In an aging society, such as present-day Japan, frailty is a critical issue related to 38 morbidity as well as mortality. Frailty is a common geriatric syndrome that embodies an 39 elevated risk of catastrophic declines in health and function in the elderly. Frailty relates to 40 many factors in an individual's life, particularly physical, psychological, and social factors.
41 Recently, it has been reported that oral health is associated with physical frailty and mortality, 42 hence the term "oral frailty" [1-3]. Oral frailty consists of factors such as the diminished 43 condition of teeth and ability to chew, swallow, and converse. It can lead to dysphagia, 44 dehydration, malnutrition, asphyxia, and aspiration pneumonia, which is one of the most life-45 threatening concerns for the elderly [4, 5] .
The tongue is one of the most important organs related to oral frailty, a condition 47 that can be life-threatening; swallowing dysfunction can lead to aspiration pneumonia and is 48 a critical concern. Dysphagia is reported to be associated with oral, physical, cognitive, and of the plane perpendicular to the Frankfurt horizontal plane of the frontal section ( Fig 1A) .
This perpendicular plane intersects the distal surfaces of the bilateral mandibular second 95 premolars. The vertical distance was measured from the surface of the mylohyoid muscle to 96 the tongue dorsum ( Fig 1B) . This measurement was performed three times, and the mean 97 value was defined as the tongue thickness for each participant. We confirmed the reliability 98 of tongue ultrasonography by calculating the intra-rater and inter-rater reliability. For 99 investigating intra-rater reliability, we measured the tongue thickness of the normal subject 100 three times per day; their mean value was defined as the tongue thickness for the day. These 101 measurements were repeated for ten days, and the resulting coefficient of variation was 150 Chronic kidney disease was defined as an eGFR < 60 ml min −1 1.73 m −2 . Grip power was 151 measured for both sides and the mean value was used for analysis. Lower leg circumference 152 was measured at the thickest place at both sides and the mean value was used for analysis.
154
The data were expressed as the mean ± standard deviation for continuous variables 166 predicting a tongue pressure < 20kPa, which suggests swallowing dysfunction. We 167 considered p < 0.05 as statistically significant.
168 Results
We evaluated 254 elderly patients, whose backgrounds are shown in Table 1 Fig 3B) . 
The potential factors associated with tongue thickness (listed in Table 1) were 187 evaluated using multifactorial regression analysis by sex. In the male group, dyslipidemia, 188 lower leg circumference, and tongue pressure were independently significant in their 189 association with tongue thickness (adjusted R 2 = 0.653, p < 0.001, n = 163) ( Tongue thickness was compared by five-year age increments for each sex (Fig 4) .
198 These data suggested that tongue thickness is significantly decreased by age in each sex (p < 199 0.001). In addition, tongue pressure was compared by five-year age increments for each sex 200 (Fig 5) . These data suggested that tongue pressure also significantly decreased by age in each