The goal of this study was to evaluate for links between the amount of TV and other screen use and weight status in kindergarteners. We analyzed data from a large cohort of children followed longitudinally as part of the Early Childhood Longitudinal Survey‐Kindergarten Cohort 2011 (ECLS‐K:2011), for whom we evaluated relationships between both TV viewing and computer and video‐game use on BMI‐ z ‐score from kindergarten to first grade. We hypothesized that even relatively small amounts of TV viewing (as little as 1 h daily) would be linked to higher weight status and higher weight gain than children viewing shorter duration of TV over time in this age range.

For kindergarten and school‐aged children, the American Academy of Pediatrics (AAP) currently recommends limiting screen time to no more than 2 h daily to avoid obesity ( 6 - 8 ). Numerous studies have supported positive associations between TV viewing and obesity as defined by body mass index (BMI) or body fat ( 9 - 18 ). The majority of these focused on a threshold of 2 h to identify excessive TV viewing ( 15 - 17 ). In one study, school‐age children who watched ≥2 h daily had 1.58 times the odds of being overweight compared with those who watched no TV ( 17 ). However, other studies reported weak or absent associations between TV and obesity ( 14 , 16 , 19 ). Studies that found associations were commonly from wide age ranges ( 9 - 11 , 13 , 18 ), were from smaller cohorts that were not nationally representative ( 11 , 12 , 18 ), or did not evaluate computer or video‐game use in their analysis ( 9 - 11 , 17 , 18 ). In addition to these limitations, it remains unclear whether shorter duration of TV (e.g., 1 h daily) is associated with higher odds of overweight or obesity in young children. An increased risk of obesity from shorter viewing amounts could have implications for updating the AAP recommendations for young children, particularly given the frequent persistence of unhealthy weight gain from early childhood ( 20 ).

Childhood obesity has more than doubled in the past 30 years to the point that 17% of children are obese and 14% overweight ( 1 , 2 ). This high prevalence is alarming due to both short‐ and long‐term effects of childhood obesity, including cardiovascular, bone and joint, sleep, and psychological problems during childhood ( 3 ) and higher risk of cancer, type 2 diabetes, and heart disease in later life ( 4 ). Reversing this trend may require lifestyle changes within communities and families in practices including television viewing and other screen time exposures ( 5 ).

All analyses were run using SAS 9.4 (SAS Institute, Research Triangle, NC). We used linear and logistic regression to examine relationships between TV and weight‐based outcomes. We adjusted for sex, race/ethnicity, highest parental education, and household income. SAS survey procedures were used to account for the complex sampling design and potential nonresponse in the ECLS‐B and appropriate weights provided by the NCES were used to give population estimates. These weights adjust for the complex sampling design and allows for the data to be representative of all children in kindergarten in the US in 2011. For linear regressions, TV viewing was assessed as a linear variable and as categories (e.g., 0‐<1, 1‐<2, and ≥2 h) when calculating adjusted means of BMI‐ z ‐score by TV‐viewing time. We chose to use BMI‐ z ‐scores because these are standardized to be able to calculate mean values of weight (and in particular, excess weight) in a group of children. To permit comparison between kindergarten and first grade, we assessed TV viewing as the number of hours on weekdays. For the kindergarten wave, we also assessed total hours over the entire week. For computer use, we assessed total times used weekly in kindergarten, and for video‐game use, we assessed total hours played daily. In cross‐sectional linear regressions, our outcome was BMI‐ z ‐score. In longitudinal linear regressions, our outcome was change in BMI‐ z ‐score (between spring of kindergarten and spring of first grade), which we examined by baseline TV‐viewing category to avoid the potential for reverse causation. For longitudinal analyses, we restricted our analysis to those who had complete data at both the kindergarten and first‐grade waves. In logistic regressions, we evaluated odds of overweight and obesity separately for TV ≥2 h daily (vs. <2 h) and ≥1 h daily (vs. <1 h). In longitudinal logistic regression, we assessed for odds of becoming overweight or obese between kindergarten and first grade by baseline TV‐viewing status, restricting the sample to children who at baseline were not overweight or obese, respectively.

In first grade, parents were again asked regarding weekday (but not weekend) TV viewing, which was categorized as above. In first grade only, parents were asked: “On any given weekday, how much time does your child spend playing video games? Please do not include time spent on educational activities.” This use was categorized as 0, <1, 1‐<2, and ≥2 h daily.

The amount of weekly TV was calculated from parental responses to several questions in the computer‐assisted interview. In kindergarten, the parents were asked to respond to the questions: “On any given weekday, how many hours (and minutes) of TV, videotapes and DVD's does your child watch at home?” Viewing time estimates were provided separately for morning, afternoon and evening. Based on parental responses, daily weekday TV viewing was then categorized as the number of hours of TV by weekday (e.g., categories 0‐<1, 1‐<2, 2‐<3 and ≥3 h). In kindergarten only, parents were also asked “On average, about how many hours a day does your child watch television or videos in your household each Saturday and Sunday?” This allowed calculation of total weekly viewing time by taking weekday and weekend estimates into account. In evaluating additional screen time, in kindergarten parents were asked, “How many times a week does your child use the computer?”

Trained field staff assessed children in their schools and collected information from parents. The majority of parent interviews were conducted by telephone, with additional interviews conducted in person for parents without telephones or who preferred an in‐person interview. These interviews followed a validated computer‐assisted interview template from which parents were presented with multiple‐choice questions ( 21 ). During these parent interviews, parents identified the gender and race/ethnicity of their child. Race/ethnicity was grouped into five categories: white, African‐American, Asian, Hispanic, and other. Parents were also asked their highest education level achieved and the family's income.

The ECLS‐K:2011 is an ongoing study sponsored by the National Center for Education Statistics (NCES) of the US Department of Education ( https://nces.ed.gov/ecls/kindergarten2011.asp ). The NCES ethics review board approved the study. The children in ECLS‐K:2011 comprise a nationally‐representative sample using multi‐stage probability sampling, with primary sampling units consisting of counties or groups of counties, second‐stage units consisting of schools and third‐stage sampling consisting of children attending kindergarten within schools. Parents gave informed consent. The study enrolled 18,174 students in kindergarten in the fall of 2010 ( 21 ). Follow‐up evaluations in kindergarten were performed in the spring of 2011 and in first grade in the fall of 2011 and the spring of 2012.

For further longitudinal analysis, the lower portion of Table 3 shows the odds of becoming overweight and obese among children who were not in these categories at baseline. Children who were normal weight in kindergarten and watched 1‐<2 and ≥2 h of TV daily (compared with those watching <1 h) had an adjusted odds of becoming overweight of 1.49 and 1.81, respectively. In the case of those watching 1‐<2 h, this association was not significant following adjustment for baseline BMI‐ z ‐score, suggesting that the increased odds of becoming overweight was due largely to a higher number of children in that viewing group who were just below the cut‐off for overweight. Similarly, among children who were not obese in kindergarten, those watching 1‐<2 and ≥2 h of TV daily (compared with those watching <1 h) had an odds of becoming overweight of 1.65 and 1.93, respectively—an association which was not significant following adjustment for BMI‐ z ‐score in kindergarten.

Adjusted mean BMI‐ z scores by TV‐viewing category between kindergarten and first grade . BMI‐ z scores were higher among those watching 1‐<2 h and ≥2 h of TV daily compared with those watching <1 h daily. The change in BMI‐ z score was less favorable for both higher TV‐viewing categories compared with those watching <1 h daily. All values adjusted for sex, race/ethnicity, household income, and parental education. P ‐values for BMI‐ z score: * P < 0.0001 compared with <1 h daily group, # P < 0.05 compared with 1‐2 h daily; P ‐value for change in BMI‐ z score kindergarten‐first grade ϕ P < 0.05 compared with <1 h daily.

Figure 1 displays mean values of BMI‐ z ‐score by weekday TV‐viewing category from kindergarten and first grade, following adjustment for sex, race, parental income, and parental education. At both time points, children watching <1 h of TV daily had lower BMI‐ z ‐scores than those watching more ( P < 0.001). This difference between children watching <1 h and those watching 1‐<2 and ≥2 h was a BMI‐ z ‐score of 0.16 and 0.2, respectively, which at these ages corresponds to differences in BMI of ∼0.28 and 0.3 and differences in weight of ∼0.28 and 0.35 kg, respectively. Children in kindergarten watching 1‐<2 and ≥2 h daily experienced a less favorable change in BMI‐ z ‐score over time as compared with those watching <1 h daily. Results were similar when first‐grade viewing time was considered (data not shown).

Regarding computer and video‐game use, the proportion of the proportion of children with overweight and obesity is shown in Supporting Information Table S2. Using linear regression there was no relationship between either computer use or video‐game use on BMI‐ z ‐scores at kindergarten and first grade, respectively (data not shown).

Table 3 compares the odds ratios for being overweight or obese by the amount of TV viewing. Compared with children who watched <1 h per day, children who watched 1‐<2 and ≥2 h per day in kindergarten had an odds ratio for overweight of 1.43 and 1.58, respectively, and for obesity of 1.47 and 1.76. The results were similar for children in first grade (see Table 3 ).

Supporting Information Table S3 displays linear regression results of TV‐viewing time on BMI‐ z ‐score, with adjustment for sex, race, parental income, and parental education. On cross‐sectional analysis at both kindergarten and first grade, TV‐viewing time was positively associated with BMI‐ z ‐score, even following adjustment for all these variables (β estimate 0.03, P < 0.0001). This is to say that for every 1‐h increase in weekday TV viewing, the BMI‐ z ‐score increased by 0.03.

Table 2 shows TV‐viewing time by weight category. In this recent cohort, there was a high proportion of both overweight and obesity overall, with 29% of children overweight or obese in kindergarten and 29.7% in first grade. When assessed by TV‐viewing category, higher weekday TV‐viewing categories were associated with higher proportions of overweight or obesity.

Participant characteristics by the amount of weekday TV are shown in Table 1 . There was no difference in mean hours of TV‐viewing time between genders. TV‐viewing differed by race/ethnicity; African‐American children were reported to watch the most TV, followed by Hispanics, whites, and Asians respectively. TV viewing time differed by parental income and parental education with both higher income levels and higher parental education associated with less TV. Frequency of computer use in kindergarten and video‐game use in first grade are shown in Supporting Information Table S2. Computer use was not significantly associated with TV‐viewing time, while video‐game use in first grade was positively associated with TV ( P < 0.001).

Of the 18,174 children originally enrolled in ECLS‐K(2011), we analyzed data from a total of 14,645 participants who had complete data regarding BMI‐ z ‐score and weekday TV‐viewing time at kindergarten and/or first grade, including cross‐sectional data from 12,389 participants in kindergarten and 11,765 participants in first grade; 10,853 of these children had data from both time points for inclusion in longitudinal analysis. An additional 3,529 children lacked pertinent data, including data regarding BMI‐ z ‐score or complete parental response to questions regarding TV. Compared to children included in the analysis, those who did not have adequate data for analysis were more likely to be male, more likely to have race/ethnicity identified as African‐American or Hispanic or “Other,” had lower family income, and had lower family education (all P < 0.0001; Supporting Information Table S1). Supporting Information Figure S1 displays percent of children by 30‐min TV‐viewing categories at kindergarten and first grade. On average, parents reported that their kindergarten children watched 2.0 ± 1.4 h of TV each weekday and 14.7 ± 8.9 h each week (including weekend viewing). Overall in kindergarten, 33% of children were reported to watch more than the 2‐h maximum recommended by the AAP ( 7 ). In first grade, children watched on average 1.6 ± 1.4 h of TV each weekday, with 15.7% of children watching >2 h daily; when video‐game use was combined with TV viewing at this age, 36.7% of children had >2 h of screen time daily. The amount of weekday TV decreased from kindergarten to first grade ( P < 0.001), though TV‐viewing time was highly correlated between evaluations ( P < 0.001).

Discussion

This study determined that weekday TV‐viewing time is associated with obesity in a dose‐response relationship, but not in a strictly linear fashion. Obesity increases with screen time per day, but the jump in BMI‐z‐score was greatest between those watching <1 and ≥1 h of TV daily. These data are novel in showing that children watching as little as 1 h of TV daily (compared with those watching <1 h daily) had multiple concerning findings: 1. higher odds (1.50‐1.73) of current overweight and obesity in both kindergarten and first grade, 2. a less favorable change in BMI‐z‐score between the two time points, and 3. higher odds (1.39, 1.86) of becoming overweight and obese over time. These findings are suggestive that the negative associations of TV viewing—including less physical activity (24, 25) and greater exposure to food‐related advertisements (7, 26)—may already be influential with relatively modest TV exposure. These data have strong implications for the recommendations from the AAP (7, 8). Given the extent of these findings in a large, recent, nationally representative cohort of early school‐aged children, it may be both reasonable and beneficial for the standard recommendation to be lowered from ≤2 to ≤1 h daily for children in this age range.

These data update other estimates of TV viewing in childhood with recent, nationally representative data for kindergarteners. Other recent studies have used a wider age range. Fakhouri et al. reported that 41% of 6‐8 year‐old children exceeded AAP recommendations for screen time (13). We found that weekday TV‐viewing time exceeded recommendations for 33% of kindergarten; this proportion decreased between kindergarten and first grade, though when total TV and video‐game use was considered together at first grade 36% exceeded screen time recommendations. It is unclear whether the lower proportion of those exceeding recommendations reflects a purposeful shift toward less viewing time or simply a decrease in viewing time as the number of hours in school increases, as occurs for some kindergarteners. In addition, it should be acknowledged that both our study and the Fakhouri study relied on parent‐reported TV, and the exact amount of TV may be greater than reported here. Prior studies have demonstrated reasonable accuracy of parental report, with some potential for underreporting of TV‐viewing time compared to more objective measures (24, 27, 28).

The majority of studies on screen time in the past have focused solely on TV viewing (14, 18), which is often cited as a weakness given the current plethora of other screens available. For this reason, we included additional assessment for computer use in kindergarten and video‐game use in first grade. We found that in contrast to the relationships between TV and weight status, neither computer use nor video games were not significantly associated with obesity. Some (29) but not all (30) prior studies have demonstrated links between video‐game use and weight status; some of the variability of these associations are related to whether the video games used are active or not (30, 31)—data which we lacked in our study. Computer use has been linked to higher adiposity in preschool children (32). The weight‐related relevance of more recent screens such as iPads, smartphones and hand‐held video games is unclear and was not assessed in our study. Our lack of findings relating other screen activities to obesity may have been because kindergarten and first‐grade students still spend much more time watching TV than they do on the computer and video games, with only 34% of kindergarteners reported as using computers more often than 1‐2 times per week and only 28% of first‐graders reported as using video games ≥1 h per day. We noted an association between video‐game use (but not computer use) and TV viewing, suggesting that a high degree of video‐game use may still be a red flag clinically for children who watch excessive TV.

This study had multiple weaknesses, including the observational nature of the data and the lack of complete data on all participants. We excluded individuals without complete data, despite the fact that these participants were not apparently missing completely at random (33), with differences regarding parental education, family income, or race/ethnicity. We attempted to deal with this in part by adjusting for these factors, and the data are weighted to account for parental nonresponse to the questionnaire. Nevertheless, there remains the possibility that those missing were also more likely to exhibit differences in the relationship between TV viewing and weight status.

Multiple mechanisms have been proposed by which TV viewing is related to unhealthy weight gain. The most prominent of these is that TV displaces physical activity, causing a reduction in energy expenditure. Studies have reported significant inverse relationships between TV‐viewing time and physical activity as measured by accelerometry (24, 25). Even in the absence of displaced physical activity, early studies reported that TV viewing involves less energy expenditure than other sedentary activities (34). Excess TV is associated with shortened sleep time (35), itself linked to weight gain (36). Finally, food advertisements during TV viewing may contribute to excess calorie intake, both during TV viewing and afterward, should these advertisements lead to purchase of unhealthy food items (7, 26).

Pediatricians should choose wisely how to spend time counseling families regarding weight control. The preponderance of data linking TV and obesity supports the need to limit viewing time at young ages before habits are more deeply ingrained. We found family income, education, and racial/ethnic disparities in TV‐viewing time, underscoring the importance of providing more support to families who are in high risk groups. Parental restriction of childhood media exposure predicted lower BMI‐z‐scores and less BMI gain from age 5‐9 years (37). Additionally, pediatricians should regularly recommend against placement of a TV in a child's bedroom, a further risk factor for unhealthy weight gain (38). Nevertheless, interventions targeting reduction in TV‐viewing time have had mixed results. Robinson assessed the effectiveness of a 6‐month curriculum to reduce TV among third‐ to fourth‐graders, demonstrating slower increases in BMI and waist circumference (39). However, one meta‐analysis of such interventions reported only nonsignificant reductions in BMI (mean change −0.1 kg/m2, P = 0.32) and time watching TV (−0.9 h/wk, P = 0.49), potentially emphasizing the difficulty in changing children's longstanding habits (40). More research is needed on ways to reduce screen time exposure in childhood, including more recent screens such as iPads, smartphones, and hand‐held video games.