Circadian regulation modulates metabolic and hormonal processes throughout the day, yet it remains unclear whether these diurnal fluctuations are reflected in exhaled volatile organic compound (VOC) profiles and whether such temporal patterns differ between individuals with and without diabetes. Previous breath analysis studies in diabetes have shown heterogeneous results, which may reflect differences in analytical approaches and the lack of standardised sampling times.

This prospective, single-centre observational study examines daytime VOC dynamics from 08:00 to 16:00 among adults without diabetes, and individuals with type 1 diabetes or type 2 diabetes. 60 participants will complete one in-person visit with repeated breath measurements using a BreathSpec^® gas chromatography–ion mobility spectrometry system (GC-IMS) device, capillary glucose testing, body composition assessment, questionnaires, and oral and stool microbiota sampling. A standardised breakfast is provided; subsequent meals follow structured timing but are not standardised. The primary outcome is temporal variation in VOC intensities. Secondary outcomes include between-group differences and associations with glucose levels, body composition and microbiota composition. Analyses will use established GC–IMS tools and exploratory multivariate approaches.

Ethics approval was granted by the Ethics Committee of the Canton of Bern (BASEC 2023-01143). Results will be shared via peer-reviewed publications, conferences and lay summaries.

NCT05984979.