Introduction

In many industrialized countries as well as in countries in economic transition, childhood overweight and obesity are increasing in alarming rates and reaching epidemic proportions (1,2,3,4). Longitudinal studies have shown that overweight children are more likely to remain overweight or obese in adulthood (5,6) resulting in an increased risk of developing acute and chronic medical conditions such as type 2 diabetes, hypertension, and cardiovascular disease when they become adults (7,8). There is evidence that genetic factors play an important role in developing obesity and its comorbidities. However, the increasing prevalence of obesity among children and adults is likely to be due to changes in environmental factors such as an increase in the energy intake and a decrease in physical activity (9,10).

Individuals and especially children spend a substantial part of their lives in front of a screen. Several studies have proposed that the excessive television (TV) viewing time is associated with a variety of adverse outcomes among preschoolers, school-aged children, and adolescents, such as violent or aggressive behaviors, several psychosomatic symptoms (i.e., attention-deficit/hyperactivity disorder–associated behaviors) (11,12,13,14) and overweight or obesity (15,16,17). In response to the growing problem of childhood obesity, the American Academy of Pediatrics has proposed a limit of no more than 2 h/day exposure of children to TV and video (18). The studies conducted to evaluate the association between the TV viewing time and adiposity among preschoolers have suggested that the prevalence of overweight is higher among children watching TV 2 h/day compared to those spending <2 h/day in front of a TV set (17,19,20,21). Moreover, Viner et al. recently reported that the TV viewing time at the age of 5 years predicts the BMI in adulthood (22).

Several investigators have proposed that the TV viewing time contributes to overweight or obesity by one or both of the following mechanisms: (i) reduced physical activity and (ii) increased caloric intake during TV watching (23,24). Few studies conducted among children and adults have revealed that the association between TV watching time and obesity is independent of physical activity levels (25,26,27). However, to the best of our knowledge, no similar study has been carried out among preschoolers, although several investigators have examined the association between TV watching time and obesity after controlling for important potential confounders.

Finally, despite the fact that the prevalence of overweight is high among Greek children (28) and TV viewing time is a proposed risk factor for childhood obesity, no studies have been conducted in Greece to investigate this potential association. Therefore, the main objective of this work was to investigate whether TV viewing time is independently associated with the prevalence of overweight among Greek preschoolers even after controlling for total energy intake and physical activity.

Methods and Procedures

Sampling

The design and rationale of the "Growth, Exercise and Nutrition Epidemiological Study in preSchoolers" (GENESIS) study have been published previously (28). In brief, 2,518 children, aged 1–5 years, were recruited in the study from a representative sample of randomly selected public and private nurseries as well as day-care centers within municipalities in five counties of Greece. The proportion of children attending day-care centers varies between 85–95% depending on age group whereas no differences are observed for these proportions between urban and rural residencies. Complete data became available for 2,374 children. Approval to conduct the study was granted by the Ethical Committee of Harokopio University of Athens and by all municipalities invited to participate in the study.

TV viewing assessment

Children's TV viewing time was assessed by parental report on their children's TV viewing time during a usual weekday and at a usual weekend. Children of this age usually spend 6–8 h/day at day care, and there is no opportunity for TV watching during that time. The mean daily TV watching time was calculated using the following equation: daily TV hours = ((weekday TV hours 5) + weekend TV hours)/7. Thereinafter, taking into account the recommendations of the American Academy of Pediatrics regarding children's TV watching time, the participants were divided into two groups as follows: children watching TV <2 h/day and those watching TV 2 h/day and more (18). Moreover, parents were asked to report the time that they spent watching TV on a usual weekday and at a usual weekend. Using the aforementioned equation, parental mean TV watching time was also calculated, separately for participating mothers and fathers.

Physical activity assessment

Children's physical activity was assessed via parental proxy reports during scheduled interviews at the nurseries (28). Parents were asked to provide information on their child's participation in outdoor organized or nonorganized physical activities whereas a research team member was recording this information on a valid, structured, proxy questionnaire. Emphasis was placed on light-to-vigorous physical activities (L-VPAs) with intensity higher than four metabolic equivalents. Typical L-VPAs for the younger age groups were playground recreational activities and taking walks with parents whereas sports participation, such as soccer, swimming, ballet, gymnastics, etc. were more commonly reported for the older age group. More information on the type of activities comprising L-VPAs is presented elsewhere (28). Preschoolers were divided into those having an L-VPA 3 h/week and those having L-VPA <3 h/week.

Anthropometric measurements

Children's body weight, recumbent length, and standing height were recorded. Details about the measures used for performing the aforementioned measurements are presented elsewhere (28). BMI was calculated as the ratio of weight (kg) with height squared (m²). The Nutstat module of EpiInfo (29) was used to determine children's age and sex-specific percentiles for weight, length, and BMI. The US Center for Disease Control age- and sex-specific growth charts and relative cutoff points were used for the definition of obesity (30). Specifically, up to 24 months the weight-for-length growth chart was used to classify children >95th percentile as "obese" whereas for children >24 months, the BMI-for-age chart was used to classify children 85th and <95th percentile as "overweight" and those 95th percentile as "obese." Finally, paternal and maternal BMIs were calculated from self-reported height and weight. These values were used to categorize parents as "normal weight" (18.5 < BMI < 25 kg/m²) and as "overweight and obese" (BMI >25 kg/m²).

Dietary assessment

Dietary intake data were obtained for two consecutive weekdays and one weekend day using a combination of techniques comprising the weighed food records (during nursery hours) and 24-h recall or food diaries (outside nurseries, under parents'/guardians' supervision). More specifically, during the two weekdays, and while the child was at the nursery, a team member weighed and recorded all foods consumed by each child. Information on the food consumed outside the nursery was obtained during a prearranged meeting with the parent or guardian the following day. Parents were advised to return the food diaries at the nursery on Monday mornings, where a team member received and checked the records for any errors. Food intake data were analyzed using the Nutritionist V diet analysis software (First DataBank, San Bruno, CA), which was extensively amended to include traditional Greek foods and recipes (31,32).

Data obtained by parents and birth certificates

A structured interview was conducted with both parents to collect additional information. All interviewers were trained prior to the beginning of the study to ensure interobserver reliability as well as accuracy of the data. Information were obtained regarding (i) parental educational level; (ii) maternal age at birth; (iii) participant's birth rank (i.e., firstborn vs. not firstborn child); (iv) the presence of brothers/sisters; (v) the region of residence (i.e., large urban/urban vs. rural/small towns); (vi) maternal employment status (i.e., employed vs. unemployed); (vii) child's size at birth; (viii) children's feeding patterns from birth to 6 months of age (i.e., breast-feeding, use of formula, and age when formula was first initiated); (ix) maternal medical history of gestational diabetes; (x) maternal active and passive smoking patterns during pregnancy; and (xi) time spent by mother and father, separately, with their children on a usual weekday and at a usual weekend.

Statistical analysis

Normality of distribution was evaluated through the Shapiro–Wilk test. Continuous variables are presented as mean s.d. and categorical variables are summarized as relative frequencies (%). Associations between categorical variables were tested by the use of contingency tables and the calculation of ²-tests without the correction of continuity. Associations between continuous variables and the time that children spent watching TV (i.e., <2 vs. 2 h/day) were evaluated through Student's t-test or Mann–Whitney when continuous variables were normally distributed or skewed, respectively.

Logistic regression analysis was performed in order to determine whether children's TV viewing time 2 h/day is independently associated with the probability of being obese. Four different models were developed to examine whether children's physical activity (i.e., L-VPA) or children's total energy intake confound the association between children's TV viewing time and the prevalence of obese. In the first model, children's TV viewing time was entered as the main independent variable and several socio-demographic and other characteristics (i.e., child's age, gender, maternal educational status (years), maternal smoking during pregnancy, child's size at birth, parental BMI, child's growth at the first 6 months, and region of residence) were also entered as potential confounders. In addition to variables considered to be established confounders, those found to be significantly associated with children's TV viewing time or children's obesity were used as potential confounders, too. Moreover, all possible two-way interaction terms were entered to examine whether the potential confounders modify the association between TV watching time and obesity. The significance of interaction terms was examined comparing the likelihood of model without interaction terms with the likelihood of model including interaction terms through the likelihood-ratio test. In the other three models, children's physical activity status (i.e., L-VPA), children's total energy intake and both physical activity and energy intake were entered as additional independent variables, respectively. All the above-mentioned analyses were performed for children aged <3 years and those aged 3–5 years, separately, as it is reasonable that the former have lower physical activity and watch TV less than the latter. All models were tested for goodness of fit by the use of the Hosmer–Lemeshow statistic (33).

A probability value of 5% is considered as statistically significant. All statistical calculations were performed on the SPSS version 14.0 software (SPSS, Chicago, IL).

Results

The overall mean (s.d.) of children's TV viewing time was 1.32 (1.12) h/day. On a usual weekday, the mean (s.d.) of children's TV watching time was 1.27 (1.13) h/day whereas at a usual weekend, this time was 2.88 (2.68) h. The vast majority of children spent <2 h/day watching TV (74.0%). More specifically, the TV viewing time was <1 h/day for 41.5% of children, 1–2 h/day for 32.5% of children, 2–3 h/day for 17.3%, 3–4 h/day for 5.6%, and 4 h/day for 3.1% of children. Figure 1 illustrates children's TV viewing time distribution by age (i.e., for children aged <3 and those aged 3–5 years, respectively). It was observed that the percentage of children watching TV <1 h/day was significantly higher among children aged <3 years compared to those aged 3–5 years (P < 0.001).

Figure 1.

Children's television viewing time by preschoolers' age in Greece: the GENESIS study (n = 2,241). GENESIS, Growth, Exercise and Nutrition Epidemiological Study in preSchoolers.

Full figure and legend (35K)

Table 1 presents children's and parents' characteristics by children's TV viewing time per day. It was observed that children who watched TV 2 h/day were older (P < 0.001), more likely to have low physical activity (i.e., L-VPA <3 h/week) (P = 0.017), to live in large urban/urban regions (P < 0.001), to have higher total energy intake (P < 0.001), and to have low-educated mothers (P < 0.001) compared to children who spent <2 h/day in front of the screen. Moreover, the parental TV viewing time was longer among children who watched TV 2 h/day compared to those who spent <2 h/day in front of TV (P < 0.001).

Table 1 - Socio-demographic and lifestyle characteristics of Greek preschoolers and their parents by children's daily TV viewing exposure: the GENESIS study (N = 2,241).

Full table (95K)

Overall, 65.2% of participants were normal weight, 17.2% were overweight, and the rest 17.6% were obese. Table 2 presents children's BMI status according to their TV viewing time. It was detected that the prevalence of obesity was significantly higher among children that spent 2 h/day in front of TV compared to those that watched TV <2 h/day (P = 0.003). Stratified analysis by gender revealed that the prevalence of overweight and obesity was significantly higher in girls watching TV 2 h/day (P = 0.002) (Table 2).

Table 2 - The prevalence of normal weight, overweight, and obese among preschoolers from Greece by their TV viewing time: the GENESIS study (N = 2,241).

Full table (29K)

Stratified analysis by age (i.e., children aged <3 years and those aged 3–5 years) revealed that TV viewing time was significantly associated with the likelihood of being obese only among children aged 3–5 years. In particular, it was found that children watching TV 2 h/day were almost 30% more likely to be obese compared to those watching TV <2 h/day (unadjusted OR: 1.30, 95% CI: 1.01–1.70, P = 0.049). Moreover, treating TV viewing time as a continuous variable, it was detected that one additional hour per day of TV viewing is associated with 12% higher probability of being obese (OR: 1.12, 95% CI: 1.03–1.26, P = 0.042). Children's TV viewing time (those aged 3–5 years) remained significantly associated with the likelihood of being obese even after controlling for potential confounders, such as children's gender, region of residence, parental BMI status, maternal educational status, birth weight for gestational age, weight gain for the first 6 months, parental TV viewing time, and mother's smoking habits during pregnancy (P = 0.032, Model 1, Table 3). Moreover, entering all possible two-way interaction terms for each one of the potential confounders with TV viewing time, it was found that no factor modifies the relationship between TV viewing time and the prevalence of obesity.

Table 3 - The association of child's TV viewing time with the likelihood of being obese after controlling for potential confounders among children aged 3–5 years from Greece.

Full table (97K)

Further adjusting for physical activity status showed that TV viewing time remained significantly related to the likelihood of being obese (P = 0.048, Model 2). In this model, physical activity was also inversely associated with the probability of being obese (P = 0.014). However, when total energy intake was added as a potential confounder in the Model 1, the association between children's TV viewing time and the likelihood of being obese was no longer statistically significant (P = 0.115, Model 3). Finally, further adjusting for both children's physical activity status and their total energy intake showed that despite the fact that TV viewing time did not remain significantly associated with the probability of being obese (P = 0.184), physical activity status did (P = 0.002, Model 4).

Finally, although no significant interaction was detected between total energy intake, physical activity status, and TV viewing time, stratified analyses by these two factors were performed in order to further examine the association between TV viewing time and the probability of being obese. In particular, children aged 3–5 years were divided into those with total energy intake higher and lower than the median of population and into those with L-VPA <3 and those with L-VPA 3 h/day, respectively. As concerns stratified analysis by energy intake, it was detected that TV viewing time was not significantly associated with the likelihood of being obese (P = 0.249 and P = 0.339 for children consuming total energy intake lower and higher than the median, respectively) in both groups. On the contrary, physical activity status was significantly related to the probability of being obese (P = 0.028 and P = 0.047 for children consuming total energy intake lower and higher than the median, respectively). Moreover, as concerns stratified analysis by physical activity status, it was observed that TV viewing time was significantly correlated with the likelihood of being obese only among children with L-VPA <3 h/day (P = 0.047). However, this association was no longer statistically significant after controlling for total energy intake (P for the effect of TV viewing time = 0.135).

Discussion

In the present study, the association between TV viewing time and the probability of being obese was evaluated among Greek preschoolers. To the best of our knowledge, this is the first study that evaluates this association taking into account not only several socio-demographic characteristics and characteristics related to the quality of home environment but also children's physical activity and their total energy intake (17,19,20). The current results indicate that both TV viewing time and physical activity status are significantly related to the prevalence of being obese after controlling for potential confounders among children aged 3–5 years. However, only children's physical activity status remained significantly associated with the probability of being obese after further controlling for total energy intake.

The mean daily TV exposure of the participants was 1.32 h/day. Moreover, 26% of the participants exceeded the limit of 2 h/day. These results are consistent with those reported from other studies. For instance, Mendoza et al. reported that 30.8% of US preschoolers aged 2–5 years exceeded the American Academy of Pediatrics recommendations to limit media viewing time to <2 h/day (17). Dannison and his colleagues found that the percentage of preschoolers watching TV 2 h/day ranged between 25.6% and 57.1% for children aged 1–4 years, respectively (20).

In addition, the current findings suggest that the prevalence of obesity was significantly higher among children spending 2 h/day compared to those watching TV <2 h/day even after adjusting for other factors that may be associated with childhood obesity. These results are in agreement with those proposed from previous works conducted among preschoolers (17,19,20), although different potential confounders were used in these studies.

Investigators have proposed that more than one mechanisms, such as reduced physical activity or increased total energy intake during TV watching, may account for the effect of TV exposure on childhood obesity risk (23). However, previous works conducted among adults, school-aged and preschool-aged children suggested that TV viewing time is related to the risk of being obese independently of physical activity status (25,26,27,34). In good keeping with these findings, the present results report that both preschoolers' physical activity status and TV viewing time were significantly associated with the likelihood of being obese in a multiple logistic regression model including these two factors and further potential confounders.

An intervention study conducted to investigate the effect of reducing TV viewing time on children's BMI revealed that changes in TV viewing time were related to changes in total energy intake but not to changes in physical activity of children (16). This finding provides significant evidence that the influence of TV viewing time on the risk of being obese may be attributed to the increased energy intake during TV watching (35,36). Several studies have demonstrated that TV viewing time is positively and strongly associated with total energy intake. Epidemiological studies carried out in children and adolescents have revealed that increased TV viewing is strongly correlated with increased consumption of snacks (37,38), that is, increased consumption of high-fat foods, fast foods, and soft drinks (39), and strongly associated with reduced intake of fruits and vegetables (40). The findings of the current study may provide further support of the hypothesis that increased total energy intake accounts for the relationship between TV viewing time and the prevalence of obesity, as the significant effect of TV watching time was no longer observed after further controlling for energy intake.

The strengths of the current study are the large sample size and the fact that data regarding children's physical activity, their diet as well as parental BMI status and parental TV viewing time (i.e., the major limitations of previous corresponding works) have been included in the present analyses (17,19,20). However, the current work has several limitations. First and foremost, a cause–effect relationship cannot be identified due to the cross-sectional design of the study. Moreover, children's TV viewing time was obtained by parental report and the validity of their report was not examined. However, significant correlation has been reported between parental reports of children's TV viewing time and TV diaries (41). Moreover, it is difficult to assess energy intake accurately. Therefore, it is possible that there is a bias in the estimation of the association between diet and obesity. However, several aspects of this study suggest enhanced reliability of the data (i.e., combination of weighed food records and 24-h recall or three days, highly qualified interviewers who checked the records for any misrecorded or missing information).

In conclusion, the present work revealed that a substantial percent (almost 26%) of Greek preschoolers exceed the limit of 2 h/day of TV viewing proposed by American Academy of Pediatrics. Moreover, it was found that the effect of TV viewing time on childhood obesity is independent of children's physical activity status. However, the association between TV viewing time and the prevalence of overweight was no longer statistically significant after adjusting for the total energy intake; thus indicating that this association could be attributed to the increased total energy intake during TV watching. Therefore, public health policy makers and health professionals should focus on increasing parental awareness on the potential consequences of both increased TV viewing time as well as children's snacking habits in front of the TV set. Further studies are needed to confirm that increased total energy intake is the potential mechanism linking TV viewing to overweight and obesity.

Disclosure

The GENESIS study was supported with a research grant from Friesland Foods Hellas. Y.M. works as a scientific consultant for Friesland Foods Hellas. None of the other authors had any personal or financial conflict of interest. The study sponsor had no interference in the study design, data collection, or writing of the manuscript.

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Acknowledgments

We thank Evdokia Oikonomou, Vivian Detopoulou, Christine Kortsalioudaki, Elina Ioannou, Margarita Bartsota, Thodoris Liarigkovinos, Manolis Birbilis, Elina Dimitropoulou, Nikoleta Vidra, Theodoros Athanasoulis, Pari Christofidou, Lilia Charila, Sofia Tzitzirika, and Christos Vassilopoulos for their contribution to the completion of the study.