BACKGROUND:

In 1974, a twin registry was established from members of the Kaiser Permanente Medical Care Program's Northern California region. In all, 9,821 twin pairs were enrolled. In 1978-1979, 434 pairs of female twins were recruited from the registry for a clinical examination of coronary heart disease risk factors. The clinical examination included an extensive health questionnaire, measurements of plasma lipids and serum chemistries, analyses of genetic markers for determination of zygosity, measurement of expired air carbon monoxide and serum thiocyanate levels, and a complete physical examination. During the ten years since the first examination, several new or more specific coronary heart disease risk factors have been recognized, including low density lipoprotein and insulin levels, and the anatomic distribution of body fat.

DESIGN NARRATIVE:

All women who participated in the first examination were invited to participate in the second examination. Medical records and/or death certificates were obtained to document coronary heart disease events. Questionnaires were used to collect information on health history, nutrition, and physical activity. A clinical examination assessed blood pressure, a variety of anthropometric measures and cardiovascular fitness. Blood specimens were collected for clinical chemistry and lipid/lipoprotein profiles. To assess genetic influence, the heritability of each of the risk factors was estimated. Blood was frozen for future studies of polymorphic DNA markers. Using the monozygotic co-twin control method, the effects of environmental or behavioral factors on these risk factors were assessed after controlling for genetic factors that could explain their association. Two hypotheses were examined that could be uniquely addressed using twin data. First, the association of body fat distribution with other coronary heart disease risk factors was mediated by serum insulin levels. Secondly, cardiovascular fitness explained in part the association of physical activity with coronary heart disease risk factors such as HDL-cholesterol and insulin levels. Lipoprotein analyses were conducted at Donner Laboratory at Berkeley.