Fat-free mass index cutoff values for reduced muscle mass in older community-dwelling adults in Japan: A descriptive cohort study.

Background: The Global Leadership Initiative on Malnutrition criteria emphasize the importance of assessing muscle mass, recommending the fat-free mass index (FFMI) as a key measure. This study aimed to establish FFMI cutoff values for Japanese individuals and examine the risk factors of falls using these values.

Methods: This descriptive cohort study included 696 older adults from a Japanese outpatient frailty clinic located within a hospital. FFMI was measured via bioelectrical impedance analysis, and the appendicular skeletal muscle index (ASMI) was measured using dual-energy X-ray absorptiometry. Receiver operating characteristic analysis identified optimal FFMI cut-offs against low ASMI (<7.0 kg/m² for men; <5.4 kg/m² for women). Logistic regression was used to analyze the relationship between reduced muscle mass and falls over 1 year.

Results: Participants (mean age 76.1 ± 7.4 years; 64.8% female) experienced falls in 180 cases (25.9%), with 244 participants (35.1%) exhibiting low ASMI. The optimal FFMI cut-offs were <17.5 kg/m² for men (area under the curve [AUC]: 0.926; 95% confidence interval [CI]: 0.893-0.958) and <14.4 kg/m² for women (AUC: 0.927; 95% CI: 0.902-0.953), yielding 84.0% sensitivity, 91.2% specificity, and 88.6% accuracy. Low ASMI was not significantly linked to falls (odds ratio [OR: 1.04; 95% CI: 0.70-1.53; P = 0.860), whereas low FFMI was predictive (OR: 1.49; 95% CI: 1.01-2.20; P = 0.044).

Conclusion: FFMI cut-offs of <17.5 kg/m² for men and <14.4 kg/m² for women effectively identified reduced muscle mass and predicted falls. The results suggest that FFMI may be a useful tool in malnutrition diagnosis.