Jump to: Page Content, Site Navigation, Site Search,
You are seeing this message because your web browser does not support basic web standards. Find out more about why this message is appearing and what you can do to make your experience on this site better.
Eva Prescott a Copenhagen Center for Prospective
Population Studies, Danish Epidemiology Science Center at the Institute
of Preventive Medicine, University of Copenhagen, Denmark, b Copenhagen City Heart Study, Bispebjerg Hospital,
University of Copenhagen, c Epidemiological Research Unit,
Bispebjerg Hospital, University of Copenhagen
Correspondence to: Dr Eva Prescott Institute of Preventive
Medicine, Kommunehospitalet, DK-1399 Copenhagen K,
Denmark eva.prescott{at}ipm.hosp.dk
| |
Abstract |
|---|
|
|
|---|
Objective: To compare risk of myocardial infarction
associated with smoking in men and women, taking into consideration differences in smoking behaviour and a number of potential confounding variables.
Ischaemic heart disease is responsible for about 40% of deaths in
Western countries, with smoking as a major modifiable risk factor.1 The steep rise in the worldwide prevalence of
smoking among women is expected to continue in the near future. At the start of the smoking epidemic, female smokers were few and differed extensively from male smokers in factors such as age of starting smoking, amount smoked, and inhalation habits, and the risk associated with smoking in women may have been underestimated.2-4
Within the past two or three decades male and female smoking habits
have become similar, and a more fair comparison of the risk associated with smoking in both sexes based on recent prospective population studies is now possible. From a public health point of view, as well as
a clinical point of view, it is important to recognise sex differences
in risk associated with smoking and elucidate possible mechanisms by
which these differences could act.
We recently found that the relative mortality from vascular disease was
higher in female smokers than in male smokers.5 Consequently, we aimed to examine sex differences in smoking related risk of myocardial infarction while simultaneously including multiple cardiovascular risk factors in a prospective population study conducted
in Copenhagen.
This study is based on data from three longitudinal population
studies: the Copenhagen city heart study, with 15 789 subjects from
central Copenhagen; the Glostrup population studies, with 6341 subjects
from Copenhagen suburbs; and the Copenhagen male study, which sampled
3355 subjects from 14 large workplaces. All datasets with sufficient
information on cardiovascular risk factors were included. The study
population is outlined in table 1. Overall response rate at first
examination was 77% (range 69-88%).
Table 1
Design: Prospective cohort study with follow up of
myocardial infarction.
Setting: Pooled data from three population studies
conducted in Copenhagen.
Subjects: 11 472 women and 13 191 men followed for
a mean of 12.3 years.
Main outcome measures: First admission to
hospital or death caused by myocardial infarction.
Results: 1251 men and 512 women had a
myocardial infarction during follow up. Compared with non-smokers,
female current smokers had a relative risk of myocardial infarction of
2.24 (range 1.85-2.71) and male smokers 1.43 (1.26-1.62); ratio 1.57 (1.25-1.97). Relative risk of myocardial infarction increased with
tobacco consumption in both men and women and was higher in inhalers
than in non-inhalers. The risks associated with smoking, measured by both current and accumulated tobacco exposure, were consistently higher
in women than in men and did not depend on age. This sex difference was
not affected by adjustment for arterial blood pressure, total and high
density lipoprotein cholesterol concentrations, triglyceride
concentrations, diabetes, body mass index, height, alcohol intake,
physical activity, and level of education.
Conclusion: Women may be more sensitive than men to
some of the harmful effects of smoking. Interactions between components of smoke and hormonal factors that may be involved in development of
ischaemic heart disease should be examined further.
![]()
Introduction
![]()
Methods
Cardiovascular risk factors were assessed by a self administered
questionnaire and various laboratory tests. Tobacco consumption was
studied in six categories: never smokers; ex-smokers; non-inhaling current smokers; and inhaling current smokers of 1-14, 15-24, and
25
g tobacco per day. Type of tobacco (cigarette, cheroot, cigar, pipe, or
mixed) was recorded for current smokers, as were years of smoking for
both current and former smokers. Current tobacco consumption was
calculated by equating a cigarette to 1 g tobacco, a cheroot to 3 g
tobacco, and a cigar to 5 g tobacco. Pack years in current smokers was
calculated as years of smoking multiplied by packs (of 20 cigarettes)
currently consumed.
Arterial blood pressure was measured with the subject in a sedentary position after at least five minutes' rest. Blood lipids were non-fasting in the Copenhagen city heart study and fasting in the remaining cohorts. Body mass index was calculated as weight (kg) divided by height squared (m2). Educational level was divided into three categories: <8 years of schooling (completed primary school), 8-11 years, and >11 years. Alcohol consumption was classified according to total weekly intake: <1 drink per week, 1-6 drinks, 7-13 drinks, 14-27 drinks, 28-41 drinks, and >41 drinks; one drink contained 9-13 g alcohol. Physical activity in leisure time was classified into three categories as sedentary; moderate activity <4 hours per week; and moderate activity >4 hours per week. Self reported diabetes was defined as an affirmative answer to the question "Do you have diabetes?"
Subjects were followed until 31 December 1993 for fatal and non-fatal myocardial infarction (ICD-8 diagnosis code 410); the information was obtained from the National Board of Health or the National Hospital Discharge Register. Subjects with myocardial infarction before enrollment were excluded; analyses therefore concern first myocardial infarction only.
Statistical analysis
Arterial blood pressure, total and high density lipoprotein
cholesterol concentrations, triglyceride concentrations, height, weight, and body mass index were divided into fifths within cohorts, by
sex and by 10 year age groups. In this way differences in methods of
measurement between the three cohorts were taken into account. Association between risk factors and myocardial infarction was analysed
by using Cox proportional hazards regression models with age as
underlying timescale and delayed entry accordingly. Relative risks for
covariates other than smoking did not differ between men and women, and
the final analyses were performed on the pooled data stratified by sex;
this assumed the same effect of covariates in men and women but allowed
for different baseline hazard in men and women. All covariates were
treated as categorical variables as described above, and tests for
interaction were done by using the likelihood ratio test. There were no
significant interactions between sex and cardiovascular risk factors
other than smoking. Both incidence rate and distribution of risk
factors differed in the three study groups, but risk estimates did not
differ. We therefore report results from the pooled data adjusted for cohort of origin. Incidence rates were based on number of events and
person years of observation in 5 year age bands. The Stata statistical
package was used for estimation.6
| |
Results |
|---|
Analyses were based on 11 472 women and 13 191 men (table 1). During follow up, 512 women and 1251 men had myocardial infarctions, of which 104 and 274, respectively, were fatal.
With the exception of alcohol consumption and physical activity, men had a more disadvantageous cardiovascular risk profile (table 2).
|
Figure 1 shows age and sex specific incidence rates of myocardial infarction. Men had higher incidence rates than women at all ages but the male-female risk ratio decreased from about 3 in the younger groups to 1.5 in older groups.
|
Systolic blood pressure, diastolic blood pressure, total and high density lipoprotein cholesterol concentrations, triglyceride concentrations, body mass index, height, education, alcohol intake, leisure time physical activity, and diabetes were all strongly associated with risk of myocardial infarction, and relative risks were similar in men and women.
Female current smokers had a relative risk of 2.24 (range 1.85-2.71) and male smokers 1.43 (1.26-1.62) relative to non-smokers. This difference was significant (ratio 1.57 (1.25-1.97), P=<0.001) and was not changed after multiple adjustment for other risk factors. Risk in ex-smokers was not increased, but in current smokers there was a clear dose-response relation from a relative risk of 1.70 (1.31-2.21) and 1.26 (0.98-1.61) in female and male non-inhalers, respectively, to a maximum of 3.31 (1.91-5.73) and 2.08 (1.60-2.69) in heavy smokers (table 3). All risk estimates were higher in women than in men and were not affected by adjustment for other major risk factors, as indicated. Similar results were seen after categorisation by pack years: maximum risk was seen in inhaling smokers of more than 30 pack years (3.26 (2.36-4.50) in women, 1.76 (1.41-2.19) in men; ratio 2.13 (1.48-3.07)). There was no interaction between smoking and other risk factors.
|
Figure 2 shows relative risk for inhaling current smokers versus never smokers by age in men and women up to age 85. The risk of myocardial infarction decreased with age but was higher in women at all ages. The interaction term between smoking and sex did not differ in the four 10 year intervals (P=0.73) and when the age dependence of the risk associated with smoking was adjusted for, the overall ratio between female and male relative risk was 2.01 (1.39 to 2.90).
|
| |
Discussion |
|---|
In this prospective study of almost 25 000 subjects the main result was that relative risk of myocardial infarction in female smokers exceeded that of male smokers by more than 50%. This difference was not affected by multiple adjustment for major cardiovascular risk factors.
Myocardial infarction
Our end point was defined as ICD-8 code 410, ascertained from the
Hospital Discharge Register and from registration of cause of death
with the National Board of Health, and included both fatal and
non-fatal myocardial infarction. Some infarctions did not lead to
hospital admission or may have been coded differently, for instance as
codes 411 (other acute ischaemic heart disease), 427 (symptomatic heart
disease), and 795 (sudden death). However, only differential
misclassification (related to both sex and smoking status) will bias
results, and there is no reason to suspect this.
on the haemodynamic system, for example
are more important than the chronic exposure in
development of coronary thrombosis.
Comparison by sex
In a previous study we showed that female smokers have about a
50% higher relative risk of dying from vascular disease.5 The present study confirmed this sex difference and found that the
difference is not affected by adjustment for other cardiovascular risk
factors.
Conclusion
Female smokers have a higher relative risk of myocardial
infarction than male smokers, even after adjustment for major
cardiovascular risk factors. This raises the question of whether
tobacco smoke may be more harmful to women with regard to ischaemic
heart disease, possibly because of constituents of tobacco smoke
exerting anti-oestrogenic effects. Results from large ongoing clinical
trials of hormonal replacement therapy may be able to elucidate this.
| |
Acknowledgments |
|---|
The Copenhagen Center for Prospective Population Studies (T I A Sørensen, G Jensen, H O Hein, T Jørgensen, N Keiding, J Vestbo, M Grøenbak) consists of the Glostrup population studies (T Jørgensen, H Ibsen, K Borch-Johnsen, P Thorvaldsen, T Thomsen), the Copenhagen male study (H O Hein, F Gyntelberg, P Suadicani) and the Copenhagen city heart study (G Jensen, P Schnohr, M Appleyard, P Lange, B Nordestgaard, M Grønbæk).
Contributors: EP had the original idea for the present study, performed the data analyses, and is guarantor for the paper. MH and JV participated in data analyses and contributed to the paper. HOH and PS participated in data collection. The paper was written jointly by EP, MH, JV, HOH, and PS.
Funding: Grants from the Danish Ministry of Health, the Health Insurance Fund, the Danish Heart Foundation, and the Danish Medical Research Council (12-1661-1).
Conflict of interest: None.
| |
References |
|---|
|
|
|---|
(Accepted 5 December 1997)
Israeli students are refusing to perform intimate examinations on anaesthetised women without their informed consent.