Three certified examiners evaluated the factors associated with the oral function and nutritional status of the subjects. We prepared the measurement protocol, trained each examiner accordingly, and confirmed that the same measurements were performed by each examiner.

2.3.1 Demographic variables

Age and sex were considered as demographic variables.

2.3.2 Factors related to oral function

We adopted oral moisture as oral secretory function, whereas the movement and force of the tongue and lip, the number of remaining teeth as an alternative index of occlusal force, masticatory performance, and swallowing function regarded as oral motor functions were adopted to assess oral hypofunction. Medical equipment typically used for the measurement in Japan was used to evaluate each function. Subjects using removable dentures wore them during the measurements.

Oral moisture was measured using an oral moisture-checking device (Mucus; Life Co., Ltd., Saitama, Japan; Fukushima et al., 2017). Mucosal wetness in the central region of the tongue dorsum at 10 mm from the tip of the tongue was considered as oral moisture. A sensor cover was placed over the oral moisture-checking device, and the sensor was pressed uniformly to the test tongue surface with a 200 gf force for 3 s. The measurement was repeated thrice, and the median was adopted as the measured value.

The subjects repeatedly pronounced /pa/, /ta/, and /ka/ syllables for 5 s each, and the oral diadochokinetic rate per second was measured using an automatic measurement device (KENKOU-KUN handy; Takei Scientific Instruments Co., Ltd., Niigata, Japan; Yamada et al., 2015). The measurement was repeated for three cycles. Each cycle involved repeating the syllable for 5 s each in the order of /pa/, /ta/, and /ka/. The maximum value of oral diadochokinetic rate per second was adopted as the measured value.

Maximum tongue pressure was measured using a tongue pressure measurement device (JMS tongue pressure measuring device, JMS Co., Ltd., Hiroshima, Japan; Tsuga et al., 2011). In the anterior region of the palate, the balloon of the tongue pressure probe was compressed voluntarily with the tongue using maximum force for several seconds, and the pressure was measured as the maximum tongue pressure. The measurement was repeated thrice, and the maximum value was adopted as the measured value.

Lip-seal strength was measured using a lip-seal strength-measuring device (Lipplekun; SHOFU, Inc., Kyoto, Japan; Sakai et al., 2019) with the subject sitting on a chair with their head positioned such that the Frankfort plane was parallel to the floor. For the measurement, a button (Lipple button, SHOFU, Inc.) was placed in the median oral vestibule and pulled in parallel to the floor in the sagittal direction. Pulling was continued with increasing force until the button left the lip. The measurement was repeated twice, and the maximum value was adopted as the measured value.

Masticatory performance as masticatory function was measured using GLUCO SENSOR GS-II (GC Corp., Tokyo, Japan; Uesugi & Shiga, 2017). Subjects chewed a piece of 2 g gummy jelly for 20 s and rinsed out the jelly from their mouth lightly with 10 ml of water, after which the eluate glucose concentration was measured.

2.3.3 Nutritional status

Subjects' height (m), body weight (kg), fat-free mass (FFM) (kg), and skeletal muscle mass (SM) (kg) were measured using InBody J10 (Biospace Inc., Seoul, Korea), which is capable of performing body composition analysis using Multi-frequency bioelectrical impedance analysis (MF-BIA) (Yamada et al., 2017) . The measurements were performed ≥1 h after bath, meals, and exercise. Other measurement conditions were in accordance with the guidelines of the European Society for Clinical Nutrition and Metabolism (ESPEN; Kyle et al., 2004). We calculated BMI (kg/m2) from body weight/height2 (Fried et al., 2001), FFMI (kg/m2) from FFM/height2 (Fried et al., 2001), and SMI (kg/m2) from SM/height2 (Fried et al., 2001). FFM is calculated by subtracting fat mass from total body weight. FFM and SM are estimated using body composition analysis.