In our study, we found a significant correlation between salivary pH and the expression of MetS components expression among women. To the best of our knowledge, this is the first study investigating the association between the MetS and salivary pH. Although numerous studies have shown correlations between serum and saliva levels for a wide range of components, none has studied saliva from this simple perspective [9, 11, 12, 25–28].
The correlation between salivary pH and MetS component expression remained significant even after the inclusion of age as a covariate but tended to be stronger among PMW. This could be explained by the facts that aging is an important risk factor for the MetS and menopause is often associated with additional metabolic alterations [2, 29, 30]. Moreover, the role of hormones in several metabolic processes suggests that the onset of menopause may influence the relationship between salivary pH and MetS component expression. Estrogen deficiency may notably influence salivary flow rates by indirect pathways. Dyslipidemia, diabetes and hypertension have been related to a decrease of the salivary flow, and their simultaneous presence could act as a confounding factor [9, 16, 24].
Menopause, as a risk factor for almost all components of the MetS , may put a woman at risk of developing salivary dysfunction. In addition, other disorders associated with aging, including but not limited to obesity , rheumatoid arthritis and fibromyalgia , depression  and nutritional deficiencies , could also reduce the salivary function. The associated permanent histological changes in the salivary glands they can trigger may also explain the differences we tended to observe between PMW and MW in the present study .
Among the different variables associated with the MetS, TG is the most significantly related to salivary pH, followed by apo B and glycemia. Our results are therefore consistent with those of previous studies that have shown associations between salivary dysfunction and high plasma lipid levels, particularly hypertriglyceridemia and increased apo B concentrations [36–38]. However, plasma levels of apo B and TG are not independently related to salivary pH. Their relations with salivary pH vary when other covariates are added to the models (Table 3). Hypertriglyceridemia is part of a complex network of interrelated metabolic abnormalities that act as confounding factors when multivariate analyses are used . This could contribute to explain the decrease of the effect of hypertriglyceridemia in the multivariate analysis. The significant result of apo B observed in MW may be explained by a redistribution of body fat to the abdominal region. Such a distribution is often associated with both menopause and increased TG levels .
Many studies have found that the saliva flow rate and pH are related to the level of glycemic control, particularly in the presence of a severely impaired control of blood glucose . There is evidence that adverse hormonal, microvascular and neuronal changes in poorly controlled diabetes could contribute to salivary gland hypofunction [11, 24, 42–46]. The present study indicates that differences in salivary pH could be linked with glucose control, particularly in PMW. In addition, studies have reported conflicting results on the relationship between salivary pH and hypertension or hypertensive therapy [24, 47–49]. Our observations are therefore consistent with those of studies that have found no difference between normal and hypertensive subjects. Finally, considering that a BMI over 25 kg/m2 in adults and obesity in childhood have been linked to hyposalivation [31, 50], pH changes could also be driven by the increase in waist circumference frequently associated with a rise in plasma TG and apo B concentrations. This is consistent with the results of the univariate analysis, while no significant effect of the waist circumference on the salivary pH was observed in the multivariate models. The lack of significant association between age and salivary pH among PMW could be due to the combination of various factors including the small size of the group, the smaller age-range and the lower variability of the sample dispersion.
Our study has some limitations. Because of the cross-sectional nature of the study, confounding factors such as medication or permanent alterations in salivary glands were not taken into consideration. Also, we didn’t have information about the percentage on which subjects had undergone a hysterectomy/bilateral oophorectomy or where late perimenopausal. Moreover, this study does not give any information about the potential causal pathway affecting the salivary function. As an individual’s flow rate and salivary pH remain relatively constant during the different life stages, salivary gland hypofunction is commonly associated with concomitant diseases and daily use of drugs. Thus, a longitudinal study design with disease or medication pre-test and post-test should be more conclusive. Finally, our study should be replicated in larger samples of women with various risk levels of MetS components to obtain the real positive (PPV) and negative predictive values (NPV) as well as specificity and sensitivity values. However, despite its limitations, our study emphasized the important association between systemic and oral health. In the near future, further scientific advances in salivary diagnosis could lead dentists to be more involved in diagnosis and monitoring of MetS components.