1 Julius Center for General Practice and Patient Oriented Research, University Medical Center Utrecht, Utrecht, the Netherlands.
2 Department of Epidemiology, Faculty of Medicine, University of Nijmegen, Nijmegen, the Netherlands.
3 Department of Internal Medicine, University Medical Center Utrecht, Utrecht, the Netherlands.
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ABSTRACT |
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cardiovascular diseases; estrogens; menopause; mortality
Abbreviations: CI, confidence interval; HR, hazard ratio; HRT, hormone replacement therapy; ICD-10, International Classification of Diseases, Tenth Revision; PAR, population attributable risk; SD, standard deviation
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INTRODUCTION |
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The aims of this study are to describe the association between age at menopause and cardiovascular mortality and to investigate whether duration of premenopausal endogenous estrogen exposure adds to the prediction of cardiovascular mortality risk in postmenopausal women. Since progestogen could attenuate the estrogen effect on cardiovascular disease, we are most interested in the duration of exposure to "unopposed" endogenous estrogen. To quantify the duration of this exposure, we will combine data on the reproductive factors age at menopause, age at menarche, number and duration of pregnancies, duration of breastfeeding, and oral contraceptive use. Furthermore, we want to study whether premenopausal duration of exposure to endogenous estrogen is a better predictor of cardiovascular mortality than age at menopause. For this purpose, we used data from postmenopausal women who participated in a breast cancer screening project in the Netherlands.
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MATERIALS AND METHODS |
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Duration of exposure
Women completed questionnaires on age at menarche, regularity, and duration of menstrual cycle and the year and month that menses had stopped. Information was obtained about the number of liveborn children, miscarriages and stillborn children, children breastfed, and years of oral contra-ceptive use and to estimate a woman's lifetime duration of exposure to endogenous estrogen during premenopausal life.
Before age at menarche, sex hormone levels are very low. During the preovulation part of the menstrual cycle, estrogen levels increase without an increase in progesterone levels. During the postovulation part of the menstrual cycle, progesterone levels are high and estrogen levels are decreasing. Variations in cycle length are almost completely determined by differences in the first part of the menstrual cycle length. Duration of the cycle after the ovulation is 2 weeks, independent of total cycle length. Therefore, only the first part of the menstrual cycle, the duration of which varies among women, contributes to the period of unopposed estrogen exposure. During pregnancy, hormone levels increase substantially, but there is no period in which the increase of estrogen is not accompanied by a progesterone increase. Pregnancy therefore is not an episode with exposure to unopposed endogenous estrogen. During breastfeeding, there is a negligible, very low exposure to estrogen and progesterone. The oral contraceptives that were used before 1975 were "first-generation" pills that contained a very high dose of synthetic estrogen plus progestogen. Therefore, the normal cycle with endogenous estrogen was suppressed. At menopause, estrogen and progesterone levels decrease substantially.
For each woman, age at menarche (in months) was subtracted from age at menopause (in months). For each liveborn child, 9 months (the average duration of pregnancy) were subtracted; for each miscarriage or stillborn child, 3 months (the average duration of pregnancy for a miscarriage/stillborn child) were subtracted, and for each child who was breastfed, 4 months (the average duration of breastfeeding per child) were subtracted. This assumption of the average duration of breastfeeding was based on calculations within a population-based screening program for breast cancer in Utrecht, the Netherlands, in 1975 (10). Furthermore, total duration of oral contraceptive use was subtracted. In this way, an estimation of the lifetime total duration of all menstrual cycles (in months) was obtained. To calculate the duration of unopposed endogenous estrogen exposure, we subtracted from this number the total duration of all postovulatory periods of this cycle (2 weeks multiplied with the total number of menstrual cycles). For example, for a woman with an age at menarche of 14 years and an age at menopause of 52 years with two children born alive, one miscarriage/stillborn child, 1 year of oral contraceptive use, and an average cycle duration of 4 weeks, we estimated a duration of exposure to endogenous estrogen of 17.7 years.
Other determinants
Age at entry was calculated by subtracting date of birth from the date of screening examination. Height and weight were measured at enrollment. Body mass index was calculated as weight (kg)/squared height (m2). Women reported whether a surgical treatment was the cause of the cessation of menstruation, and if so, what kind of treatment they received. They were asked about the use of medication for menopausal complaints during the 12 months prior to enrollment. Menses were reported as regular or irregular, on average. Women were classified as a prevalent cardiovascular disease case if they had used medication for cardiovascular disease (heart glycosides, antiarrhythmic drugs, nitrates, other antianginal drugs, or anticoagulants) during the year prior to enrollment. They were classified as hypertensive if they had used antihypertensive drugs during the previous year or as diabetic if they reported that they were diabetic and/or that they had used antidiabetic medication in the year prior to enrollment. Socioeconomic class was divided in four categories mainly based on the average educational level in the area in which they were living.
Cardiovascular disease mortality
The municipal registry informed the Department of Epidemiology at the University of Nijmegen weekly about migration and deaths of cohort members before December 31, 1996. We linked the death certificate numbers from the municipal registry in Nijmegen with the cause-of-death registration at the Netherlands Central Bureau of Statistics, and so, we obtained the primary cause of death for women who had died during the 22 years of follow-up since enrollment.
The primary endpoint of the analysis was cardiovascular mortality (International Classification of Diseases, Tenth Revision (ICD-10) codes 100199 (11)). Subgroup analyses were done separately for ischemic heart disease mortality (ICD-10 codes 2025) and cerebrovascular disease mortality (ICD-10 codes 6069). Death from other causes, loss to follow-up due to moving outside Nijmegen, and loss to follow-up due to a changing registration system at the municipal registries in Nijmegen were considered censoring events.
Data analysis
All analyses were performed with the statistical package SPSS (SPSS for Windows, Release 8.0.0., SPSS, Inc., Chicago, Illinois). Correlation statistics were used to quantify the association between the different reproductive factors and the composite measure, duration of exposure to endogenous estrogen, calculated from these factors. Pearson correlation was used for the association between numeric reproductive factors and duration of endogenous estrogen exposure (continuous), and Spearman correlation was utilized for the nominal and ordinal reproductive factors. Cox proportional hazards regression analysis was used to quantify the effect of age at menopause and the duration of exposure to endogenous estrogen on cardiovascular mortality. The time of the event in the Cox analysis was marked by the time since enrollment in 1975 and by an event ((cardiovascular) death, moving, or end of the study (1996)). Since cardiovascular disease mortality increases with age, all analyses were adjusted for age at entry. Age at menopause and duration of exposure to endogenous estrogen were analyzed in quartiles, with the lowest quartiles as reference groups. Hazard ratios are presented with 95 percent confidence intervals. We repeated all analysis with multivariate adjustment for HRT use, hypertension, body mass index, and social economic status.
To study whether duration of exposure to endogenous estrogen is a better predictor of cardiovascular mortality than age at menopause, we compared the Receiver Operating Curves of the two determinants. Using the Cox model, a probability of dying of cardiovascular disease was calculated twice for each woman, first by a prob-ability based on the Cox model with age at menopause (in quartiles) as an independent variable and, second, by a probability based on the Cox model with duration of exposure to endogenous estrogen (in quartiles) as an independent variable. The linear predictor (ß1X1 + ß2X2 +...+ ßnXn) was compared with the outcome, ignoring censoring (12, 13
). Receiver Operating Curves were constructed by plotting sensitivity versus one minus specificity at various cutpoints computed for both models. The area under the curve was calculated by using the nonparametric trapezoidal rule, with its standard error according to Hanley and McNeil (14
). The standard errors were used to compute 95 percent confidence intervals.
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RESULTS |
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DISCUSSION |
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Before these results are interpreted, some issues concerning internal validity need to be addressed. Women had to be healthy enough to attend screening and were probably more motivated compared with women who did not attend. Thus, women who attended the screening were probably at lower cardiovascular risk. We do not think that, because of an early menopause and a short duration of exposure to endogenous estrogen, women were less likely to attend screening. Therefore, selection bias due to the use of a screening population is not very likely in this study.
Recall bias did not apply to this study because all history of exposure was collected before the investigated event. If any misclassification has occurred in the reporting of reproductive factors, it will tend to be random. Although, in a rare case, random misclassification has resulted in bias in the estimates away from the null (15), in general, it will lead to a dilution of the estimate of the true association between age and menopause or duration of exposure to endogenous estrogens and cardiovascular mortality (16
19
).
The potential for information bias due to a limited reliability and validity of self-reported information about cardiovascular and diabetes medication is recognized. On the other hand, it is not very likely that there is an association between the validity of self-reported medication and age at meno-pause or duration of endogenous estrogen exposure. Misclassification due to the limited validity of self-reported information will therefore be nondifferential.
In our model, the assumption is made of an average duration of pregnancy and breastfeeding. With this model, we do not have the possibility of measuring the differences in duration of exposure to endogenous estrogen more precisely. This will decrease our ability to find an association between our exposure measure and cardiovascular disease mortality.
It is very unlikely that our results are due to confounding. Age at entry is strongly related to the risk of cardiovascular mortality, and in our data, it is also related to the duration of exposure to endogenous estrogen (table 2). This is probably an artifact of the data, due to the inclusion of only postmenopausal women and, therefore, including relatively more young women with an early menopause compared with older women with a late menopause. A cohort effect as an explanation for this relation between age at entry and duration of exposure to endogenous estrogen is less likely. If any cohort effect had been present, it would have been in the opposite direction, with the youngest birth cohort having a longer duration of exposure due to an earlier age at menarche, a later age at menopause, and fewer pregnancies. The entire postmenopausal age range is represented in the cohort, so selection bias is unlikely. To adjust for confounding bias, we corrected for age at entry in all analyses.
Only 7 percent of the women used HRT during the year before the start of the study. Exclusion of these women from the analysis did not substantially alter the estimates of the hazard ratios. Exclusion of women with a surgical menopause (hysterectomy, ovariectomy, or unknown type of surgical menopause) or of women with an irregular menses did not alter the results either. Adjustment for all potential confounders was achieved by adding them simultaneously into the Cox model. These confounders did not materially influence the estimation of the hazard ratio for age at menopause or that of duration of exposure to endogenous estrogen. Residual confounding by other factors might be possible. Information on a lipoprotein profile was not available. A favorable change in this profile is one of the mechanisms through which estrogens influence the cardiovascular risk. Therefore, lipoprotein profile is part of the causal chain, and it would be inappropriate to adjust for this factor (20). Diabetes and prevalent cardiovascular disease can be considered as mediators in the pathway between estrogen exposure and cardiovascular death as well, so we did not control for these variables. Information on smoking was not available either. However, in comparable studies of age at menopause and (cardiovascular) mortality, smoking was not a strong confounder (6
, 8
, 9
, 21
). Hu et al. (9
) also studied age at menopause and cardiovascular risk in strata of smoking and suggested that age at natural menopause was related only to cardiovascular disease in (past) smokers, although the null association among nonsmokers could well be caused by chance because of the low number of cases in this stratum.
In the Netherlands, causes of death are registered with the Netherlands Central Bureau of Statistics. This registration was used to collect the information on primary causes of death of the women in the cohort. Although misclassification may have occurred, we do not believe this was related to the determinants of interest, therefore causing most likely an underestimation of the true effect.
We studied cardiovascular mortality instead of cardiovascular disease. Studies on age at menopause and cardio-vascular mortality (6) and on age at menopause and cardiovascular disease (9
) showed very comparable results.
The Cox regression model is based on the assumption that women who were withdrawn from the study did not differ in their outcome risk compared with women in the cohort. Censoring affected comparable percentages of the duration of exposure categories (approximately 16 percent in each quartile), and mean duration of endogenous estrogen exposure for women who were withdrawn was similar compared with the mean of the whole cohort.
We analyzed in quartiles the variables age at menopause and duration of exposure to endogenous estrogen. The results seem to indicate a quadratic or threshold protective effect for cardiovascular mortality that was particularly pronounced in the highest quartile of age at menopause and duration of exposure to endogenous estrogen . This indicates that an increasing exposure does not reduce the risk of cardiovascular mortality until a older age at menopause or a high level of exposure to endogenous estrogen.
Our model of duration of exposure to endogenous estrogen allows us to combine all data on reproductive endogenous estrogen-related events in one measure that we can use to address the association between endogenous estrogen and estrogen-related diseases. To our knowledge, no other studies have been performed using this or comparable endogenous estrogen exposure algorithms. Our model has not been validated in other studies yet. Theoretically, it is still possible that duration of endogenous estrogen exposure is a better predictor of cardiovascular disease risk compared with age at menopause. This will be the case when the model is not valid or when the model can be improved by a more precise measurement of duration of endogenous estrogen exposure. It would be interesting to validate the algorithm in observational studies with other estrogen-related diseases such as breast or endometrial carcinoma or osteoporosis.
Indirect evidence of a role of endogenous hormones on cardiovascular disease risk comes from studies relating cardiovascular risk to the timing or frequency of events associated with endogenous sex hormone levels. These reproductive events have been studied in relation to known cardiovascular disease risk factors as well as cardiovascular disease events. There was much variation in the study design, domain, and endpoints of the studies considering these reproductive factors in association with cardiovascular risk (22). Age at menopause was the only reproductive factor that was clearly associated with cardiovascular disease risk in these studies. Therefore, we compared the predictive value of duration of exposure to endogenous estrogen with that of the single reproductive factor age at menopause by the use of Receiver Operating Curves. It is possible that the duration of endogenous estrogen exposure does not affect cardiovascular disease risk but that lack of exposure to estrogen does. In that case, age at menopause is the only reproductive factor associated with cardiovascular disease risk because this age demarcates the transition from high to low exposure to endogenous estrogen. The duration of exposure to endogenous estrogen is, by definition, strongly related to age at menopause (Pearson correlation coefficient = 0.73, p < 0.001), which could explain the equality of predictive values of cardiovascular mortality.
Research on the protective cardiovascular effects of estrogen has focused mostly on the effects of HRT. A recent meta-analysis estimated a 50 percent reduction in coronary heart disease risk (4). Most studies in this meta-analysis addressed the use of unopposed, conjugated equine estrogens. The only randomized controlled trial published so far of which we are aware, assessing secondary prevention by HRT (Heart and Estrogen/Progestin Replacement Study (5
)), did not show an effect of combined therapy on cardiovascular disease outcome after 4 years of treatment. A possible explanation for these results was the adverse effect of conjugated estrogens combined with medroxyprogesterone on arterial thrombosis, annulling the positive effects on lipoproteins. The favorable association in observational studies between hormonal replacement therapy and cardiovascular disease risk could also be explained by selection bias, introduced by the "healthy estrogen user effect" (23
). Ongoing primary prevention trials should provide us with more definitive answers on the beneficial effects of HRT.
The population attributable risk (PAR) represents the proportion of women who died of a cardiovascular event that was attributable to a specific risk factor. A 20 percent reduction in cardiovascular mortality risk in the highest quartile of endogenous estrogen exposure and age at menopause means a PAR of -5 percent. The PAR of a body mass index above 29 in this population is 14 percent, and the PAR for hypertension is 11 percent. Cardiovascular deaths account for an important proportion of all deaths among postmenopausal women (43.6 percent of total deaths in this population). A PAR of -5 percent for cardiovascular mortality in the highest quartile of endogenous estrogen exposure and age at menopause can therefore be clinically relevant.
In conclusion, this prospective study shows that age at menopause is related to cardiovascular disease mortality. However, a newly developed composite measure of endogenous estrogen exposure did not add to the predictive value for cardiovascular mortality of age at menopause. Therefore, age at menopause will provide the clinician sufficient information on history of endogenous estrogen exposure and its associated cardiovascular disease risk.
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ACKNOWLEDGMENTS |
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The authors thank E. Brummelkamp and H. Otten for collecting data. The authors are grateful to the Netherlands Central Bureau of Statistics, who allowed them to work in their office with Central Bureau of Statistics data on causes of death.
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NOTES |
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REFERENCES |
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