(Stroke. 1996;27:204-209.)
© 1996 American Heart Association, Inc.
Articles |
From the Departments of Neurology (A.J.O.) and Preventive Medicine (A.J.O., M.L.D., A.R.D., J.S.), Northwestern University Medical School, Chicago, Ill, and the Department of Epidemiology, School of Public Health, University of Texas Health Science Center (Houston) (R.B.S.).
Correspondence to Anthony J. Orencia, MD, PhD, Indiana University, 541 Clinical Drive CL365, Indianapolis, IN 46202.
| Abstract |
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Methods A cohort of 2107 men aged 40 to 55 years from the
Chicago Western Electric Study who were free of coronary heart
disease and stroke through their first annual reexamination was
investigated in relation to baseline fish intake and 30-year risk of
fatal and nonfatal stroke. Data on baseline fish intake, categorized
into four levels (
35 g/d, 18 to 34 g/d, 1 to 17 g/d, and 0 g/d), were
available for 1847 men. Average values of macronutrients and
micronutrients from the first two examinations and major
coronary and stroke risk factors were assessed in relation to
fish consumption. Stroke mortality was ascertained from death
certificates and nonfatal stroke from records of the Health Care
Financing Administration.
Results During 46 426 person-years of follow-up,
76 stroke deaths occurred. Men consuming
35 g/d of fish (highest
level) had a higher age-adjusted death rate from stroke (23.5 per
10 000 person-years) than men in the three other categories of
fish consumption. Based on a Cox proportional hazards regression model
with adjustment for age, systolic blood pressure, cigarette
smoking, serum cholesterol level, diabetes,
electrocardiographic abnormalities, and table salt use, hazards ratios
(and 95% confidence intervals) for fish consumers compared with
nonconsumers were 1.34 (0.53 to 3.41) for
35 g/d, 0.96 (0.41 to 2.21)
for 18 to 34 g/d, and 1.00 (0.43 to 2.33) for 1 to 17 g/d.
Age-adjusted and multivariate analyses for
fatal and nonfatal strokes (n=222) yielded similar results.
Conclusions With stroke rates highest in the subgroup reporting highest fish intake, these data do not support the hypothesis of an inverse association of fish consumption with strokes.
Key Words: diet epidemiology risk factors
| Introduction |
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Experimental studies in vitro and in humans16 17 18 19 20 indicate that fish abundant in eicosapentaenoic (20:5 n-3) and docosahexaenoic (22:6 n-3) acids may have an inhibitory effect on the n-6 polyunsaturated acid pathway synthesis of eicosanoids. n-3 polyunsaturated fatty acids reduce platelet aggregability and thrombosis via inhibition of cyclooxygenase, which leads to a diminution of thromboxane A2 content. n-3 polyunsaturated fatty acids possibly stabilize the phospholipid membrane through alteration of the cell membrane moiety and its enzymes and receptors16 and consequently protect the blood vessel wall from ischemic damage.
This study is a prospective population-based investigation of the association of fish intake with stroke. Its a priori hypothesis is that fish consumption is inversely related to stroke risk.
| Subjects and Methods |
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Dietary data were obtained at the initial examination and at the second examination 1 year later by two nutritionists who used standardized interviews and questionnaires based on Burke's diet history method.23 The interview, which lasted about 1 hour, asked about the usual eating pattern (what, when, and where) on a typical workday and on a typical weekend, special diets followed now and previously, and changes in eating habits during the preceding 20 years. This was followed by a detailed cross-check of 195 specific food items to determine the number of times in the previous 28 days each food item had been eaten and the quantity per serving. As an aid in determining portion sizes, wax models of commonly used foods and dishes of varying sizes were used for reference. This was further assisted by information obtained from the dietary department of the Western Electric Co regarding standard portions and types of foods served in the cafeteria. Supplementary information regarding food preparation was obtained from a questionnaire mailed to wives and returned by participants at the time of interview. Information on food preparation was also obtained from neighborhood restaurants and bakeries. The list of 195 specific foods was compiled so that it would be adequate for most participants, but when a man reported habitual consumption of a dish not on that list, the recipe was obtained and analyzed into its component parts so that the contribution of that dish to his nutrient intake would be represented.
Dietary information was coded by the nutritionists to indicate kinds and quantities of foods and beverages (alcoholic and nonalcoholic) consumed by each participant during the past 28 days. These data were analyzed based on a food table derived from several sources24 25 26 27 to obtain each man's usual daily caloric intake and consumption of animal and vegetable protein, animal and vegetable fat, total carbohydrate, total saturated fatty acids, total unsaturated fatty acids, linoleic acid, linolenic acid, arachidonic acid, cholesterol, calcium, phosphorus, iron, vitamins (A, C, and D), thiamine, riboflavin, and niacin. Quantities of linoleic, linolenic, and arachidonic acid were added to estimate total polyunsaturated fatty acids.
The dietary histories are no longer available, but food profile scores
were used at the second examination to indicate consumption of 26
separate foods or food groups: soft drinks; whole milk; skim milk;
cream; cheese; eggs; ice cream; puddings and custards; soups; fish;
beef, veal, and lamb; pork, ham, and bacon; liver; poultry; mixed
dishes; vegetables; breads and cereals; potatoes; fruits; pastries;
sweets and sugars; butter; margarine; and fried foods. Each was coded
on a 4-point scale on which 0 indicated none, and codes 1 through 3
indicated increasing levels of consumption. For instance, consumption
of fish in 120-g units per 28 days was coded as 0 (none), 1 (<4), 2 (4
to 8), and 3 (>8). Averaged over a 28-day period, these categories
corresponded to average daily intakes of none, 1 to 17 g/d, 18 to 34
g/d, and
35 g/d. Since each participant was classified only by this
score, no actual mean grams per day of fish was recorded. During
that survey year (calendar year 1959), per capita consumption of fish
(expressed in edible weight) in the United States was 13.3
g/d28 ; this national data estimate deals with fish sales
rather than intake.
Men continuing to participate in the study were reexamined annually until 1966. Afterward, vital status was determined periodically by mailed questionnaire and telephone interview through the 25th anniversary of the initial examination, when vital status was known for all 2107 men. Subsequently, vital status at the 30th anniversary of the participant's initial examination was ascertained through the National Death Index. Death certificates of all decedents were obtained and coded for multiple causes.
Ascertainment of cause of death from death certificates was made without knowledge of other information on the participant. Transient ischemic attack was considered a nondefinitive end point for purposes of this study, particularly since ascertainment of transient ischemic attack through medical records (including death certificates) is not comprehensive.29 After exclusion of one case of transient ischemic attack, all death certificates with cerebrovascular disease codes (ICD codes [adapted for use in the United States, 8th Revision] 430 through 434 and 436 through 438)30 were reviewed. Only causes of death in Part I of the death certificate were considered definitive end points. Stroke mentioned as immediate or underlying cause of death was excluded as a definitive end point for any of the following reasons: cerebrovascular insufficiency or ischemia without further qualification (n=1), strokes due to secondary coagulopathies such as myeloblastic leukemia or lymphoma (n=2), progressive cerebral degeneration (without any mention of stroke) (n=1), subdural hematoma (n=1), cerebral arteriosclerosis (n=4), and intracerebral hemorrhage due to brain tumor or trauma (n=2). After review of death certificates, 76 stroke end points were identified for inclusion in the analyses.
HCFA tapes (Part A, inpatient, and Part B, outpatient) were obtained for study participants for the period 1984 through 1989. These tapes contain a record of each Medicare-reimbursed healthcare encounter. Because all surviving study participants were Medicare eligible (age 65 years or older) by 1983, costs for essentially all major events were eligible for Medicare reimbursement by the HCFA. The data available for each encounter included date of service and up to five ICD (9th revision; ICD-9) diagnostic codes. Stroke was defined as any mention of cerebrovascular disease (ICD-9 codes 430 through 438), with the exception of transient cerebral ischemia (ICD-9 435), in either HCFA Part A or Part B encounter records (n=147). With events on the death certificate combined with nonoverlapping events from the HCFA, there were 222 men with nonfatal or fatal strokes.
Exclusions from the original 2107 participants were made for one or more of the following reasons: coronary heart disease or stroke at initial examination (n=44) and/or missing data for dietary nutrients (n=187), serum cholesterol (n=6), or body mass index (n=9) at initial examination and/or reexamination. At baseline, five participants included in the analysis had atrial fibrillation, a well-established risk factor for stroke and stroke mortality.31 32 None of these men had a stroke (nonfatal or fatal) during the 30-year follow-up. Analyses were based on fish consumption data available for 1847 men. Rates of stroke incidence and mortality per 10 000 person-years of follow-up were computed for each category of fish consumption. Fish consumers were compared with nonfish consumers in regard to average baseline dietary intake and stroke risk factor profile from the 1958 and 1959 examinations. Multivariate proportional hazards regression models were used to adjust for potential confounding variables, including age, systolic blood pressure, cigarette smoking, serum cholesterol, diabetes (yes or no), ECG abnormalities (present or absent), table salt use (salt before and after food tasting versus no salt use at table), and macronutrients and micronutrients.
| Results |
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During the 30-year follow-up, 76 stroke deaths meeting inclusion
criteria occurred among the 1847 men. The large proportion of
participants without information about stroke types (namely,
ischemic stroke and intracerebral
hemorrhage) and the sparse number of
intracerebral hemorrhagic stroke deaths precluded
stratified analyses according to stroke type. The cohort
experience represented a total of 46 426 person-years
of follow-up. Table 2
presents mortality rates
of stroke per 10 000 person-years of observation by categories of
fish consumption. Persons consuming at least 35 g/d of fish had a
higher age-adjusted rate of stroke death (23.5 per 10 000
person-years) than persons consuming 1 to 17 g/d of fish or 18 to
34 g/d of fish and than nonconsumers. Mortality rates among men
consuming 1 to 17 g/d of fish or 18 to 34 g/d (15.5 and 15.3 per
10 000 person-years, respectively) were similar to those of
nonconsumers (15.6 per 10 000 person-years).
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Results on fish intake and stroke mortality were adjusted for age and
then for age, systolic blood pressure, cigarette smoking, serum
cholesterol, diabetes, ECG abnormalities, table salt use,
macronutrients, and micronutrients (Table 2
). Study
participants who
never consumed fish served as the comparison group. Age-adjusted
risk of stroke death among men who consumed
35 g/d of fish was 47%
greater than that of those who never ate fish (95% CI for hazards
ratio, 0.60 to 3.61). In the risk factoradjusted model, risk for
stroke death among consumers of
35 g/d was 34% higher compared with
participants who never ate fish (95% CI for hazards ratio, 0.53 to
3.41) (Table 2
). Neither the age-adjusted analyses nor the
multivariate risk factoradjusted analyses
showed a significant increase or decrease in risk of stroke death with
higher levels of fish consumption.
Analyses were repeated for nonfatal plus fatal strokes derived
from both HCFA data and death certificates (Table 2
). The risk
factoradjusted model for fatal and nonfatal strokes showed a
relative risk of 1.26 for fish consumers in the
35-g/d category
compared with participants who never ate fish (95% CI, 0.74 to 2.16).
Fish intake was not significantly related to nonfatal and fatal
strokes.
As expected, baseline age, blood pressure, and cigarette smoking were significantly related to stroke. Multivariate proportional hazards coefficients (rounded to three decimal places) for stroke mortality were 0.129±0.029 for age, 0.028±0.006 for systolic pressure, and 0.020±0.010 for cigarette smoking. Hazards ratio estimates and 95% CIs were as follows: age (per 5 years older) adjusted for systolic blood pressure and cigarette use, 1.91 (95% CI, 1.43 to 2.53); systolic blood pressure (per 20 mm Hg higher) adjusted for age and cigarette use, 1.75 (1.38 to 2.22); and cigarette smoking (per 20 cigarettes higher) adjusted for age and systolic pressure, 1.49 (1.01 to 2.21).
| Discussion |
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35 g/d, as noted by a dietary intake evaluation at the beginning
of the study in 1957, were not significantly associated with increased
or decreased risk of stroke in the Western Electric sample of 1847
middle-aged men followed up for 30 years. There was no significant
relationship between fish consumption and stroke; however, the lower
limits of the CIs (0.53 and 0.74, Table 2These findings have to be interpreted with respect to the potential limitations in the study. Although trained interviewers who were nutritionists collected dietary data 2 years in a row with standardized interviews and questionnaires based on Burke's in-depth diet history, measurement of fish intake was available from one examination only. These estimates of fish consumption by the individual men would have been more stable if two or more measurements were available. Fish consumption may also have increased or decreased over the course of follow-up. Trends toward a more healthy lifestyle that include exercise, a modified fat diet, reduced salt intake, and avoidance of cigarette smoking and excessive alcohol intake did not begin until the 1970s and beyond in the United States. Long-term data on such possible trends are not available for this cohort. However, the available data did show a significant inverse association between baseline fish intake and 19-year coronary heart disease mortality (probability value for trend, P=.008),6 a finding that persisted with 30-year follow-up.
As to potential limitations in stroke ascertainment, death certificates are a main source for stroke deaths in epidemiological investigations.33 34 35 36 The follow-up methods used in the Western Electric cohort were comprehensive (eg, annual examinations of the participants during the early years of the study, letters and questionnaires, and death-certificate requests through the National Death Index). The finding that the established major risk factors for stroke (age, blood pressure, and cigarette use) were significantly related to long-term risk of stroke death in this cohort lends credence to the judgment that ascertainment of stroke as a cause of death was of high validity.
Annual examinations for the Western Electric men were conducted through 1966; between 1966 and 1983, no follow-up on stroke incidence was done. In 1984, HCFA data became available for research purposes. Although the HCFA method of outcome ascertainment has limitations because it is difficult to verify incident stroke cases, analyses for fatal and nonfatal strokes (n=222) yielded hazards ratios similar to those for fatal stroke (n=76).
The results on fish intake and stroke risk prevailed with adjustment for multiple possible confounders and were not accounted for by significant differences across the groups in several dietary variables. The nonsignificantly higher stroke rates for the group with highest fish intake may thus be a chance finding. In any case, these data lend no support to the hypothesis that fish intake is associated with lower stroke risk.
This study is one of the few population-based investigations of
fish consumption and stroke. Two other studiesone in Zutphen, The
Netherlands, and one in Perth, Australiahave reported findings
seemingly different from the Western Electric cohort
results.3 4 The prospective Zutphen Study, of 552 men
experiencing 42 strokes during 15-year follow-up, presented
data indicating a nonsignificant inverse relationship between fish
intake (
20 versus <20 g/d, or "always" versus
"never"
ate fish) and stroke incidence. Compared with men in the Zutphen Study,
the Western Electric men were younger at initial examination. The
method used in the two studies to assess diet was essentially the same,
with relatively minor modifications for local
use.3 5 21 22 37
With respect to baseline dietary profiles
for fish consumers and nonconsumers, total energy, total fat, saturated
fatty acids, monounsaturated fatty acids, and
carbohydrate intake did not differ in the Zutphen and Western Electric
cohorts. Intake of polyunsaturated fatty acids was lower in the Western
Electric than the Zutphen men. These minor differences do not seem
capable of explaining the possibly diverse findings of the two studies
on fish and stroke. Given that results in each of the two studies were
not statistically significant, the seemingly disparate trends may be
due to chance.
The case-control study of lifestyle risk factors for stroke in the Perth, Western Australia, region reported an inverse relation of fish consumption (more than twice per month compared with less than this amount) to first strokes (odds ratio, 0.60; 95% CI, 0.36 to 0.99).4 In identical multivariate risk-factor models for ischemic stroke and for intracerebral hemorrhage, a significant inverse relation with fish consumption was found for intracerebral hemorrhage but not for ischemic stroke. Possible reasons are not apparent for the difference in the findings of the Chicago and Perth studies.
The Physicians' Health Study38 reported that men who had five or more servings of fish per week had a nonsignificantly lower risk of fatal and nonfatal stroke compared with men consuming less than one serving per week (relative risk, 0.6; 95% CI, 0.3 to 1.6). Because this study is a clinical trial related to myocardial infarction and cancer outcomes, findings may not be generalizable to the natural history of stroke. In addition, the study population for this clinical trial is more restrictive and homogeneous (eg, persons with liver or renal disease, peptic ulcer, and gout or who were currently using aspirin, other platelet-active drugs, or nonsteroidal anti-inflammatory agents were excluded at baseline screening). Therefore, results from the Physicians' Health Study are not directly comparable with the Zutphen, Chicago, and Perth studies.
In summary, the Chicago Western Electric Study data show no significant relationship between fish intake and stroke mortality. With stroke rates highest in the subgroup reporting highest fish intake, these data do not support the hypothesis of an inverse relationship between fish consumption and risk of stroke.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received July 20, 1995; revision received September 26, 1995; accepted October 9, 1995.
| References |
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