We have often observed increases in plasma glucose and HbA1c levels in patients with type 2 diabetes during the winter in Japan. This phenomenon has been explained in part by weight gain due to increases in food intake and decreases in exercise during the winter. However, HbA1c levels are also increased during the winter season in most patients with type 2 diabetes without weight gain. In this study, we examined the relationship between seasonal changes in body composition and blood HbA1c levels in patients with type 2 diabetes treated with insulin over a 2-year period.
Eleven male patients with type 2 diabetes treated with insulin were investigated. The mean (±SD) age was 60.4 ± 10.1 years, and BMI was 20.9 ± 1.9 kg/m2. The daily dosage of insulin was 17.5 ± 5.5 units (0.31 ± 0.07 units/kg). All patients had accomplished stable plasma glucose control for >4 years without any changes in insulin dosage. They had no diabetes complications and no other diseases aside from diabetes. The subjects were taking no other medication. The dosage of insulin was not changed during the experimental period.
The patients attended as outpatients every month. Body fat was determined by bioelectrical impedance analysis using a body composition analyzer (TBF-305; Tanita). Body composition was measured 3 h after breakfast every month for 2 years. The data obtained over the 2-year period were analyzed by ANOVA.
Body weight did not change during the period (56.4 ± 2.6 kg in January 2000, 57.4 ± 2.9 kg in December 2000, and 55.9 ± 3.0 kg in July 2001). As shown in Fig. 1, body fat began at 20.5 ± 1.1% in January 2000 (mean ambient temperature 4.5°C), decreased to 18.3 ± 1.3% in April 2000 and 17.5 ± 3.8% in September 2000 (24.0°C), increased to 20.4 ± 3.3% in January 2001 (4.5°C), and decreased again to 17.6 ± 1.3% in June 2001 (26.2°C). Blood HbA1c levels were 6.81 ± 0.16% in February 2000, decreased to 6.39 ± 0.15% in April 2000, increased to 6.65 ± 0.12% in February 2001, decreased to 6.25 ± 0.21% in July 2001, and increased to 6.60 ± 0.25% in December 2001.
Body fat and HbA1c levels were increased in winter and decreased in summer without any appreciable change in body weight. Previous reports have suggested that there is no seasonal variation in body fat in healthy subjects (1). Lower HbA1c levels have been observed in the spring and summer and higher levels in the autumn and winter in patients with type 1 diabetes undergoing intensive insulin treatment (2). The most plausible explanations for the seasonal variations in HbA1c could be the increases in insulin resistance in winter. Plasma cortisol and tissue sensitivity to glucocorticoids are higher in winter (3), which could contribute to increased body fat. However, there have been no studies to determine the precise relationship between HbA1c and body fat in type 2 diabetes treated with fixed-dose insulin. In conclusion, seasonal changes in body fat rather than body weight might attribute to seasonal variations in plasma glucose control in patients with type 2 diabetes.
Seasonal changes in body fat (upper panel) and blood HbA1c levels (lower panel). *P < 0.05 vs. January 2000; #P < 0.05 vs. February 2000.
Seasonal changes in body fat (upper panel) and blood HbA1c levels (lower panel). *P < 0.05 vs. January 2000; #P < 0.05 vs. February 2000.
