The primary aim of the study was to investigate the effects of salivary flow and mucosal moisture on oral malodor. The results revealed that resting salivary flow is an important modulating factor of oral malodor.
Salivary flow is considered to be affected by age and sex. The functional failures of the salivary glands cause a reduction in salivary flow and increase in dryness, which are common in elderly people [22]. A recent meta-analysis of salivary flow rates in young and older adults reported that whole and submandibular and sublingual salivary flow rates were reduced significantly in older participants, whereas parotid and minor gland salivary flow rates were not significantly reduced with increasing age [8]. The results of the current study are consistent with these findings: the flow rate of resting saliva (i.e., submandibular and sublingual saliva) showed a negative correlation with age, whereas the flow rate of stimulated saliva had no correlation with age. In addition, a sex difference was observed in the flow rate of resting saliva. Inoue et al. [23] investigated the resting salivary flow rate and salivary gland size in healthy young adults and reported that both were smaller in women than in men.
The moisture level of the skin is commonly used to measure aging. The moisture level of the oral mucosa reportedly decreased with aging in a study that assessed the midline of the lower labial mucosa [24]. However, the changes in the moisture levels of the tongue and buccal mucosa with age or sex were unclear. In the current study, the moisture levels of the tongue and buccal mucosa were negatively correlated with age. Sex had no effect on the moisture levels of the tongue and buccal mucosa.
The major causes of oral malodor are tongue coating, poor oral hygiene, and periodontitis; therefore, the clinical parameters related to these, such as TCS, PlI, average PPD, and BOP, are associated with strong oral malodor. Concerning the wetness and moisture of the oral cavity, the flow rate of resting saliva in subjects with oral malodor was significantly lower than in subjects with no or weak oral malodor. Stimulated salivary flow and the moisture of the tongue surface and buccal mucosa were not associated with strong oral malodor. Several studies investigated the relationship between salivary flow and oral malodor. Koshimune et al. [11] obtained results similar to those in the current work. On the other hand, another report stated that the flow rate of resting saliva did not differ between subjects with oral malodor and those with no or weak oral malodor [12]. The resting whole saliva was collected by a draining method in those studies, whereas the saliva collected in the current study was limited to the submandibular and sublingual saliva obtained by the cotton roll method. Several other studies also reported that the stimulated salivary flow is not associated with oral malodor [13].
Few studies have assessed the relationship between oral mucosal moisture and oral malodor. The moisture checker used in the current study measures the moisture of the submucosal layer, about 50 μm under the mucosal surface, and is useful for evaluating the dryness in patients with xerostomia [14, 15]. The patients in the current study did not have xerostomia, and their moisture levels of the tongue and buccal mucosa were higher than the threshold of dryness (=25) defined by the manufacturer. The moisture levels of the tongue and buccal mucosa may not be associated with strong oral malodor in generally healthy populations. This instrument may be useful for investigating the oral malodor in patients with xerostomia.
Logistic regression analysis included clinical parameters that were significantly related to strong oral malodor (TCS, PlI, the presence of a ≥5-mm PPD with BOP, and the flow rate of resting saliva). It revealed that TCS, the presence of a ≥5-mm PPD with BOP, and the flow rate of resting saliva were important modulating factors for strong oral malodor. The method of saliva testing used in the current study is simple and can be performed in a short time. It may thus be suitable for use in clinical practice. In addition, our results indicate that it is advisable to consider resting salivary flow as an important modulating factor in future research concerning oral malodor.
The current study is limited by the numbers of men and women and the age groups of the subjects. The numbers of men and women were 48 and 71, respectively. Women tend to present with the complaint of halitosis more often than men [21, 25]. More than half of the subjects (53.8 %) were ≥50 years of age in the current study, and there were very few young people: the proportion under 30 years of age was only 10.9 %. Therefore, it can be said that analyses were performed on the middle-aged and elderly age groups. These shortcomings should be considered when interpreting the obtained results.