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Coronary Heart Disease Incidence, by Sex -- United States, 1971-1987
Coronary heart disease (CHD) is the leading cause of death in the United States for both men and women, although the rates are lower among women (1). The incidence of CHD, however, has not been as well characterized among women as it has among men (2). This report presents data on sex-specific incidence of CHD and the risks associated with smoking, diabetes, hypertension, total cholesterol, body mass, and age using data from the Epidemiologic Follow-up Study of the First National Health and Nutrition Examination Survey (NHEFS) (3).
The NHEFS is the first prospective cohort study of a representative sample (n=14,407) of the noninstitutionalized U.S. adult population. During 1971 through 1975, members of the First National Health and Nutrition Examination Survey (NHANES I) cohort completed an extensive interview regarding demographic characteristics and medical history and received a standardized physical examination. During 1982-84, 1986, and 1987, the NHEFS attempted to trace and re-interview NHANES I participants aged greater than or equal to 25 years during the baseline exam. Death certificates and records of hospitalizations also were obtained. As of 1987, more than 96% of the initial study participants either had been recontacted at least once or had died.
This analysis was limited to persons without CHD at baseline as ascertained through self-report and physical examination. Because of the small number of persons of other races in the sample, results are presented for the 12,402 white participants. Information obtained during the baseline physical examination and in-person interview as well as subsequent interviews was used to characterize each person's exposure to various CHD risk factors at baseline. Hospitalizations in which a diagnosis of CHD (International Classification of Diseases, Ninth Revision, Clinical Modification, codes 410-414) was listed on the discharge summary provided the measure for CHD morbidity. CHD mortality was defined as deaths for which CHD was listed as the underlying cause of death on the death certificate. The first diagnosis of CHD, either at hospitalization or death, was used as the measure of CHD incidence.
Incidence rates were calculated as incidence densities; each person could contribute person-years of follow-up to more than one age category (e.g., a 42-year-old person who was followed for 11 years contributed 3 years to the 25-44-year age group and 8 years to the 45-54-year age group). The mean length of follow-up for CHD incidence was 12.4 years. Sex-specific Cox proportional hazards models that included all the risk factors being analyzed were used to obtain relative risks (RRs), controlling for all the other variables. These adjusted RRs were then used to obtain the population-attributable risk percentage (PAR%) (i.e., the proportion of risk in the study population attributable to each factor, controlling for all others).
The age-adjusted CHD incidence rate for men was 110 per 10,000 person-years; for women, the rate was 64 per 10,000 person-years. Within each age group, men had a higher rate of CHD incidence (Figure 1). While the rate generally increased by the same amount with age for men aged 25-74 years, the rate of increase of CHD incidence among women accelerated after age 65 years. Thus, the RR for CHD for women increased from 0.3 for the youngest two age groups (25-44 years and 45-54 years) to 0.8 for the oldest (greater than or equal to 75 years).
Men were more likely than women to be first diagnosed with an acute form of CHD. Death was the incident CHD event among 18.6% of men, compared with 12.5% of women. Myocardial infarction was diagnosed in 41.3% of incident CHD events among men and 29.7% of incident events among women. Although women were more often first diagnosed with chronic CHD, specific CHD diagnoses could not be reliably analyzed individually.
Most of the risk factors studied appeared to have similar effects on CHD incidence in men and women when assessed in sex-specific models (Table 1). Cholesterol is the only risk factor for which the effect was greater among men. Furthermore, when data for both sexes were combined and interactions between sex and all risk factors were included, only age and cholesterol had significantly different effects on incidence among women than among men (p less than 0.05).
When sex-specific models were stratified by age (25-64 years and 65-75 years), the associations between most risk factors were weaker among the older participants in both sexes. The only exception was the relation between education and CHD incidence among men, which was stronger among older participants.
Most of the risk factors were associated with a similar proportion of CHD incidence among women and men as measured by PAR% (Table 2). The exceptions to this were high cholesterol, which accounted for a higher proportion of CHD incidence among men, and having less than 9 years of education, which accounted for a higher proportion of CHD incidence among women.
Reported by: Div of Analysis, Office of Analysis and Epidemiology, National Center for Health Statistics, CDC.
Editorial Note: Since the mid-1960s, national death rates for CHD have been decreasing for both sexes (4). However, the proportional decrease has been greater among men than women, even though the decline began first among women. During that time, women in other developed countries (e.g., Finland and Sweden) have had larger proportional reductions in CHD mortality than men, even though in these countries, as in the United States, men aged 40-69 years have been at least 2.5 times more likely to die from CHD than women (5). Thus, better understanding of risk factors may assist in reducing the incidence of CHD among women in the United States.
The rates of CHD incidence measured in this study may differ somewhat from the true incidence rates in the United States. In particular, the rates may be higher because of oversampling of persons of lower socioeconomic status during NHANES I, or they may be lower because of incomplete ascertainment of hospital stays or because persons diagnosed with but not hospitalized for CHD were not included (e.g., silent myocardial infarctions, which tend to be more prevalent among women (6), may not be identifiable from hospital records). Nonetheless, this database provides estimates of nationally representative CHD morbidity rates that are unavailable elsewhere.
Most women with CHD in the NHANES I cohort were diagnosed with a chronic form of CHD when they were hospitalized or when CHD was identified as the cause of death; some form of acute CHD was more likely to be diagnosed in men. Potential explanations for this difference include differential diagnosis of CHD by physicians or differential treatment whereby physicians do not treat CHD as aggressively in women, allowing it to progress to advanced stages that are less successfully treated (7). Although data regarding severity of disease were not obtained in this study, the findings suggest the need to further assess possible differentials in recorded diagnoses.
In this analysis, age was the strongest risk factor and the factor differing the most by sex. Although for persons aged 25-64 years, the ratio of male-to-female CHD incidence was 2:1, the risk for CHD in older women was approximately the same as in older men. This effect of age is only partially explained by cessation of estrogen production after menopause (2,8), and additional causes of the effect of age must be identified and studied.
In this analysis, risk factors often targeted in health education programs (i.e., hypertension, high cholesterol, cigarette smoking, and overweight) all appear related to incidence of CHD in both sexes. Cigarette smoking and hypertension are especially important modifiable factors that increase the risk for CHD. Although a weak association between CHD and cholesterol was found among women, this may reflect the need to study component elements of cholesterol (e.g., low-density lipoprotein). Finally, the substantial impact of low education levels on the incidence of CHD among women and older men suggests that low socioeconomic status has an additional influence on CHD not mediated through other risk factors.
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