
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

39251647
68427
10.1038/s41598-024-68427-2
Article
The association between tooth brushing during lunchtime and obesity in South Korean adolescents
Cho Yoonjin 1
Shin Heejae 1
Lee Seunghoon 1
Kim Jinhyun 23
Shin Jaeyong 23
Kim Hyunkyu healbot@yuhs.ac

2345
1 https://ror.org/01wjejq96 grid.15444.30 0000 0004 0470 5454 Premedical Courses, Yonsei University College of Medicine, Seoul, 03722 Republic of Korea
2 https://ror.org/01wjejq96 grid.15444.30 0000 0004 0470 5454 Department of Preventive Medicine, Yonsei University College of Medicine, Seoul, Republic of Korea
3 https://ror.org/01wjejq96 grid.15444.30 0000 0004 0470 5454 Institute of Health Services Research, Yonsei University, Seoul, Republic of Korea
4 https://ror.org/01wjejq96 grid.15444.30 0000 0004 0470 5454 Department of Psychiatry, Yonsei University College of Medicine, Seoul, Republic of Korea
5 https://ror.org/01wjejq96 grid.15444.30 0000 0004 0470 5454 Institute of Behavioral Science in Medicine, Yonsei University, Seoul, Republic of Korea
5 9 2024
5 9 2024
2024
14 207301 9 2023
23 7 2024
© The Author(s) 2024
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ Open Access This article is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, which permits any non-commercial use, sharing, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if you modified the licensed material. You do not have permission under this licence to share adapted material derived from this article or parts of it. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.
Obesity in adolescents is a global health concern associated with several diseases and complications in adulthood. Obesity has been reported to be strongly associated with oral habits. To investigate the association between tooth brushing during lunchtime and the prevalence of obesity among adolescents in a nationally representative sample of South Korean adolescents. Data from the 2018–2021 Korea Youth Risk Behavior Web-Based Survey were used in this cross-sectional study. Obesity was measured based on the body mass index. The chi-square test was used to investigate and compare tooth brushing habits during lunchtime in the study population, and multivariate logistic regression analysis was performed to analyze the relationship between tooth brushing during lunchtime and the prevalence of obesity. The odds of obesity were lower in the group that brushed their teeth during lunchtime (adjusted odd ratio: 0.90, 95% confidence interval: 0.88–0.93). Furthermore, the odds of obesity were significantly higher in high school students; boys; those with a low household income, low subjective health status awareness, medical history (asthma or atrophy), and history of sealant use; and those who responded after the coronavirus disease 2019 pandemic. A significant inverse relationship was found between tooth brushing habits during lunchtime and the prevalence of obesity. This relationship was consistent regardless of socioeconomic and lifestyle factors.

Subject terms

Obesity
Risk factors
issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

With improvements in living standards, weight gain and obesity are increasingly becoming global health concerns. Obesity, a chronic disease that affects both adults and children, is prevalent in both developed and developing nations, including South Korea1,2. Notably, obesity prevalence has almost surpassed traditional public health concerns such as undernutrition and infectious diseases and is one of the most significant contributors to poor health3. Obesity has been associated with various diseases, including cardiovascular disease and diabetes4, and a correlation between poor oral health and obesity has been reported in previous studies5. Notably, individuals with obesity tend to experience periodontal disease and lose more teeth than those without obesity6,7.

In addition to the direct adverse effects of obesity on general health, research has focused on numerous comorbidities that have common risk factors and complex health implications8. Previous studies have reported an association between obesity and oral diseases, including periodontal disease, dental caries, tooth erosion, xerostomia, and dentinal hypersensitivity. These oral diseases are categorized as health issues that are often accompanied by obesity and exhibit similar underlying risk factors among adolescents9,10. Furthermore, individuals’ self-perceived oral health is associated with general health11, and this association has crucial implications for healthcare professionals specializing in obesity management and dental practitioners who oversee the oral health of overweight individuals5,12. Tooth brushing is one factor in the favorable health cluster of public school children, owing to its primary role in maintaining good oral hygiene, ultimately leading to better oral health13,14. Oral and general health are not independent of contextual influences, as evidenced by the clustering of body mass index (BMI) and tooth brushing in the health and lifestyle clusters among the children, indicating their influence in a larger social environment15,16.

The present study aimed to investigate the correlation between lunchtime tooth brushing habits and the prevalence of obesity among Korean adolescents, which is a potentially under-explored aspect of adolescent health. We specifically focused on the unique setting of schools, where adolescents are more likely to manage their oral hygiene independently, unlike at home, where parental supervision often plays a significant role. This distinction is crucial because it allows for an evaluation of how self-initiated oral hygiene practices, particularly in a school environment, impact adolescent health. The correlation between oral health and obesity in adolescents has been investigated in numerous studies; however, only a few studies have specifically targeted Korean adolescents. Therefore, our study represents a novel contribution to this field.

We primarily aimed to explore the association between consistent tooth brushing habits after lunchtime in school and the prevalence of obesity among Korean adolescents. In this study, obesity was measured based on BMI, providing an overall indicator of general obesity. In addition, we aimed to investigate whether the observed associations are influenced by various factors, such as socioeconomic status, lifestyle choices, dental anxiety, and other relevant covariates. This multifaceted approach seeks to provide a comprehensive understanding of the interplay between personal oral hygiene practices and broader health outcomes, including obesity.

By delving into these research areas, we hope to not only establish an association between individual health practices, such as tooth brushing after lunch, and overall well-being, particularly regarding obesity, but also to explore how this association is influenced by different demographic and lifestyle factors in Korean adolescents. Our findings may significantly contribute to the fields of health education and policy, offering new perspectives on promoting healthy lifestyle habits and understanding health status among Korean adolescents.

Methods

Source of data analyzed

Data obtained from the Korea Youth Risk Behavior Web-based Survey (KYRBS) between 2018 and 2021 were retrospectively analyzed in this study. The KYRBS is an ongoing national cross-sectional survey used to assess health-risk behaviors among middle and high school students in South Korea and monitor progress toward achieving the national health objectives outlined in Korea’s National Health Plan in 2020. Furthermore, it provides data essential for the development and evaluation of school health policies and programs in South Korea. The target population for this survey was nationally representative middle and high school students aged 12–18 years in South Korea. Approximately 75,000 students from 400 middle and 400 high schools participated in the survey, which was conducted between June and July of each year. Students anonymously completed a self-administered questionnaire in the computer laboratory at each sampled school. A steering committee representing the Office of Education from 17 provinces in South Korea managed the survey17. All procedures contributing to this study comply with the ethical standards of the relevant national and institutional committees on human experimentation and with the Helsinki Declaration of 1975, revised in 2008. The KYRBS was approved by the Institutional Review Board (IRB) of the Korea Centers for Disease Control and Prevention (KCDC; approval number: 2014-06EXP-02-P-A) in 2014. From 2015 onward, the requirement for ethical approval for the KYRBS was waived by the IRB of the KCDC under the Bioethics and Safety Act and opened to the public for academic use. All participants signed an informed consent form before participating in the KYRBS.

Variable definitions

In this study, BMI was calculated using participants’ self-reported height and weight and classified according to the age- and sex-specific equations from the 2017 Korean National Growth Charts for Children and Adolescents. BMI was classified as follows: underweight (BMI < 5th percentile), normal weight (5th ≤ BMI < 85th percentile), overweight (85th ≤ BMI < 95th percentile), obesity (95th ≤ BMI < 99th percentile), and severe obesity (BMI ≥ 99th percentile). This classification reflects the nationally recognized standards, ensuring a robust and contextually relevant assessment of weight status among Korean adolescents. Regarding our primary variable of interest, “tooth brushing after lunch,” we first investigated the student’s engagement in this habit based on a “yes” or “no” response. To provide deeper insights using a subgroup analysis, we expanded this variable by introducing a more detailed classification based on the frequency of tooth brushing. This classification was based on the number of times students brushed their teeth after lunch in the last 7 days. The frequency of tooth brushing was categorized as follows: (a) Always: if the participant brushed their teeth after lunch seven times (daily) in the last 7 days; (b) Mostly: if the participant brushed their teeth after lunch 4–6 times in the last 7 days; and (c) Sometimes: if the participant brushed their teeth after lunch 1–3 times in the last 7 days.

Covariates

Common possible confounders associated with body weight, including socioeconomic and lifestyle factors18 and oral health9, should be considered in studies on oral health and obesity. Grade level (middle or high school) was considered to reflect the academic characteristics of the students. Sex (boys and girls) was recorded as a significant variable. Economic status was determined using the variable “household income,” categorized as “high,” “middle,” or “low.” Similarly, the variable “living companions” reflected the living arrangements of the students, categorized as “with family,” “live alone,” or “in a childcare facility.” The students’ perceived stress levels were evaluated using the variable “perceived stress level,” categorized as “high,” “middle,” or “low.” In addition, students’ subjective assessment of their overall health was measured using the variable “subjective health status,” categorized as “high,” “middle,” or “low.” The variable “physical activity” was used to assess the level of physical activity among students and was categorized as “high,” “middle,” or “low.” Students with 0 days of physical activity of ≥ 60 min weekly were classified as having a “low” physical activity level, those with 7 days of intense physical activity or who underwent strength training for ≥ 3 days weekly were classified as having a “high” physical activity, and the remaining students were classified as having a “middle” physical activity19,20. Furthermore, students’ sleep duration was examined using the variable “sleep duration” and classified into “sleep deprivation,” “appropriate sleep,” or “excessive sleep.” Specific classification criteria were applied, and sleep duration was classified as “insufficient” (0–480 min), “sufficient” (480–720 min), and “excessive” (720–1200 min). Inappropriate values of > 1200 min were excluded from the analysis21. The variable “asthma/atopic dermatitis” was used to investigate whether students had previously been diagnosed with asthma, atopic dermatitis, or both based on a “yes” or “no” response. Students’ body shape perception compared with what is considered normal or below normal and overweight was measured using the variable “body shape perception,” categorized as “below normal,” “normal,” or “overweight.” The variable “nutrition education” was used to determine the impact of nutrition education based on a “yes” or “no” response. The variable “carbonated drinks” was used to assess students’ consumption of such beverages based on a “yes” or “no” response. Students’ self-reported oral health status was examined using the variable “oral health status,” categorized as “high" or "low.” Furthermore, the variable “sealant history” was used to determine whether students had a history of dental sealant application based on a “yes” or “no” response. Finally, the variable “corona period” was used to distinguish between the pre-coronavirus 2019 disease (COVID-19) and post-COVID-19 pandemic periods, categorized as “pre-corona” or “post-corona.”

Statistical analysis

Chi-square tests were used to analyze and compare variables. To examine the relationship between tooth brushing after lunch and obesity in adolescents, we performed a survey logistic regression analysis using strata, weight, and clusters after adjusting for covariates. Subgroup analyses were performed to investigate the combined association of tooth brushing and other covariates with obesity. Participants with obesity were divided into three groups according to the severity of obesity, and the difference in the relationship between tooth brushing after lunchtime and obesity was analyzed in each of these groups. The results were presented as odds ratios (ORs) and 95% confidence intervals (CIs) to compare the prevalence of obesity. The analyses were performed using stratified sampling variables (strata) and weighted variables as suggested by the KYRBS. All analyses were performed using SAS software (version 9.4; SAS Institute, Cary, NC, USA), and statistical significance was set at p < 0.05.

Results

This study included 137,734 participants (71,080 boys and 66,654 girls) from the KYRBS survey conducted between 2018 and 2021. Table 1 presents the distribution of the participants’ responses, with 24% of respondents classified as having obesity and 76% as having normal weight. Among them, 58.9% brushed their teeth during lunchtime (22.7% had obesity), whereas 41.1% did not (25.8% had obesity).Table 1 Socioeconomic and health-related characteristics of the study participants according to obesity status.

Variables	Obesity	Normal	Total	
N	(%)	N	(%)	N	
Toothbrushing during lunch time	
	 Always	6,764	(19.9)	27,147	(80.1)	33,911	
	 Mostly	4,735	(24.0)	15,033	(76.0)	19,768	
	 Sometimes	6,919	(25.3)	20,481	(74.7)	27,400	
	 No	14,643	(25.8)	42,012	(74.2)	56,655	
Age	
	 Middle school	11,078	(22.0)	39,366	(78.0)	50,444	
	 High school	21,983	(25.2)	65,307	(74.8)	87,290	
Gender	
	 Male	20,278	(28.5)	50,802	(71.5)	71,080	
	 Female	12,783	(19.2)	53,871	(80.8)	66,654	
Household income	
	 High	11,925	(23.3)	39,313	(76.7)	51,238	
	 Middle	15,887	(23.4)	52,110	(76.6)	67,997	
	 Low	5,249	(28.4)	13,250	(71.6)	18,499	
Living companion	
	 With family	31,252	(24.0)	98,914	(76.0)	130,166	
	 Live oneself	1,730	(24.0)	471	(76.0)	7,201	
	 Childcare facility	79	(21.5)	288	(78.5)	367	
Perceived stress level	
	 High	6,052	(24.0)	19,133	(76.0)	25,185	
	 Middle	13,590	(23.1)	45,195	(76.9)	58,785	
	 Low	13,419	(25.0)	40,345	(75.0)	53,764	
Subjective health status	
	 High	21,049	(22.1)	74,293	(77.9)	95,342	
	 Middle	8,594	(27.1)	23,076	(72.9)	31,670	
	 Low	3,418	(31.9)	7,304	(68.1)	10,722	
Physical activity	
	 High	11,057	(22.2)	38,663	(77.8)	49,720	
	 Middle	20,775	(25.3)	61,338	(74.7)	82,113	
	 Low	1,229	(20.8)	4,672	(79.2)	5,901	
Sleep duration	
	 Sleep deprivation	27,713	(24.1)	87,428	(75.9)	115,141	
	 Appropriate sleep	4,666	(23.5)	15,208	(76.5)	19,874	
	 Excessive sleep	682	(25.1)	2,037	(74.9)	2,719	
Past medical history(asthma, atopy)	
	 Yes	23,424	(23.7)	75,253	(76.3)	98,677	
	 No	9,637	(24.7)	29,420	(75.3)	39,057	
Nutrition education	
	 Yes	19,066	(24.1)	60,072	(75.9)	79,138	
	 No	13,995	(23.9)	44,601	(76.1)	58,596	
Carbonated drinks	
	 Yes	26,327	(24.2)	82,364	(75.8)	108,691	
	 No	6,734	(23.2)	22,309	(76.8)	29,043	
Oral health status (self-reported)	
	 High	15,784	(24.1)	49,689	(75.9)	65,473	
	 Low	17,277	(23.9)	54,984	(76.1)	72,261	
Sealant history	
	 Yes	25,019	(24.9)	75,358	(75.1)	100,377	
	 No	8,042	(21.5)	29,315	(78.5)	37,357	
Corona period	
	 Pre-corona	16,164	(22.6)	55,443	(77.4)	71,607	
	 Post-corona	16,897	(25.6)	49,230	(74.4)	66,127	
Year	
	 2018	7,990	(21.9)	28,444	(78.1)	36,434	
	 2019	8,174	(23.2)	26,999	(76.8)	35,173	
	 2020	8,020	(25.0)	24,008	(75.0)	32,028	
	 2021	8,877	(26.0)	25,222	(74.0)	34,099	

Table 2 presents the results of the multivariate logistic regression analysis of the association between each variable and obesity. The hypothesis testing for the coefficients was implemented using a chi-squared test based on the Wald statistic. The odds of obesity were lower in the group that brushed their teeth during lunchtime than in the group that did not (adjusted odds ratio [OR] 0.90, 95% confidence interval [CI] 0.88–0.93). Furthermore, the odds of obesity were significantly higher in high school students; boys; those with a low household income, low subjective health status awareness, medical history (asthma or atrophy), and history of sealant use; and those who responded post-corona.Table 2 Results of the multiple logistic regression analysis for the association between lunchtime tooth brushing and obesity.

Variables	Obesity	
Adjusted OR	95% CI	
Toothbrushing during lunch time	
	 No	1.00		
	 Yes	0.90	(0.88–0.93)	
Age	
	 Middle school	1.00		
	 High school	1.25	(1.21–1.29)	
Gender	
	 Male	1.00		
	 Female	0.57	(0.55–0.59)	
Household income	
	 High	1.00		
	 Middle	0.99	(0.96–1.02)	
	 Low	1.20	(1.15–1.26)	
Living companion	
	 With family	1.00		
	 Live oneself	0.97	(0.91–1.04)	
	 Childcare facility	0.81	(0.61–1.09)	
Perceived stress level	
	 High	1.00		
	 Middle	0.97	(0.94–1.01)	
	 Low	1.06	(1.02–1.11)	
Subjective health status	
	 High	1.00		
	 Middle	1.39	(1.34–1.43)	
	 Low	1.73	(1.65–1.83)	
Physical activity	
	 Middle	1.00		
	 High	0.90	(0.87–0.92)	
	 Low	0.74	(0.68–0.80)	
Sleep duration	
	 Appropriate sleep	1.00		
	 Sleep deprivation	0.98	(0.94–1.02)	
	 Excessive sleep	1.05	(0.81–1.35)	
Past medical history(asthma, atopy)	
	 Yes	1.00		
	 No	1.07	(1.04–1.10)	
Nutrition education	
	 Yes	1.00		
	 No	1.02	(1.00–1.05)	
Carbonated drinks	
	 Yes	1.00		
	 No	1.01	(0.98–1.05)	
Oral health status (self-reported)	
	 High	1.00		
	 Low	0.99	(0.97–1.02)	
Sealant history	
	 Yes	1.00		
	 No	0.87	(0.84–0.90)	
Corona period	
	 Pre-corona	1.00		
	 Post-corona	1.16	(1.12–1.19)	
The hypothesis testing for the coefficients was conducted using a chi-squared test based on the Wald statistic.

Table 3 presents the results of the subgroup analysis. The hypothesis testing for the coefficients was implemented using a chi-squared test based on the Wald statistic. For all subgroups of all variables, the odds of obesity were lower in the group that brushed their teeth during lunchtime than in the group that did not. This difference was significant for all subgroups except for middle school students, those with low household income, those who lived alone, and those with low physical activity.Table 3 Subgroup analysis of the association between lunchtime tooth brushing and obesity stratified by demographic variables.

			Obesity	
		No brushing	Toothbrushing during lunch time	
		Adjusted OR	Adjusted OR	95% CI	
Age	
	 Middle school	1.00	0.95	(0.91–1.00)	
	 High school	1.00	0.88	(0.85–0.91)	
Gender	
	 Male	1.00	0.91	(0.87–0.94)	
	 Female	1.00	0.89	(0.85–0.94)	
Household income	
	 High	1.00	0.88	(0.84–0.93)	
	 Middle	1.00	0.91	(0.87–0.95)	
	 Low	1.00	0.95	(0.88–1.03)	
Living companion	
	 With family	1.00	0.91	(0.88–0.93)	
	 Live oneself	1.00	0.93	(0.80–1.09)	
	 Childcare facility	1.00	0.52	(0.29–0.94)	
Perceived stress level	
	 High	1.00	0.91	0.85–0.97)	
	 Middle	1.00	0.91	(0.87–0.95)	
	 Low	1.00	0.91	(0.87–0.95)	
Subjective health status	
	 High	1.00	0.93	(0.90–0.96)	
	 Middle	1.00	0.89	(0.84–0.95)	
	 Low	1.00	0.81	(0.74–0.90)	
Physical activity	
	 Middle	1.00	0.88	(0.84–0.92)	
	 High	1.00	0.91	(0.88–0.95)	
	 Low	1.00	1.00	(0.86–1.16)	
Sleep duration	
	 Appropriate sleep	1.00	1.10	(1.06–1.14)	
	 Sleep deprivation	1.00	1.14	(1.06–1.22)	
	 Excessive sleep	1.00	1.03	(0.61–1.74)	
Past medical history(asthma, atopy)	
	 Yes	1.00	0.91	(0.88–0.94)	
	 No	1.00	0.90	(0.85–0.95)	
Nutrition education	
	 Yes	1.00	0.90	(0.87–0.94)	
	 No	1.00	0.91	(0.87–0.95)	
Carbonated drinks	
	 Yes	1.00	0.90	(0.87–0.93)	
	 No	1.00	0.91	(0.85–0.97)	
Oral health status (self-reported)	
	 High	1.00	0.89	(0.86–0.93)	
	 Low	1.00	0.92	(0.88–0.95)	
Sealant history	
	 Yes	1.00	0.91	(0.88–0.94)	
	 No	1.00	0.89	(0.84–0.94)	
Corona period	
	 Pre-corona	1.00	0.89	(0.86–0.93)	
	 Post-corona	1.00	0.91	(0.87–0.95)	
The hypothesis testing for the coefficients was conducted using a chi-squared test based on the Wald statistic.

Figure 1 shows the results of the multinomial multivariate logistic regression analysis between tooth brushing during lunchtime and obesity subgroups. In cases of tooth brushing, the ORs generally decreased as the prevalence of obesity increased. Figure 2 shows the results of the multivariate logistic regression analysis between the subgroups regarding the frequency of tooth brushing during lunchtime and obesity. The ORs decreased as the frequency of tooth brushing increased. As shown in Figs. 1, 2, tooth brushing, sex, age, residential type, living standard, exercise, stress, subjective health status, asthma or atopy, sleep, soda consumption, self-reported oral health, sealant status, nutrition education at school, and pre- and post-corona were all considered variables.Figure 1 Results of multivariate logistic regression analysis between tooth brushing during lunchtime and obesity severity subgroups.

Figure 2 Results of the logistic regression analysis between subgroups regarding the frequency of tooth brushing during lunchtime weekly and obesity.

Discussion

The present study, with its large sample size of 137,734 participants, presents substantial evidence of a novel inverse association between oral hygiene practices, particularly tooth brushing during lunchtime, and the prevalence of obesity among adolescents. This association remained even after adjusting for covariates and subgrouping according to sociodemographic variables and general health status, including physical activity and sleep duration. To our knowledge, the present study is the first to investigate the relationship between tooth brushing during lunchtime and obesity in the Korean adolescent population. Our results, supported by multivariate logistic regression and subgroup analyses, suggest a potential causal relationship; however, further research is required to verify this relationship. This could be either a direct effect of oral hygiene on metabolic processes or an indication that tooth brushing during lunchtime is a marker of a more health-conscious lifestyle, encompassing better dietary choices and increased physical activity.

Regarding the analysis of the primary variables and covariates, the detailed findings and implications derived from our data are presented as follows: The primary variable of interest, “tooth brushing during lunchtime,” was inversely associated with obesity. Participants who brushed their teeth during lunchtime had an adjusted OR of 0.90 for obesity, indicating a 10% lower odds compared with the adjusted OR of those who did not brush. Regarding demographic variables such as sex, age, and household income, boys and high school students exhibited higher odds of obesity. This disparity might reflect developmental and hormonal differences that influence body composition and lifestyle choices during adolescence. Furthermore, lower household income was associated with higher odds of obesity, possibly due to limited access to healthier food options and recreational activities. For health and lifestyle variables, participants with lower subjective health status and a history of asthma or atopy had increased odds of obesity, highlighting the intricate relationship between chronic health conditions and weight management. Notably, a history of dental sealant application was associated with higher odds of obesity, which may reflect broader patterns of health behavior. Regarding societal and environmental factors, we observed that respondents in the post-corona era had higher odds of obesity, indicating the impact of the pandemic on lifestyles, possibly due to reduced physical activity and altered dietary habits during lockdowns. Finally, the results of our subgroup analysis further supported the primary findings. The reduced odds of obesity among those who brushed their teeth at lunchtime were consistent across most subgroups. However, the finding was not significant among middle school students, those with low household income, those living alone, and those with low physical activity levels. This suggests that the relationship between tooth brushing and obesity might be influenced by age, socioeconomic status, and lifestyle factors.

The results of the present study are consistent with those of previous studies, which reported an association between tooth brushing and obesity. In a study in which the association between oral health behavior and obesity was confirmed based on the 2008–2010 National Health and Nutrition Examination Survey, the frequency of tooth brushing was lower in individuals with obesity22. In a cross-sectional study based on data from 21 public schools in Ichikawa City, Chiba, Japan, the frequency of tooth brushing daily, as a secondary factor in addition to physical activity, influenced obesity in adolescents23. In a cohort study of 1,619 workers in Tokyo, individuals who brushed their teeth three or more times daily had considerably lower ORs for obesity than those who brushed their teeth once or less daily24.

Unlike previous studies, in the present study, we limited our age group to teenagers and focused on tooth brushing behaviors during adolescence, when oral health habits are developed. We also examined tooth brushing behaviors during weekday school lunchtime rather than focusing on the number of times children brushed their teeth, as in previous studies. As caregivers strongly influence health habits, such as tooth brushing during adolescence, investigating tooth brushing habits at school in the absence of caregivers, which is associated with autonomy, is imperative. Furthermore, unlike previous studies limited to a specific region, our study represented adolescents in South Korea as a whole and covered the post-corona period.

The exact mechanism underlying the relationship between tooth brushing during lunchtime and obesity remains unclear; however, based on previous studies and biological data, the following hypotheses may explain the mechanism: First, inflammation may contribute to the association between tooth brushing and obesity25,26. Unhealthy tooth brushing behavior can cause inflammation, increasing the concentration of inflammatory markers such as C-reactive protein and fibrinogen27,28. A negative correlation between fibrinogen levels and weight reduction has been reported in previous studies29, and the relevance between C-reactive protein and an increase in the prevalence of obesity30 may strengthen our explanation. Second, tooth brushing may reinforce the overall health-enhancing behaviors of adolescents, such as safety, sleep, diet, exercise, low sedentary behavior, and dental care31,32, causing a significant decrease in the prevalence of obesity33. Furthermore, insufficient tooth brushing during lunchtime implies a lack of willingness to maintain health, possibly due to a decreased awareness of the importance of weight management. Notably, individuals with obesity often lack self-awareness regarding the importance of weight management and have distorted and unhealthy perceptions regarding their health. In a study, students who failed to maintain or manage a healthy oral condition tended to neglect weight management34,35. Nevertheless, further research is required to understand the reasons for these correlations.

This study has some limitations. First, this was a cross-sectional study, and only the correlation between tooth brushing and the prevalence of obesity could be determined; however, a causal relationship could not be established. Second, a risk of false responses persists in this study because the KYRBS, which forms the basis of this study, contains self-reported online data. Furthermore, our data may have consistency and reliability issues, as some variables in this study were based on the subjective judgment of adolescents. Third, in this study, only the data obtained using the KYRBS were used as independent variables; therefore, other factors that could influence obesity, such as the onset of adult disease, were not considered. Finally, we focused on the presence or absence of tooth brushing for most analyses in this study, except that shown in Fig. 2. As Fig. 2 shows a large difference even among teenagers who indicated brushing their teeth after lunch, the dichotomous interpretation can be regarded as a limitation of this study. Therefore, further research may suggest a better analysis using levels.

Despite these limitations, this study has several strengths. Although the KYRBS is limited by cross-sectional data, it provides nationally representative data, enabling us to generalize our findings to the entire adolescent population in South Korea. Our study distinguishes itself from previous studies based on the statistical results that encompass the entire adolescent population of South Korea. Furthermore, we focused on lunchtime meals in schools in South Korea, where factors such as mealtime duration and meal amount tend to remain relatively constant compared with other settings. Notably, as tooth brushing during lunchtime is not compulsory at school, it is more directly associated with students’ healthcare as a measure reflecting students’ autonomy. This approach allowed us to effectively control for confounding variables, ensuring the reliability of our statistical analysis.

In conclusion, a significant negative correlation was observed between tooth brushing during lunchtime and the prevalence of obesity, particularly among the student population with the highest BMI. This correlation was consistent regardless of socioeconomic and lifestyle factors.

Acknowledgements

The authors would like to thank the Korea Disease Control and Prevention Agency and Korean Youth Health Risk Behavior Survey for approving the use of KYRBS data.

Author contributions

Y.C., H.S., and S.L. had full access to all the data in the study and take responsibility for the integrity of data and accuracy of data analysis. Concept and design: Y.C., H.S., and H.K. Acquisition, analysis, and interpretation of data: Y.C., H.S., and S.L. Drafting of the manuscript: Y.C. and H.S. Critical revision of the manuscript for important intellectual content: J.K., J.S., and H.K. Statistical analysis: H.S. and H.K. Supervision: J.K., J.S., and H.K. All authors reviewed the manuscript.

Data availability

Publicly available datasets, obtained from the 2018–2021 KYRBS, were analyzed in this study. All data can be found and downloaded from the KYRBS official website: https://www.kdca.go.kr/yhs/home.jsp.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

These authors contributed equally: Yoonjin Cho and Heejae Shin.
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