North-South Gradients in Britain for Stroke and CHD
Are They Explained by the Same Factors?
R.W. Morris, PhD; P.H. Whincup, FRCP; J.R. Emberson, MSc; F.C. Lampe, MSc;
M. Walker, MA; A.G. Shaper, FRCP
Background and Purpose—The geographic variation in CHD and stroke within Great Britain is well known. We aimed
to quantify these variations and to determine the contribution of established risk factors.
Methods—This prospective study consisted of 7735 men 40 to 59 years of age in 24 British towns who were followed up
for 20 years from screening in 1978 to 1980. We compared age-adjusted incidences of major stroke and CHD events
in southern England and the rest of Britain before and after adjustment for established cardiovascular risk factors.
Results—At least 1 episode of stroke occurred in 467 men (3.54 per 1000 person-years, age standardized) and of CHD in
1299 men (10.05 per 1000 person-years). Event rates varied between towns, from 2.00 to 5.45 per 1000 person-years
for stroke and from 6.16 to 12.21 per 1000 person-years for CHD. Incidence for both diseases was highest in Scottish
towns and lowest in southern English towns (“north-south gradient”). The hazard ratio for stroke in the rest of Britain
compared with southern England was 1.44 (95% confidence interval [CI], 1.16 to 1.78); for CHD, it was 1.32 (95% CI,
1.14 to 1.53). After adjustment for baseline systolic blood pressure, smoking status, physical activity, social class, and
height, the hazard ratio was 1.24 (95% CI, 1.00 to 1.54) for stroke and 1.17 (95% CI, 1.02 to 1.35) for CHD.
Conclusions—Similar north-south gradients were observed for major stroke and major CHD events. The magnitude of
these gradients was considerably diminished when individual risk variables were taken into account. (Stroke. 2003;34:
2604-2611.)
Key Words: cerebrovascular accident 䡲 cohort studies 䡲 confounding factors (epidemiology)
䡲 coronary heart disease 䡲 geography 䡲 incidence
G
eographic variations in coronary heart disease (CHD)
and stroke death rates have long been observed in
Britain, with lower mortality in the south of England and
higher mortality in the north of England and Scotland.1
Attempts to explain these variations have usually consisted of
analyzing aggregated data on suspected risk factors over
geographical areas. When such data for risk factors are
analyzed in relation to disease rates, the relationships may be
overestimated because of the ecological fallacy.2 In contrast,
the British Regional Heart Study (BRHS) has obtained
individual data on risk factors for 7735 individual men in 24
British towns who were followed up for 20 years. It is thus
possible to examine differences in disease rates between
towns in relation to risk factors measured at an individual
level.
The BRHS has now followed up these men for ⬎20 years.
We have already demonstrated the geographical variation in
incidence of CHD over the first 15 years of follow-up,3 and
have shown that 77% of this variation may be explained by 5
established risk factors: smoking, systolic blood pressure,
physical activity, social class, and height.
See Editorial Comment, page 2609
Here, we report and compare the magnitude of the difference
for the 2 disease end points, namely stroke and CHD, between
men living in southern England and men in the rest of Britain.
We also report the extent to which such gradients may be
explained in terms of established risk factors for the 2 diseases.
Methods
The design of the BRHS has been described in detail.4 In the main
phase of the BRHS, 24 towns were selected to represent the range of
CHD mortality rates and to include all the major regions of Great
Britain. Random samples of ⬇400 men 40 to 59 years of age drawn
from a single general practice in each town were invited for
screening. The general practice was chosen to be representative of
the socioeconomic composition of the town. A 78% response rate
was obtained, and 7735 men were screened between 1978 and 1980.
Physical Measurements
A single team of 3 trained research nurses visited all towns in
succession. Towns in close proximity were visited at different times
of year. The London School of Hygiene and Tropical Medicine
sphygmomanometer was used to measure blood pressure twice in
Received March 6, 2003; final revision received June 26, 2003; accepted July 8, 2003.
From the Department of Primary Care and Population Sciences, Royal Free and University College Medical School (R.W.M., J.R.E., F.C.L., M.W.,
A.G.S.), and Department of Public Health Sciences, St George’s Hospital Medical School (P.H.W.), London, UK.
Correspondence to Dr R.W. Morris, Department of Primary Care and Population Sciences, Royal Free and University College Medical School, London
NW3 2PF UK. E-mail [email protected]
© 2003 American Heart Association, Inc.
Stroke is available at http://www.strokeaha.org
DOI: 10.1161/01.STR.0000092489.98235.1D
2604
Morris et al
North-South Gradient in Britain for Stroke and CHD
succession. The mean of the 2 readings was used in analyses, with
adjustment for observer variation within each town.5 Height was
measured to the nearest millimeter in subjects without shoes and
weight to the nearest 0.1 kg in subjects wearing trousers and socks.
Questionnaire Data
The research nurses administered a standard questionnaire that
included questions on smoking habits, physical activity, and social
class based on the longest-held occupation. Smoking was defined as
1 of 5 categories: never smokers, ex-smokers, and smokers of 1 to
19, 20, or ⱖ21 cigarettes per day. For physical activity, an established 6-category classification was used6 but was reduced to 4
categories for this analysis: none, occasional, light, and moderate or
more (active). Social class was defined by use of the Registrar
General’s Classification of Occupations and concerned the longestheld job. Seven categories were defined: I, II, III nonmanual, III
manual, IV, V, and armed forces.7 History of diagnosed CHD or
stroke was defined as subject recall of ever having had a doctor’s
diagnosis of “angina,” “heart attack,” “myocardial infarction,” or
“coronary thrombosis” for CHD and “stroke” for stroke.
Follow-Up
All men have been followed up for major nonfatal and fatal CHD
events (myocardial infarction and sudden cardiac death) and major
fatal and nonfatal stroke.8 Deaths have been flagged through the
National Health Service Central Registers in Southport for England
and Wales and in Edinburgh for Scotland. Fatal events resulting from
CHD or stroke were recorded if the International Classification of
Diseases, ninth revision, codes were 410 to 414 and 430 to 438,
respectively. Regular reviews of general practice records have been
carried out biennially throughout the follow-up period. Nonfatal
myocardial infarctions were defined according to standard criteria.8
Nonfatal strokes comprised all those cerebrovascular events that
produced a neurological deficit present for ⬎24 hours.9 Follow-up
rates have been ⬎99%. A cross-check between medical records and
subjects’ recall of diagnoses confirmed that 97% of CHD diagnoses
and 77% of stroke diagnoses were correctly identified by medical
records.10 Also, only 5% of strokes and ⬍1% of CHD events initially
identified by medical records were false-positives. The database was
updated accordingly. A record review carried out in 2000 completed
at least 20 years of follow-up for every subject. We have therefore
calculated event rates for all 24 towns over a 20-year follow-up
period for both CHD and stroke events.
Statistical Methods
Event rates for first CHD or stroke were calculated per 1000
person-years; follow-up time was counted as the elapsed years
between screening and the first event for men who experienced the
event and as 20 years for other men unless they died of other causes
before 20 years, in which case they were censored at that time. For
each town, these rates were calculated within the age groups of 40 to
44, 45 to 49, 50 to 54, and 55 to 59 years and then averaged to obtain
an age-standardized event rate.
Five risk factors (smoking, systolic blood pressure, physical
activity, social class, and height) were selected. These factors had
explained 77% of the variation in incidence of CHD over a 15-year
follow-up in the BRHS.3 The first 2 have well-established relationships with both CHD and stroke.11 The latter 3 have been demonstrated to be related to either CHD or stroke in the BRHS and other
studies.12 Serum total cholesterol, although a classic risk factor for
CHD, is not related to stroke incidence,12 and our previous work
found that it failed to explain geographical variation in CHD.3 Height
was taken as a proxy marker for deprivation in various life stages
before adulthood, and social class was a marker for social disadvantage in adult life.
Pearson correlations of age-standardized CHD and stroke incidence with mean levels of systolic blood pressure and height and
prevalence of current cigarette smoking, moderate or vigorous
physical activity, and manual social class were calculated for the 24
towns.
2605
Multilevel Modeling
Because a sample of 24 towns was chosen from a larger number of
possible towns and because subjects were chosen from each of the
towns, the data formed a multilevel structure.3,13 The statistical
package MLwiN was used, including a macro for fitting Cox’s
proportional-hazards model for survival data grouped into years of
follow-up. All models were adjusted for subject age as a continuous
variable and included region. The 2 chief subject-level variables
(smoking status, systolic blood pressure) were entered. Then, physical activity, social class, and height were added, and the residual
between-town variance was noted.
Each town was classified according to whether it was in southern
England or the rest of Britain. This dichotomous classification was
included in our models as a town-level variable rather than an
individual-level variable. Hazard ratios were calculated for the rest
of Britain compared with the south of England.
Results
Over 20 years of follow-up, major CHD events occurred in
1299 of 7735 men (16.8%), equivalent to 10.05 first events
per 1000 person-years of follow-up. Stroke events occurred in
467 men (6.0%, 3.54 first events per 1000 person-years). The
average age at first stroke was 65.1 years and at first CHD
event was 62.3 years. For 97 men, both a CHD event and a
stroke event were recorded.
Association Between Incidences for 2 End Points
in 24 Towns
Table 1 shows the age-standardized 20-year event rates of
major CHD and stroke in the 24 towns ordered by region.
CHD event rates varied from 6.16 per 1000 person-years in
Guildford to 12.21 per 1000 person-years in Dewsbury,
whereas stroke event rates varied from 2.00 per 1000 personyears in Guildford to 5.45 per 1000 person-years in Falkirk.
There was a correlation of 0.49 between the rates for the 2
diseases (the Figure). Table 1 also shows risk factor distributions for each town. For CHD incidence, strong correlations
were observed with the baseline prevalence of current smoking (positively) and mean height (negatively), whereas moderate positive correlations existed with mean systolic blood
pressure and prevalence of manual social class. For stroke
incidence, moderate correlations were observed with baseline
prevalence of current smoking and manual social class
(positively) and mean height (negatively). A weak positive
correlation existed with mean systolic blood pressure, and no
correlation existed with physical activity.
Role of Individual Risk Factors
Complete data on smoking, blood pressure, physical activity,
social class, and height were available for 7609 men. Table 2
shows hazard ratios for each individual risk factor for the 2
diseases after adjustment for each other and age. Strong
associations were found with smoking and blood pressure for
both CHD and stroke, with physical activity and height for
CHD, and with social class for stroke. Only weak, nonsignificant associations with social class for CHD and with
physical activity and height for stroke were noted.
Tables 3 and 4 show the hazard ratios for men living in the
rest of Britain compared with southern England for CHD and
stroke, respectively. After adjustment only for age, the hazard
ratio for the rest of Britain compared with the south of England
was 1.32 (95% confidence interval [CI], 1.14 to 1.53) for CHD
2606
Stroke
November 2003
TABLE 1.
Risk Profile of Men in the 24 Towns
Subjects,
n
20-Year
Incidence
of CHD*
20-Year
Incidence
of Stroke*
Mean SBP,
mm Hg
Current
Smokers,
%
Social Class,
% manual
Physical
Activity,
% active
Mean
Height,
cm
Exeter
332
9.83
3.20
139
38
45
40
173.6
Guildford
335
6.16
2.00
136
24
23
45
175.8
Maidstone
318
6.49
2.61
146
43
55
34
174.2
Gloucester
311
10.76
4.04
145
45
70
39
172.2
Bedford
298
8.83
2.71
148
28
46
38
174.1
Ipswich
362
9.90
2.39
143
32
42
47
174.5
Lowestoft
324
6.49
2.82
142
37
63
43
174.3
Merthyr Tydfil
283
11.85
4.30
149
48
72
27
170.9
Shrewsbury
311
12.21
4.35
136
34
39
45
174.0
Newcastle-u-Lyme
293
12.01
3.47
149
48
68
34
173.6
Mansfield
321
9.72
4.24
144
41
58
37
173.0
Scunthorpe
332
10.26
4.12
140
49
77
42
172.2
Grimsby
318
11.97
3.74
148
60
81
28
172.8
Wigan
337
11.34
3.83
148
40
65
30
172.8
Southport
322
10.01
3.86
147
36
43
45
173.8
Dewsbury
325
12.21
2.33
151
50
64
29
172.7
Burnley
287
10.60
2.84
146
46
69
22
171.7
Harrogate
280
10.01
2.96
139
32
33
44
174.5
Darlington
382
6.87
4.28
147
34
41
42
174.1
Hartlepool
313
11.47
3.36
148
42
73
30
173.2
Carlisle
389
11.88
4.50
150
41
58
32
173.2
Ayr
301
11.14
3.92
143
51
63
35
171.2
Falkirk
309
12.16
5.45
148
49
74
37
172.1
Dunfermline
352
11.86
4.02
152
46
61
43
172.8
0.49
0.40
0.59
0.52
⫺0.41
⫺0.63
0.23
0.40
0.40
⫺0.05
⫺0.53
Town (Ordered
by Region)
South
Midlands/Wales
North
Scotland
Correlation with CHD incidence
Correlation with stroke incidence
0.49
SBP indicates systolic blood pressure.
* Age standardized per 1000 person-years.
and 1.44 (95% CI, 1.16 to 1.78) for stroke. When smoking and
blood pressure were included in the model, the magnitude of the
log (hazard ratio) was reduced by 29% for CHD and by 28% for
stroke. After additional adjustment for physical activity, social
class, and height, the magnitude was reduced by 42% and 40%
for CHD and stroke, respectively. For CHD, smoking and
systolic blood pressure were the 2 variables most effective in
reducing the magnitude of the regional difference (although the
other 3 variables were almost as effective), whereas for stroke,
systolic blood pressure and social class were most effective for
reducing this difference.
Exclusion of Subjects With Prior Diagnosis of
CHD or Stroke
At the initial screening, 322 of the 7609 men recalled a doctor
diagnosis of CHD or stroke, and for another 6, information on
recall of these diagnoses was not available. The analysis was
repeated for each end point including only those 7281 subjects
who did not recall a doctor diagnosis of CHD or stroke. For
stroke, the results were very similar to those obtained before
exclusion of the 328 subjects. Hazard ratios for living outside the
south of England were 1.46 (95% CI, 1.23 to 1.68; P⫽0.001)
and 1.27 (95% CI, 1.04 to 1.50; P⫽0.037) before and after
adjustment for the 5 variables, and the magnitude of the log
(hazard ratio) was reduced by 36% when the 5 variables were
included in the model. For CHD, the hazard ratios were
somewhat reduced when the 328 subjects were omitted. The
hazard ratio was 1.25 (95% CI, 1.10 to 1.40; P⫽0.003) when
adjusted only for age. This was reduced to 1.10 (95% CI, 0.96 to
1.24; P⫽0.18) when the 5 variables were included. The magnitude of the log (odds ratio) was reduced by 59%.
Discussion
Main Findings
The present analysis has shown that the incidence of stroke
was increased in regions of Britain outside the south of
Morris et al
North-South Gradient in Britain for Stroke and CHD
2607
Scatterplot of stroke incidence vs CHD incidence in 24 towns.
England, just as for CHD. The increased incidence outside
the south of England was slightly greater for stroke both
before and after adjustments for individual risk factors and
after exclusion of subjects who recalled a doctor diagnosis of
CHD or stroke at baseline. Generally, towns with a high
incidence of 1 disease also had high incidence for the other.
However, only 97 men experienced both types of disease over
the 20-year follow-up; this amounted to about one fifth of the
strokes and ⬍1 in 13 of the CHD events. Lower mean blood
pressure was the variable that most accounted for the decreased incidence of stroke in the south (18%), whereas lower
TABLE 2. Hazard Ratios (95% CIs) of Individual Risk Factors
for 20-Year Incidence of CHD and Stroke After Adjustment for
Each Other and Age
CHD
Stroke
1.0
1.0
Ex-smokers
1.34 (1.13–1.59)
1.12 (0.85–1.49)
1–19 daily
1.73 (1.42–2.10)
1.76 (1.29–2.41)
20 daily
1.79 (1.45–2.22)
1.71 (1.21–2.42)
21–39 daily
2.16 (1.76–2.65)
1.91 (1.36–2.67)
Smoking
Never smokers
ⱖ40 daily
Systolic blood pressure
(per 10 mm Hg)
1.60 (1.18–2.18)
1.36 (0.80–2.32)
1.13 (1.11–1.15)
1.21 (1.16–1.26)
Physical activity
None
1.0
1.0
Occasional
0.73 (0.61–0.87)
0.87 (0.64–1.19)
Light
0.68 (0.56–0.82)
0.77 (0.56–1.07)
Moderate/vigorous
0.57 (0.48–0.69)
0.72 (0.52–0.99)
Social class
I
1.0
1.0
II
0.93 (0.72–1.21)
1.57 (0.94–2.61)
III nonmanual
1.02 (0.76–1.36)
1.57 (0.90–2.75)
III manual
0.99 (0.77–1.26)
1.60 (0.98–2.61)
IV
1.09 (0.82–1.45)
1.70 (0.98–2.64)
V
0.84 (0.58–1.21)
2.71 (1.51–4.86)
Armed forces
1.24 (0.85–1.79)
2.05 (1.05–4.01)
Height (per 5 cm)
0.90 (0.86–0.93)
0.97 (0.91–1.04)
mean blood pressure and lower smoking prevalence accounted to a similar extent for the decreased incidence of
CHD (15% and 14%, respectively). Physical activity and
height made very little contribution to explaining the geographic variation in stroke, but their contribution, and that of
social class, was almost as great as for smoking and blood
pressure in explaining variations in CHD.
Unadjusted hazard ratios for the 2 diseases were 1.32 and
1.44 for men living in the rest of Britain compared with the
south of England or, inverting, 0.76 and 0.69 for those living
in the south of England compared with the rest of Britain.
This suggests the potential to avoid about one quarter of CHD
and one third of stroke events occurring in the rest of Britain
if conditions enjoyed by men in the south of England could be
replicated elsewhere. Decreases in rates of smoking and
levels of blood pressure to levels prevalent in the south would
reduce the inequality by almost one third.
Strengths and Weaknesses of the Present Study
The BRHS represents towns in all major regions of Britain.
The cardiovascular mortality for these towns differed markedly when the men were screened.4 Other major British
prospective studies of CHD or stroke are based on a restricted
geographical location. The known geographic variation in
disease rates potentially allowed the study to explore the
contribution of known risk factors that may differ in their
geographic distribution. Thus, the potential benefit when risk
factors are modified at the population level may be estimated.
Other studies have examined geographic variation by use of
aggregated data. Overall rates of disease have been calculated
from routinely collected statistics and related to prevalence of
risk factors in different areas.14 –17 In contrast, the BRHS has
been able to avoid possible ecological biases inherent in
aggregated data2 by relating relationships at the individual
level to variations in incidence at the town level through
multilevel modeling.
In using stroke data ascertained from death certificates (for
fatal strokes) and general practice medical records (for
nonfatal strokes), we have been unable to distinguish between
ischemic and hemorrhagic strokes. It has been suggested that
height may be more strongly related to hemorrhagic strokes
than ischemic strokes.18 However, the vast majority of strokes
2608
Stroke
November 2003
TABLE 3. Comparison of British Regions With South of England for Incidence of
CHD After Adjustment for Explanatory Variables Specified*
Reduction in
Regional
Effect,† %
Hazard
Ratio
(95% CI)
P
0.075
0
1.32 (1.14–1.53)
0.0002
0.070
14
1.27 (1.10–1.41)
0.0007
0.074
15
1.26 (1.09–1.46)
0.0016
Loge
(Hazard
Ratio)
SE
None
0.275
Smoking
0.236
Blood pressure
0.234
Variables Included
Physical activity
0.246
0.072
11
1.28 (1.11–1.47)
0.0006
Occupational social class
0.247
0.070
10
1.28 (1.12–1.47)
0.0004
Height
0.249
0.072
9
1.28 (1.11–1.48)
0.0005
Smoking, blood pressure
0.194
0.071
29
1.21 (1.05–1.40)
0.006
Smoking, blood pressure, physical
activity, social class, height
0.159
0.071
42
1.17 (1.02–1.35)
0.025
*All models adjust for age.
†Taken as percentage of loge (hazard ratio) for model with no variables except age included.
admission rates for stroke were associated with prevalence of
hypertension, alcohol consumption, and absence of fruit
consumption.22 Common to both is the importance of raised
blood pressure. Our study has shown that this single factor
was the strongest explanation for the variation in stroke rates
between towns.
Occupational social class was the second-most-effective
variable in explaining geographic variation. The relationship
has previously been demonstrated at an individual level both
in the BRHS cohort and in a Scottish cohort (Renfrew/Paisley
study), but in both cases, its effect has been at least partially
explained by other risk factors.23 Because height has not
explained geographic variation in stroke rates in the present
study, the contribution of adult social class may reflect
exposures to risk factors acting in adult life that occur more
commonly outside the south of England.
This study was limited to British middle-aged men, but
a recently mounted study of British women 60 to 79 years
of age drawn from almost the same general practices
demonstrated similar findings using cross-sectional data
on geographical variation in cardiovascular disease
prevalence.24
of unknown origin are probably ischemic in older British
populations.19
The ecological correlations observed between towns for
risk factors with stroke events were lower than with CHD,
probably because the lower stroke event rates led to incidence
estimates that were more liable to sampling variation. The
real strength of our analysis was in quantifying the geographic gradient within a multilevel model.
It has been shown that associations between individual risk
variables and outcome may be underestimated by taking
baseline measures to represent usual levels over the follow-up
period.20 An analysis that could account for such imprecision
may explain the regional differences even more fully than
demonstrated by these findings. However, the use of baseline
measurements themselves would be more relevant for prediction of regional differences.
Comparison With Findings of Other Studies
Other studies have pointed out trends in risk factor distribution among areas where stroke mortality was known to differ.
Hypertension prevalence and glucose intolerance mirrored a
gradient observed for stroke mortality in 3 areas in the United
States.21 Among men in 22 Scottish health districts, hospital
TABLE 4. Comparison of British Regions With South of England for Incidence of
Stroke After Adjustment for Explanatory Variables Specified*
Loge
(Hazard
Ratio)
None
Smoking
Blood pressure
Variables Included
SE
Reduction in
Regional
Effect,† %
Hazard
Ratio
(95% CI)
P
0.364
0.109
0
1.44 (1.16–1.78)
0.0008
0.333
0.109
9
1.40 (1.13–1.73)
0.002
0.297
0.109
18
1.35 (1.09–1.67)
0.006
Physical activity
0.344
0.109
5
1.41 (1.38–1.75)
0.002
Occupational social class
0.308
0.110
15
1.36 (1.09–1.69)
0.005
Height
0.353
0.109
3
1.42 (1.15–1.76)
0.001
Smoking, blood pressure
0.263
0.110
28
1.30 (1.05–1.61)
0.017
Smoking, blood pressure, physical
activity, social class, height
0.219
0.110
40
1.24 (1.00–1.54)
0.046
*All models adjust for age.
†Taken as percentage of loge (hazard ratio) for model with no variables except age included.
Morris et al
North-South Gradient in Britain for Stroke and CHD
Public Health Implications
Although differences in the prevalence of the established risk
factors appear to make a substantial contribution to the
modest increase in risk of stroke and CHD associated with
living beyond southern England, it should be recognized that
mean levels of serum total cholesterol and body mass index
are uniformly high throughout Britain and that the prevalence
rates of other risk factors even in southern England are far
from desirable. The relatively small changes in blood pressure, cigarette smoking, and physical activity required to
bring the rest of Britain down to the levels of CHD and stroke
encountered in southern England cannot be seen as a major
public health objective when the levels of cardiovascular risk
in southern England are still high by international standards.
Clearly, any public health actions on diet, body weight,
smoking, physical activity, and control of blood pressure
must be directed to the whole population throughout Britain
and not targeted at specific geographical groups.
Acknowledgments
The BRHS is funded by the British Heart Foundation with additional
support from the Department of Health. Opinions expressed in the
article are those of the authors and not necessarily those of the
funding bodies.
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factors for stroke in middle aged British men. BMJ. 1991;302:1111–1115.
10. Walker MK, Whincup PH, Shaper AG, Lennon LT, Thomson AG. Validation of patient recall of doctor-diagnosed heart attack and stroke: a
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stroke mortality: an analysis of temporal patterns by sex, race, and
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16. Rodriguez AF, Guallar-Castillon P, Gutierrez-Fisac JL, Ramon BJ, del
Rey CJ. Socioeconomic level, sedentary lifestyle, and wine consumption
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Editorial Comment
North, South: Changing Directions in Cardiovascular Epidemiology
Despite impressive declines in cardiovascular disease (CVD)
over the last half-century, stroke and coronary heart disease
(CHD) still constitute the greatest disease burden in the
developed world. Moreover, there is accumulating evidence
that developing countries will be faced with stroke and CHD
epidemics in the relatively near future.1 Much of our understanding of the etiology of CVD has been gained from
prospective cohort studies such as the British Regional Heart
Study (BRHS), and in this issue of Stroke, Morris and
colleagues2 supplement a previous report from this study of
geographical patterns of CHD incidence3 by extending the
length of follow-up and examining geographical variations in
stroke. The BRHS is an ideal study for investigating the
geographical inequalities in CVD. Indeed, the study was
established with this aim in mind, specifically to test the
hypothesis that water quality was a determinant of CVD risk,
which proved not to be the case.4 However, despite the fact
that the study generated ⬎250 articles, mostly on the causes
2610
Stroke
November 2003
and consequences of CVD, it was not until 2001 (⬎20 years
after the initiation of the study) that the “definitive paper
about the causes of regional variation in coronary heart
disease appeared.”5 This most recent contribution is therefore
welcome. The authors found that among men the risks of both
CHD and stroke were greater in the rest of Britain compared
with the south of England and that this difference was
substantially, although not completely, explained by adjustment for a number of adult CVD risk factors: systolic blood
pressure, smoking status, physical activity, social class, and
height. Had they adjusted for other established risk factors, in
particular diabetes status and dyslipidemia, which are associated with both CHD and occlusive stroke, the residual
variation may have been completely removed. These findings
are in line with those of a similar previous study of British
men.6 What then does this work add to our epidemiological
and public health knowledge? In part, the answer to this
question requires a greater understanding of the factors
responsible for geographical variations in CVD risk factors
and thus CVD.
The results essentially confirm the association between
several established adult risk factors and CVD, and the
authors acknowledge in their concluding paragraphs that their
findings have little to contribute to public health practice:
“Clearly, any public health actions on diet . . . must be
directed to the whole population throughout Britain and not
targeted to specific geographically located groups.” The
article refers to the north-south gradient, but in fact the
comparisons are dichotomized between the south of England
and the rest of Britain. From Table 1, it can be seen that in this
particular study there is not a clear north-south gradient:
Merthyr Tydfil, Gloucester, and Shrewsbury, all of which
have southern latitudes, have high incidences of both CHD
and stroke, whereas Darlington in the north has a relatively
low risk of CHD, and Harrogate, also in the north, has a
relatively low risk of stroke. This reflects in part the selection
of towns in the BRHS, and although a north-south gradient
was not observed, there is substantial geographical variation,
with incidences of CHD varying between 6.16 and 12.21/
1000 person-years and of stroke between 2.00 and 5.45/1000
person-years across the 24 towns. The between-town variation in both CVD and risk factor occurrence is likely to be
explained by area- and/or individual- level deprivation; the
authors might therefore have explored this geographic diversity within the BRHS and sought explanations beyond established adult risk factors.
Many studies have demonstrated geographical inequalities of the sort presented here.7–12 Importantly, many
investigators have now moved beyond these simple ecological designs and examined individual- and area-level
measures to determine whether the physical and social
aspects of where people live influence health independently of the characteristics of the people themselves.13–19
The relevance of this issue is that if variations in health
between areas can be entirely explained by the personal
characteristics of the inhabitants of these areas, then policy
makers need act only on improving the circumstances of
individuals. Conversely, the demonstration of independent
area-level effects would be key in emphasizing the need to
focus attention on features of the areas where people live
and not just the individuals living there. This is important
because the widening gap between the rich and poor
appears to be mirrored by a growing divergence of their
residential environments, so that affluent people are increasingly living and interacting with other affluent people
while the poor increasingly live and interact with other
poor people.20 Such studies could go even further; rather
than simply use census-derived contextual effects, the
environments in which study participants live could and
should be examined.21 Both the BRHS and the newly
formed British Women’s Heart and Health Study22 could
in the future incorporate an examination of the neighborhoods in which their participants live by assessing, for
example, the following: the local availability of affordable
fruits and vegetables, green areas, and physical activity
facilities; area levels of criminal activity; and other indicators of
environmental adversity.
That the baseline measurements in this study were taken 20
years before the final period of follow-up reaffirms the need
to combat adverse risk factor profiles at least as early as
middle age. However, the findings also prompt a wider
consideration of cardiovascular risk. Blood pressure in middle age may be a strong risk factor but is itself set in train in
early life, as evidenced by the declines seen in several regions
over the last 50 years in young people who were not taking
antihypertensive medication.23 Height is also a measure of
early life exposures, and its association with CVD (demonstrated in the BRHS and other studies) may represent the role
of genes, early nutrition or infection, or other socioeconomic
exposures that become embodied over the years.24 It has been
claimed that the major risk factors (smoking, dyslipidemia,
hypertension, inactivity) for CVD are known and that emphasis should now be placed on tackling these rather than
searching for other risk factors.25 However, these risk factors
do not explain socioeconomic variations in CVD and are
themselves determined by social, environmental, and biological exposures acting throughout the course of life.26,27
Finally, it is opportune to consider the growing burden of
CVD in developed countries and the potential for it to greatly
widen the global north-south gradient in health.28 With
respect to exposures over the life course, the greatest risks to
public health are likely to be seen in developing countries
where the effects of extreme poverty in early life and in
earlier generations are being exacerbated through the adoption of adverse Western diets and lifestyles in adulthood.29
Cardiovascular research is increasingly carried out in developing countries, and continued support for this body of work
is required. Similarly, it is imperative that interventions
aimed at preserving the cardiovascular health of young
individuals in developed countries are also implemented in
more deprived parts of the globe. Geography provides a
valuable tool for a more comprehensive investigation of
disease etiology; it is essential that it is not used merely as an
indicator of health inequality.
Peter McCarron, MFPHM, PhD, Guest Editor
Department of Epidemiology and Public Health
Queen’s University
Belfast, UK
Morris et al
North-South Gradient in Britain for Stroke and CHD
Debbie A. Lawlor, MFPHM, PhD, Guest Editor
Department of Social Medicine
University of Bristol
Bristol, UK
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