Skeletal Muscle Mass Index and Body Fat Percentage Reflect Different Nutritional Markers Independent of BMI in Underweight Women

<b>Background/Aim:</b> Skeletal muscle mass index (SMI) and body fat percentage (BF%) are components of body mass index (BMI) but are considered to play independent roles. We aimed to clarify whether SMI and BF% are associated with nutritional markers independent of BMI in underweight women. <b>Methods:</b> This retrospective observational study included a total of 102 women aged 20-65 years who were referred to the outpatient nutrition evaluation clinic from 2022 to 2024 with a body mass index (BMI) < 17.5. We performed a multivariate analysis with SMI and BF% as independent variables and BMI, BMI ratio (present-to-age 20 ratio), grip strength, and biochemical nutritional indicators (vitamin B₁ level (ng/mL), cholesterol level (mg/dL), lymphocyte count (/μL), and HbA1c (%) level) as dependent variables, adjusting for age. <b>Results:</b> Women aged 30.9 ± 10.2 years (yo) with a BMI of 17.0 ± 0.7 participated in this study. BMI (kg/m²) was positively associated with SMI (kg/m²) (β (95% CI): 1.6 [1.4, 1.9], <i>p</i> < 0.001) and BF% (0.2 [0.1, 0.2], <i>p</i> < 0.001), and the BMI ratio (present-to-age 20 ratio) was positively associated only with BF% (0.5 [0.05, 0.9], <i>p</i> = 0.03). Grip strength was positively associated with SMI (4.0 [1.4, 6.6], <i>p</i> = 0.003), and lymphocyte count was positively associated with BF% (β (36.2 [6.0, 66.5], <i>p</i> = 0.02). BMI was not associated with grip strength or lymphocyte count. Vitamin B₁, cholesterol, and HbA1c were not associated with SMI, BF%, or BMI. <b>Conclusions:</b> These results indicate that SMI reflects BMI and grip strength, whereas BF% reflects BMI, the BMI ratio (present to age 20), and lymphocyte count. In addition to BMI and SMI, changes in BF% should also be noted in underweight women.