Physical activity and myocardial infarction : epidemiological studies on the association between various types of physical activity and the risk of myocardial infarction, including certain aspects of methodology

Sammanfattning: The aim of this thesis was to study the associations between physical activity during leisure time, occupational and household work, and established risk factors for cardiovascular disease, as well as the risk of acute myocardial infarction. Methodological aspects concerning the presence of recall bias in epidemiological case-control studies on physical activity and myocardial infarction were also considered. The associations between physical activity and hypertension, cholesterol levels, and plasma fibrinogen were studied through a large cross-sectional study including 10,413 persons. In this study we observed that regular leisure-time physical activity was associated with markedly lower prevalence of the cardiovascular risk factors studied. Several aspects of occupational physical activity were related to favourable HDL-cholesterol levels, especially among men. However, overweight women who perceived their occupational workload as strenuous had an increased prevalence of hypercholesterolemia. Physical activity related to household work was not as strongly associated with the risk factors that were studied. However, perception of household work as physically strenuous was associated with higher plasma fibrinogen and unfavourably low HDL-cholesterol levels in women. A combination of regular exercise, a job involving a lot of standing or walking, and physically demanding household work was associated with markedly decreased prevalence of the cardiovascular risk factors studied, in both men and women. The relationship between physical activity and risk of acute myocardial infarction was investigated in a large case-control study comprising 1,754 persons with myocardial infarction and 2,315 control subjects. We found that leisure-time physical activity was inversely related to the risk of myocardial infarction. In analyses stratified by body mass index, we observed this inverse relationship between leisure-time physical activity and myocardial infarction among lean, normal-weight and overweight persons, but not in the group of obese persons. A job that involved a lot of standing or walking was also associated with decreased risk of myocardial infarction, especially among women; whereas repetitive or heavy lifting at work, or perceiving occupational physical workload as strenuous, seemed to be associated with an increased risk. However, leisure-time physical activity was found to confer protection against myocardial infarction, irrespective of occupational physical activity level. The combination of regular exercise, standing or walking a lot at work, and having demanding household tasks was strongly related to a decreased risk of myocardial infarction. In a methodological study including 78 persons who had suffered from a myocardial infarction and 243 control subjects, we found some differences in how individuals in these two groups remembered and reported previous physical activity levels. This kind of recall bias had the largest effect on the estimated associations between myocardial infarction, and repetitive or heavy lifting at work and perception of occupational physical workload. These findings should be taken into account when interpreting the results of retrospective casecontrol studies on physical activity and myocardial infarction.