Underweight in Young Adult Women as a Dynamic Nutritional State: Evidence from Four Complementary Longitudinal
Katsumi Iizuka1, Hitomi Matsuura1,2, Kotone Yanagi3
1Department of Clinical Nutrition, School of Medicine, Fujita Health University, Toyoake 470-1192, Japan.
Background:
Underweight (BMI < 18.5 kg/m2) remains prevalent among young Japanese women but lacks standardized measurement approaches. We compared four analytical methods and identified discrepancies.
Methods:
A retrospective analysis of 883 underweight women aged 20-29 years followed for 6.1 ± 4.2 years was performed. We compared (1) year-to-year transitions, (2) state occupancy, (3) the Aalen-Johansen estimator, and (4) Kaplan-Meier Survival Analysis. We performed bidirectional flow analysis quantifying inflow/outflow rates, BMI distribution analysis, and time-weighted classification.
Results:
Methods 1 and 4 showed 31-point discrepancies (78.1% vs. 47.1% in women). In bidirectional flow, inflow exceeded outflow at ages 22-27 (35.7%/yr vs. 20.7%/yr, outflow/inflow ratio: 0.58), balanced at ages 27-37 (ratio: 1.02) and showed outflow-dominant pattern at ages 37-47 (ratio: 4.92). BMI clustered at 18.0-19.0 kg/m2 (42.7%); 69.4% crossed the threshold once. Time-weighted classification revealed four phenotypes: persistent (≥75% time underweight; 40.1%, BMI: 17.54 kg/m2), moderate (50-74%; 17.6%, BMI: 18.40 kg/m2), intermittent (25-49%; 17.6%, BMI: 18.97 kg/m2), and transient (<25%; 24.8%, BMI: 19.49 kg/m2). The moderate + intermittent group showed yo-yo phenotypes (35.2%).
Conclusions:
Underweight in young Japanese women should be viewed as a heterogeneous dynamic nutritional state. The methodological discrepancy, threshold crossing, and phenotypic classification show that BMI-defined underweight comprises distinct patterns. Cross-sectional data evaluation may lead to incorrect assessments. Future research examining relationships between longitudinal low body weight subgroups and clinical outcomes could identify at-risk populations within the underweight group.
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