Objective: To examine the role of nonfasting serum insulin level and components of the insulin resistance syndrome in the relationship between physical activity and the incidence of coronary heart disease and type 2 diabetes.
Methods: Prospective study of 5159 men aged 40 to 59 years with no history of coronary heart disease, type 2 diabetes, or stroke drawn from general practices in 18 British towns. During an average follow-up period of 16.8 years, there were 616 cases of major coronary heart disease events (fatal and nonfatal) and 196 incident cases of type 2 diabetes.
Results: After adjustment for potential confounders (lifestyle characteristics and preexisting disease), physical activity was inversely related to coronary heart disease rates, with the lowest rates in the men undertaking moderate physical activity and with no further benefit thereafter. For type 2 diabetes, risk decreased progressively with increasing levels of physical activity. Physical activity was associated with serum insulin level and with factors associated with insulin, ie, heart rate, hyperuricemia, diastolic blood pressure, and high-density lipoprotein cholesterol level, and with gamma-glutamyltransferase level, a possible marker of hepatic insulin resistance. Adjustment for insulin and associated factors made little difference to the relationship between physical activity and risk of coronary heart disease. By contrast, these factors together with gamma-glutamyltransferase level appear to explain a large proportion of the reduction in risk of type 2 diabetes associated with physical activity.
Conclusions: The relationship between physical activity and type 2 diabetes appears to be mediated by serum true insulin level and components of the insulin resistance syndrome. However, these factors do not appear to explain the inverse relationship between physical activity and coronary heart disease.