Masseter Muscle Volume and Its Association with Sarcopenia and Muscle Determinants with Insights from ACTN3 Polymorphism in Older Japanese Adults: the Bunkyo Health Study

Aim Sarcopenia has been associated with a decrease in masseter muscle (MM) thickness in high-risk older populations. However, the relationship to sarcopenia and determinants of MM volume (MMV) in the general older population remain unclear. Mabethod In a cross-sectional study of 1484 older adults of Tokyo, we evaluated MMV using 3D MRI scanning, appendicular skeletal muscle mass (ASMM), handgrip strength, dietary intake, smoking, insulin-like growth factor 1 (IGF-1) levels, and the ACTN3 R577X polymorphism. Participants were divided into quintiles based on MMV (Q1-5). Results Our study of participants with a mean age of 73.0 ± 5.3 years, MMV (Men:35.3 ± 7.8 ml, Women: 25.0 ± 5.1 ml) was significantly larger in men than in women. A significant association between MMV and sarcopenia was observed, with the lowest quintile (Q1) showing a higher risk compared to the highest quintile (Q5) in both sexes. Body mass index (BMI) and age were independent determinants of ASMM in both sexes, while BMI, but interestingly not age, was a determinant of MMV. Moreover, IGF-1 was positively correlated with MMV in both sexes; smoking negatively correlated with MMV in women. The ACTN3 577X polymorphism was associated with only smaller MMV in men. Conclusion Low MMV increased the risk of sarcopenia, particularly in men. BMI and age strongly influenced ASMM, while MMV was only weakly associated with BMI and not with age. Notably, the IGF-1 level was positively correlated to only MMV, and the ACTN3 genotype was linked to reduced only MMV in men. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study was supported by Strategic Research Foundation at Private Universities (S1411006) and KAKENHI (18H03184) grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, the Mizuno Sports Promotion Foundation, and the Mitsui Life Social Welfare Foundation. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Ethics committee of Juntendo University gave ethical approval for this work. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors