Sarcopenia is associated with frailty, quality of life, morbidity, and mortality. Low muscle strength and muscle mass are the hallmarks of sarcopenia
8, and population-specific cut-off values for muscle strength and muscle mass need to be established in different populations
9.
The accuracy of muscle mass measurement depends on the population examined and the method of analysis. MRI, CT, and DEXA are difficult to access and implement and impractical for evaluating large populations in different settings 10. BIA is the method of choice for measuring muscle mass. It is a widely available, rapid, non-invasive, inexpensive, and easy-to-use analysis that does not require advanced training. It can be used in both outpatients and inpatients 3. It uses the electrical permittivity of tissues, and the bioelectrical impedance consists of resistance (R) and reactance (Xc). EWGSOP recognizes it as a favorable alternative to DEXA (2,10). However, its accuracy can be low in certain instances, like obesity and edema, and different devices can give different results. Recently, multifrequency BIA devices have provided more accurate results regarding body water distribution, lean body mass, fat mass, tissue resistance, and reactance measurements 8. In our study, the mean SMM and SMMI of the participants and their cut-off values for low muscle mass (2-SD below the mean values) were significantly higher with direct mBIA measurements when compared to Janssen’s formula calculation using R of mBIA. Higher cut-off values lead to false positives, overestimation, and unnecessary treatment. As direct BIA assessments falsely overestimate FFM cut-off values, population-specific formulae for the BIA have been developed. These formulae are usually determined by comparison with the gold standard DXA. In the Caucasus, a BIA equation was developed by Kyle et al. in 2003 using multiple regression, and in 2014, Sergi et al. developed a more efficient equation for ASMM 11,12. Also, in 2014, a prediction equation was developed by Yoshida et al. 13. A very recent study compared many BIA equations with DXA and suggested that population-specific cut-offs need to be established in older adults 14.
Many populations have reported different results regarding muscle mass cut-off values using BIA. As the Janssen formula is used in many ethnic groups, we wanted to evaluate the cut-off values of this formula in our study. SMMI cut-off values for older men and women were 8.87 kg/m2 and 6.42 kg/m2 in Taiwan (Maltron BioScan 920, Rayleigh, UK), 8.6 kg/m2 and 6.2 kg/m2 in France (Impedimed, Brisbane, Australia), 8.3 kg/m2 and 6.7 kg/m2 in Spain (RJL Systems BIA 101) using the Janssen formula 15-17. In the NHANES IV study, healthy individuals aged 20-30 years were used as the reference population to determine the low SMMI threshold, defined as 2-SD below the mean SMMI, which was 6.81 kg/m2 for men and 5.18 kg/m2 for women using DEXA 18. BIA typically has higher cut-off values than DXA.
Previous studies in young reference groups from two different provinces of Turkey have reported SMMI thresholds of 9.2 kg/m2 for males and 7.4 for females using direct BIA measurement of SMM, and 8.33 kg/m2 for males and 5.70 kg/m2 for females using the Janssen formula 8,19. In our study, using the Janssen formula, the SMMI cut-off values for low muscle mass were 8.23 kg/m2 for males and 5.94 kg/m2 for females (Table 2). The difference in cut-off values between our study and Ates Bulut et al. 19 is due to the use of a more advanced Tanita device in our study.
The study's limitations were that it was a single-center study, and there was no comparison with DEXA or MRI, considered gold standards for measuring muscle mass.
In conclusion, the Janssen formula using resistance obtained by mBIA shows better results than previous data. Direct BIA measurement of SMM may overestimate muscle mass. Muscle mass cut-offs obtained with direct BIA measurements may lead to false positive diagnoses of sarcopenia in clinical practice.
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