Associations of incompetent lip seal and mouth breathing with stress in Japanese young adults: roles of oral function and nasal health

Mouth breathing (MB) and incompetent lip seal (ILS) are common oral dysfunctions with potential repercussions for dentofacial development, systemic health, and psychological well-being. Although related, MB and ILS represent distinct conditions: ILS denotes difficulty maintaining lip closure at rest, whereas MB concerns the route of respiration. Despite growing recognition of their physical impacts, few studies have examined how MB and ILS are related to psychological stress in young adults. A cross-sectional study was conducted among 567 Japanese university students aged 18–27 years using a self-administered questionnaire. The survey included 53 items on oral conditions, lifestyle habits, and nasal symptoms, and 29 items from Section B of the Brief Job Stress Questionnaire to assess psychological stress. Exploratory factor analysis (EFA) was used to identify latent constructs. Extreme Gradient Boosting (XGBoost) was applied to classify high-stress individuals and highlight important predictors, while direct and indirect associations among ILS, MB, nasal disease, and stress were examined using structural equation modeling (SEM). EFA extracted five latent factors: nasal disease, ILS and MB, oral hygiene and bad breath, condition of the teeth and gums, and lip dryness. XGBoost achieved moderate predictive performance (accuracy = 0.69, F1 = 0.74), with top predictors corresponding to ILS- and MB-related items (“Do you sleep with your mouth open?” and “Is your mouth often open during the day?”). SEM indicated that nasal disease had a direct association with stress, whereas ILS and MB indirectly influenced stress through oral symptoms such as lip dryness and bad breath. Lip dryness also showed a direct path to stress, suggesting its dual role as both a symptom and a potential stressor. ILS and MB are associated with higher stress levels in young adults primarily through indirect pathways involving oral discomfort. Lip dryness may mediate this relationship. Their partial independence from nasal disease supports the view that ILS and MB are distinct but interrelated conditions. These findings highlight the need for early screening and integrated management of oral and nasal function to promote psychological well-being.