## 2 Materials and Methods
### Measurements  
Data were collected on the sociodemographic characteristics (age and four others), health-related characteristics (smoking status and two others), oral health conditions (number of remaining teeth and eight others), and orofacial muscle strength (anterior tongue elevation, cheek compression, and lip compression strengths).  
For assessing oral health conditions, unstimulated whole saliva was collected in an upright position for 5 min using the spitting method to diagnose xerostomia [37], with the amount of collected saliva converted to the salivary flow rate (mL/min). Participants were instructed not to eat or brush their teeth at least 2 h before saliva collection. Using  
Figure 1: Flowchart of participant recruitment and selection.  
a cut-off value of <0.1 mL/min, participants were divided into two groups: xerostomia and normal [38]. To evaluate chewing ability, the number of remaining teeth and number of functional tooth units (FTUs) were identified [39]. FTUs are defined as pairs of upper and lower natural or artificial teeth that occlude during chewing [40]. Two molars were counted as 2 FTUs and two premolars were counted as 1 FTU; therefore, a participant with complete dentition was recorded as having 12 FTUs (excluding the third molars) [39; 41]. To assess orofacial muscle strength, the anterior tongue elevation strength, cheek compression strength, and lip compression strength were measured using the Iowa Oral Performance Instrument (IOPI) (IOPI Medical, Redmond, WA, USA), following the manufacturer's guidelines [10; 42]. The IOPI consists of a pressure bulb, connecting tube, and main body. The bulb, made of soft rubber, is approximately 3.5 cm long, 1.5 cm wide, filled with approximately 2.8 mL of air, and connected to the pressure port of the main body. Each muscle strength was measured three times at 1 min intervals, with the highest value (kPa) recorded. The position of the pressure bulb changed depending on which muscle was being measured. To measure the anterior tongue elevation strength, the bulb was positioned longitudinally on the participant's hard palate just posterior to the alveolar ridge [43]. Then, the participant was instructed to lightly close their lips and press against the bulb as hard as possible with the anterior part of their tongue for 2 s. The cheek compression strength was measured by positioning the bulb laterally between the cheek of the habitual chewing side and the buccal surface of the molar. Participants were asked to squeeze the bulb as hard as possible with their cheek muscle for 2 s, with the upper and lower teeth lightly occluded. When measuring the lip compression strength, two disposable wooden tongue depressors (150 \(\times\) 19 \(\times\) 1.6 mm) were used to evenly distribute the pressure exerted on the bulb to obtain an accurate pressure reading [10]. The bulb, inserted between two tongue depressors, was positioned between the upper and lower lips at the dental midline. Then, participants were instructed to slightly close their lips, protrude them slightly to prevent the use of the jaw muscles, and squeeze the bulb with their lips as hard as possible for 2 s.