Oral health, and more specifically, the functioning of the mouth is closely related to the overall health of the elderly. Digestion of foods begins with chewing and depends largely on the condition of the teeth. Compared to older individuals with their own set of teeth, older individuals with complete dentures consume fewer calories and eat lower-quality foods, possibly leading to various diseases.^1,2) Consequently, those with more than 20 teeth are more physically active.³⁾ Poor oral health, such as fewer teeth, has been associated with disability. Dental function is crucial to the older adults, and the most important reflection of function is the number of remaining teeth.⁴–⁶⁾

Decreasing skeletal muscle mass is an important physical change associated with aging that can be objectively measured. A decrease in skeletal muscle mass can lead to improper responses to external stressors and vulnerabilities, which can further be associated with falls and trauma, functional disorders, increased hospital admission rates, decreased quality of life, and increased mortality.⁷–¹⁰⁾

A previous study reported that unfavorable oral health, including a reduced number of teeth, is associated with disability,^6,11) decreased grip strength,¹²⁾ nutritional intake,^2,4) and weight.¹³⁾ However, there is no study investigating the correlation between the number of teeth and skeletal muscle mass in the older adults. Thus, we aimed to determine the correlation between the number of remaining natural teeth (NRT) and appendicular skeletal muscle mass (ASM) in elderly Korean individuals using data collected from the Korea National Health and Nutrition Examination Surveys (KNHANES).

Of the 2,378 participants, there were 984 men (41.4%) and 1,394 women (58.6%) with a mean age of 71.9 years. The mean NRT was 16.3, and 486 men (49.4%) and 766 women (54.9%) had NRT<20. In both men and women, the proportion of the group with NRT<20 increased with age, and 84.62% of the men and 81.48% of the women over 85 years of age had NRT<20. In women, there were differences in age, marital status, income, smoking, BMI, prostheses, nutrition, and muscle mass between those with NRT≥20 and those with NRT<20. In men, age, smoking, BMI, prostheses, nutritional status, and muscle mass differed between the 2 groups. Marital status and low family income were associated with NRT<20 only in women. The mean ASM was 20.17 and 21.21 kg in men with NRT<20 and NRT≥20, respectively (p<0.001) and 14.02 and 14.48 kg in women with NRT<20 and NRT≥20, respectively (p<0.001). The mean SMI was 7.47 and 7.74 kg/m² in men with NRT<20 and NRT≥20, respectively (p<0.001) and 6.25 and 6.26 kg/m² in women with NRT<20 and NRT≥20, respectively (p=0.782) (Table 1).

Fig. 1 illustrates the difference in SMI between NRT<20 group and NRT≥20 group. The positive correlation between the NRT and SMI was significant in men even after adjusting for age and BMI, smoking, alcohol, physical activity, protein intake, energy intake, calcium intake, marital status, income, fasting basal glucose, medication administration, and prostheses. We also found functional NRT (NRT≥20) well discriminated the SMI. But in women, the correlation was inverse after adjustment for smoking, alcohol, physical activity, protein intake, energy intake, calcium intake, marital status, income, fasting basal glucose, medication administration, and prostheses (Table 2).

This is, to the authors’ knowledge, the first study to identify the correlation between the NRT and SMI, both of which are important for the overall health and quality of life in the elderly. In both men and women, a lower NRT was related to a lower SMI.

A reduction in the number of teeth can lead to nutritional deficiencies, resulting in muscle loss. Hildebrandt et al.¹⁹⁾ report that older adults with reduced functional dentition are more likely to avoid food with certain characteristics, such as meat (tough to chew), raw carrots (hard), and dry foods. The resulting malnutrition and decrease in protein intake may affect the metabolic function of the body and can lead to the loss of muscle mass.²⁰⁾ However, interestingly, the relation between the number of teeth and SMI was still significant after controlling for protein intake in men. The relation between low SMI and low NRT in men may also be due to other reasons. For example, a decreased number of teeth may also be related to reduced mental capacity and/or reduced gastrointestinal function resulting in limited food choices and malnutrition.²¹⁾ Furthermore, the relationship between the reduction in NRT and the decrease in skeletal muscle mass may be confounded by periodontitis.²²⁾

However, in women, a relationship between the NRT and SMI was not found after controlling for smoking, alcohol, physical activity, protein intake, energy intake, calcium intake, marital status, income, fasting basal glucose, medication administration, and prostheses.

It is well known that in older adults, both the number of teeth and ASM are related to social and economic factors; those with a higher income and higher educational level as well as those who live with a spouse and live in a city have been known to possess a higher number of functional teeth.^23,24) Furthermore, skeletal muscle mass in Korean women is much more affected by BMI, lifestyle, intake, marital status, and socioeconomic status than it is in men.²⁵⁾ Another possible explanation is that the female participants were older than the male participants.

Older women had much less skeletal muscle mass than men. The choice of food may differ between men and women,^26,27) as men tend to consume more beans and proteins containing fats than women, while women tend to consume more fruits and vegetables than men. This tendency may cause restrict protein intake in women, leading to a difference in ASM between the sexes.⁴⁾

Furthermore, functional dentition (NRT≥20) showed a negative relationship with SMI in females, unlike in males. Inui et al. also reported negative correlation between skeletal muscle mass and NRT in females only. Additionally, females presented lower mean skeletal muscle mass in class A group with good occlusal support which implies that occlusal support, as well as NRT, affects sarcopenia.^28,29)

Some previous studies reported higher prevalence of sarcopenia in Korean men than in women when height adjusted skeletal muscle mass was used to define sarcopenia,^30,31) because ASM adjusted by height showed an insignificant trend associated with age in female.³²⁾ The reason may be due to reduction in height in females due to osteoporosis after menopause.³³⁾

A limitation of this study is its cross-sectional design, and the results do not indicate causal relationships. A second limitation is that gait speed or handgrip strength were unavailable, and therefore could not present the relationship between teeth loss and sarcopenia. Nevertheless, this study is based on a general population representative survey and is the first study to identify a correlation between oral health and ASM. Such correlation indicates that maintenance of oral health must be considered in managing skeletal muscle mass in older people.

In conclusion, this study showed a correlation between the NRT and SMI in participants ≥65 years of age in Korea. The relationship persisted in men, even after adjusting for confounders, but not in women.