Seasonal Variation of Glycemic Control in Type 2 Diabetic Patients

Medical nutrition therapy is integral to diabetes care and management (1). Balance between dietary intake and energy consumption through daily physical activities is the most influential factor in the glycemic control of type 2 diabetic patients. The nutritional prescription made for a diabetic individual is usually determined by taking into consideration the expected physical activity, diabetes complication(s), and age. The dietary advice based on this prescription seems to be valid in many cases for at least a few years; for some diabetic patients, its validity is lifelong. Such a dietary prescription is made by an implied understanding that eating habits and physical activity do not change throughout the year. Here, we show a seasonal variation of HbA_1c levels in type 2 diabetic patients.

Fukushima province is a large agricultural area surrounded by mountains, and it has a relatively low population density compared with central Japan. The climate is typical of any valley area; the people experience very warm and humid Asian summers (>34°C) and icy cold winters from January to early March. Generally, the people here are active outdoors, with some patients engaging in field work from spring to fall, but not as frequently during winter. During winter, when it gets dark around 4:00 p.m. and the roads are icy and slippery, the people customarily enjoy salty meals prepared in a pot and alcoholic beverages.

We calculated the mean HbA_1c levels of 39 type 2 diabetic patients (27 women and 12 men, mean age 65.6 years) in each month. The mean HbA_1c level was elevated by ∼0.5% in winter compared with the period between spring and autumn, ranging from 6.42 ± 0.65% (mean ± SD) in July to 6.96 ± 0.90% in March, P < 0.01.

This observed seasonal variation in HbA_1c levels is likely caused by an increased dietary calorie intake and decreased physical activity during the cold winter months. It is rare for doctors to prescribe different nutritional prescriptions in winter, and we did not find any diabetes textbook that discussed this seasonal change in lifestyle. It seems reasonable for diabetologists and dietitians to modify the nutritional prescription for those diabetic patients whose opportunities for physical exercise are reduced during the winter months.