Although mortality due to coronary heart disease (CHD) has declined considerably in the United States in the past 30 years, it is not clear whether a reduced incidence of disease or improved survival is responsible. This study examined incidence trends in CHD from 1980 to 1994 in 85,941 women participating in the Nurses' Health Study. The participants, ranging in age from 34 to 59 years, had not received a diagnosis of cardiovascular disease or cancer when admitted to the study cohort. There were 946 nonfatal myocardial infarctions and 358 deaths due to CHD during the 14-year follow-up interval. The incidence of CHD declined over time in all age groups, mostly during the latter half of the study period. At the same time the proportion of women who smoked decreased by more than 40% from 1980 to 1992, but the proportion who were overweight increased from 37 to 51%. The number of postmenopausal women currently receiving hormone therapy rose from 16 to 44%. Dietary changes included a 31% decrease in daily trans fat intake from 1980 to 1990, a 69% increase in the polyunsaturated-to-saturated fat ratio, and a 90% rise in average daily intake of cereal fiber. Women tended over time to eat less red meat and more poultry and fish. Their diets contained progressively less high-fat and more low-fat dairy products.
Age-adjusted analysis confirmed a significant trend toward less CHD over time. The relative risk in 1992-1994 compared with 1980-1982 was 0.69, representing a 31% overall decrease in incidence. The relative risk was 0.90 after adjusting for smoking, dietary score, postmenopausal hormone use, and body mass index. Taken together, the dietary and lifestyle factors explained 21% of the total 31% decline in CHD or 68% of the overall change. Less smoking explained a 13% drop in age-adjusted CHD incidence, and dietary improvements explained 16%. Increased use of postmenopausal hormones contributed a 16% decline in incidence. Increased body mass was associated with an 8% increase in CHD incidence.
Much of the decline in CHD observed in women in the 1980s and early 1990s can be ascribed to less smoking, dietary improvements, and more prevalent hormone use by postmenopausal women. On the other hand, an increasing incidence of obesity seems to have slowed this trend.