Low muscle mass has been associated with increased morbi-mortality and should be identified for optimizing preventive and therapeutic strategies. This study evaluates the prevalence of bioelectrical impedance analysis (BIA)-derived low muscle mass in older persons using definitions found through a systematic literature search and determines the link between body mass index (BMI) and low muscle mass.We performed a systematic search of trials involving ≥100 persons that derived low muscle mass from BIA and reported cut-offs for low muscle mass normalized for body height or weight. These cut-offs were applied to all adults ≥65 y who underwent a BIA measurement at Geneva University Hospital between 1990 and 2011 (N = 3181). The association between BMI and low muscle mass was evaluated through multivariate logistic regressions.We identified 15 cut-offs based on the fat-free mass index (FFMI), skeletal muscle index (SMI), or skeletal muscle percentage (SMP). Depending on the definition, the prevalence of low muscle mass was 17% to 68% in women and 17% to 85% in men. The risk of low muscle mass increased with a BMI <18.5 kg/m2 when using cut-offs based on FFMI (odds ratio [OR] ♀ 14.28-24.04/♂ 25.42-50.64) or SMI (OR ♀ 3.56-4.56/♂ 7.07-8.87) and decreased with a BMI ≥25 kg/m2 (FFMI: OR ♀ 0.03-0.04/♂ 0.01-0.04; SMI: OR ♀ 0.18-0.25/♂ 0.14-0.18). The opposite association appeared between BMI and cut-offs based on SMP.The prevalence of low muscle mass varies widely depending on the definition, especially in persons with BMI <18.5 or ≥25 kg/m2.