Seasonal fluctuations of glycated hemoglobin levels in Japanese diabetic patients

AimsWe examined whether glycated hemoglobin (HbA1C) levels of Japanese diabetic patients showed seasonal fluctuations.MethodsSubjects included 2511 diabetic patients who regularly visited a single diabetic outpatient clinic for 10 years. A total of 253,477 HbA1C 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 HbA1C 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.ResultsSubjects showed a clear seasonal fluctuation of HbA1C 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 HbA1C levels. The time-series analysis showed that 78.3% of patients had complete or incomplete seasonal fluctuations. HbA1C levels were highest in winter–spring and lowest in summer–autumn in most patients; however, some patients showed a reverse pattern.ConclusionsSeasonal fluctuations of HbA1C 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 HbA1C levels.