In our homeless population from the Toronto site of the At Home/Chez Soi trial, we found moderate positive associations (AOR: ≥ 3.5) between DPs and heart disease, stomach or intestinal ulcer, and anemia, and small-to-moderate positive associations (AOR: ≥ 2.0 and < 3.0) between DPs and diabetes, chronic bronchitis/emphysema, IBD, arthritis, and kidney/bladder problems. Less intense but statistically significant associations (AOR: ≥ 1.5 and < 2.0) were found for DPs with migraine and the number of CDs.
These positive associations may be a result of the potential exposure of our participants to risk factors shared by both DPs and those CDs such as age, low socioeconomic conditions, unhealthy lifestyles (smoking, alcohol and drug use), obesity, and poor mental health. However, we adjusted for these key variables in our analysis, and upon adjustment, no substantial changes in the magnitude of the effects were observed.
Our study findings support the hypothesised biological relationship between poor dental health and some inflammatory-based CDs such as heart diseases, chronic bronchitis/emphysema, IBD, arthritis and diabetes. For instance, a positive association between DPs and heart problems such as ischemic heart disease, heart failure, and atrial fibrillation have been reported in numerous studies in the general population [7, 40]. Related to bronchitis/emphysema, DPs such as periodontitis has been reported to be more prevalent and incident in people suffering from chronic obstructive pulmonary disease (COPD) [11, 41]. Moreover, people with COPD tend to have worse DPs manifestations (e.g., deeper periodontal pockets, gingival inflammation and bleeding) . Regarding IBD, a higher prevalence of oral problems including buccal mucosa and gingiva disorders have been found in people who have Crohn’s disease and colitis . Associations of reverse directionality between dental disorders with IBD have been reported as well . With respect to arthritis, having a higher occurrence of DPs such as gingival problems, pockets depth, periodontitis, and poor oral hygiene index have been found among people with rheumatoid arthritis than those without . Likewise, a positive association between DPs and high risk of incident palindromic-rheumatism has been reported . The coexistence of DPs with diabetes and chronic kidney conditions has also been documented [46, 47].
The association observed between DPs with specific CDs might be explained by linked infective, inflammatory and immune mechanisms. The inflammatory and immune response originating from tissue damage, alteration of the local microbiota, or infections in the oral cavity and dental structures could propagate through the body, triggering the activation, deregulation, or damage of other local body tissues and physiological functions (e.g., infecting local structures, increasing the production of antigens and neutrophils and cytokines release) [48,49,50]. These underlying mechanisms might contribute to the development of inflammatory-based CDs. The reverse biological pathway might also be likely, where damages in other body structures trigger or exacerbate an inflammatory and immune response contributing to oral disorders including DPs. It is also possible that the observed associations are due to the poor mental health status  combined with the disadvantaged socioeconomic conditions, social exclusion, systemic barriers, and enduring discrimination and stigma faced by homeless people [5, 19, 25, 27, 52], which increase their likelihood of having a neglected physical health status and higher burden of chronic comorbidities, including dental diseases.
Our study also showed a positive relationship between DPs and less studied CDs such as stomach or intestinal ulcer, migraine and anemia. DPs such as periodontitis and tooth loss have been found to be associated with an increased risk of gastric and duodenal ulcer in prior literature [53, 54]. The potential presence of Helicobacter pylori in the oral cavity might be one of the explanations for our observed findings, as it is likely that due to their disadvantaged socioeconomic conditions, people experiencing chronic homelessness are more exposed to these bacteria. In fact, Helicobacter pylori infection is associated with poverty and unhygienic environments . Studies have suggested that the oral cavity is also a reservoir for Helicobacter pylori, the main contributor of gastro-duodenal ulcers . Indeed, Helicobacter pylori has been found simultaneously in the oral and stomach cavities . Other authors have also suggested that microorganisms different to the Helicobacter pylori such as P. Gingevalis (a common bacteria associated with periodontitis) can alternate the gut microbiota and intestinal-relate functioning, inducing several local harmful responses such as inflammation or damage in the stomach and duodenum . Related to migraine, our results lend support to the hypothesis that poor dental health may be a risk factor for a chronic migraine as documented in few prior studies , where particularly, the neurological inflammation, or endothelial, immunity and matrix protease dysfunctions can act as the potential linked pathways . Concerning the association between DPs and iron-deficiency anaemia, a recent meta-analysis showed that individuals with chronic periodontitis have lower levels of hemoglobin, erythrocytes and hematocrit biomarkers , suggesting that the inflammatory response in chronic DPs might leading to anemia.
The positive associations between DPs and all the above discussed CDs could reflect inequalities in accessing and using dental care services and prescription drugs by individuals who are poor and socially excluded, such as homeless individuals, even within the context of universal health care systems such as in Canada . Although universal health care systems are built around the principle of providing access according to health need rather than the ability to pay , they do not cover many of the health services needed to meet the many care needs of disadvantaged individuals . For example, in Canada, the majority of dental care services are provided by private clinics and funded by private insurance or out-of-pocket payments [62, 63] and are therefore poorly accessible to low-income and socially excluded people. Moreover, Canada is the only socioeconomically developed country that provides universal coverage for health care services but not for prescription drugs [62, 64]. All of this contributes further to a high economic and comorbidity burden and health inequities for low-income people without private insurance. In other developed countries such as the United Kingdom, low-income households, people with chronic conditions, and individuals aged less 16 years or more than 60 years are exempt from paying for drugs prescriptions, as well as certain dental care services, or are entitled to receive co-payment for those services . Thus, enhancing free access to dental care and prescription drugs for managing chronic comorbidities can mitigate the high burden of CDs and DPs.
In our study, we found no associations between chronic DPs and stroke, hypertension, asthma, thyroid problems and liver disease, which contrasts with some studies carried out in the general population [8, 66, 67]. It has been suggested that exposures to unhygienic conditions, overcrowding, pets, outdoor spaces, and diversity of microorganisms might have a potential protective effect for asthma and asthma symptoms in some individuals . In our homeless population, these kind of exposures are indeed common. However, it is also possible that the DPs and their related inflammatory and immune responses previously discussed have no role in the pathogenesis of these diseases; or that the few events observed especially for stroke, thyroid and live problems did not allow enough statistical power for detecting significant associations.
Overall, our study findings suggest that DPs and several CDs are strongly associated in homeless adults, possibly contributing to their poor health profiles. Further, they strongly support the need for proper medical and dental management of these conditions, with primary, secondary and tertiary prevention measures, highlighting the necessity for appropriate, comprehensive and continuous medical care for this population [18, 25, 27] even within high-income countries such as Canada. Moreover, getting access to dental treatments to mitigate DPs, an area that is not well covered with public insurance in many settings, is also a priority. Better yet, dental health that is integrated into the social and health programs serving individuals experiencing homelessness such as supportive housing and social programs, outreach programs, and shelter services would be beneficial as it may ease access to these services.
In addition, structural and administrative barriers that homeless people experience, such as lack of health coverage, limited access to and provision of both preventive and treatment services for dental and overall health conditions [15,16,17,18,19, 25, 27], should be eliminated to reduce the premature morbimortality of this population. Furthermore, to reduce experiences of discrimination and stigma, it is essential that health care professionals, including dentists, [16, 17] receive training on serving homeless people in their settings.
Although our study is one of the few in examining the associations between DP and several CDs in the homeless population, it is not excluded from the following potential limitations that should be considered when interpreting the results. First, except for hypertension, we used self-reported CDs (lasted 6 months or more) rather than medical examination or biomarkers, which may be biased by the unawareness of our population on whether or not they were suffering from the studied conditions. Second, we did not have specific information on the type of DP (e.g., periodontitis, number of teeth, damage in the oral bone structures), which may have allowed us to assess the effect of specific DPs on the studied CDs. However, the criterion used for reporting the dental problem and CDs was that used in many survey-based health studies, which was whether a participant had experienced DPs for at least 6 months or more. This is a robust proxy for serious dental health problems rather than for relatively brief and less severe disorders. Due to the cross-sectional nature of our analysis, it was not possible to separate the temporal relationship between DP and the studied CDs. Therefore, causal inference cannot be drawn. Third, we used MI to handle the missing data in exposure, outcomes and some co-variates, which may not represent the associations that would have been observed if all data was complete. However, we carefully specified and tested the imputed models, indicating high appropriateness and efficiency in estimating accurate results. Fourth, for CDs (heart disease, stroke, thyroid problems, and liver disease) that affected a smaller sample of our population (less than 30 subjects), we only adjusted for key variables . Therefore, the observed findings for those outcomes may be affected by the lack of control for other potential confounding factors. Fifth, we were unable to adjust our findings for use of dental health care, as this information was not collected. Thus, the observed associations between DPs and CDs may have differed if we were able to account for dental care utilization. Finally, our results may not be generalizable to other homeless populations outside our study setting or to different health care systems.