Social determinants of health associated with underweight among middle-aged Japanese men and women: a cross-sectional study †

Objective This study examined the gender-specific associations between social determinants of health and underweight status among middle-aged Japanese adults. Methods This cross-sectional study analyzed data from a postal survey of 2799 healthy middle-aged Japanese adults. Underweight was defined as a body mass index (BMI) <18.5 kg/m 2 based on self-reported height and weight. The following variables were included as covariates in the multivariable-adjusted models: age, economic status, educational level, health literacy, living alone status, daily working hours, smoking status, drinking status, physical activity, sleep quality, and self-reported mental health. Results Of the 2799 participants, the prevalence of underweight was 5.0% in men (59/1170) and 14.9% in women (242/1629). Among men, living alone was significantly associated with underweight (adjusted risk ratio [RR]: 1.97, 95% confidence interval [CI]: 1.16–3.36), as was current smoking (RR: 1.75, 95% CI: 1.04–2.95). Self-reported average and poor/very poor mental health also had strong associations with underweight among men: average mental health (RR: 2.51, 95% CI: 1.21–5.19) and poor/very poor mental health (RR: 3.05, 95% CI: 1.36–6.87) versus very good/good mental health. Among women, no significant associations were observed. However, sensitivity analyses indicated an association between smoking and being underweight. Conclusions This study identified gender-specific factors associated with underweight in healthy middle-aged Japanese adults, highlighting the importance of social and psychological influences on nutritional status. Gender-specific approaches may be essential for early identification of underweight and appropriate interventions.