**Background:** Undernutrition among adolescent girls is a critical public health issue, particularly in low- and middle-income countries (LMICs). In Nepal, 30% of females aged 15–19 are thin (BMI < 18.5 kg/m²), and the intergenerational cycle of malnutrition is evident. However, data on undernutrition and its determinants among late adolescent females in rural districts like Darchula are limited. This study aimed to assess the prevalence of thinness and identify associated factors among female adolescent students in Darchula District, Nepal.
**Methods:** A school-based cross-sectional study was conducted from November 2021 to February 2022 in four rural municipalities of Darchula District. A total of 400 female students aged 15–19 years were recruited using quota sampling from six schools. Data were collected via a structured questionnaire covering socio-demographics, dietary diversity (using FAO guidelines), household food insecurity (using FANTA tool), knowledge/attitude/practice (KAP), and health-related factors. Anthropometric measurements (height and weight) were taken to calculate BMI. Thinness was defined as BMI < 18.5 kg/m². After excluding 17 overweight students, 383 students (284 normal BMI, 99 thin) were analyzed. Chi-square tests and multiple logistic regression (forward stepwise) were used to identify factors associated with thinness. Qualitative data from 9 key informant interviews (school principals, health focal persons, ward chairpersons) and document review of national policies were analyzed thematically.
**Key Results:** The prevalence of thinness was 24.7% (99/400), normal BMI 71% (284/400), and overweight 4.3% (17/400). In bivariate analysis, factors significantly associated with thinness included: age (15–16 years vs. 17–19 years, p=0.016), father's occupation (p=0.005), household food insecurity (p=0.002), frequency of food purchase (p=0.001), type of food mostly bought (p=0.002), initiation of menstruation (p=0.012), and water treatment (p=0.028). In the adjusted logistic regression model, the following factors remained significantly associated with thinness:
- Father's occupation: monthly paying job (AOR=4.384; 95% CI: 1.135–16.928; p=0.032) and foreign employment (AOR=6.961; 95% CI: 1.649–29.377; p=0.008) compared to 'other' occupations.
- Household food insecurity (AOR=2.079; 95% CI: 1.182–3.658; p=0.011).
- Type of food mostly bought: grains/roots/tubers (AOR=9.487; 95% CI: 1.182–76.138; p=0.034) compared to dairy/eggs.
- No initiation of menstruation (AOR=5.015; 95% CI: 1.257–20.011; p=0.022).
Qualitative findings revealed that existing programs (IFA distribution, deworming, nutrition education) were perceived as beneficial but faced implementation gaps such as budget delays, duplication, and lack of contextual adaptation. Key informants recommended involving guardians, restricting junk food in school canteens, and scaling up school health nurses.
**Clinical Implications:** The high prevalence of thinness (24.7%) among female adolescents in this rural setting underscores the urgent need for regular nutritional screening and targeted interventions. The significant associations with household food insecurity and poor dietary diversity (grains/roots/tubers as staple) highlight the importance of food security and nutrition-sensitive agriculture. The link between father's occupation (especially foreign employment) and thinness suggests that economic migration may disrupt household food systems and care practices. Delayed menarche as a risk factor for thinness indicates that undernutrition may impair pubertal development. These findings call for multi-sectoral approaches—integrating health, education, agriculture, and social protection—to address the root causes of adolescent undernutrition. Policy recommendations include scaling up school health programs, contextualizing interventions, and ensuring sustained funding and coordination.