Background: Little is known of the periodontal status of the New Zealand (NZ) adult population, despite the availability of data from three national oral health surveys undertaken in 1976, 1988 and 2009. Although the first two surveys provided an epidemiological overview of the NZ population's oral health, they did not provide precise estimates because of shortcomings in the periodontal indices used and the relatively small sample sizes. The latest national oral health survey provided extensive data on the periodontal status of the NZ adult population; however, there are no published articles describing the periodontal status of NZ adults in detail.While diabetes is a commonly-accepted risk factor for chronic periodontitis, substantial heterogeneity exists in the literature. In fact, the body of the evidence suggesting a strong association between diabetes and periodontitis originates from studies of Pima Indians (an indigenous U.S. population with the world's highest prevalence of diabetes - 50% in those aged 30+ years). Other studies, however, have challenged such an association. The nature and strength of such an association have not yet been clearly defined. There is compelling evidence for tobacco smoking as a risk factor for periodontitis, but there are limited NZ population data. Further, most of the data on the association between diabetes/smoking and periodontitis come from small observational studies, and only a handful of studies have examined these associations using nationally representative data. PurposeThe objectives of this study were to examine: (1) the extent, prevalence and severity of periodontitis in NZ adults; (2) the strength of the association between self-reported diabetes and periodontitis; and (3) the association between self-reported tobacco smoking status and periodontitis. MethodsData from two NZ national surveys (n=2048) were analysed to compare estimates of the strength of the association between diabetes/smoking status and periodontitis using two epidemiological approaches (cohort and matched case-control studies). This was possible because the sample for the first survey provided participants for the subsequent survey. Severe periodontitis cases were chosen using six different definitions. The cohort study identified those who were self-reported diabetic in 2006/07 and those who had periodontitis in 2009, to determine the diabetics' relative risk for severe periodontitis, using logistic regression modeling (adjusting for smoking status, socio-demographic and dental characteristics). The matched case-control study identified cases of severe periodontitis in 2009 and compared their 2006/07 diabetic status with that of controls individually matched on age group, sex and socio-economic status. Conditional logistic regression modelling was used for the case-control study, adjusting for ethnicity, smoking status and dental characteristics. The same methods were used to investigate the strength of the association between self-reported smoking status and periodontitis, except that longitudinal smoking status could be generated, because smoking status was reported in both 2006/07 and 2009.ResultsOverall, half and one-third of NZ adults were affected by mild and moderate periodontitis, respectively. Estimates for the prevalence of severe periodontitis ranged from 5 to 14%, depending on the definition used. There were substantial disparities in the prevalence, extent and severity of periodontitis in different socio-demographic subgroups in NZ. The prevalence, extent and severity of periodontitis were greater in males, and an age gradient was evident. All ethnic minority groups (Māori, Pacific Islanders and Asians) had a significantly greater prevalence, severity and extent of periodontitis than European/Other groups, and this was consistently observed with all periodontitis definitions. Depending on the definition of periodontitis used, there were different estimates of the self-reported diabetes-periodontitis association. The odds of having periodontitis ranged from 1.91 (P=0.011) to 3.51 (P=0.22) in diabetics, using the cohort study approach; however, self-reported diabetes was not associated with a majority of the periodontitis definitions. No association was observed using the matched case-control study. Using cohort and matched case-control study approaches, self-reported smoking was consistently weakly associated with various definitions of periodontitis. Chronic smokers (those who were current smokers in both 2006/07 and 2009) had a 2 to 3 times greater risk of having periodontitis than never smokers.ConclusionPrevalence estimates for mild/moderate and severe periodontitis among NZ adults fall within the internationally reported range, regardless of periodontitis definition. There were substantial socio-demographic disparities in the occurrence of periodontitis in the NZ adult population. There was little support for an association between self-reported diabetes and periodontitis, while self-reported smoking was weakly associated with periodontitis in the NZ population overall. This study confirmed that the determination of the strength of a putative association is method-dependent: whether there is an apparent association between periodontitis and its putative risk factors depend on the epidemiological approach which is taken. Readers of the literature must be aware of the methodological limitations (including periodontitis case definitions and the analytical approaches used) when interpreting studies and evaluating the literature on the risk factors for periodontitis.