Results of the search and characteristics of the included studies
A total of 1387 studies were identified from the primary search after removing duplicate studies. After screening the titles and abstracts, 73 studies were identified for further evaluation. After browsing the full text, 50 studies were considered eligible for inclusion. Twenty-three studies were excluded for various reasons. Among these, 16 studies were excluded because of the study type (7 meta-analyses, 6 review articles and 3 case series reporting periodontal treatment for T2DM patients); 4 studies were excluded because the reported outcomes were insufficient; 2 studies were excluded because they included type 1 DM. Additional reference checking revealed 3 studies that were then included. Journal searching did not add any new studies. Finally, a total of 53 studies were included in the present work. Figure 1 shows the search and inclusion process. Appendix Tables S1 and S2 summarize the characteristics of 43 [19-61] cross-sectional studies and 12 [62,23,63-68,39,69-71] cohort studies, respectively. Appendix Tables S3 and S4 summarize the AHRQ and NOS scores of cross-sectional studies and cohort studies. All included cross-sectional studies and cohort studies had scored higher than 3.
After systematically reviewing the included studies, we found that they answered 3 questions (questions 1-3, Q1-3). Specifically, cross-sectional studies gave the answer “Q1: Are periodontitis and T2DM associated with each other?” Cohort studies gave the answer to the other two questions: “Q2: Does T2DM increase the risk of developing periodontitis?”, and “Q3: Does periodontitis increase the risk of developing T2DM?”
Results of meta-analyses
Q1: Are periodontitis and T2DM associated with each other?
A total of 43 cross-sectional studies were included to answer Q1. Evidence was from some national large-scale population-based studies, such as the SHIP, NHANES and KCIS, and some small-sample studies recruiting participants from communities or hospitals. Among these studies, only 14 studies reported adjusted outcomes (Table 1). Six meta-analyses were conducted as follows.
Strength of association between periodontitis and T2DM
A total of 15 cross-sectional studies with 17924 participants reported the unadjusted OR between these two diseases (Table S1). Since the original data were not directionally adjusted, a meta-analysis was not undertaken. Among the 15 studies, except for 4 studies [21,34,43,51] that reported that the presence of periodontitis was not different between T2DM patients and non-T2DM controls, all the other studies acknowledged that there was a strong connection.
Directional adjusted T2DM prevalence (periodontitis versus nonperiodontitis)
A total of 6 cross-sectional studies were included, and all had T2DM prevalence as an outcome. Three studies with 1956 participants were included in a meta-analysis that included a periodontitis diagnosis as an outcome. The included studies had no significant heterogeneity. The results showed that periodontitis patients had significantly higher odds of T2DM prevalence than participants with no periodontitis (OR=4.04, 95% CI 2.48-6.59, p=0.000, Figure 2a). Influence analysis showed that the pooled result was stable (Figure S1a). Other exposure factors included CAL, PPD, LOT, tooth mobility and alveolar bone loss. The results all proved that T2DM was more prevalent in participants with worse periodontal health (Table 1).
Directional adjusted periodontitis prevalence (T2DM versus non-DM)
A total of 8 cross-sectional studies were included, and all took T2DM as exposure. Three studies with 11459 participants were included in a meta-analysis evaluating periodontitis prevalence. No significant heterogeneity was detected. The results showed that T2DM patients had a significantly higher ORs for PD prevalence (OR=1.58, 95% CI 1.38-1.81, p=0.000, Figure 2b). Influence analysis indicated that the pooled result was stable (Figure S1b). In addition to periodontitis prevalence, other outcomes were divergent. In brief, all studies demonstrated that periodontitis-related parameters were more prevalent in T2DM patients, although some of the differences were not statistically significant. The results are summarized in Table 1.
CAL level differences between T2DM and DM-free participants
Eighteen cross-sectional studies with 9571 participants were included. Significant heterogeneity was detected (p=0.000; I²=92.5%). Pooled results showed that T2DM patients had a 0.89 mm higher CAL than controls (WMD=0.89, 95% CI 0.64-1.15, p=0.000, Figure 2c). Influence analysis demonstrated that the pooled result was stable (Figure S1c). Publication bias was detected by Egger’s and Begg’s tests (Egger, p=0.003; Begg, p=0.015). Then, we employed the trim and fill method to further evaluate publication bias and found that the results were still significantly positive after adding the hypothesized studies (Table S5).
PPD differences between T2DM and DM-free participants
Seventeen cross-sectional studies with 8982 participants were included. Significant heterogeneity was detected (P=0.000; I²=94.5%). Pooled results showed that the periodontal pockets of T2DM patients were 0.61 mm deeper than those of controls (WMD=0.61, 95% CI 0.42-0.79, p=0.000, Figure 2d). Influence analysis demonstrated that the pooled result was stable (Figure S1d). Publication bias was detected by Egger’s and Begg’s test (Egger, p=0.015; Begg, p=0.006). However, adding hypothesized studies by the trim and fill method still resulted in strong significance (Table S5).
NOT differences between T2DM and DM-free participants
Nine cross-sectional studies with 4415 participants were included. Significant heterogeneity was detected (p=0.000; I²=86.6%). Pooled results showed that T2DM patients had, on average, 2.01 fewer teeth remaining than controls. (WMD=-2.01, 95% CI -3.20--0.82, p=0.000, Figure 2e). Influence analysis demonstrated that the pooled result was stable (Figure S1e). No publication bias was detected (Egger, p=0.723; Begg, p=0.917).
LOT differences between T2DM and DM-free participants
Eleven cross-sectional studies with 3405 participants were included. Significant heterogeneity was detected (P=0.000; I²=90.7%). Pooled results showed that T2DM patients had, on average, lost 2.22 more teeth than controls. (MD=2.22, 95% CI 0.94-3.49, p=0.000, Figure 2f). Influence analysis demonstrated that the pooled result was stable (Figure S1f). No publication bias was detected (Egger, p=0.230; Begg, p=0.755).
Meta-regression for meta-analyses with huge heterogeneity
Huge statistical heterogeneity existed in the above 4 meta-analyses, and the I2 ranged from 86.3% to 94.5%; thus, we performed meta-regression to find the possible sources of heterogeneity. The available covariates included the number of participants, mean age, sex composition of the participants, geographic area and AHRQ scores. However, single variable regression did not find any significant covariates; multiple regression of these covariates only explained approximately 10% of the heterogeneity of all meta-analyses (data not shown). The significant heterogeneity might be caused by statistical heterogeneity or other potential clinical diversity not included in the meta-regression.
Q2: Does T2DM increase the risk of developing periodontitis?
A total of 6 cohort studies were considered eligible. The results are summarized in Table 2. Two meta-analyses on periodontitis incidence were performed as follows. In addition to periodontitis incidence, other outcomes, including LOT, PPD, CAL and alveolar bone loss, were also reported. The results are summarized in Table 2.
Four studies investigating whether manifested T2DM increases periodontitis incidence were included in one meta-analysis. In total, 46191 participants, including 2548 T2DM patients, were included, with a follow-up period ranging from 2.6 to 20 years. A total of 6361 incident periodontitis cases were detected. The results showed that T2DM led to a 34% elevated risk for incident PD (RR=1.34, 95% CI 1.11-1.61, p=0.002, Figure 3a). Slight heterogeneity among studies was detected (I2=54.7%, p=0.085). Influence analysis found that this result was stable (Figure S2a).
Another meta-analysis was carried out to investigate the impact of well-controlled and poorly controlled T2DM on periodontitis incidence. In total, two studies with 2791 participants were included. Ninety-four well-controlled and 89 poorly controlled T2DM patients at the baseline were selected as the exposure group. The follow-up was 2.3 (1.2-6.9) and 5 years, respectively. Two included studies [23,69] both indicated that well-controlled T2DM did not increase the risk of periodontitis, and poorly controlled T2DM significantly promoted the incidence of periodontitis. The meta-analysis showed the same trend (Figure 3b), but the results were non-significant for both well-controlled T2DM (RR=1.22, 95% CI 0.63-2.39, p=0.548) and poorly controlled T2DM (RR=3.42, 95% CI 0.43-26.98, p=0.243). The non-significant result of the latter might be caused by a high level of heterogeneity (p=0.007; I²=86.3%).
Q3: Does periodontitis increase the risk of developing T2DM?
A total of 7 cohort studies were included. The results are summarized in Table 3. In total, 27498 participants were included. Among these participants, 8701 had mild periodontitis, while 3994 had severe periodontitis. A total of 1772 incident T2DM cases were detected during a follow-up period ranging from 5 to 18 years. Interestingly, all the included studies reported their results based on periodontitis severity. Thus, we performed two meta-analyses according to periodontitis severity as follows.
The impact of mild periodontitis on T2DM incidence
A meta-analysis on this topic showed that mild periodontitis led to a 28% elevated risk for incident T2DM (RR=1.28, 95% CI 1.07-1.54, p=0.007, Figure 4a). No significant heterogeneity (I2=20.4%, p=0.27) or publication bias (Egger, p=0.133; Begg, p=0.133) among studies was detected. Influence analysis found that this result was unstable (Figure S2b). Deleting Demmer’s study  would reduce the effect size and obtain a marginally significant result (RR=1.17, 95% CI 0.99-1.39, p>0.05). Due to this unstable result, we used the trim and fill method. After adding 3 hypothetical studies, the results became significant (RR=1.14, 95% CI 0.92-1.41, p=0.23, Table S5). The above results indicate that the effect of mild periodontitis on T2DM incidence was not very robust.
The impact of severe periodontitis on T2DM incidence
Pooled results showed that severe periodontitis increased the risk of T2DM incidence by 53% (RR= 1.53, 95% CI 1.27-1.83, p=0.000, Figure 4b). The heterogeneity was very low (I2=0%, p=0.649). No publication bias (Egger, p=0.104; Begg, p=0.230) was detected. In contrast to mild periodontitis, influence analysis found that the impact of severe periodontitis was very stable (Figure S2c). To further confirm this, we used the trim and fill method. After adding 2 hypothetical studies, the results were still significant (RR=1.46, 95% CI 1.23-1.73, p=0.000, Table S5). The above results indicated that the effect of severe periodontitis on T2DM incidence was strong.