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(Hypertension. 2001;37:187.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Department of Epidemiology and Public Health (P.P., E.F., M.G.M.), Royal Free and University College Medical School, London, UK; Health Promotion Division (S.G.), Department of Health, London, UK; and Imperial College School of Medicine (N.R.P.), London, UK.
Correspondence to Dr P. Primatesta, Department of Epidemiology and Public Health, Royal Free and University College Medical School, 1-19 Torrington Place, London WC1E 6BT, UK. E-mail paolap{at}public-health.ucl.ac.uk
| Abstract |
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16 years old).
Randomly selected adults (33 860; 47% men) with valid body mass index
(BMI) and BP measurements provided data on smoking status (never, past,
or current) and were stratified into younger (16 to 44 years old) and
older (
45 years old) age groups. Analyses provided between
89% and 94% power to detect a difference of 2 mm Hg
systolic BP between smokers and nonsmokers in the 4 age/gender
strata (
=0.05). Older male smokers had higher systolic BP
adjusted for age, BMI, social class, and alcohol intake than did
nonsmoking men. No such differences were seen among younger men or for
diastolic blood pressure in either age group. Among women,
light smokers (1 to 9 cigarettes/d) tended to have lower BPs than
heavier smokers and never smokers, significantly so for
diastolic BP. Among men, a significant interaction between
BMI and the BP-smoking association was observed. In women, BP
differences between nonsmokers and light smokers were most marked in
those who did not drink alcohol. These data show that any independent
chronic effect of smoking on BP is small. Differences between men and
women in this association are likely to be due to complex
interrelations among smoking, alcohol intake, and BMI.
Key Words: blood pressure smoking epidemiology
| Introduction |
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Smoking causes an acute increase in blood pressure (BP) and heart rate and has been found to be associated with malignant hypertension.4 Nicotine acts as an adrenergic agonist, mediating local and systemic catecholamine release and possibly the release of vasopressin.5 Paradoxically, several epidemiological studies have found that BP levels among cigarette smokers were the same as or lower than those of nonsmokers.6 7 However, in a study of 24-hour ambulatory BP monitoring, smokers maintained a higher mean daytime ambulatory systolic BP (SBP) than nonsmokers, even though office BP levels were similar.8 These findings reflect the fact that patients do not smoke during measurement of office BPs and hence the BP that is recorded may not represent the subjects usual BP.
Given the importance of smoking, BP, and their interaction in the determination of cardiovascular risk, we investigated BP levels among smokers and nonsmokers with data from the Health Survey for England (HSE).
| Methods |
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78% overall but
slightly lower in men and in inner city residents. Data collection
involves an interviewers visit (which includes height and weight
measurements), followed by a visit from a nurse, who measures BP,
records current use of prescribed medicines, and takes a blood
sample. Overall, 48 307 adults aged
16 years were interviewed in the
1994, 1995, and 1996 surveys. BP was measured with the automated Dinamap 8100 monitor.10 With an appropriately sized cuff, 3 BP readings were taken on the right arm with the informant in a seated position after a 5-minute. Informants who had eaten, drunk alcohol, or smoked in the 30 minutes before the measurement were excluded from analysis. Data used in this study are based on the mean of the second and third measurements.
Detailed information on smoking habits was collected. Past
smokers were defined as subjects who had stopped smoking >1 year ago;
those who stopped within 1 year (n=868) and those with missing data on
smoking status (n=81) were excluded. Current smokers were further
categorized according to number of cigarettes smoked per day (light 1
to 9, moderate 10 to 19, heavy
20). In 1994 and 1996 (but not 1995),
blood samples were analyzed for serum cotinine
levels.
Body mass index (BMI) was defined as weight (kg)/height
(m)2. Respondents were classified into 1 of
4 categories of BMI:
20 kg/m2,
underweight; >20 to 25 kg/m2, normal
weight; >25 to 30 kg/m2, overweight; and
>30 kg/m2, obese.
Respondents were asked about the frequency, quantity, and type of alcoholic drink consumed during the past 12 months. Consumption in units per week was categorized as (1) none, past drinking, or <1 unit; (2) 1 to 10 units for men or 1 to 7 units for women; (3) 11 to 21 units for men and 8 to 14 units for women; and (4) >21 units for men or >14 units for women.
Social class was assigned on the basis of occupation of the head of the household, with the Registrar Generals standard classification.11 Social classes were further grouped into manual (skilled manual, partly skilled, and unskilled occupations) and nonmanual (professional, managerial and technical, and skilled occupations).
The psychosocial measures included in the HSE have varied
each year. The self-administered General Health Questionnaire (GHQ12),
designed to detect possible psychiatric morbidity, was included in 1994
and 1995. A score of
4 was used to identify possible psychiatric
disorders.
In 1994, physical activity levels and serum
cholesterol levels were assessed. Physical activity was
categorized as inactive (those informants who reported <12 occasions
of moderate or vigorous activity during the 4 weeks before the
interview) and active (the remainder). Total serum
cholesterol was categorized as normal (<6.5 mmol/L)
and raised (
6.5 mmol/L). Separate analyses that included
these covariates were run only for 1994.
Analyses are presented for the 33 860 persons (70% of those interviewed) who provided data on smoking status and had valid BMI and BP measurements, of whom 11 222, 14 063, and 8575 were never, past, and current smokers, respectively.
Statistical Methods
The relationship between smoking and BP was assessed
with linear and logistic regression. The dependent variables in the
linear regression were the continuous variables SBP and DBP,
whereas that for the logistic regression was the odds of being on
antihypertensive medication.
The explanatory or independent variables used in both models were age (used both as a continuous variable and in 10-year age groups), BMI, smoking status, social class, and alcohol consumption, plus cholesterol levels, physical activity, and mental health status when data were available.
Data from HSE show that although cigarette smoking prevalence decreased with age, mean consumption per smoker increased up to age 45 to 54 before decreasing.12 Given these differences in smoking patterns by age and that SBP remained fairly constant up to the mid 40s,13 SBP and DBP were adjusted for age (through linear regression) after stratification into 1 of 2 age groups: 16 to 44 years (younger) and >44 years (older). The analyses provided 89% to 94% power to detect at 5% significance a difference in SBP of 2 mm Hg between smokers and nonsmokers in each of the 4 age/gender strata studied.
Tests for interaction were carried out with multiple regression analyses, fitting a smokingxBMI interaction term in the appropriate model, with smoking fitted as a 5-category variable (never, past, light, moderate, and heavy) and BMI as a binary variable (overweight and obese or underweight and normal).
| Results |
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Table 1 shows age-adjusted characteristics of the
men and women studied according to smoking status. Similar rates of
current smoking were found in men and women, although among current
smokers, more men than women were heavy smokers. On average, among men
and women, current smokers had significantly lower BMIs than did never
and past smokers. However, heavy smokers had the same mean BMI as never
smokers. In both genders, there was a higher proportion of heavy
drinkers among current smokers than among never smokers, whereas
alcohol consumption among past smokers was between that of never and
current smokers. The age-adjusted proportion of heavy alcohol
consumption was higher among heavy smokers than among light smokers.
The proportion of men and women from manual social classes was higher
among current smokers than among never or past smokers, and the
proportion increased with the number of cigarettes smoked. The
proportion of both men and women who scored
4 on the GHQ12
questionnaire was higher in smokers than in nonsmokers.
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Table 2 shows the effect of various characteristics on age-adjusted BPs. In men, increasing alcohol consumption was positively associated with BP. In women, mean SBP and DBP showed a U-shaped relationship with alcohol intake. Increasing BMI, manual social class, raised cholesterol level, physical inactivity, and normal GHQ12 score were associated with increasing BP.
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Overall, age-adjusted BPs did not differ importantly among never, past, and current smokers (Table 3), although in men, a small significant difference was observed in mean SBP between never smokers (139.9 mm Hg) and current smokers (140.7 mm Hg) (P<0.05), and heavy smokers (141.4 mm Hg) (P<0.05). In women, the lowest BPs were observed among light smokers, who had significantly lower levels than never smokers (135.5/72.4 and 136.8/73.6 mm Hg, respectively; P<0.05).
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Mean BP levels after adjustment for age, BMI, alcohol intake, and social class are shown stratified by age and smoking status in Table 4. Among women of both age groups, the lowest BPs were found among light smokers, whereas in men, the lowest mean BPs were found among past smokers. In most age/gender strata, the highest BPs among current smokers were found in the heaviest smokers. However, only among older men and women (>44 years) were SBPs of current smokers greater than SBPs of never smokers and only significantly so for men. No significant differences were seen between DBPs of never and heavy smokers for either gender. The data shown in Table 4 were unaffected by the exclusion of social class from the analytic models.
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Among men with normal weight, no association was seen between smoking and SBP, whereas in women of normal weight, past smokers had a mean SBP that was 0.9 mm Hg lower than that of nonsmokers (P=0.04). Among overweight or obese people, male moderate and heavy smokers had an SBP that was 2.1 and 1.8 mm Hg, respectively, higher than that of nonsmokers (P<0.01), whereas no significant differences were observed among women (Figure). A formal test for interaction between BMI and the smoking-SBP relationship was significant in men (P=0.02).
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In both never and heavy smoking men, alcohol intake showed a graded effect on mean SBP (P<0.05), and smoking was associated with higher SBP levels only among alcohol drinkers, although the association did not reach statistical significance (Table 5). Further adjustment of these data for BMI did not alter BP patterns by alcohol intake and smoking strata. In nonsmoking women, SBP was lower in moderate than in nondrinkers and heavy drinkers, whereas among light smokers, SBP increased with increasing alcohol intake. Light smokers had lower SBPs than never smokers in all 3 strata of alcohol intake but only significantly so among nondrinkers. The difference in SBP levels between never and light smokers was reduced after adjustment for BMI, regardless of alcohol intake but remained statistically significant among nondrinkers.
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Patterns in BPs in the 1994 survey, including additional adjustment for cholesterol levels, physical activity, and GHQ12 scores, were essentially the same as those seen in Tables 4 and 5.
No association between smoking and use of antihypertensive agents was observed in either gender after adjustment for age, alcohol consumption, BMI, and social class, and the results shown in Table 4 did not change substantially when the analyses were repeated and excluded those on antihypertensive treatment (data not shown).
| Discussion |
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Among smokers, a U-shaped relationship between the number of cigarette smoked and relative body weight has been found in several studies, with those smoking 10 to 20 cigarettes/d being the leanest.16 Although this seems paradoxical given that smoking is associated with increased 24-hour energy expenditure,17 heavy smokers may weigh more because of other habits, such as high alcohol and saturated fat intakes. Other dietary variables, such as electrolyte intake, may further confound the BP-smoking association, because smokers have been reported to have different diets than nonsmokers.18 We did not have dietary data but showed that heavy smokers tend to drink more than nonsmokers or lighter smokers and, in the 1994 survey, had higher cholesterol levels.
Alcohol intake, which is strongly associated with smoking,12 also appears to affect the smoking-BP relationship: a difference in the relationship between alcohol consumption and BP among smokers and nonsmokers was observed in 3 German cross-sectional studies.19 20 In 1 of these studies,19 effect modification by smoking was stronger in men than in women. Moreover, BMI, which has well-established effects on BP,21 is also affected by alcohol intake, and this, too, must be taken into account when assessing the relationship between smoking and BP because of the apparent opposing effects that alcohol and smoking exert on body weight.
Given these complex interrelationships, we examined the relationship between smoking and SBP separately by alcohol intake status, before and after adjustments for BMI. Among men, we observed higher levels of SBP with increasing alcohol intake at each level of smoking, whereas in nondrinkers, heavy smokers did not have higher SBPs. In contrast, among women, SBP was lower among light smokers than among never smokers regardless of alcohol intake, and alcohol showed only a small dose-response effect on SBP among light smokers and not among never smokers. Hence, differences in the BP-smoking association observed between men and women appear to be due to the stronger interrelation between smoking and drinking in men. For any given smoking category, women tend to consume smaller amounts of alcohol than men. Among men, adjustment for BMI did not affect the increase in SBP with increasing alcohol intake among never or heavy smokers: the effect of smoking on SBP was small, variable in direction, and nonsignificant. In women, the difference in SBP levels between never and light smokers was reduced, but still significant, after adjustment for BMI in nondrinkers, appearing to be in part due to the confounding effect of body weight.
One of the possible limitations of the study is that the HSE
collects information by means of a questionnaire, and therefore some of
the risk factor data are self-reported, which may be
inaccurate.22 However,
agreement between self-reported smoking status and serum cotinine
levels was good: only 3.5% of men and 0.8% of women who reported
never smoking had a cotinine level of
20 ng/mL, a level that is
strongly suggestive of smoking. Reclassification of these respondents
as smokers did not affect the observed results for SBP and DBP (data
not shown). The use of the Dinamap 8100 for BP measurement has been
challenged.23 However, this
machine is considered to be suitable and sufficiently accurate in the
setting of a large multicenter
survey.10
In summary, these data from a large nationally representative database suggest that there are no consistent independent differences of clinical significance in BP values between smokers and nonsmokers. The BP differences associated with smoking that were observed in this study differed with age and between men and women and may well be explained at least in part by differential confounding effects of BMI and alcohol intake. However, because smoking and BP have been shown to exert a synergistic adverse effect on the risk of coronary heart disease,3 it is critical that persons with raised BP are advised to stop smoking. Furthermore, because BP levels in smokers are rarely recorded during or immediately after smoking when acute rises in BP occur, usual BP levels of smokers tend to be systematically underestimated.
| Acknowledgments |
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Received April 18, 2000; first decision May 18, 2000; accepted August 21, 2000.
| References |
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