Abstract
The association between screen time (ST), including that for smartphones, and overweight/obesity in children was examined separately for boys and girls, considering the influence of lifestyle factors. A cross-sectional study was conducted in 2,242 Japanese children (1,278 girls) aged 10–14 years. Overweight/obesity was defined by the International Obesity Task Force. Logistic regression analysis showed that only for girls, total ST (≥4 h), smartphone ST (≥3 h), and non-smartphone ST (≥2 h) were all independently and significantly associated with overweight/obesity compared to <2 h total ST, non-use of smartphones, and <1 h non-smartphone ST. Thus, smartphone ST ≥3 h and non-smartphone ST ≥2 h were additively associated with overweight/obesity in girls only. Girls having smartphone ST ≥3 h and non-smartphone ST ≥2 h were 6.79 times (95% CI: 3.11–14.81) more likely to have overweight/obesity than girls with less usage of both. In girls, when total ST was ≥4 < 5 h or smartphone ST was ≥2 h, the significant association with overweight/obesity disappeared when physical activity was ≥60 min/day and sleep time was ≥8.5 h. In addition, none of these associations was significant in boys. In Japanese girls, smartphone ST, non-smartphone ST, and total ST were all significantly associated with overweight/obesity. To avoid overweight/obesity, it is suggested to keep smartphone ST, non-smartphone ST, and total ST to <3 h, <2 h, and <4 h, respectively, and to engage in sufficient physical activity and sleep time.
CHILDHOOD OBESITY mostly transitions into adulthood and leads to non-communicable diseases such as type 2 diabetes mellitus and hypertension [1]. Thus, prevention is important.
It has long been problematic that increased TV viewing promotes obesity in children [2, 3]. In recent years, with the rapid spread of the Internet and use of personal computers (PCs) and cell phones, the increased time spent on PCs [4] and mobile phones also has been associated with overweight and obesity in children [5]. Time spent using TV and electronic devices is collectively referred to as screen time (ST) [6]. A meta-analysis indicated that increased ST was positively associated with overweight and obesity in children [4]. However, it has been pointed out that ST discussed in previous studies mostly referred to that for TV, PCs, and electronic games and that there is a paucity of studies on smartphone ST [4, 7].The smartphone has exploded in popularity over the past few years. A positive correlation between the frequency of smartphone daily use and body mass index (BMI) was reported [8]. Also, smartphone addiction, defined as smartphone use that causes physical and mental problems, among middle school students was associated with obesity [9]. Although the degree of use of mobile screen devices, including smartphones, was associated with obesity in children [10, 11], few studies have quantitatively examined the effects of smartphone ST on overweight/obesity nor have considered the effects of lifestyle factors such as sleep duration and physical activity in relation to smartphone ST.
Regarding the influence of lifestyle, it was reported that ST above a certain level was significantly associated with overweight and obesity even when physical activity took place for at least 60 minutes on the 7 days preceding data collection [12]. ST has been associated with shorter sleep duration [13], and insufficient sleep duration was associated with obesity [14]. In addition, portable electronic devices such as smartphones were shown to have a stronger effect on sleep time than nonportable devices [15].
Previous studies examining ST and pediatric overweight/obesity by sex reported that TV viewing was associated with overweight in both boys and girls, but electronic games were not associated with overweight/obesity in boys [16, 17], suggesting that sex differences may exist. In addition, problematic smartphone use was indicated to be more likely in females than in males with the risk being particularly high in adolescents [18].
In this study, total ST was divided into smartphone ST and non-smartphone ST, and the relationship between each and overweight/obesity was examined separately according to sex considering lifestyle factors such as physical activity and sleep time in Japanese adolescents.
Materials and Methods
Participants
Study participants were 3,776 people (1,848 girls) aged 10–14 years who participated in part of the non-communicable disease prevention project in Agano and Sanjo cities, Niigata Prefecture, in 2018–2019. Written informed consent was obtained from all parents. The study was conducted according to the Japanese Government’s Ethical Guidelines for Medical and Health Research Involving Human Subjects and in accordance with the Declaration of Helsinki. The ethics committee of the Niigata University faculty of medicine also reviewed the protocol (No. 2015-1864).
Determinations of overweight/obesity, ST, and lifestyle
Height and weight were determined using data from a health checkup conducted at schools. Overweight/obesity was diagnosed according to the International Obesity Task Force (IOTF) [19] definition based on BMI.
Information on smartphone ST and non-smartphone ST per day on weekdays and weekends, respectively, was obtained for the following categories: none, <1 h, ≥1 < 2 h, ≥2 < 3 h, ≥3 < 4 h, ≥4 < 5, and ≥5 h. The middle value of each category was used to calculate smartphone ST = (weekday smartphone ST × 5 + weekend smartphone ST × 2)/7, non-smartphone ST = (weekday non-smartphone ST × 5 + weekend non-smartphone ST × 2)/7, and total ST = smartphone ST + non-smartphone ST. Values were also calculated for non-smartphone ST in the same way.
Lifestyle habits, such as sleep time and physical activity, were surveyed using a self-reported questionnaire distributed to the participants or their guardians. We asked about bedtimes and wake-up times, respectively, for weekdays and weekends separately and used the following formula: sleep time = (weekday sleep time × 5 + weekend sleep time × 2)/7. Physical activity was calculated as the metabolic equivalent of task (MET) using the Japanese version of the International Physical Activity Questionnaire (IPAQ) for early adolescents [20]. Energy intake was calculated by a brief-type self-administered diet history questionnaire (BDHQ) 15y [21]. Both have been validated in Japanese children.
Statistical analysis
Total ST, smartphone ST, and non-smartphone ST were each divided into five groups according to hours of ST usage for both boys and girls. Then, continuous variables and categorical variables were examined using the Jonckheere-Terpstra trend test and the Cochran-Armitage trend test, respectively. Furthermore, the associations between total ST, smartphone ST, and non-smartphone ST, respectively, and overweight/obesity were examined by logistic regression analysis.
In addition, each sex was divided into four groups based on whether they did or did not meet the criteria for smartphone ST ≥3 h and non-smartphone ST ≥2 h, respectively. Overweight/obesity was examined when criteria for one or the other were met by logistic regression analysis.
Next, when girls were divided into three groups according to total ST (<4, ≥4 < 5, ≥5 h) or smartphone ST (non-use, >0 < 2, ≥2 h), we examined how the combination of physical activity and sleep time was related to the prevalence of overweight/obesity. Logistic regression analysis was performed to examine how the combination of physical activity and sleep time was associated with the prevalence of overweight/obesity, with reference to the group with the shortest ST and the highest physical activity and sleep time values. Physical activity was based on the Japanese Ministry of Health, Labour and Welfare’s recommended 23 METs h/w (60 min/day) for adults [22], and sleep time was based on the minimum recommended by the American Academy of Sleep Medicine [23] (9 h for 6- to 12-year-old children and 8 h for 13- to 18-year-old adolescents), with a cutoff value of 8.5 h, the median of those values, based on the participants’ ages (10–14 y) in this study.
Analyses were performed using SPSS (version 19.0, Chicago, IL, USA). Statistical significance was considered for p < 0.05.
Results
Study participants
Excluded from the analysis were participants who had missing physical measurement data (n = 77), missing data on the lifestyle questionnaire (n = 1,187), no dietary survey (n = 1), and possible under- or over-reporting of energy intake on the dietary survey (energy intake <600 kcal or ≥4,000 kcal) (n = 269). After these exclusions, 2,242 people (1,278 girls, mean age 11.8 years) were analyzed. The proportion of study participants with overweight/obesity was 14.5% of boys and 9.9% of girls. The median ST for smartphones, non-smartphones, and the combined total ST were in the ≥2 < 3 h, non-user, and ≥3 < 4 h categories, respectively, with only slight differences between sexes. The overall smartphone usage rate was 30.1% (25.3% for boys and 34.5% for girls). In the total ST ≥5 h group, the percentage of those using smartphones was the highest for both sexes compared to the other groups based on ST (Supplemental Fig. 1).
Table 1 shows the characteristics of boys and girls in relation to total ST, smartphone ST, and non-smartphone ST for five categories of hours of ST. As total ST increased, there was a significantly decreased trend in physical activity and sleep time for both boys and girls. Energy intake was not significantly associated with ST in either sex. There was no clinically meaningful difference in protein, fat, and carbohydrate energy intake ratios.
Table 1
Characteristics of study participants by five groups according to total ST (A), smartphone ST (B), and non-smartphone ST (C). Data are presented as mean (SD) or numbers (percentage). Bold font indicates statistical significance.
| (A) Total ST (hours) |
Total |
<2 |
≥2 < 3 |
≥3 < 4 |
≥4 < 5 |
≥5 |
p for trend |
| Boys n |
1,164 |
160 |
239 |
295 |
302 |
168 |
|
| Age (years) |
11.8 (1.2) |
11.7 (1.2) |
11.7 (1.1) |
11.6 (1.2) |
11.7 (1.2) |
12.5 (1.2) |
<0.001 |
| Overweight/obesity (%) |
169 (14.5) |
17 (10.6) |
31 (13) |
51 (17.3) |
42 (13.9) |
28 (16.7) |
0.126 |
| Physical activity (METs h/w) |
68 (79) |
82 (78) |
67 (85) |
64 (72) |
56 (62) |
84 (105) |
0.035 |
| Sleep time |
8:33 (0:52) |
8:37 (0:43) |
8:37 (0:53) |
8:35 (0:34) |
8:32 (0:51) |
8:21 (1:18) |
<0.001 |
| Energy intake (kcal/day) |
2,274 (594) |
2,358 (600) |
2,261 (579) |
2,261 (557) |
2,277 (594) |
2,230 (670) |
0.188 |
| Protein (%energy) |
14.1 (2.3) |
14.7 (2.3) |
14.4 (2.4) |
14.1 (2.3) |
13.9 (2.1) |
13.8 (2.4) |
<0.001 |
| Fat (%energy) |
29.9 (5.5) |
29.4 (5.9) |
29.5 (5.6) |
30.2 (5.6) |
30.2 (5.1) |
29.6 (5.3) |
0.288 |
| Carbohydrate (%energy) |
56 (6.8) |
55.8 (7.3) |
56.1 (7.2) |
55.7 (7/0) |
55.9 (6.2) |
56.6 (6.6) |
0.603 |
| Girls n |
1,278 |
230 |
291 |
282 |
268 |
207 |
|
| Age (years) |
11.9 (1.3) |
11.5 (1.2) |
11.7 (1.2) |
11.7 (1.2) |
11.9 (1.2) |
12.8 (1.1) |
<0.001 |
| Overweight/obesity (%) |
126 (9.9) |
12 (5.2) |
25 (8.6) |
21 (7.4) |
36 (13.4) |
32 (15.5) |
<0.001 |
| Physical activity (METs h/w) |
48 (57) |
49 (56) |
49 (53) |
44 (47) |
45 (61) |
58 (68) |
0.294 |
| Sleep time |
8:33 (0:53) |
8:44 (0:36) |
8:38 (0:39) |
8:33 (0:36) |
8:33 (1:11) |
8:12 (1:10) |
<0.001 |
| Energy intake (kcal/day) |
1,929 (538) |
1,882 (509) |
1,965 (534) |
1,935 (532) |
1,858 (515) |
2,012 (597) |
0.471 |
| Protein (%energy) |
14.3 (2.3) |
14.4 (2.1) |
14.7 (2.4) |
14.3 (2.1) |
13.9 (2.3) |
13.8 (2.4) |
<0.001 |
| Fat (%energy) |
30.9 (5.4) |
30.2 (5.3) |
31.2 (5.4) |
31.2 (5.2) |
30.9 (5.2) |
30.8 (6) |
0.501 |
| Carbohydrate (%energy) |
54.8 (6.7) |
55.4 (6.5) |
54 (6.8) |
54.4 (6.4) |
55.2 (6.6) |
55.4 (7.3) |
0.319 |
| (B) Smartphone ST (hours) |
|
Non-user |
>0 < 1 |
≥1 < 2 |
≥2 < 3 |
≥3 |
p for trend |
| Boys n |
|
869 |
65 |
73 |
67 |
90 |
|
| Age (years) |
|
11.5 (1.1) |
12.1 (1.3) |
12.7 (1.2) |
12.7 (1.1) |
12.9 (1.1) |
<0.001 |
| Overweight/obesity (%) |
|
135 (15.5) |
7 (10.8) |
7 (9.6) |
7 (10.4) |
13 (14.4) |
0.238 |
| Physical activity (METs h/w) |
|
61 (70) |
92 (115) |
89 (103) |
95 (110) |
82 (69) |
<0.001 |
| Sleep time |
|
8:37 (0:40) |
8:21 (0:47) |
8:19 (0:42) |
8:31 (1:47) |
8:13 (1:24) |
<0.001 |
| Energy intake (kcal/day) |
|
2,256 (574) |
2,428 (668) |
2,374 (648) |
2,280 (642) |
2,251 (635) |
0.287 |
| Protein (%energy) |
|
14.2 (2.3) |
14.6 (2.1) |
14.5 (2.8) |
13.9 (2.3) |
13.5 (2.3) |
0.158 |
| Fat (%energy) |
|
30 (5.5) |
29.9 (5.8) |
30.5 (5.6) |
29.6 (5.3) |
28.6 (5.8) |
0.171 |
| Carbohydrate (%energy) |
|
55.9 (6.7) |
55.5 (7.0) |
55 (7.5) |
56.5 (6.7) |
57.9 (7.1) |
0.150 |
| Girls n |
|
837 |
98 |
111 |
102 |
135 |
|
| Age (years) |
|
11.4 (1.1) |
11.9 (1.3) |
12.6 (1.2) |
13 (0.8) |
13 (0.8) |
<0.001 |
| Overweight/obesity (%) |
|
78 (9.3) |
8 (8.6) |
7 (6.3) |
9 (8.8) |
24 (17.8) |
0.043 |
| Physical activity (METs h/w) |
|
42 (50) |
50 (46) |
65 (80) |
65 (69) |
62 (66) |
<0.001 |
| Sleep time |
|
8:41 (0:35) |
8:28 (0:45) |
8:35 (1:37) |
8:06 (0:45) |
8:03 (1:19) |
<0.001 |
| Energy intake (kcal/day) |
|
1,926 (522) |
1,872 (484) |
1,902 (540) |
1,958 (554) |
1,981 (647) |
0.936 |
| Protein (%energy) |
|
14.4 (2.2) |
14.5 (2.2) |
14.3 (2.3) |
13.8 (2.4) |
13.6 (2.2) |
0.002 |
| Fat (%energy) |
|
31.1 (5.2) |
31.5 (5.5) |
30.5 (5.5) |
29.8 (6.1) |
30.5 (6.0) |
0.048 |
| Carbohydrate (%energy) |
|
54.5 (6.5) |
53.9 (6.6) |
55.2 (7.0) |
56.4 (7.5) |
55.9 (7.1) |
0.009 |
| (C) Non-smartphone ST (hours) |
|
<1 |
≥1 < 2 |
≥2 < 3 |
≥3 < 4 |
≥4 |
p for trend |
| Boys n |
|
56 |
198 |
288 |
292 |
330 |
|
| Age (years) |
|
12.5 (1.2) |
12 (1.3) |
11.8 (1.2) |
11.7 (1.2) |
11.6 (1.2) |
<0.001 |
| Overweight/obesity (%) |
|
6 (10.7) |
21 (10.6) |
37 (12.8) |
53 (18.2) |
52 (15.8) |
0.033 |
| Physical activity (METs h/w) |
|
95 (73) |
76 (73) |
72 (90) |
66 (87) |
56 (64) |
<0.001 |
| Sleep time |
|
8:28 (1:37) |
8:36 (0:57) |
8:30 (0:41) |
8:37 (0:46) |
8:31 (0:52) |
0.425 |
| Energy intake (kcal/day) |
|
2,351 (630) |
2,346 (603) |
2,262 (597) |
2,244 (545) |
2,254 (621) |
0.108 |
| Protein (%energy) |
|
14.5 (2.5) |
14.4 (2.3) |
14.3 (2.5) |
14.1 (2.2) |
13.8 (2.2) |
0.001 |
| Fat (%energy) |
|
28.9 (6.5) |
29.5 (6.0) |
29.5 (5.5) |
30.3 (5.4) |
30.2 (5.2) |
0.017 |
| Carbohydrate (%energy) |
|
56.5 (8.1) |
56.1 (7.4) |
56.2 (7.2) |
55.7 (6.5) |
56 (6.3) |
0.465 |
| Girls n |
|
126 |
274 |
347 |
257 |
274 |
|
| Age (years) |
|
12 (1.3) |
12 (1.3) |
11.9 (1.2) |
11.6 (1.2) |
11.9 (1.2) |
0.010 |
| Overweight/obesity (%) |
|
7 (5.6) |
16 (5.8) |
38 (11) |
25 (9.7) |
40 (14.6) |
<0.001 |
| Physical activity (METs h/w) |
|
54 (55) |
52 (60) |
52 (62) |
45 (49) |
40 (55) |
<0.001 |
| Sleep time |
|
8:28 (0:46) |
8:33 (0:42) |
8:34 (1:06) |
8:36 (0:34) |
8:29 (1:00) |
0.918 |
| Energy intake (kcal/day) |
|
1,858 (566) |
1,901 (508) |
1,963 (548) |
1,963 (531) |
1,913 (545) |
0.260 |
| Protein (%energy) |
|
14.3 (2.3) |
14.2 (2.1) |
14.5 (2.4) |
14.2 (2.1) |
13.9 (2.4) |
0.058 |
| Fat (%energy) |
|
30.4 (5.7) |
30.2 (5.5) |
30.9 (5.2) |
31 (5.3) |
31.7 (5.5) |
0.001 |
| Carbohydrate (%energy) |
|
55.4 (6.9) |
55.5 (6.8) |
54.5 (6.6) |
54.8 (6.4) |
54.3 (7.0) |
0.044 |
Table 2 shows the odds ratios (ORs) for overweight/obesity in the remaining categories when the lowest total ST, smartphone ST, and non-smartphone ST categories were used as references. In boys, there was no significant association with overweight/obesity in any ST category. On the other hand, in girls, total ST ≥4 < 5 h, smartphone ST ≥3 < 4 h, and non-smartphone ST ≥2 h all showed a significant 3-fold increase in ORs. Both smartphone ST and non-smartphone ST were found to be significant when adjusted for each other, and both were found to be independently and significantly associated with overweight/obesity. Furthermore, the combined examination of smartphone ST and non-smartphone ST was additively associated with overweight/obesity in only girls (Fig. 1). Compared to the group with <3 h of smartphone ST and <2 h of non-smartphone ST, the association with overweight/obesity was about 3 times greater when either ≥3 h of smartphone ST or ≥2 h of non-smartphone ST was exceeded, and it was 7 times greater when both criteria were met.
Table 2
Odds ratios (95% CI) of total ST (A), smartphone ST (B), and non-smartphone ST (C) and overweight/obesity with ST categorized into 5 categories each. Bold font indicates statistical significance.
| (A) Total ST (hours) |
<2 |
≥2 < 3 |
≥3 < 4 |
≥4 < 5 |
≥5 |
| Boys |
| Model 1 |
1.00 (ref.) |
1.25 (0.67–2.35) |
1.75 (0.98–3.15) |
1.36 (0.75–2.47) |
1.75 (0.91–3.37) |
| Model 2 |
1.00 (ref.) |
1.24 (0.66–2.33) |
1.79 (0.99–3.24) |
1.39 (0.75–2.56) |
1.83 (0.94–3.56) |
| Girls |
| Model 1 |
1.00 (ref.) |
1.75 (0.86–3.56) |
1.49 (0.72–3.10) |
2.96 (1.49–5.86) |
3.81 (1.85–7.85) |
| Model 2 |
1.00 (ref.) |
1.71 (0.83–3.50) |
1.48 (0.71–3.10) |
2.99 (1.50–5.98) |
4.16 (1.99–8.69) |
| (B) Smartphone ST (hours) |
Non-use |
<1 |
≥1 < 2 |
≥2 < 3 |
≥3 |
| Boys |
| Model 1 |
1.00 (ref.) |
0.65 (0.29–1.47) |
0.57 (0.25–1.30) |
0.63 (0.28–1.43) |
0.91 (0.48–1.73) |
| Model 2 |
1.00 (ref.) |
0.67 (0.30–1.52) |
0.59 (0.26–1.34) |
0.67 (0.29–1.52) |
0.94 (0.49–1.80) |
| Model 3 |
1.00 (ref.) |
0.74 (0.33–1.69) |
0.67 (0.29–1.54) |
0.72 (0.31–1.66) |
1.00 (0.52–1.94) |
| Girls |
| Model 1 |
1.00 (ref.) |
0.95 (0.44–2.04) |
0.71 (0.31–1.62) |
1.06 (0.49–2.31) |
2.37 (1.33–4.21) |
| Model 2 |
1.00 (ref.) |
0.91 (0.42–1.98) |
0.71 (0.31–1.62) |
1.14 (0.52–2.52) |
2.60 (1.44–4.68) |
| Model 3 |
1.00 (ref.) |
1.20 (0.54–2.63) |
0.89 (0.38–2.05) |
1.47 (0.66–3.30) |
2.99 (1.63–5.47) |
| (C) Non-smartphone ST (hours) |
<1 |
≥1 < 2 |
≥2 < 3 |
≥3 < 4 |
≥4 |
| Boys |
| Model 1 |
1.00 (ref.) |
0.98 (0.38–2.57) |
1.22 (0.49–3.05) |
1.83 (0.74–4.52) |
1.54 (0.62–3.81) |
| Model 2 |
1.00 (ref.) |
0.99 (0.37–2.59) |
1.20 (0.48–3.01) |
1.87 (0.75–4.66) |
1.57 (0.63–3.92) |
| Model 4 |
1.00 (ref.) |
0.96 (0.36–2.54) |
1.16 (0.46–2.93) |
1.80 (0.72–4.51) |
1.51 (0.60–3.80) |
| Girls |
| Model 1 |
1.00 (ref.) |
1.05 (0.42–2.63) |
2.10 (0.91–4.82) |
1.85 (0.77–4.41) |
2.92 (1.27–6.71) |
| Model 2 |
1.00 (ref.) |
1.08 (0.43–2.72) |
2.17 (0.94–5.01) |
1.97 (0.82–4.73) |
3.02 (1.30–7.01) |
| Model 4 |
1.00 (ref.) |
1.13 (0.45–2.85) |
2.45 (1.05–5.71) |
2.30 (0.95–5.57) |
3.55 (1.52–8.32) |
Model 1: adjusted for age
Model 2: adjusted for Model 1 + sleep time, physical activity, energy intake, fat, carbohydrate
Model 3: adjusted for Model 2 + non-smartphone ST
Model 4: adjusted for Model 2 + smartphone ST
In girls, neither physical activity nor sleep time had a significant effect on the OR of overweight/obesity when total ST was <4 h or ≥5 h and smartphone ST <2 h, but when total ST was ≥4 < 5 or smartphone ST ≥2 h, both physical activity and sleep time had a significant effect on the OR for overweight/obesity. However, for total ST ≥4 < 5 h or smartphone ST ≥2 h, the OR for overweight/obesity did not increase significantly if physical activity was ≥23 METs h/w or sleep time was ≥8.5 h (Figs. 2, 3).
Discussion
The current study revealed that total ST, smartphone ST, and non-smartphone ST were all independently and significantly associated with overweight/obesity only in girls, and their cut-off values for being associated with a significantly increased risk of overweight/obesity were ≥4 h, ≥3 h, and ≥2 h, respectively. Non-smartphone ST and smartphone ST were additively associated with overweight/obesity. In boys, none of these associations was significant. However, total ST ≥4 < 5 h and smartphone ST ≥2 h were not associated with overweight/obesity in participants with physical activity ≥23 METs h/w (i.e., ≥60 min/day) or sleep time ≥8.5 h in girls.
Although previous studies reported that ST, including TV and PC viewing [4], and cell phone use were associated factors for overweight/obesity [5], in the present study, increased non-smartphone ST was associated with overweight/obesity independently of smartphone ST among girls. Furthermore, only for girls, smartphone ST was significantly associated with overweight/obesity independently of non-smartphone ST.
It is assumed that people sit for long periods while engaging in TV, PC, games, etc., within a limited location, but the smartphone can be used while moving about. Also, smartphones have applications that allow people to play games while exercising. Thus, there are fewer limitations as to situations in which smartphone ST can be measured. Therefore, we speculated that the association of smartphone ST is less associated with overweight/obesity than non-smartphone ST when smartphones are used over longer periods of time.
The reason for the sex difference in the association between total ST, non-smartphone ST, and smartphone ST and overweight/obesity in this study was that the girls spent a longer time sitting and engaging in less physical activities than the boys. Sedentary time is a risk for obesity independent of time spent on physical activity [24, 25]. A longitudinal study in the United States showed that prolonged ST presented a risk for obesity in girls [26] and that girls spent more time sitting compared to boys [27]. Furthermore, the activity level was reported to be consistently lower in girls than in boys [28, 29]; similarly, the physical activity level was significantly lower in girls compared to boys in our study (p = 0.001). This suggests that ST and sedentary time should be reduced, especially in girls, and that the longer the ST, the greater should be the level of physical activity. Also, these results suggest that excessive energy intake, rather than long ST, may be the cause for overweight in boys. Therefore, we think that dietary guidance is more important than reducing ST as a lifestyle guidance for overweight/obesity boys.
We found that as total ST and smartphone ST increased, sleep duration decreased. Furthermore, our results suggested that in girls, even in case of total ST and smartphone ST were prolonged, if it is up to a certain level such as total ST <4 h and smartphone ST <2 h, and if sufficient physical activity or sleep time is secured, the risk of overweight/obesity does not increase. Katzmarzyk and colleagues reported [30] that compared to children with ≥60 min/day of moderate to vigorous physical activity, those with lesser amounts of physical activity had a higher rate of obesity regardless of sedentary behavior. In the present study, those in the ≥4 < 5 h group for total ST and in the smartphone ST ≥2 h group who engaged in physical activity for ≥60 min/day were not affected by overweight/obesity. This suggests that such physical activity may help prevent overweight/obesity in those with high ST. In addition, frequent nighttime awakening occurs when bedtime and awakening time are later for those who spend greater amounts of time on social media [31]. Poor sleep quality has been associated with obesity [18, 32]. In the present study, as ST increased, sleep duration decreased, and sleep quality may have decreased. Although we did not investigate the quality of sleep, it is necessary to reduce total ST to <4 h and smartphone ST to <2 h to ensure sufficient sleep time. Even if total ST ≥4 < 5 h or smartphone ST is ≥2 h, its effect on overweight/obesity was not necessarily significant if physical activity and sleep time were sufficient. Therefore, further research is needed in the future.
An increase in energy intake with increased TV viewing was reported [33] but no such trend was observed in this study. Furthermore, in this study, there was a statistically significant trend in fat and carbohydrate intakes with increased ST, but it was not clinically meaningful. The reason is that it is possible to eat while watching TV, but with smartphones and non-smartphone devices (excluding TV), it is difficult to eat while watching or operating the device. Therefore, it can be considered that even when smartphone ST and non-smartphone ST increases, energy intake does not increase.
The first strength of this study is that we investigated the effects of smartphone ST and non-smartphone ST separately on individuals with overweight/obesity, whereas previous studies mostly focused on total ST and TV and PC usage. Our results showed that both types of ST independently and additively increased the risk of overweight/obesity in girls. Second, both physical activity and diet were measured as were aspects of energy expenditure and energy intake. As a result, we found that only physical activity was significantly associated with ST and overweight/obesity.
Several limitations must be mentioned. First, we could not identify causal relationships because this was a cross-sectional study. In the future, longitudinal studies could be expected to prove the causal relationship between ST including that for smartphones and overweight/obesity. Second, the proportion of smartphone users was too small to analyze the effects of ST on smartphones alone. Third, the assessment of physical activity and sedentary behavior was by questionnaire rather than by an accelerometer. As a result, the actual physical activity level was not determined. Furthermore, we had no information on blood pressure, inflammatory markers, glucose, insulin, and lipid concentrations that are potentially associated with overweight/obesity [7]. The lack of these covariates may exaggerate the effect estimate.
In conclusion, the present results suggest that ST including that on smartphones is a significant factor associated with overweight/obesity in girls, and that even if total ST and smartphone ST are long, up to a certain level the risk of overweight/obesity can be reduced by ensuring sufficient physical activity and sleep time.
Acknowledgements
The authors thank the students and their parents who participated in this study, the teachers at each school and the city officials who cooperated in the implementation of this study, and Agano City for their financial support. We also thank Secretary Sakiko Sato who helped us create the database.
Disclosure
S.H. is a member of Endocrine Journal’s Editorial Board. Except for S.H., other authors, I.I., F.K., M.Y.S., T.Y., I.H., Y.H.M., H.C., M.Y., Y.T., O.Y., declare no conflict of interest with this article. The authors declare that there is no duality of interest associated with this manuscript.
Author Contributions
I.I. and S.H. developed the study design, researched the data, contributed to discussions, wrote the manuscript, and reviewed and edited the manuscript. F.K., M.Y.S., T.Y, I.H., Y.H.M., H.C., M.Y., Y.T., and O.Y. researched the data, contributed to discussions, wrote the manuscript, and reviewed and edited the manuscript. S.H. planned and supervised this research, developed the study design, researched the data, contributed to discussions, wrote the manuscript, and reviewed and edited the manuscript. I.I. and S.H. are the guarantors of this work and, as such, had full access to all the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis.
Funding
This work is supported in part by the Japan Society for the Promotion of Science (22H03529, 21K11569), JST SPRING (JPMJSP2121) and Taiju Life Social Welfare Foundation.
Informed Consent
Written informed consent was obtained from all participants.
Data Availability Statement
Data cannot be shared for privacy or ethical reasons.
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