-report of physical activity can lead to overreport the physical activity due to a social desirability bias, and therefore the number of inactive individuals may be lower than that reported [53, 54], especially among children and adolescents, and also among obese [53]. Moreover, sedentary behaviours may be more difficult to remember than activities of higher intensity [55] and recall bias in self-reported sedentary behaviours have been reported among adolescents [56]. Therefore, objective methods are generally preferable in assessing dimensions of physical activity in young people [4]. An extensive range of instruments for measuring physical activity has been reported in the literature, but critical elements in the utility of an instrument to measure physical activity are to be relatively inexpensive, cause minimal inconvenience to the participant, and be able to be administered with relative ease [57]. However, in epidemiological studies, self-reporting is usually the most feasible method of assessing physical activity, especially when a high number of participants are interviewed [4].