18. Seasonal variation in physical activity and plasma fibrinogen levels

Background: The observation that plasma fibrinogen tends to be higher in winter than in summer has not been adequately explained. Since physical activity has been shown to be negatively associated with plasma fibrinogen levels, we studied the possibility that the seasonal variation in plasma fibrinogen is at least partly due to seasonal changes in the levels of physical activity. Methods: A group of 110 adult males aged 20-55 years and engaged in very light physical work, were examined twice, in winter and summer. Plasma fibrinogen was measured in mg/dl. Leisure time physical activity was determined on the basis of a standard questionnaire and converted to estimated energy expenditure in kcal/week. Results: Mean physical activity was estimated as 159 kcal/week in summer compared with 118.5 kcal/week in winter (p=0.06). Mean plasma fibrinogen was 367.3 mg/dl in summer compared with 390.8 mg/dl in winter (p=0.001). However, after controlling for changes in body mass index, total and HDL cholesterol, and cigarette smoking, there was no association between the seasonal change in physical activity and the changes in plasma fibrinogen levels. Furthermore, there was a direct association between physical activity and plasma fibrinogen in winter, while there was no association in summer. Conclusions: These findings suggest that the seasonal variation in plasma fibrinogen is not explained by changes in physical activity. Nevertheless, since the activity levels in this study group were generally low, one cannot exclude the possibility that at higher levels, changes in physical activity may explain some of the variation in plasma fibrinogen.