Objectives To compare oral health (dental caries, periodontal status, and erosive tooth wear (ETW)), diet and oral hygiene habits between obese and normal weight adolescents, and to explore possible risk associations.

Materials and Methods In this case-control study, a convenient sample of 81 obese adolescents (age range 11-18) from a rehabilitation centre, and 81 age-sex-matched normal weight adolescents were selected. Groups were defined using the Body Mass Index and growth curves for Flemish. Oral health was measured using DMFT, gingival, plaque and BEWE index. A validated questionnaire was utilized to assess diet and oral hygiene habits.

Participants and ethical aspects For this case-control study, a total of 162 adolescents residing in Belgium were examined between July 2015 and October 2015.

Height and weight were measured to calculate the Body Mass Index (BMI) of the participants. Obese and normal weight groups were defined using the BMI according to the European Childhood Obesity Group (ECOG) and the growth curves for Flemish boys and girls (2-20years). Only the participants with obesity were included in the case group (having a BMI above the 97th percentile + 2.17 to+2.18 SDS), whereas only the participants with normal weight were included in the control group (BMI between the 10th and 90th percentile -1.64 to +1.64 SDS).

Clinical examination Anthropometrical and dental examinations were performed in the educational institutions. Before scoring, dental surfaces were dried using sterile gauze. Standard illumination (LED head lamps), sterile number five mouth mirrors and CPI periodontal probes (WHO 1997) were used. Caries experience was screened using DMFT index (Decayed, Missing, Filled Teeth) with a subdivision for D1 (early caries, 'white spots'), D2 (enamel caries) or D3 (dentine caries). The periodontal status was evaluated by measuring the Gingivitis index (Silness and Loë, 1963), Plaque index (Silness and Loë, 1964) and the presence of periodontal pockets (≥4mm) in all Ramfjord teeth (1.6, 2.1, 2.4, 3.6, 4.1 and 4.4). Data were obtained from the neighbouring tooth in cases where one of these teeth were not present. Deciduous teeth and teeth with developmental disturbances were excluded. Regarding ETW, prevalence and severity was recorded using the Basic Erosive Wear Examination (BEWE index). According to this index, every permanent tooth surface is evaluated and is classified into a 4 score criteria, "0" being an indication for the absence of ETW, "1" being initial loss of surface texture, "2"* being distinct defect, hard tissue loss affecting less than 50% of tooth surface and "3"* being an indication for hard tissue loss and presence of ETW in more than the 50% of the surface (*usually involving dentin). The final BEWE sum score was the result of the sum of the highest score recorded per sextant .

Reproducibility Two paediatric dentists (FM and SD) were educated, instructed and calibrated for DMFT index. The examiners scored twice several clinical cases in two sessions separated by a two-week period. Then, the examiners scores were compared against an experienced benchmark (LM).

Questionnaire A validated questionnaire was applied in order to assess oral hygiene and dietary habits. Prior to initiation of the study, a content validation of the questionnaire was performed by five professors with experience in epidemiology. For the reliability of the questionnaire, a group of 20 students (15-year-old) were asked to complete the questionnaire twice separated by a two-week period. Once validated, the questionnaire was applied to each participant of the study on the same day of dental examination. Participants were asked to answer the questionnaire according to their habits just before they enter the rehabilitation centre.

Inclusion Criteria:

• obesity in case group (having a BMI above the 97th percentile + 2.17 to+2.18 SDS),
• normal weight in the control group (BMI between the 10th and 90th percentile -1.64 to +1.64 SDS). Obese and normal weight groups were defined using the BMI according to the European Childhood Obesity Group (ECOG) and the growth curves for Flemish boys and girls (2-20years).

Exclusion Criteria:

• smoking
• chronic disease
• receive treatment with antibiotics three months prior to the study
• consume medications with side effects on the gums
• wearing orthodontic appliances
• other ethnicity (not Belgian) were excluded as ethnicity is an influential factor in the relation between caries and obesity.

Cases were all the obese children/adolescents (N=81; age range 11 to 18 years-old) who were recently participating in a treatment program for obese patients in a rehabilitation centre for chronic diseases (Zee Preventorium, De Haan, Belgium). Based on the number of cases, 81 normal weight students of two Belgian secondary schools (Deinze, Belgium) were recruited as controls. Controls were sex-age-matched, had the same education level and socioeconomic status (SES).