This systematic review was aimed to analyze the association between tooth loss and hypertension by systematically summarizing the scientific evidence derived from clinical studies during the last decade. Twenty studies were included, with a total of 745,140 adults. To the best of our knowledge, this is the first systematic literature review to examine this association. The results of this review may help elucidate the influence of oral health on blood pressure.
Although the majority of included studies were conducted in Asia, while few studies from Europe were included, countries where the study was conducted were prevalent in a wide range of the world. In Asia, such countries are prevalent in many areas (East Asia 4, South and South East-South Asia 3, Middle East 2). Since hypertension and tooth loss are common all over the country, these diseases may be targets for study in many countries. Since hypertension and tooth loss are common all over the country, these diseases may be targets for study in many countries.
Most of the included studies have demonstrated an association between tooth loss and hypertension. Individuals who had greater tooth loss exhibited a higher prevalence of hypertension and higher blood pressure than those who had less tooth loss. However, only two studies reported no association between tooth loss and hypertension without adjusting for possible confounding factors. According to our literature review, tooth loss is thought to have a considerable association with hypertension. However, a few studies reported that the significant association disappeared after adjusting with possible confounding factors and the odds ratios for this association in most studies, which ranges between 1 and 2, are not very large. Therefore, the extent of the association between tooth loss and hypertension may not be so great.
There are two possible cascades related to the association between tooth loss and hypertension. One is that periodontitis, a major cause of tooth loss, has been reported to be associated with hypertension. It is speculated that periodontitis progresses and consequently tooth loss and hypertension develop. The mechanisms by which periodontitis elicits hypertension are complex and not fully elucidated. It is likely that the major mechanism by which increase of blood pressure occurs in patients with periodontitis is spread of inflammation and secondary damage to the vascular endothelium53–56. Periodontal tissue covers a wide area of the oral cavity. The influence of local inflammation of periodontitis occurred in a large extent of the oral cavity may significantly contribute to systemic inflammation mediated by C-reactive protein and main inflammatory cytokines such as tumor necrosis factor alpha, interleukin 1b and interleukin 657–58. Increase of nitrate-reducing bacteria, which is observed in patients with periodontitis may induce a reduction of nitric oxide, which may consequently lead to an increase in blood pressure59–61. It was also reported that an intervention of non-surgical periodontal treatment lead to an improvement of hypertension, accompanied with an improvement of periodontal status62. Another reason is that tooth loss causes a decrease in masticatory function, thereby inducing obesity. There are two possible explanations for the association between mastication and obesity. One is that the eating habits of people with poor masticatory function, and decreased consumption of vegetables and fruits, and higher consumption of high energy food, tend to cause obesity compared to those with adequate mastication63–66. Another is that less chewing leads to a decrease in diet-induced thermogenesis and inactivation of neuronal histamine, which may consequently leading to obesity67–69.
In patients with metabolic syndrome, other factors, such as diabetes and hyperlipidemia, add to this complex relationship. Obesity is one of the causes of diabetes and diabetes and periodontitis have a bidirectional relationship. In some of the studies reviewed in this article, the association between tooth loss and hypertension disappeared after adjustment for confounders. This suggests that other factors do have a considerable influence on this association. It is thought that various factors interact with each other in a complicated cascade from tooth loss to hypertension.
A significant association between SBP and tooth loss was observed, although it disappeared after adjustment for confounders, in studies included, however, the association between DBP and tooth loss was not significant. The reasons why SBP exhibits a higher association with tooth loss than DBP is unclear. The proportion of individuals with diastolic hypertension (systolic–diastolic hypertension [SDH] or isolated diastolic hypertension [IDH]) decreased, while those with systolic hypertension increased with age. Accumulation of advanced glycation end products (AGE) with aging leads to increased arterial stiffness and contributes to the development of ISH70, 71. Excessive intake of animal-derived foods that are rich in AGE may increase the risk of hypertension and other chronic disease 72–75. In general, individuals who suffer from decreased masticatory function due to tooth loss tend to eat foods that are high in fat76–79. Tooth loss may induce AGE accumulation, consequently contribution to increased SBP.
Two prospective cohort studies demonstrated that individuals with more tooth loss exhibited a higher incidence of hypertension than those with less tooth loss during the observation period49, 50. On the contrary, one study reported that subjects with hypertension experienced lower tooth loss than those without hypertension51. The association between tooth loss and hypertension may not be bidirectional. In other words, hypertension may not cause tooth loss.
Overall, the studies included in this review have a large number of subjects. Eight of 20 included studies have investigated large-scale community dwelling of > 1000 33, 37, 41, 44, 46–48, 51, which enhances the credibility of the results of the studies. Several studies have investigated many specialized subjects, including patients from clinics or hospitals 32, 35, menopausal women 34, 40, 49, and male health care specialists50. Although it is problematic to apply the results of these studies to the general population, the large number of subjects in their studies increases the reliability of the study results.
Of the 20 included studies, 14 (70%) adjusted the association between blood pressure and tooth loss with all possible confounding factors (demographic factors, socio-economic factors, health behavior and general health). Of the remaining six studies, 4 studies lacked only one confounding factor in adjustment. The most important confounding factor was obesity, and almost all studies used obesity in the adjustment. These warrant the reliability of the evidence obtained in this review. However, three studies that investigated patients from university hospitals failed to adjust for socio-economic factors. The characteristics of these studies’ settings might make it difficult to obtain data of socio-economic factors.
Eight studies employed self-reported data on the number of teeth and/or hypertension. Some studies have shown that the validity of self-reported number of tooth loss in high-income countries is strong76, 77. One study examined in health professionals in the US, expecting high validity against clinically measured results50. However, the validity of the self-reported number of lost teeth has not been evaluated in lower and middle-income countries. Moreover, the validity of self-reported hypertension in developing countries is not high78, 79. Since most of the studies that employed self-reported data on the number of teeth and/or hypertension in this review were performed in developing countries, self-reported data may deviate from true values. Since subjective measurements have the possibility to give considerable optimistic results compared with practitioners’ measurements80, self-reported data may often be underestimated.
There are several limitations in this study. First, all studies included in this review were observational studies. Intervention studies are necessary to analyze the causal relationships. Although lost teeth cannot be regenerated, they can be restored by prosthetics. Provision of prosthetics can improve both masticatory function and diet. When decreased mastication elicits obesity and subsequently hypertension, restoration of mastication by the provision of prosthetics may improve increased blood hypertension. The number of teeth is just an anatomical indicator. There is an indicator that is the sum of the number of natural teeth and the number of lost teeth that are restored by prosthetics. It is of interest to investigate the association between the number of functional teeth and hypertension.
Second, the grouping of participants according to the number of teeth differed among the studies. Because of this problem, only small-scale meta-analyses were performed. Moreover, the only meta-analyses that were carried out were to compare the hypertension rate for tooth loss vs. no tooth loss and for dentate vs. edentate. Pooled odds ratio data based on a cut-off value for the number of teeth is valuable for estimating the association between the number of remaining teeth and hypertension. Moreover, it may be a rough indication for maintaining oral health to prevent hypertension.
The strength of our study is that it included many studies with a large number of subjects. The greater the sample size, the smaller the error, which makes the results more reliable.