Aims: We examined whether glycated hemoglobin (HbA(1C)) levels of Japanese diabetic patients showed seasonal fluctuations.
Methods: Subjects included 2511 diabetic patients who regularly visited a single diabetic outpatient clinic for 10 years. A total of 253,477 HbA(1C) measurements, as well as sex, age, BMI, type of diabetes, and mode of therapy were extracted from a hospital-based database. For the cross-sectional and longitudinal analyses, average monthly HbA(1C) values of subjects and amplitudes of seasonal fluctuations were calculated. For the time-series analysis, seasonal adjustment factors of each subject were classified as complete, incomplete, or no fluctuation.
Results: Subjects showed a clear seasonal fluctuation of HbA(1C) levels, with highest levels in March (7.69%) and lowest levels in August (7.46%; p<0.001). The amplitudes of the seasonal fluctuations were associated with the mean HbA(1C) levels. The time-series analysis showed that 78.3% of patients had complete or incomplete seasonal fluctuations. HbA(1C) levels were highest in winter-spring and lowest in summer-autumn in most patients; however, some patients showed a reverse pattern.
Conclusions: Seasonal fluctuations of HbA(1C) levels were recognized in most of the Japanese diabetic patients. Physiological or metabolic factors related to temperature may be the main cause of seasonal fluctuations in HbA(1C) levels.