The oral health environment and the equal environment assumption (EEA) among 1-8-year-old twins

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1 D. Holst, J.A. Aas Institute of Clinical Dentistry, Section of Community Dentistry and Department of Prosthodontics University of Oslo, Faculty of Dentistry, Oslo, Norway The oral health environment and the equal environment assumption (EEA) among 1-8-year-old twins abstract Aim The purpose of the present study was to describe the oral health environment in preschool children and to examine the extent to which paired twins experience the same oral health environment. Methods In collaboration with The Medical Birth Registry of Norway (MBRN) 100 twin families who participated in the Norwegian Mother and Child Cohort, were invited to take part in ongoing studies on oral health. Participating twin families lived in Oslo and the surrounding counties of the capital. The age range of the participating twins was 1-8 years. A clinical examination took place at The Institute of Clinical Dentistry, University of Oslo in The oral health environment was measured in two ways: 1) Interview. Mothers were interviewed by trained interviewers about oral health related habits of each of the twins. 2) Weekend diet log. Parents listed 84 different deserts, ice cream, sweets, cakes, cookies, fruits, snacks, and biscuits for each twin that were consumed on during the weekend. The statistical analyses comprised frequency distributions of the environmental variables and correlations between the variables within the pair of twins. Results The results showed a parental involvement in early tooth brushing and also an indication of tooth brushing not always being easy. Use of fluoride toothpaste started early, and two thirds of the children also used fluoride tablets. Use of pacifier was prevalent; the duration of use of pacifier and feeding bottle was relatively long. Nearly 75% of the parents indicated that they had no problems relatively to the twins meals, and 53% mentioned that the twin pairs were different with regard to meals. Nearly 70% of the kindergartens had a clear health profile. The correlations varied between r= 0.45 and Conclusion The children in the present work were young, and the detailed information in this paper therefore adds to the knowledge of parental involvement in children s oral health. Generally the findings indicate a high level of involvement from the parents in the oral health environment at home. Altogether the results showed that the assumption of identical oral health environment cannot be supported by these data. Keywords Equal environment assumption; Oral health environment; Preventive habits; Twins. Introduction The oral health of pre-school children is to a large extent dependent upon the parents involvement in a sound oral health environment [Feierskov et al., 2008]. We had the opportunity to study parents involvement in oral health of siblings in this case twins. Research of the role of genetic and environmental factors on dental caries in children has shown a genetic contribution to caries (heritability) reaching to 0.67% [Shuler, 2001; Bretz et al., 2005; Bretz et al., 2006; Townsend et al., 2006]. The classical twin study is based on the knowledge that twins are of two kinds. Monozygotic (MZ) twin pairs derive from a single fertilised ovum that divides and develops into two individuals. Dizygotic (DZ) twin pairs result from two eggs, and these twins share their genes to the same extent as ordinary siblings, i.e. they have on average 50% of the genes in common [Spector et al., 2000]. In analyses of genetic contribution to a trait both type of twin pairs are believed to share the environment to the same degree, usually referred to as Equal Environment Assumption [Kendler et al., 1993; Eriksson et al., 2006]. To our knowledge the EEA has not previously been explored with regard to oral diseases. The data used in the present study stem from the first phase of a twin cohort study. The purpose of the present study was to describe the oral health environment in young children and to examine the extent to which twins pairwise experience the same oral health environment. Material and methods The Medical Birth Registry of Norway (MBRN) is a 41

2 Holst D. and Aas J.A. national health register of all newborns in Norway. The register is a part of the National Institute of Public Health, a government agency administrated by Norwegian Directorate of Health [Medical Birth Registry of Norway, 2011]. In collaboration with the MBRN 100 twin families who participated in the Norwegian Mother and Child Cohort, were invited to take part in the ongoing studies on oral health. Participating twin families lived in Oslo and the surrounding counties. The age range of the participating twins was 1-8 years (Table 1). Fourteen of the mothers and 10 of the fathers were of non- Norwegian ethnicity. Protocols and patient consent forms were approved by the Regional Committee for Medical Research Ethics (REK). The clinical examinations and the collection of epidemiological data took place at The Institute of Clinical Dentistry, University of Oslo in The oral health environmental was measured in two ways. 1) Interview. Mothers were interviewed by trained interviewers about oral health related habits related to each of the twins using a structured interview guide. The questions were developed in order to obtain a detailed picture of the oral health environment and included oral hygiene habits, use of fluorides, meals, drinking and eating during the day, tooth eruption, bottle feeding and finger sucking. One interview was made for each twin. The interviewers were instructed to assist the interviewee to present the most relevant information. The interview comprised 25 pre-tested questions. 2) Weekend diet log. The majority of the preschool children attended kindergartens. Therefore in order to obtain valid information about each family s diet a separate a weekend diet log was prepared. We assumed that a family-specific profile of diet would appear during the weekend when family members share free time together. At the end of the first weekend after the clinical examination the parents listed 84 different categories of deserts, ice cream, sweets, cakes, cookies, fruits, snacks, and biscuits for each twin consumed on Saturday and on Sunday. The inventory was prepared and tested [Norwegian Directorate of Health, 2002]. From the weekend diet log the number of times an item was ticked were summarised into sum-scores of four additive indexes of the meals, between meals and drinking and sweets. These indices were added to a total score ranking from The higher the value of the total scores the higher the exposure to risk factors. Analyses The distribution of the twins on the variables is presented in Table 2. The statistical analysis comprised frequency distributions of all twins according to the categories of the variables (Table 2). In order to analyse the agreement of the answers within each pair of twins Pearson's correlation coefficient was used (Table 3). While correlation analysis usually tests whether correlations are statistically different from zero, we tested deviations from 1.00 that represents a complete agreement within pair of twins. This test is not included in the SPSS program and was modelled in SAS statistical program. In order to control the effect of age and sex on the bivariate correlations, partial correlations were used in addition. Results Table 2 shows the distribution of answers on categories of the variables for all the twins. The number of variable categories varied from two to five. For most of the twins tooth brushing started when the twins were younger than one year. Most parents brushed their children s teeth and 60.1% brushed once a day. Nearly 40% of the parents said that brushing was not easy, and on a scale from 1-5 where 5 is very difficult, 8% stated that brushing was very difficult. Nearly 40% of the twin pairs reported that the twins behaved differently with regard to tooth brushing. Nearly all twins use toothpaste and the majority started during their first year. A little more than two thirds used fluoride tablets, and they were mostly taken in the evening (88.1%). Nearly 75% of the mothers indicated that there were no problems related with the twins meals. They also Age Age Age Age total Gender n % n % n % n % n % Boys 18 18, , ,5 6 6, Girls 22 21, , , , Total 40 20, , , , tab. 1 Distribution of children according to age and sex. 42

3 Oral health environment in norwegian twins Toothbrushing % Fluoride tablets, meals and habits % Who brushes Twins used fluoride tablets The twin him/her self 2,1 Yes 69,2 Parents 97,9 No 30,8 When fluoride tablets When during the day Morning 3,2 Once per day 60,1 Evening 88,1 Twice per day 39,9 Morning and evening 8,7 Ever used pacifier Twin's age at start of parents brushing Yes 91,5 Less than one year 85,1 No 8,5 1-2 years 12,2 Pacifier duration in months Cannot remember 2,7 1-6 months 9, months 3,8 Twins' age at start of self brushing months 7,0 Less than two years 20,2 18 months + 32,7 2-3 years 9,6 Does not use 29,6 3 years and more 3,2 Parents brush 62,3 Feeding bottle duration in months Brushing the twin's teeth is difficult 1-6 months 3,8 No 61, months 29 Now and then 26, months 16,7 Often 7,4 18 months + 28,1 Every time 4,8 Does not use 22,6 Brushing difficulty on a scale from 1-5 Easy (1) 51,6 Description of meals Middle (2-3) 40,4 No problem 74,5 Difficult (4-5) 8,0 Now and then a problem 21,8 The twins are different regarding tooth brushing Often a problem 3,7 Yes 39,3 Twins are different regarding meals No 60,7 Yes 53,6 Use of toothpaste No 46,2 Always 98,4 Kindergarden's oral health profile Irregularly 1,0 No health focus 6,7 Some health focus 23,6 Age when twins started to use fluoride toothpaste Has a clear health focus 69,7 Less than 1 year 69,1 Finger sucking 1-2 year 26,6 Yes 75,6 2-3 year 4,2 No 24,4 tab. 2 Distribution of children according to oral health environment variables. All children. 43

4 Holst D. and Aas J.A. said that 53% of the twin pairs behaved differently with regard to meals. Nearly 70% of the kindergartens were described as having a clear health profile. Table 3 shows the environmental variables and the correlations between Twin 1 and Twin 2 of each pair on the variables. In column three the correlation was controlled for age (partial correlations). The correlations varied between r= 0.45 and The correlations were relatively high on questions related to tooth brushing (r= ) except the question about age of the twin when tooth brushing began (r=0.45). The correlation between the mothers judgement of the difficulty of brushing each of the twins teeth was All the correlations but one were significantly different from 1.00 showing that within pairs of twins there was a difference with regard to exposure to the environmental variables. The partial correlations showed only small changes from the bivariate correlations indicating that the age and sex of the twins had minor influence. Discussion The present study has examined the oral health environment of young children and the extent to which the oral health environment is identical within pair of twins. The results have shown that parents were involved early in tooth brushing and that tooth brushing was not always easy. Use of fluoride toothpaste started early and two thirds of the children also used fluoride tablets. None of the parents added sugar to the pacifier and the drinks of the children (not shown). Generally there is sparse knowledge about the oral health environment among young children. Recently the importance of cultural background for diet, hygiene and dental caries was shown [Skeie et al., 2010; Wigen and Wang, 2010]. The children in the present work were younger, and the detailed information in this paper therefore adds to the knowledge of parental involvement in children s oral health. The generalisation potential from twin data is considered to be limited if there is an association between the twinning itself and the disease or trait studied [Neale and Maes, 1992]. There is no obvious reason to assume that the oral health environment of the twins studied is different from other children of the same age, sex and background. The detailed information from the oral health environment indicators therefore has an informative value even though the generalisation based on statistical theory can only be made to a theoretical universe of twins with the same characteristics as in this study. n (pairs) Intrapair correlation 95 % Confidence int Part. correlation Who brushes the twins' teeth 94 1,00 1,00 When toothbrushing during the day during the day 94 0, ,85 Age of start toothbrushing 94 0, ,45 Age start of self brushing 94 0, ,9 Toothbrushing is difficult 94 0, ,64 Difficulty of toothbrushing, scale 94 0, ,66 Twins are different wrt toothbrushing 94 0, ,85 Use of toothpaste 94 0, ,69 Age start toothpaste 94 0, ,96 Use of fluoride tablets 91 0, ,91 When flouride tablets, morning or evening 64 1,00 1,00 Description of meals 94 0, ,46 Twins are different wrt meals 93 0, ,86 Kindergarden's oral health profile 88 0, ,52 Questions-weekend diary 85 0, ,72 tab. 3 Correlation within pair of twins. Environmental oral health variables. 44

5 Oral health environment in norwegian twins The results have shown that the concordance of oral health behaviours within the twins varied by type of indicator of oral health environment. With regard to tooth brushing the age when tooth brushing started varied considerably between and within the twins. The practical management of meals was no problem for the parents of three fourths of the twins, yet the mothers answered that nearly half of the twin pairs were different with regard to the carrying out the meals. The weekend diet log also indicated that the twins pairwise had a different number of sweets, fruits and drinks during the weekend. Examining the equal environment assumption usually requires having a known zygocity of the twins. However, since the EEA applies to both monozygotic and dizygotic twins, the disadvantage of not knowing zygocity was limited to not knowing whether zygocity affects the size of the correlations (Table 3). Other challenges relate to validity and reliability of the measurements of the oral health environment. In the present study the indicators were measured by interviewing mothers and by diet logs. Mothers opinion about differences between twins with regard to tooth brushing and meals management was elucidated by two questions. Even though it can be argued that difficulties related to tooth brushing and meals are not solely environmental influences these observations provide indications of home situations that disturb the EEA. The chosen indicators represent most aspects of the measurable environmental input close to risks of oral health such as tooth brushing, use of fluoride toothpaste, eating habits and selection of food. It was important that anticipated causal oral health environment variables were included. Based on recent literature there is a scientific evidence for the choice of the variables. They are all shown to have a direct and close role in the development of caries [Fejerskov and Kidd, 2008]. Environmental oral health risks more distant to the family situation were assumed to be mediated through the family environment variables. The parents assessment of the health profile of the kindergartens was included because most children attend a kindergarten and have two-three meals during the day there. The weekend diet log questions have been tested for their validity and reliability prior to this study and found satisfactory [Norwegian Directorate of Health, 2002]. Parents of twins have shown a tendency to have a twin-affected memory of health habits during the day [Spector et al., 2000]. Lack of reliability can be caused by memory failures by the parents, a tendency to believe a priori that twins are treated equally, and disturbances during the interviews, etc. So rather than overestimating differences between the twins, one might expect underestimation of twin differences. Lack of reliability of the measurements results in weaker correlations. This may affect the fulfilment of the EEA requirements in a negative way. Generally the findings indicate a high level of involvement from the parents in the oral health environment at home. The extent to which twin pairs share the environmental effects is trait specific, and must be considered anew for each trait, in each environment. Altogether the results showed that the assumption of identical oral health environment cannot be supported by these data. The consequence of the failing assumption is that the genetic effects on a phenotype will be overestimated in a twin analysis. Acknowledgement We want to thank all the participants, twins and parents for their patience and willingness to participate in the study. References Bretz WA, Corby PMA, Hart TC, Costa S, Coelho MO, Weyant RJ, Robinson M, Schork NJ. Dental caries and microbial aid production in twins. Caries Res 2005; 39: Bretz WA, Patricia MA, Corby MA, Melo MR, Coelho MQ, Costa SM, Robinson MR, Schork NJ, Drewnowski A, Hart TC. Heritability estimates for dental caries and sucrose sweetness preference. Arch Oral Biol 2006; 51: Eriksson M, Rasmussen F, Tynelius P. Genetic factors in physical activity and the equal environment assumption the Swedish Young Male Twins Study. Behav Genet 2006; 36: Fejerskov O, Kidd E. Dental Caries. The disease and its clinical management.oxford: Blackwell Munksgaard; Kendler KS, Neale MC, Kessler RC, Heath AC, Eaves LJ. A test of the equal-environment assumption in twin studies of psychiatric illness. 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