Seasonal Variations in HbA1c and Body Composition: A Sex-Specific Analysis in Adolescents With Type 1 Diabetes

Background: Circannual fluctuations in glycated haemoglobin (HbA1c) levels are recognized among adults, but comparable changes and contributing factors in adolescents with type 1 diabetes (T1D) have not been investigated in depth. Objectives: To examine the seasonal changes in HbA1c and their association with body composition in youth with T1D. Methods: This retrospective observational study included adolescents with T1D followed at our paediatric diabetes centre (2021–2023). Seasonal means were calculated for two periods (winter-spring: December to May and summer-autumn: June to November), and ΔHbA1c was calculated as the difference between the values. The patients' body composition (via bioelectrical impedance analysis (BIA)) data were reviewed, and correlation analyses were performed between sex- and age-adjusted z-scores of body composition components [appendicular muscle mass (ASMM) and fat mass (FATM)] and ΔHbA1c. Results: The seasonal means of HbA1c calculated for 259 adolescents with T1D (56% male, mean age: 16.01 ± 2.23 years) were significantly higher during winter-spring compared to summer-autumn (7.75% vs. 7.24%, p < 0.001, CI: 0.42–0.57), (61.16 mmol/mol vs. 55.72 mmol/mol, respectively p < 0.001, CI: 4.64–6.23). ΔHbA1c displayed a sex-specific association with body composition components in 102 patients (50% males) who underwent BIA. The correlation was significant for only ASMM z-scores in boys (r = 0.277, p = 0.049), while both the ASMM and FATM z-scores significantly correlated with ΔHbA1c (r = 0.301, p = 0.032 and r = 0.284, p = 0.043, respectively) in girls. Conclusions: There is a seasonal variation in HbA1c levels in adolescents with T1D, with higher values during winter-spring. The link between seasonal variability and body composition components varied by sex, indicating a need for sex-specific strategies in adolescent diabetes management.