Changes in tooth brushing frequency and its associated factors from 2006 to 2014 among French adolescents: Results from three repeated cross sectional HBSC studies.

OBJECTIVES: This study aimed to evaluate in the changes in the percentage of adolescents who brush their teeth twice a day and the association with socio-economic status and health behaviors between 2006, 2010 and 2014 among adolescents from the French cross-sectional studies of the Health Behavior in School-aged Children (HBSC) survey.
METHODS: Our sample included 18727 adolescents aged 11, 13 or 15 years old (y/o). The relationship between toothbrushing frequency (TBF) and eating habits, health and socio-economic status markers, family status, school perception, substance use, sedentary lifestyle and physical activity, together with their evolution over the 3 studies, were investigated using multivariate logistic regression.
RESULTS: The proportion of adolescents brushing twice a day increased from 68.8% in 2006 to 70.8% in 2010 and 78.8% in 2014 (p<0.0001). Notable associated factors (p<0.0001) were: being a girl (adjusted Odds Ratio = 1.5) and, even more, an older girl (aOR 1.5 for 15 y/o vs 11 y/o girls), having breakfast (aOR 1.4) and eating fruits daily (aOR 1.6), excellent perceived health (aOR 1.2), obesity or overweight (aOR 0.6), being bullied at school (aOR 0.8), and perceived family wealth (aOR 1.4 for High vs Low). No impact from any associated factor changed over the 3 studies.
CONCLUSIONS: Among French adolescents, TBF improved from 2006 to 2014. TBF was significantly associated with other health behaviors. These associations stayed similar in 2006, 2010 and 2010. This increase in TBF may be linked with global prevention programs developed during this time period. These programs should be maintained and associated with more specific ones targeting and adapted to disadvantaged populations, in order to reduce inequalities in oral hygiene and oral health.

Introduction

Oral diseases such as dental caries and periodontal diseases are among the most frequent chronic diseases around the world [1]. Dental caries is the most frequent childhood disease and the vast majority of adults in developed countries, if not worldwide, suffer from at least one oral pathology [1, 2]. Among 12 years old (y/o) French children, the DMFT index (Decayed, Missing or Filled Teeth) decreased from 4.20 to 1.23 between 1987 and 2006, while the proportion of children free of dental caries increased from 12% to 55.9% over the same period [3, 4]. Despite these encouraging observations, around half of the children still suffered from carious diseases.

The last study on the oral status of (12 year old) French children described an improvement in the proportion of children free from dental caries, from 56% in 2006 to 66% in 2010, among children participating in a national voluntary prevention program with an attendance of only 33% [4, 5]. Thus, it is likely that the oral status of the population, qualified as good overall, is overestimated in this study. Another study, in French schools in 2009, showed only 56% of adolescents to be caries-free, a proportion around 10% lower than the 2010 sample previously described [6].

Oral hygiene, whose main element is toothbrushing, is an efficient, low-cost and easy-to-implement method to prevent almost all oral diseases, especially caries and gingivitis [7]. A recent meta-analysis confirmed that an increase in tooth brushing frequency is associated with a lower incidence and increment of carious lesions [8]. There is overall national and international consensus to advise a twice a day frequency of tooth brushing to prevent oral diseases [7, 9, 10]. It has also been shown that brushing more than once a day before the age of 12 leads to a stable tooth brushing frequency during the following years [11]. Tooth brushing is not only a preventive act; its frequency also provides an easy-to-assess indicator of oral health habits.

While necessary to improve oral health, changes in dietary habits and tooth brushing frequency and efficiency are hard to implement [9, 12]. A low socio-economic status (SES) during childhood is associated with a lower tooth brushing frequency and a higher caries prevalence among adults, as are other aggravating factors for oral health such as smoking or unhealthy diet [13-15]. Upward changes in SES in adulthood are not sufficient to recover from these adverse effects [16].

A previous work on data from the 2010 French HBSC study (Health Behavior in School-aged Children) highlighted the strong relationships between tooth brushing frequency, health behaviors (mostly dietary), health and body perception, and environmental socio-economic factors [17].

We wanted to evaluate the trends in toothbrushing frequency over successive studies, as well as the evolution of the relationships between toothbrushing frequency and previously identified associated factors. Our main hypothesis was that adolescent’s behavior regarding toothbrushing frequency may have improved. Our secondary question was “Among adolescents, what is the effect of associated factors (Diet, Health and body perception, School life, SES) on toothbrushing frequency in 2006, compared to 2010 and 2014”.

The objectives of the present study were (1) to provide an update on the evolution of the tooth brushing frequency among French adolescents, and (2) to determine if the relationship between toothbrushing frequency and its socio-economic and behavioral associated factors changed over the years, using the data from the successive French HBSC studies of 2006, 2010 and 2014.

Materials and methods

We used the data from the representative HBSC cross-sectional studies of 2006, 2010 and 2014 on French adolescents from 11 to 15 years old. Details on the survey can be found at http://www.hbsc.org.

Briefly, the HBSC studies are cross sectional studies, repeated every 4 years, describing adolescent’s well-being, health behaviors and their social context. Our sample consists of adolescents from 11 to 15 years old selected using the international standardized HBSC sampling protocol. A two-stages cluster sampling over schools (public and private) and classes (two classes by school) was used. We used only the French cases included in the HBSC international database, which targeted adolescents of 11, 13 and 15 years old.

The full protocol of the HBSC studies has been described in previous articles and is designed to collect the most representative data and limit the risk of bias [18-20]. The questionnaire includes mandatory international questions, optional international thematic questions, and some specific national questions. It was validated on around 100 adolescents prior to the study, and double checked for translation from English. The minimal sample size (around 4500 for each study) was determined by the international HBSC protocol.

Information was collected over 2 months (22th April to 22th June in 2010) using an anonymous self-reported questionnaire. Data collectors were mostly teachers and nurses. The HBSC study received approval from the French data collection authority (CNIL) and the French ministry of education. Parents and students were given a consent form with the option to refuse before the adolescent’s participation.

Tooth brushing frequency was assessed using the question “How often do you brush your teeth?” with five levels of response: More than once a day, Once a day, At least once a week but not daily, Less than once a week, Never. Data were pooled into two groups: “Tooth brushing at least twice a day” and “Tooth brushing once a day or less”. This cutoff was chosen to match the international recommendations for tooth brushing and to avoid groups of subjects being too small. Toothbrushing frequency was our main outcome; other variables were considered as potential associated factors. Missing answers for this question led to exclusion from the analysis.

Despite being fairly constant over the years, the questionnaire has evolved slightly, especially as far as questions specific to the French version are concerned. Variables previously identified as associated with toothbrushing frequency and coming from items that remained similar in each study from 2006 to 2014 were considered for this paper (Table 1). They were divided in 4 main categories: Diet, Health and body, School life, Socio-economic status (SES).

Table 1

Proportion of children brushing at least twice a day according to explanatory variables over the whole sample (bivariate analyses), significant variables at p<0.05 in bold.

N = 18727.

Variable (% of the sample)	%	p-value
DIET
Daily breakfast		<0.0001
Yes (58.4%)	73.3
No (41.6%)	67.3
Daily fruit consumption		<0.0001
Yes (35.3%)	77.4
No (64.7%)	67.3
Daily vegetable consumption		<0.0001
Yes (42.8%)	74.5
No (57.2%)	68.1
Daily sweets consumption		0.001
Yes (25.1%)	69.0
No (74.9%)	71.5
Daily soft drinks consumption		<0.0001
Yes (26.3%)	67.3
No (73.7%)	72.1
Being on a diet		0.14
Yes (9.9%	72.3
No (90.1%)	70.6
HEALTH AND BODY
Excellent self-reported health		<0.0001
Yes (35.9%)	74.0
No (64.1%)	69.0
Overweight or obese		<0.0001
Yes (10.6%)	59.6
No (89.4%)	72.6
Cantril score > = 6		<0.0001
Yes (84.1%)	71.6
No (15.9%)	66.6
Body Image		<0.0001
About the right size (57.4%)	72.5
Too thin (13.4%)	69.0
Too fat (29.2%)	68.2
Variable (% of the sample)	%	p-value
SCHOOL LIFE
Academic delay		<0.0001
Yes (14.6%)	65.4
No (85.4%)	71.7
Liking school		<0.0001
Yes (69.6%)	72.0
No (30.4%)	68.0
Stressed by school		0.06
Yes (68.3%)	70.4
No (31.7%)	71.8
Victim of violence at school		<0.0001
Yes (21.0%)	66.0
No (79.0%	72.1
Perceived school grades		<0.0001
Low (11.8%)	66.1
Mid (36.8%)	68.4
High (51.4%)	73.8
Support from classmates		<0.0001
Low (12.1%)	66.6
Mid (54.6%)	70.5
High (33.3%)	73.0
School too demanding		0.28
Low (36.2%)	70.7
Mid (48.8%)	71.2
High (15.0%)	69.6
Bullying others		<0.0001
Yes (36.0%)	66.9
No (64.0%	73.2
Being bullied		<0.0001
Yes (32.8%)	66.5
No (67.2%)	73.0
Variable (% of the sample)	%	p-value
SES
Perceived wealth		<0.0001
Low (7.4%)	63.1
Mid (27.4%)	67.6
High (65.2%)	72.9
Parental activity		<0.005
Both parents working (75.1%)	71.6
One parent working (20.8%)	69.2
None working (3.1%)	66.8
Family Affluence Scale		<0.0001
Low (8.8%)	66.1
Mid (35.5%)	69.3
High (55.6%)	72.8

The Body Mass Index (BMI) was used with Cole’s age and sex specific cut-offs for overweight or obesity which allow the use of BMI for children. The Cantril score was used to measure overall life satisfaction and dichotomized at the usual threshold of 6 or over for good life satisfaction [21].

Analyses were performed using R [22] with the RStudio interface [23]. Chi square tests were used to assess the relationship between tooth brushing frequency and co variables.

Three variables were used to assess SES: Perceived wealth, Parental employment and the Family Affluence Scale (FAS), a composite variable used in HBSC studies since 1990 to measure SES [24]. Due to very strong collinearity, we only used the most significant and discriminating variable in regression models: Perceived wealth.

Three regression models were used with toothbrushing frequency as the response variable to:

Test for interactions between the year of study and each significant variable in order to assess any changes of the impact of these variables.

Control for SES (measured by Perceived wealth) which was considered as the main potential causal factor associated with toothbrushing frequency while other groups of variables (Diet, Perceived health and body, school life) were considered as potential confounding factors (Table 2a, 2b and 2c).

Table 2

Results from the three different logistic regressions on the whole sample with the adjusted odds-ratios (aOR) of brushing at least twice a day.

A. Dietary behavior. B. Health and body. C. School life.

	aOR	p-value	CI 95%
A.
Study year 2010 versus 2006	1.01	0.7393	0.94	1.09
Study year 2014 versus 2006	1.86	<0.0001	1.7	2.02
Boy of 13 y/o versus 11 y/o boy	1.11	0,0598	1	1.24
Boy of 15 y/o versus 11 y/o boy	1.18	0,0037	1.06	1.32
Girl of 11 y/o versus 11 y/o boy	1.51	<0.0001	1.35	1.69
Girl of 13 y/o versus 11 y/o boy	2.36	<0.0001	2.09	2.65
Girl of 15 y/o versus 11 y/o boy	2.71	<0.0001	2.39	3.07
Eating breakfast daily	1.38	<0.0001	1.29	1.48
Eating fruits daily	1.52	<0.0001	1.41	1.65
Eating vegetables daily	1.13	0.0010	1.05	1.22
Eating sweets daily	0.91	0.0194	0.83	0.98
Drinking soft drinks daily	0.91	0.0181	0.84	0.98
Wealth perceived: Mid vs Low	1.25	0.0008	1.1	1.43
Wealth perceived: High vs Low	1.66	<0.0001	1.47	1.89
B.
Study year 2010 versus 2006	1.07	0.1041	0.99	1.16
Study year 2014 versus 2006	1.96	<0.0001	1.79	2.15
Boy of 13 y/o versus 11 y/o boy	1.07	0.2311	0.96	1.20
Boy of 15 y/o versus 11 y/o boy	1.04	0.4680	0.93	1.17
Girl of 11 y/o versus 11 y/o boy	1.57	<0.0001	1.39	1.77
Girl of 13 y/o versus 11 y/o boy	2.31	<0.0001	2.03	2.62
Girl of 15 y/o versus 11 y/o boy	2.54	<0.0001	2.23	2.89
Cantril ≥ 6	1.19	0.0007	1.08	1.31
BMI: Overweight or obese	0.65	<0.0001	0.58	0.73
Excellent perceived health	1.30	<0.0001	1.21	1.41
Body image: too thin vs normal	0.93	0.1880	0.84	1.04
Body image: too fat vs normal	0.88	0.0037	0.80	0.96
Wealth perceived: Mid vs Low	1.16	0.0456	1.00	1.33
Wealth perceived: High vs Low	1.47	<0.0001	1.29	1.69
C.
Study year 2010 versus 2006	1.02	0.6111	0.94	1.11
Study year 2014 versus 2006	1.86	<0.0001	1.70	2.04
Boy of 13 y/o versus 11 y/o boy	1.15	0.0147	1.03	1.29
Boy of 15 y/o versus 11 y/o boy	1.18	0.0049	1.05	1.33
Girl of 11 y/o versus 11 y/o boy	1.56	<0.0001	1.39	1.76
Girl of 13 y/o versus 11 y/o boy	2.32	<0.0001	2.06	2.61
Girl of 15 y/o versus 11 y/o boy	2.62	<0.0001	2.31	2.97
Having academic delay	0.80	<0.0001	0.72	0.88
Perceived school grades: Mid	1.03	0.6189	0.92	1.15
Perceived school grades: High	1.24	0.0002	1.11	1.40
Liking school	1.02	0.6955	0.94	1.10
Demand from school: Mid	0.94	0.1313	0.87	1.02
Demand from school: High	0.90	0.0633	0.81	1.01
Classmate’s support: Mid	1.17	0.0049	1.05	1.30
Classmate’s support: High	1.29	<0.0001	1.15	1.45
Being bullied	0.84	<0.0001	0.78	0.90
Bullying others	0.86	<0.0001	0.80	0.93
Wealth perceived: Mid vs Low	1.19	0.0112	1.04	1.36
Wealth perceived: High vs Low	1.56	<0.0001	1.37	1.77

For variables with 3 levels, the “Low” level was used as reference. Due to the interaction, age and sex are described as one variable with 6 levels, the reference level being a boy of 11 y/o. Significant values at p<0.0001 in bold.

The reference level is a boy of 11 y/o in 2006. For example, on Table 2a, being a boy of 11 y/o in 2014 gives an aOR of 1.86 in favor of brushing at least twice a day versus a boy of 11 y/o in 2006. No significant interactions were identified, so the aOR of brushing at least twice a day for a 15 y/o girl in 2014 versus a 15 y/o girl in 2006 is not significantly different from 1.86.

Three final models were obtained, each including the Year of the study, Age, Sex, Perceived wealth, and the significant (p<0.001) variables describing either Diet (Table 2a), Health and body (Table 2b) or School life (Table 2c), as well as significant interactions if any. The impacts of variables were measured using adjusted odds ratio (aOR).

Interactions were tested between age, sex and the other significant variables, since age and sex are commonly involved in interactions regarding health behaviors.

As is often done in HBSC analyses, we used a significance level α of p<0.001.

Results

From the 29226 questionnaires of the 2006, 2010 and 2014 studies, keeping only the questionnaires of students of 11, 13 and 15 years old who were included in the international HBSC studies (10309 exclusions) and who answered the question about toothbrushing (352 exclusions), 18727 questionnaires were analyzed, 7135 for 2006, 6103 for 2010 and 5489 for 2014 (Fig 1).

Fig 1

Flow chart.

In 2006, the study was designed to comply with the international requirements while, in 2010 and 2014, the sample was larger to better describe a specific French population.

The sample was almost perfectly balanced, with 9352 boys (49.9% overall, 49.7% in 2006, 50.1% in 2010 and 2014) and 9375 girls. 6294 students were 11 years old (33.6%), 6609 were 14 (35.3%), and 5824 were 15 (31.1%) (Fig 2).

Fig 2

Population according to age, sex and year of study.

Toothbrushing frequency improved significantly in 2014 compared to 2010 and 2006. While in 2006, 66.8% of students reported brushing twice a day or more, their percentage was 68.0% in 2010 (not significantly different from 2006) and 78.8% in 2014 (significantly different from 2006 and 2010, p<0.0001).

Age and sex are strong factors associated with tooth brushing frequency, and their impact changed little over the years (Fig 3). Brushing frequency among boys is not associated with their age, while toothbrushing frequency increased among girls as they grow older. However, this improvement with age among girls tended to diminish in the 2014 study due to a faster increase of the twice a day brushing among younger girls (+11.4% from 2010 to 2014 for the 11 year old girls) than among older girls (+9.3% over the same period for the 15 year old girls). Being a girl (and, even more, an older girl) remained the most important associated factor over the 3 studies with around 10% more girls than boys brushing twice a day.

Fig 3

Proportion of adolescents brushing at least twice a day according to age, sex and year of the study.

The proportion is significantly higher (***: p>0.0001) among girls than boys, and in 2014 compared to 2010 and 2006 in every age/sex group (***). A slightly increased toothbrushing frequency among older girls (13 y/o vs 11 y/o, and 15 y/o vs 13 and 11 y/o) can also be seen.

Among the whole sample (2006, 2010 and 2014 grouped together), almost all studied variables were associated with tooth brushing frequency (Table 1).

In particular, healthy eating habits (daily consumption of fruit, vegetables or breakfast) were associated with an increased proportion of twice a day tooth brushing among students; up to a 10% increase for fruit consumption (Table 1). High socio-economic status markers (Family affluence scale, perceived family wealth, parental employment), having no problem in school life (no involvement in bullying or violence, not lagging behind academically, liking school), excellent self-reported health, good body perception, good life satisfaction and not being overweight or obese were also associated with a higher toothbrushing frequency (Table 1).

The vast majority of the missing values in regression models were due to the BMI item (2074 missing values). Among the adolescents with a missing BMI, 67.2% brushed twice a day. Since this value is between the “Normal weight” (72.6% of them brush twice a day) and the “Overweight or obesity” (59.6%) groups, we assumed that those missing value were from adolescents with normal weight as much as overweight or obese adolescents. The most probable reason for those missing value is adolescents not knowing their height or weight. Those missing values were considered as missing at random.

Results from the regressions (Table 2a, 2b and 2c) confirmed the findings on the impact of age and sex, including an interaction term. No significant effect of age on toothbrushing frequency among boys was identified, while there was a slight significant effect among girls. Adjusted OR (aOR) at 11 y/o ranged from 1.40 to 1.57 in the models, and from 2.54 to 2.71 at 15 y/o. Results shown are the cumulative effect of being a girl and being 11, 13 or 15 y/o, the base level being an 11 y/o boy. In each model, being a girl (especially an older girl) was the strongest factor associated with an appropriate toothbrushing frequency. The diminution over the studies of the impact of age on girls’ toothbrushing frequency, identified in bivariate analysis, was not strong enough to be kept in the regression model.

The year 2014 was the second strongest associated factor and no interaction between the year of the study and any other variable associated with tooth brushing frequency was found. All associated factors stayed almost constant over the years.

SES, measured via perceived wealth, was the 3rd strongest associated factor.

In the “Dietary behavior” model (Table 2a), having a breakfast (aOR 1.38, CI95% [1.29–1.48]) and eating fruits every day (aOR 1.52, CI95% [1.41–1.64]) were the strongest positive factors associated with twice a day tooth brushing.

In the “Health and body” model (Table 2b), being overweight (aOR 0.65, CI95% [0.58–0.73]) was a strong negative factor associated with toothbrushing frequency, while an excellent perceived health (aOR 1.30, CI95% [1.21–1.41]) was the strongest positive associated factor.

In the “School life” model (Table 2c), Classmate’s support and high perceived school grades were positive associated factors, while having academic delay or being part of bullying as a bully or a victim were unfavorable factors associated with twice a day toothbrushing.

Discussion

We observed an increase of twice a day tooth brushing among adolescents from around 70% in 2006 and 2010 to 80% in 2014. It is likely that knowledge about the appropriate toothbrushing frequency led to a response (social desirability) bias overestimating the real toothbrushing frequency. It is however likely that this bias stayed constant over the 3 studies as no important changes were made to the protocol of the studies, and thus that the increase of twice a day toothbrushing reflect a real trend. Societal pressure may have increased, but would be more likely to be associated with an increase in missing answers to the question in 2014, than to false declaration to this extent. The national voluntary prevention program mentioned in introduction was updated to be more efficient on the same period, however it still only reaches one third of the targeted children and likely can’t be the only cause of the changes we observed. A detailed investigation on the French national and regional prevention programs as well as societal changes could help understand this evolution.

The strongest factor explaining differences in toothbrushing behavior is being a girl, which has already been known for a long time [14]. A high SES was a strong factor associated with adequate toothbrushing in each model.

A low tooth brushing frequency is associated with poor perceived health, unhealthy eating habits and overweight or obesity, all of these elements being likely consequences of unhealthy lifestyles and poor health knowledge. This should be a major concern since the association of these unfavorable factors leads to an increased risk of oral and general health diseases. Difficulties in school life and low perceived family wealth are often associated with low SES, which is often found as a socio-environmental factor associated with poor oral hygiene. Our findings match previous results [17].

An adequate toothbrushing frequency comes from the combination of knowledge (Health literacy) and motivation [25]. Age, gender and SES are most likely associated with both elements and possibly causal factors for toothbrushing frequency. On the other hand, the “Health and body” and “Dietary behavior” variables are consequences of health literacy and should be considered as confounding factors considering toothbrushing frequency. They are however of interest due to the cumulative effect they may have with toothbrushing frequency in preventing or facilitating oral diseases.

Variables describing “School life” are harder to interpret. They may be consequences of SES (a low SES is associated with lower academic results and a higher risk of bullying), and thus confounding factors, but could also be causal factors for knowledge (better academic results linked to a better health literacy) and motivation (via better relationships with classmates). Those two interpretations are likely coexisting.

The improvement in tooth brushing frequency is shared among the whole population with no exception. All associated factors stayed constant in 2006, 2010 and 2014. Traditional populations showing a high risk of low oral hygiene (boys, populations with low social and economic status, cumulating other risk factors such as unhealthy lifestyles) benefited from this change as well and were not left behind. Although they still have more room for improvement than the other adolescents, this shows that there are no inevitable consequences for those populations, which are often considered to be too hard to reach for prevention.

Most missing values were linked to BMI. Adolescents may not have known their height or weight, or did not want to disclose it. A lack of knowledge seems more likely since both variables were missing at roughly the same ratio, and while weight is a sensitive question, height isn’t. This is backed by the fact that their brushing frequency is between the frequencies of the “overweight” group and the “normal weight” group, advocating for a mixed group.

Global population-based prevention programs should be maintained to address the broadest proportion of children and adolescents. Some high-risk groups should be targeted with more specific programs, such as multidisciplinary counselling or therapeutic patient education to improve their knowledge and behavior considering oral health [12, 26].

Cohort studies are needed to confirm the causality hypothesis we developed in discussion, which cross-sectional studies like HBSC can’t. While this sample is highly representative of the French population, it would be interesting to check if the associations identified during this study are similar in other countries, which could be done using the result from other HBSC studies.

Conclusion

Tooth brushing frequency increased among adolescents from 2010 to 2014, probably due to an efficient improvement of the prevention programs over recent decades. Although socio-economic inequalities in oral hygiene habits persists, populations at risk still benefitted from the improvement in toothbrushing frequency like the rest of the population.

Oral hygiene stays closely associated with other healthy behaviors and should be considered as part of a whole when addressing health education. It is possible to improve oral hygiene and while the whole population could benefit from it, some populations are more in need and could use oral hygiene and diet improvement to reduce their risk for oral diseases.

Comparison with the other countries undertaking the HBSC study could now offer a broader vision of oral hygiene in western countries.

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PONE-D-20-30886

Changes in toothbrushing frequency and its associated factors from 2006 to 2014 among French adolescents

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Reviewer #2: No

Reviewer #3: N/A

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3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: Yes

Reviewer #3: Yes

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5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thank you very much for submitting your research to Plos One. The reviewer would like to make several comments on your article.

1.What is the main message the authors would like to deliver?

2.Pleaes comment on the accuracy of the self-reported questionnaire.

3.Please provide the full paragraph of self-reported questionnaires.

4.Please comment on this.

This cutoff was chosen to match the international recommendations for tooth brushing and to avoid groups of subjects being too small.

5.Please comment on confounding factor.

6.Can you provide the data for more recent years.

7.Can the individual be followed up.

8.What is the rationale for toothbrushing frequency?

9.Please provide the possible explanation.

10. Please provide the clinical relevance.

Thank you very much.

Reviewer #2: The paper shows that among French adolescents, toothbrushing frequency changed from 2006 to 2014 using the data of Health Behavior in School-aged Children (HBSC) survey.

This is an interesting study. However, I would like to make some points regarding the manuscript. The article needs to be revised. First, the authors should follow the STROBE guideline if this study is an observational study. Second, the logic is unclear. The authors answered to a reviewer in the last revision as below.

However, the current form is not appropriate.

1. I actually wasn’t entirely sure what the research question was. I believe the question was to determine whether the percentage adolescents who brushed their teeth twice a day changed over time. What was confusing was that the predictors were all individual/family level variables. So there seemed to be a mismatch between the question and the factors examined. To determine predictors of change over time, I would look to broader social factors. The authors seemed to recognize this, because in their discussion, they speculated about the role of a social program on the outcome. In their study, however, they did not examine the effect of this social program analytically. Thus, there is a mismatch between the research question and the predictors included in the study. To me, this is a fatal flaw of the paper.

Answer: It seems our secondary objective was misunderstood by this reviewer which led to many criticisms. We wanted to determine if the relationship between toothbrushing frequency and its predictors changed over time among French adolescents. We did NOT aim to determine predictors of changes in toothbrushing frequency; a goal that would require a longitudinal study. We did NOT aim to offer an analytical model. We reformulated our objectives : “The objectives of the present study were (1) to provide an update on the evolution of the tooth brushing frequency among French adolescents, and (2) to determine if the relationship between toothbrushing frequency and its socio-economic and behavioral predictors changed over the years, using the data from the successive French HBSC studies of 2006, 2010 and 2014.”

2. The authors seemed to have ideas about the importance of different classes of predictors, but they failed to describe any theoretical underpinning for their ideas. I believe we have passed the time when atheoretical research is acceptable. At this time, the best way for research to make a contribution is if it is theory-driven. The authors should ground their work in theory. Doing so would help the authors address the point above. This is a flaw that is addressable, but addressing it would turn this into a new paper.

Answer: We indeed have theory about the roles of the identified predictors, which are discussed in the paper. We rewrote the discussion to make it clearer. However, we insist on the fact that this article’s main goal was to identify changes in adolescent toothbrushing, and is thus a descriptive article whose goal is to offer information about the situation, not to offer a full theory on the relationships between toothbrushing and all its associated factors.

3. The analytic strategy is weak. There are serious drawbacks to using stepwise regression, one being that results using this approach are not generalizable. The results are contingent on the specific sample on which the analysis is run. See: https://www.stata.com/support/faqs/statistics/stepwiseregression-problems/ This is a flaw that is addressable, but addressing it would turn this into a new paper. My advice would be to start with theory and figure out the ways in which their study can help inform our understanding of that theory.

Answer: We rewrote the article, deleted the part about stepwise regression and choose to present only descriptive models to make it clearer that this is a descriptive article. We never aimed to offer more than a description of toothbrushing frequency and its associated factors among French adolescents. Our discussion mentions the possible consequences and origin of these associations, but we did not aim to offer a full theory about the mechanisms leading to these associations.

Furthermore, to be accepted for publication in PLOS ONE, research articles must satisfy the following criteria:

1. The study presents the results of original research.

2. Results reported have not been published elsewhere.

3. Experiments, statistics, and other analyses are performed to a high technical standard and are described in sufficient detail.

4. Conclusions are presented in an appropriate fashion and are supported by the data.

5. The article is presented in an intelligible fashion and is written in standard English.

6. The research meets all applicable standards for the ethics of experimentation and research integrity.

7. The article adheres to appropriate reporting guidelines and community standards for data availability.

The current form is not acceptable in this journal policy.

TITLE

1) Please add the study design following the STROBE guideline if it is a cross-sectional study.

INTRODUCTION

1) The logic is unclear. If this is a descriptive epidemiology, the authors should need hypothesis formulation. The structure of this paper should be changed dramatically. If it’s a cross-sectional study, they need to add hypothesis before aim. The PECO model should be considered.

2) The topic of dental caries occupies most of introduction section. The authors should make it short.

MATERIALS & METHODS

1) Please add some comments following the STROBE checklist.

2) Children or adolescents? Please unify the expression.

3) The authors stated that “Details on the survey can be found at http://www.hbsc.org.” and “The full protocol of the HBSC studies has been described in previous articles (17-19).” However, the authors should add more details for readers following the STROBE checklist. Furthermore, I could not find any supplemental files. 4) If the authors would like to compare toothbrushing frequency in 2010 and 2014 against 2006, and to investigate whether a temporal association between toothbrushing frequency and its related factors changed over time, the statistical analyses should be changed. The logistic regression models should be revised based on the hypothesis/PECO model.

RESULTS

1) Please add the characteristic table following the STROBE guideline.

2) Please the results based on new methods.

DISCUSSION

1) The data will be changed by new results.

2) The authors cannot use “predictive” and “improve” because this is a cross-sectional study and not cohort study.

3) What is the sentence, “An adequate toothbrushing frequency is the fruit of knowledge (Health literacy) and motivation”? Please revise it appropriately.

4) In a paragraph, there is only one sentence, “All those results match previous results (16).” The authors should revise the part.

5) Please add some comments about limitations, such as a cross-sectional study, no data of important confounders, no generalizability, bias, etc.

6) Please revise the conclusions because this is a cross-sectional study.

Reviewer #3: - it is based on questionnaire data (survey) that may over or under estimate the real tooth brush frequency.

- If the authors presume the incresced toothbrushing frequency in 2014 is related to the educational program, I recommend to mention the subjective evidence uch as statistical analysis.

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6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

9 Feb 2021

All comments from reviewers are answered in the response file. Additionally, here are our answers to the Journal requirements:

1) Concerning style requirements, we updated the organization of the article, the naming of figures and the organization of tables according to PLOS ONE's style requirements. We used page break to keep tables organized since they are quite long.

2) Concerning consent, we corrected the sentence to include more details: "Parents and students were given a consent form with the option to refuse before the adolescent’s participation."

3) Concerning data availability, we realized that the data are now old enough to be on public access, and we gave the link to the databases, both on editorial manager and in the manuscript: "Data are fully available on: https://www.uib.no/en/hbscdata/113290/open-access"

Submitted filename: response reviewers.docx

Click here for additional data file.

4 Mar 2021

PONE-D-20-30886R1

Changes in tooth brushing frequency and its associated factors from 2006 to 2014 among French adolescents: results from three repeated cross sectional HBSC studies

PLOS ONE

Dear Dr. Fernandez de Grado,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Apr 18 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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We look forward to receiving your revised manuscript.

Kind regards,

Frédéric Denis, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: (No Response)

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2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: No

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: No

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: No

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Dear Authors

I extent my sincere thanks for submitting your revised manuscript for the further review. Authors have answered all the queries very nicely. This paper is of excellent merit and it seems fully acceptable for publication in Medicine.

Thank you very much

Reviewer #2: The paper was overall improved. However, there are some issues. The logic is still unclear.

1) The authors answered;

‘Our secondary question was “Among adolescents, what is the effect of associated factors (Diet, Health and body perception, School life, SES) in 2006, compared to 2010 and 2014 on toothbrushing frequency”’.

However, the hypothesis is inappropriate, because the participants were not same. We can only investigate the association between tooth brushing frequency in 2006 and related factors in 2006 but not 2010/2014.

2) The title of table 2 is unclear. Please add more detail comments. What is the odds ratio for? Furthermore, if the dependent value is “Tooth brushing at least twice a day” vs. “Tooth brushing once a day or less” in 2014, the independent values should be the data in 2014 but not 2006/2010. The analyses are inappropriate, even though “all predictors stayed almost constant over the years”. The authors can’t separate the table 2 such as 2a, 2b and 2c, too.

3) What is the mechanism? Do the authors mean that eating breakfast daily, eating fruits daily, eating vegetables daily, wealth perceived, BMI, excellent perceived health, body image, having academic delay, perceived school grades, classmate’s support, being bullied, bullying others is associated with TBF? Are these factors risk for decreasing TBF? If yes, they should show the appropriate references in the introduction and discuss the mechanisms. If no, the logistic regression analyses should be deleted or revised.

4) The authors can’t use the words, “predictors” and “predictive factors”, because this is a cross-sectional study but not a cohort study. If the new results are similar after re-analyses, they may change the conclusion; “Among French adolescents, TBF improved from 2006 to 2014. The TBF was significantly associated with some health behaviors in each year.”

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: Yes: Jun-Beom Park

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

10 Mar 2021

Answer to the reviewers

Dear Reviewers,

Thank you for your constructive comments. You will find in this text, in red, all the changes we have made (re-written sentences, corrections, and adjunctions), following your remarks and advices. Changes in the article are in revision mode.

Please find below our answers concerning some of your questions and points:

1) The authors answered;

‘Our secondary question was “Among adolescents, what is the effect of associated factors (Diet, Health and body perception, School life, SES) in 2006, compared to 2010 and 2014 on toothbrushing frequency”’.

However, the hypothesis is inappropriate, because the participants were not same. We can only investigate the association between tooth brushing frequency in 2006 and related factors in 2006 but not 2010/2014.

Of course, in our study we compared the association between TBF in 2006 and its related factors in 2006, with the association of TBF in 2010 and its related factors in 2010 (and the same in 2014). We did not compare related factors of 2010 or 2014 with TBF in 2006 which would make no sense.

We rewrote the sentence to make this clearer.

“Among adolescents, what is the effect of associated factors (Diet, Health and body perception, School life, SES) on toothbrushing frequency in 2006, compared to this association in 2010 and 2014 “

Actually, when studying a specific age group (children from 11 to 15 in our study), it is impossible to keep the same sample over years. However, using successive representative samples, it is possible to compare those samples and then describe an evolution over the years.

2) The title of table 2 is unclear. Please add more detail comments. What is the odds ratio for? Furthermore, if the dependent value is “Tooth brushing at least twice a day” vs. “Tooth brushing once a day or less” in 2014, the independent values should be the data in 2014 but not 2006/2010. The analyses are inappropriate, even though “all predictors stayed almost constant over the years”. The authors can’t separate the table 2 such as 2a, 2b and 2c, too.

We added more details to this table’s title which was unclear. The new title better explains the analyses that were done, without the need to refer to the manuscript’s text.

“Tables 2: Results from the three different logistic regressions on the whole sample with the adjusted odd-ratios of brushing at least twice a day: The reference level is a boy of 11 y/o in 2006. For example, on table 2a, being a boy of 11 y/o in 2014 gives an aOR of 1.86 in favor of brushing at least twice a day. No significant interactions were identified, so the aOR of brushing at least twice a day for a 15 y/o girl in 2014 versus a 15 y/o girl in 2006 is not significantly different from 1.86.”

3) What is the mechanism? Do the authors mean that eating breakfast daily, eating fruits daily, eating vegetables daily, wealth perceived, BMI, excellent perceived health, body image, having academic delay, perceived school grades, classmate’s support, being bullied, bullying others is associated with TBF? Are these factors risk for decreasing TBF? If yes, they should show the appropriate references in the introduction and discuss the mechanisms. If no, the logistic regression analyses should be deleted or revised.

We discuss those elements in discussion, stating that most of the elements associated with toothbrushing frequency are probably confounding factors sharing common causal factors (SES, health knowledge, lifestyle) with toothbrushing frequency. This is however of interest since some of these factors are also predictors of oral health, just like TBF. Of course, those are only hypothesis in discussion since we have no way to determine causality links.

“A low tooth brushing frequency is associated with poor perceived health, unhealthy eating habits and overweight or obesity, all of these elements being likely consequences of unhealthy lifestyles and poor health knowledge. This should be a major concern since the association of these unfavorable factors leads to an increased risk of oral and general health diseases. Difficulties in school life and low perceived family wealth are often associated with low SES, which is often found as a socio-environmental predictor for poor oral hygiene. Our findings match previous results (17).

An adequate toothbrushing frequency comes from the combination of knowledge (Health literacy) and motivation (25). Age, gender and SES are most likely predictors of both elements and possibly causal factors for toothbrushing frequency. On the other hand, the “Health and body” and “Dietary behavior” variables are consequences of health literacy and should be considered as confounding factors considering toothbrushing frequency. They are however of interest due to the cumulative effect they may have with toothbrushing frequency in preventing or facilitating oral diseases.

Variables describing “School life” are harder to interpret. They may be consequences of SES (a low SES is a predictor of lower academic results and a higher risk of bullying), and thus confounding factors, but could also be causal factors for knowledge (better academic results linked to a better health literacy) and motivation (via better relationships with classmates). Those two interpretations are likely coexisting.”

4) The authors can’t use the words, “predictors” and “predictive factors”, because this is a cross-sectional study but not a cohort study. If the new results are similar after re-analyses, they may change the conclusion; “Among French adolescents, TBF improved from 2006 to 2014. The TBF was significantly associated with some health behaviors in each year.”

This term still seems confusing despite our explanation in our previous answer that we used “predictor” with the statistical meaning of “associated factor” with no causality implication. Therefore, and to make it clearer to the readers, we replaced every instance of “predictor” and “predictive factor” by “associated factor” or “factor associated with…”.

Following your suggestion, we changed our conclusion in the abstract to “Among French adolescents, TBF improved from 2006 to 2014. TBF was significantly associated with other health behaviors. These associations stayed similar in 2006, 2010 and 2014.”

We really hope that these answers and corrections will meet your expectations.

Best regards,

The authors.

Submitted filename: response reviewers2.docx

Click here for additional data file.

12 Mar 2021

Changes in tooth brushing frequency and its associated factors from 2006 to 2014 among French adolescents: results from three repeated cross sectional HBSC studies

PONE-D-20-30886R2

Dear Dr. Fernandez de Grado,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Frédéric Denis, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

17 Mar 2021

PONE-D-20-30886R2

Changes in tooth brushing frequency and its associated factors from 2006 to 2014 among French adolescents: results from three repeated cross sectional HBSC studies

Dear Dr. Fernandez de Grado:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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Thank you for submitting your work to PLOS ONE and supporting open access.

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on behalf of

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Academic Editor

PLOS ONE