RESUMO
In order to evaluate seasonal changes in hemoglobin A1c (HbA1c) values, we examined HbA1c values among 34,590 patients in 2010, and calculated the monthly average of HbA1c values through the year. HbA1c values were the highest in March and the lowest in October with a difference of 0.30%. The similar annual pattern was observed in HbA1c values from 2006 to 2009. Then we selected 453 diabetic patients whose treatment did not change through the year, and calculated average HbA1c values in four seasons each. There were also significant seasonal changes in diabetic patients, which were the highest in the spring and the lowest in the autumn, especially found in patients with insulin therapy. These effects may be caused by cold climate, decreased physical activity, over food intake and body weight gain in the winter. These seasonal changes in HbA1c should be concerned in the case of health service research, clinical trials and evaluation of the effects of medical treatment.