Characterization of individuals in whom body weight loss precedes diabetes onset: a retrospective, observational, longitudinal cohort study based on health checkup in Japan

East Asians are known to develop diabetes mellitus at a lower body weight than Caucasians, potentially because of the different mechanisms underlying disease development. This study aimed to evaluate the variation in weight transition leading to diabetes onset in two subtypes of individuals (obese and non-obese) in a Japanese population. We conducted a retrospective, observational, longitudinal cohort study using health checkup data from 9, 260 participants in Japan. Individuals who developed diabetes within three years of the start of the observation period were excluded. Among the participants, 61.4% were men, and 259 developed diabetes. In the obesity group (body mass index [BMI] ≥25 kg/m²), the average BMI increased prior to the diabetes onset and subsequently decreased. Conversely, in the non-obesity group (BMI <25 kg/m²), the average BMI decreased and then stabilized before the onset of diabetes. Notably, a greater number of participants in the non-obesity group exhibited a BMI change of ≤-0.15 kg/m² per year compared with those with a BMI change of ≥0.15 kg/m² per year before diabetes onset (p = 0.003). Our findings indicate that body weight loss precedes the onset of diabetes in the non-obesity group. We recommend that non-obese individuals with elevated blood glucose levels who do not meet the criteria for diabetes should be considered a high-risk group for diabetes development. Therefore, it is imperative to identify these individuals and provide lifestyle guidance that does not focus on weight loss to prevent the onset of diabetes.