Abstract
In Japan, approximately 20% of women in their twenties are underweight (BMI < 18.5 kg/m(2)), representing one of the highest prevalences among developed nations. Underweight and undernutrition are associated with decreased bone mass, menstrual abnormalities, impaired glucose tolerance, sarcopenia-like changes, and systemic symptoms including depression and fatigue, significantly impacting health from young adulthood through the life course. This situation is influenced by complex factors including the societal internalization of thinness ideals, social media and mass media influences, the off-label use of GLP-1 receptor agonists, and socioeconomic factors such as poverty. However, while current healthcare and screening systems have advanced in addressing obesity, systematic approaches to underweight remain inadequately developed. Based on discussions by an expert working group established by the Japan Society for the Study of Obesity in collaboration with multiple related academic societies, this paper proposes a novel disease concept: Female Underweight/Undernutrition Syndrome (FUS). FUS encompasses the diverse physical and psychological health disorders arising from underweight or undernutrition, aiming to establish clear diagnostic criteria and frameworks for early detection and preventive intervention. Comprehensive measures are required, including stigma prevention, collaboration with educational and industrial sectors, integration into health screening systems, and addressing underlying social determinants. Through the proposal of FUS, we anticipate progress toward comprehensive interventions addressing the societal challenges surrounding women's health and body image.