Association of self-reported dental caries with sex, lifestyle, and problematic Internet use among elementary school children in Japan
According to the World Health Organization, almost half of the world’s population suffer from oral diseases , and dental caries is the most prevalent disease (39.0%) in elementary school children in Japan, followed by myopia (36.9%) . Dental caries in children negatively affects their quality of life and school performance , as well as the development of permanent teeth . This implies that prevention of dental caries in childhood is crucial. Dental caries is a result of bacterial activity in dental plaque, with lifestyle being the main contributor because it disrupts good dental hygiene . Infrequent tooth brushing, irregular meal consumption tendency, and high consumption of snacks and sweetened drinks are associated with the development of dental caries [ – ]. Given that dental caries is highly preventable through good oral hygiene, such as avoiding frequent snacking and brushing teeth twice a day, exploring other possible lifestyle-related risk factors may contribute to the improvement of public oral health. The Internet has become an essential part of children’s lives. It is useful for accessing unlimited information and exchanging communication without delay. However, according to national statistics, the average time of Internet usage among elementary school children was 213.7 min a day in 2022, and it was used mostly for hobby or entertainment, not academic . Excessive or poorly controlled Internet use can cause Internet addiction or problematic Internet use (PIU), accompanied by symptoms of preoccupation, tolerance, and relapse . PIU is a serious behavioral problem because of its detrimental effect on health and disturbing lifestyles such as physical inactivity, sleep shortage, and unhealthy dietary habits [ – ]. However, to date, only one small-scale study of 153 children has shown the association between poor dental health and PIU in children . Mustuloğlu et al. showed a signigicant association between PIU and the presence of caries (decayed/filled/missing), but its independent association was not shown because other factors, including tooth brushing, sleep habits, and family environments, were not adjusted for. We previously showed that the existence of family rules restricting screen time was significantly associated with prolonged screen time and pathological gaming [ , , ]. Therefore, we hypothesized that setting family rules against any unhealthy behaviors in children could affect their health. Sex differences in caries prevalence in mature adults have been widely observed in previous studies, with females being at a higher risk of developing caries than males because of hormonal and reproductive factors . In children, several studies reported a higher prevalence in girls [ – ]; however, the difference was inconclusive . Therefore, we aimed to explore (1) the sex differences in dental caries and (2) the association of self-reported dental caries with lifestyle and PIU among elementary school children in a large-scale study in Japan. 

Study design and participants In 2018, we conducted a school-based self-administered questionnaire survey, the Toyama Safe Internet Use Project. Toyama Prefecture is located in the central part of the Hokuriku region of Japan and has a population of approximately 1 million. Called upon by the Toyama Prefecture Education Board, 110 elementary schools out of 185 in Toyama Prefecture agreed to join the project (59.5%, as of 2018). The rest of the schools (40.5%) which had already conducted a similar survey on problematic Internet use among children, did not join the project. After excluding children and their parents (or guardians) who were illiterate in Japanese, 13,413 children (boys, 50.9%; mean age, 10.5 years) in the 4th-6th grades were included. The school teachers explained the purpose of the survey to the children and their parents and told them that participation was voluntary, with the opportunity to opt out of the study. A paper questionnaire was distributed to all schoolchildren via school teachers, and the children and their parents answered the paper questionnaire. Demographic and explanatory variables The questionnaire was used to assess the children’s basic characteristics, lifestyle, anthropometric data, and caries experience. The lifestyle variables included daily breakfast consumption, physical activity, and sleep habits. Responses to breakfast consumption were dichotomized: “every day” or “skipping breakfast.” Physical activity responses were answered at three levels: “often,” “not so much,” or “rarely to almost never.” Regarding sleeping habit, the children’s bedtime on weekdays was assessed and categorized into < 10 p.m., 10 to 11 p.m., or ≥ 11 p.m. The validity of the lifestyle questions on physical activity and sleep behaviors has been assessed in previous studies, and they demonstrated good agreement between subjective and objective measures . In these studies, frequent physical activity was significantly associated with daily energy expenditure, mean steps, and mean activity count on the Actiwatch ( p < 0.05, for a linear trend test) , and the correlation coefficient between subjective and objective sleep records was 0.97 ( p < 0.001) for the assumed amount of sleep . We obtained the children’s anthropometric data, which were measured every April by trained school nurses following the School Health and Safety Act in Japan. Age- and sex-specific cutoff points equal to the adult BMI value of 25–18.5 kg/m 2 for classification as overweight or thin, respectively, were applied to determine whether the children were overweight or thin. These cutoff points were developed by the Childhood Obesity Working Group of the International Obesity Task Force . Regarding PIU, we used a Japanese version of the Young Diagnostic Questionnaire (YDQ, Cronbach’s alpha: 0.725), comprising eight items with “yes” or “no,” answers, and the scores range from 0 to 8. A higher score indicates a more serious degree of PIU. Based on previous studies , we categorized each child into three groups: adaptive Internet use (0–2), mild PIU (3–4), and severe PIU ( ≥ 5). Regarding child-parent relationships, we asked “Are there any rules at home to restrict recreational screen time (viewing TV, DVD, and Internet contents, and playing video game online or offline). The answer was dichotomized into “yes” or “no.” Outcome variable We asked for the cumulative number of treated dental teeth of both primary and permanent teeth to date. Referring to the school’s health checkups, parents (or guardians) reported the answer. The answers were categorized from 1 to 4: 1, none; 2, one treated tooth; 3, two treated teeth; and 4, ≥ 3 treated teeth. According to the national survey in 2016, the mean number of persons with decayed (d) and filled (f) primary teeth was 1.3 (standard deviation [SD] = 2.5) among children under 13 years old, and the decayed (D), missing (M), and filled (F) teeth (DMFT) for permanent teeth was 0.4 (SD = 1.0) among 10 to 14 years old children in Japan . Therefore, we defined ≥ 3 treated teeth as the presence of “multiple caries. Previous studies have shown that maternal perceptions (self-report) of oral health are significantly correlated with their children’s DMFT scores . Statistical analysis Collective data analyses and chi-square tests were performed for all variables according to sex (Table ). We fitted the data using Poisson regression analysis with robust variance (modified Poisson regression analysis) and explored the prevalence ratios (PR) with 95% confidence intervals (95% CI). Poisson analysis is often used when the prevalence is not rare . We calculated univariable (crude) PR and multivariable (fully adjusted) aPR for the presence of multiple caries in all children (Table ) and separately by sex (Table ). Data analyses were performed using the STATA software (version 16.0; STATA Corporation, College Station, TX, USA). Statistical significance was set at p < 0.05. 

In 2018, we conducted a school-based self-administered questionnaire survey, the Toyama Safe Internet Use Project. Toyama Prefecture is located in the central part of the Hokuriku region of Japan and has a population of approximately 1 million. Called upon by the Toyama Prefecture Education Board, 110 elementary schools out of 185 in Toyama Prefecture agreed to join the project (59.5%, as of 2018). The rest of the schools (40.5%) which had already conducted a similar survey on problematic Internet use among children, did not join the project. After excluding children and their parents (or guardians) who were illiterate in Japanese, 13,413 children (boys, 50.9%; mean age, 10.5 years) in the 4th-6th grades were included. The school teachers explained the purpose of the survey to the children and their parents and told them that participation was voluntary, with the opportunity to opt out of the study. A paper questionnaire was distributed to all schoolchildren via school teachers, and the children and their parents answered the paper questionnaire. 

The questionnaire was used to assess the children’s basic characteristics, lifestyle, anthropometric data, and caries experience. The lifestyle variables included daily breakfast consumption, physical activity, and sleep habits. Responses to breakfast consumption were dichotomized: “every day” or “skipping breakfast.” Physical activity responses were answered at three levels: “often,” “not so much,” or “rarely to almost never.” Regarding sleeping habit, the children’s bedtime on weekdays was assessed and categorized into < 10 p.m., 10 to 11 p.m., or ≥ 11 p.m. The validity of the lifestyle questions on physical activity and sleep behaviors has been assessed in previous studies, and they demonstrated good agreement between subjective and objective measures . In these studies, frequent physical activity was significantly associated with daily energy expenditure, mean steps, and mean activity count on the Actiwatch ( p < 0.05, for a linear trend test) , and the correlation coefficient between subjective and objective sleep records was 0.97 ( p < 0.001) for the assumed amount of sleep . We obtained the children’s anthropometric data, which were measured every April by trained school nurses following the School Health and Safety Act in Japan. Age- and sex-specific cutoff points equal to the adult BMI value of 25–18.5 kg/m 2 for classification as overweight or thin, respectively, were applied to determine whether the children were overweight or thin. These cutoff points were developed by the Childhood Obesity Working Group of the International Obesity Task Force . Regarding PIU, we used a Japanese version of the Young Diagnostic Questionnaire (YDQ, Cronbach’s alpha: 0.725), comprising eight items with “yes” or “no,” answers, and the scores range from 0 to 8. A higher score indicates a more serious degree of PIU. Based on previous studies , we categorized each child into three groups: adaptive Internet use (0–2), mild PIU (3–4), and severe PIU ( ≥ 5). Regarding child-parent relationships, we asked “Are there any rules at home to restrict recreational screen time (viewing TV, DVD, and Internet contents, and playing video game online or offline). The answer was dichotomized into “yes” or “no.” 

We asked for the cumulative number of treated dental teeth of both primary and permanent teeth to date. Referring to the school’s health checkups, parents (or guardians) reported the answer. The answers were categorized from 1 to 4: 1, none; 2, one treated tooth; 3, two treated teeth; and 4, ≥ 3 treated teeth. According to the national survey in 2016, the mean number of persons with decayed (d) and filled (f) primary teeth was 1.3 (standard deviation [SD] = 2.5) among children under 13 years old, and the decayed (D), missing (M), and filled (F) teeth (DMFT) for permanent teeth was 0.4 (SD = 1.0) among 10 to 14 years old children in Japan . Therefore, we defined ≥ 3 treated teeth as the presence of “multiple caries. Previous studies have shown that maternal perceptions (self-report) of oral health are significantly correlated with their children’s DMFT scores . 

Collective data analyses and chi-square tests were performed for all variables according to sex (Table ). We fitted the data using Poisson regression analysis with robust variance (modified Poisson regression analysis) and explored the prevalence ratios (PR) with 95% confidence intervals (95% CI). Poisson analysis is often used when the prevalence is not rare . We calculated univariable (crude) PR and multivariable (fully adjusted) aPR for the presence of multiple caries in all children (Table ) and separately by sex (Table ). Data analyses were performed using the STATA software (version 16.0; STATA Corporation, College Station, TX, USA). Statistical significance was set at p < 0.05. 

In total, 13,092 children filled and returned the questionnaire (response rate: 97.6%), and 12,146 (90.6%) children who answered all relevant questions were included in our analyses. The mean age of the children was 10.5 years (SD = 0.96). Table describes the distribution of the basic characteristics and explanatory and outcome variables in total and stratified by sex. Boys were more likely to skip breakfast, be physically active, be obese, have family rules, and have higher PIU than girls. Girls were more likely to be thin and brush their teeth more frequently. Almost all the girls (95.6%) brushed their teeth twice daily. In total, 25.7% of the children (boys 24.4% and girls 27.0%) were defined as having multiple caries ( ≥ 3 treated teeth). Table shows the results of the Poisson regression analysis exploring the association between the demographic and explanatory variables and the presence of multiple caries. In the multivariable (full adjusted) analysis, being a girl (aPR 1.13), being in a higher grade (5th grade: aPR 1.13and 6th grade: aPR 1.14), skipping breakfast (aPR 1.11), having a late bedtime (10 to 11 p.m.: aPR 1.17 and ≥ 11 p.m.: aPR 1.23), tooth brushing (once a day: aPR 1.14 and not every day: aPR 1.41), having no family rules for screen time at home (aPR 1.20), and PIU (YDQ score 3–4: aPR 1.17) were significantly associated with multiple caries. Poisson regression analysis was conducted according to sex (Table ). Boys and girls showed similar trends in all variables except PIU. While a dose-response relationship was observed between PIU and presence of multiple caries in boys (YDQ score 3–4: aPR 1.19 and YDQ score ≥ 5: aPR 1.22), there was no such a trend in girls. 

We conducted a cross-sectional, school-based study to explore the prevalence of self-reported dental caries in Japanese elementary school children. Multiple caries were prevalent (25.7%) in children and being a girl, unhealthy lifestyles including skipping breakfast, late bedtime, and infrequent tooth brushing, having no family rule for screen time, and PIU, were significantly associated with having multiple caries. To the best of our knowledge, this is the largest study to explore dental caries and its association with PIU in elementary school children. Sex differences in dental caries in children have been inconclusive. Several previous studies showed no sex difference in the dental caries prevalence [8 24], however, many studies have reported a higher prevalence in girls [ , , ]. In line with these reports, our study showed that multiple caries were more prevalent in girls. Moreover, in our study, we showed that, even after adjusting for the frequency of tooth brushing (less than twice a day: 10.2% in boys and 4.4% in girls; Table ) and other lifestyle factors, being a girl was significantly associated with having multiple caries. Therefore, girls’ dominance may be attributable to mechanisms rather than lifestyle factors. There are several plausible explanations for the girls’ dominance. First, teeth eruption occurs earlier in girls, exposing permanent teeth to a potential caries-inducting environment for longer durations than in boys . Joshi et al. demonstrated dmft (for primary teeth) and DMFT (for permanent teeth) scores by sex, and they showed boy’s dominance in dmft scores and girl’s dominance in DMFT scores. This could explain why many studies sampling older children showed girl’s dominance . Second, difference in the salivary microbiome has been reported . Ortiz S et al. showed that acid-generating, cariogenic Lactococcus lactis and Alloprevotella species HOT 126 are more abundant in caries-active girls than in caries-active boys . Moreover, genetic variations that influence taste, enamel proteins, and enamel structure are also considered to contribute to sex differences . Both biological (genetics and hormones) and anthropological (behavioral) factors such as gender-based dietary preferences are thought to cause the sex differences. In our study, unhealthy lifestyle habits such as skipping breakfast, infrequent tooth brushing, and late bedtime were significantly associated with multiple dental caries, which is in line with previous findings [ , , , ]. Plausible explanations for skipping breakfast and infrequent tooth brushing could come from having breakfast daily being an indication of a regular lifestyle, meaning regular tooth brushing and less need for snacks . Since dental caries are a result of bacterial activity in dental plaque , poor oral hygiene (irregular tooth brushing) may accelerate dental plaque accumulation and bacterial activity in dental plaque, resulting in dental caries. Regarding sleep habits, there is evidence that sleeping for a short duration can cause dental caries . The first plausible explanation is that insufficient sleep can increase appetite, following increased levels of ghrelin and decreased levels of leptin . Nedeltcheva et al. reported that healthy people with sleep curtailment were more likely to have an increased caloric intake by snacking more frequently, thereby contributing to the development of dental caries . In addition, sleep shortage has been linked to increased production of salivary Streptococcus mutans , which ferments dietary carbohydrates and contributes to dental caries by lowering the pH of dental plaque. Getting sufficient sleep is an established form of protection against caries. In our study, having no family rules regarding screen time was significantly associated with the presence of multiple dental caries. As a plausible explanation, family rules can be a proxy variable for parental health awareness of children or socioeconomic status (SES). According to a meta-analysis, dental caries was significantly more prevalent in low-SES household . Moreover, Asaka et al. assessed dental neglect, which refers to parents who do not take children to a dental clinic, even if their children have untreated dental caries. They showed that fathers’ prolonged Internet and game use, not being affluent, and non-engagement in afterschool activities were significantly associated with dental neglect . Therefore, in addition to biological and behavioral aspects, familial or social aspects should be considered for caries prevention. Higher PIU rates were significantly or marginally associated with the presence of multiple caries (Table ). Our findings were in line with those of previous studies , and we found that PIU was independently associated with the presence of multiple dental caries, even after adjusting for tooth brushing and other lifestyle factors. There are several possible explanations for the association. First, PIU can lead to frequent consumption of sugar. Iwasaki et al. demonstrated that high school students with Internet addiction were more likely to consume soft drinks and sweet snacks frequently and have irregular meal time . In addition, it is probable that children with PIU do not have sufficient time to brush their teeth seriously. In addition to behavioral factors, biological factors are also considered plausible reasons. Kaess et al. reported that patients with Internet gaming disorder had greater daily and chronic stress, as well as psychopathological comorbidity . Under chronic stress, a prolonged reduction of salivary immunoglobulin A (sIgA) levels, which is thought to serve as a first line of defense against invading organisms (e.g., pathogens), was reported . Thus, we can suggest that PIU cause dental caries through behavioral as well as biological pathways. Moreover, from the analysis stratified by sex, we found a dose-response relationship between PIU and dental caries only in boys (Table ) and no relationship in girls. To our knowledge, this is the first study to clarify the association between PIU and caries in children according to sex. A plausible reason is that sex-specific applications may contribute to sex differences . Prevalence of PIU (or Internet gaming disorder) has been reported to be predominantly higher in males , and Kawabe et al. showed that male students played games more than did female students, while female students used mobile phones (such as cell phones and smartphones) for communication on social networking sites more than did male students . Moreover, in Taiwanese junior high school students, lower self-esteem and lower satisfaction with daily life were reportedly associated with more severe gaming addiction among males, but not among females . Consequently, boys’ enthusiasm for gaming and lowered psychological health may result in poor oral hygiene and cause sex differences in the prevalence of dental caries. Understanding these mechanisms will provide better management strategies for both boys and girls. The strength of this study is that, to the best of our knowledge, we conducted the largest-scale study assessing dental caries among elementary school children, with an extremely high response rate (97.6%), minimizing sampling bias. However, this study has some limitations. First, it used subjective rather than objective data regarding the number of dental caries. Second, the study design was cross-sectional; therefore, the causal relationship between dental caries and PIU could not be clarified. We cannot deny the possibility that children with many dental caries tend to lead unhealthy lifestyles such as having late bedtimes. A longitudinal study is required to elucidate the effect of PIU on dental caries. Third, our study was conducted in only one prefecture in Japan; thus, caution is needed when generalizing our findings. Finally, we did not explore other potential risk factors such as brushing time (duration), regular dental check-ups, and family SES. These factors should be considered in future studies. In conclusion, our large-scale study found that multiple self-reported caries were prevalent (25.7%) among elementary school children and that being a girl, unhealthy lifestyle habits such as skipping breakfast and infrequent tooth brushing, having no family rule for screen time, and PIU were significantly associated with multiple caries. In addition to establishing healthy lifestyles in children, restricting PIU is also important for caries prevention. 

