DLM: Omega-3 Fatty Acids (2009-2010)

Citation:
 
Study Design:
Class:
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Quality Rating:
Research Purpose:

To examine the association between fish consumption and the 30-year risk of death from myocardial infarction (sudden and non-sudden), coronary heart disease, cardiovascular diseases and all causes of death in the Chicago Western Electric Study.

Inclusion Criteria:
  • Age 40 to 55 years old in 1957
  • Employed at the Western Electric Company Hawthorne Works in Chicago for at least two years in occupations related to the manufacture of telephones.
Exclusion Criteria:
  • Did not attend second examination
  • Prior coronary heart disease
  • Missing data on diet, education, body mass index, serum total cholesterol level, smoking status or blood pressure.
Description of Study Protocol:

Recruitment

Not described: Data was taken from Chicago Western Electric Study data set.

Design

Prospective cohort design.

Statistical Analysis

  • Average baseline (1957 to 1959) dietary intake and cardiovascular risk factors were calculated for four strata of fish consumption: None, one gram to 17g, 18g to 34g and 35g or more per day
  • Age-adjusted mortality rates for fatal myocardial infarction (all deaths, sudden deaths and non-sudden deaths), all deaths from coronary heart disease, all deaths from cardiovascular diseases and deaths from all causes were calculated per 10,000 person-years for each stratum of fish consumption
  • Cox proportional-hazards regression was used to estimate the relative risk of death and 95% confidence interval for each stratum of fish consumption (as compared with no consumption), with adjustment for possible confounding variables and to test for linear trend (for the four strata of fish intake classified as indicator variables). This model was also used to test for possible interactions (the statistical power was limited in this regard).
Data Collection Summary:

Timing of Measurements

  • Dietary information was obtained at the first and second examinations performed one year apart by two nutritionists using standard questionnaires
  • Vital status was determined at the annual examination for the first 10 years and by means of mailed questionnaires or telephone interviews for the next 15 years
  • For the 31st year, data on vital status were obtained from the National Health Index, the Health Care Financing Administration and surviving participants' responses to questionnaires.
Dependent Variables
  • Variable One: Death from myocardial infarction (measured by ICD-8 code 410), classified as sudden or not sudden, according to duration of illness and the place of death as recorded on the death certificate
  • Variable Two: Death from coronary heart disease (ICD-8 codes 410 through 414)
  • Variable Three: Death from cardiovascular diseases (ICD-8 codes 400 through 445).

Independent Variables

Dietary intake [measured by the creation of food profile scores based on information obtained by two nutritionists using standard questionnaires, which included typical a workday eating pattern, a typical weekend eating pattern, the time and place of meals, special diets, changes in eating habits over the previous 20 years, and the consumption (frequency and quantity) of 195 foods during the previous 28 days; each participant's daily intake of nutrients was determined with the use of a food composition table] of soft drinks, coffee, decaffeinated coffee, whole milk, skim milk, cream, cheese, eggs, ice cream, puddings or custards, soups, fish, beef, veal or lamb, pork, ham or bacon, liver, poultry, mixed dishes, vegetables, breads or cereals, potatoes, fruits, pastries, sweets or sugars, butter, margarine and fried foods. Each item was coded on a four-point scale (0 for none and 1, 2 or 3 for a low, moderate or high level of consumption, respectively.

Fish consumption, measured by the creation of food profile scores based on information obtained by two nutritionists using standard questionnaires during the second examination: Fish consumption, in 120-g units per 28 days, was coded as 0 for none, 1 for less than four units, 2 for four to eight minutes and 3 for more than eight units, corresponding to the following average daily intakes: None, one gram to 17g, 18g to 34g and 35g or more.

Description of Actual Data Sample:
  • Initial N: 2,107 men.
  • Attrition (final N): 1,822 (62 were excluded because they did not attend the second examination; 223 men were excluded because of poor coronary heart disease or missing data on diet, education, body mass index, serum total cholesterol level, smoking status or blood pressure).
  • Age: Participants were between the ages of 40 and 55 years old in 1957
  • Ethnicity: Not reported in this article

Other Relevant Demographics

  • 68% of study participants were blue-collar workers
  • 42% were Catholic (religion was included as a variable because of religious dietary practices)
  • Over half the participants were smokers (58%), with a mean of 18 cigarettes smoked per day
  • Most (85%) were drinkers, who consumed an average of 18.8ml of alcohol per day (4.2% of kcal).

Anthropometrics

On average, the study participants were overweight with higher-than-desirable levels of blood pressure, serum cholesterol and intake of total fat, saturated fat and cholesterol.

Location

Chicago, Illinois.

Summary of Results:

Baseline: Religion and the intake of total energy, ethanol, several macronutrients and several micronutrients, differed significantly among the four strata. For all the nutritional variables except carbohydrate and saturated and monounsaturated fatty acid, the highest levels were in the men with the highest level of fish consumption. Age, education, body-mass index, blood pressure, serum cholesterol level, smoking status and number of cigarettes smoked, heart rate, presence or absence of a history of diabetes and presence or absence of electrocardiographic abnormalities did not differ significantly among the four strata.

30-year mortality: During 30 years of follow-up, there were 293 deaths from myocardial infarction (196 sudden deaths, 94 non-sudden deaths, and three that were not classifiable), 430 deaths from any type of coronary heart disease, 573 from any type of cardiovascular disease and 1,042 from any cause. Of the 137 deaths from coronary heart disease other than myocardial infarction (ICD-8 codes 411 through 414), few were sudden.

Age-Adjusted Mortality Rate at 30 Years, According to Baseline Fish Consumption*

Cause of Death 
Total Deaths
Fish Consumption
0g/day
1-17g/day
18-34g/day
>35g/day
No. of deaths
Death rate
No. of deaths
Death rate
No. of deaths
Death rate
No. of deaths
Death rate
No. of Men/Person Years
 
189/4,754
646/16,681
745/19,350
242/6,368
Overall
MI
293
36
78.6
115
69.2
113
57.9
29
45.3
All CHD
430
48
104.2
157
94.6
179
91.8
46
71.2
All CVD
573
61
132.4
208
125.5
235
120.6
69
108.2
All causes
1042
105
226.4
377
227.0
432
221.7
128
203.6
MI
Non-sudden death
94
12
27.2
44
26.5
32
16.3
6
9.7
Non-sudden death (>12hours)
70
8
18.0
33
19.8
23
11.7
6
9.7
Sudden death
196
24
51.5
69
41.6
80
41.1
23
35.6
Sudden death (<12hours)
164
15
32.5
61
36.7
66
34.0
22
34.2
Excluding MI
CHD
137
12
25.6
42
25.4
66
33.9
17
25.9
CVD
280
25
53.8
93
56.3
122
62.7
40
62.9
All causes
749
69
147.8
262
157.8
319
163.8
99
158.3
Excluding Non-Sudden Death from MI
CHD
336
36
77.1
113
68.1
147
75.5
40
61.6
CVD
479
49
105.3
164
99.0
203
104.3
63
98.6
All causes
948
93
199.2
333
200.6
400
205.5
122
193.9

*Myocardial infarction (MI) was defined as ICD-8 code 410, coronary heart disease (CHD) as ICD-9 codes 410 through 414 and cardiovascular diseases (CVD) as ICD-8 codes 400 through 445.

The trend toward an association between higher fish consumption and lower rates of death from coronary heart disease, cardiovascular diseases and all causes was attributable to the inverse relationship between fish consumption and death from myocardial infarction, particularly non-sudden death from myocardial infarction. Thus, when deaths from myocardial infarction were subtracted from deaths from coronary heart disease, deaths from cardiovascular diseases and deaths from all causes, there was no significant graded relationship between the four strata of fish consumption and death rates. The result was similar with the subtraction of non-sudden deaths from myocardial infarction.

Three Cox proportional-hazards models were used to calculate the relative risks of death for the men in the four strata of fish consumption.

  • Model One was adjusted only for age
  • Model Two was adjusted for:
    • Age
    • Education
    • Religion
    • Systolic blood pressure
    • Serum cholesterol
    • Number of cigarettes smoked per day
    • Body-mass index
    • Presence or absence of diabetes
    • Presence of absence of electrocardiographic abnormalities
    • Daily intake of energy, ethanol, macronutrients and cholesterol
    • Daily intake of iron, thiamine, riboflavin, niacin, vitamin C, beta carotene and retinol.
  • Model Three was adjusted for all the non-nutrient variables in Model Two plus daily intake of energy, alcohol and the four categories of food correlated with fish consumption and death from myocardial infarction or other coronary causes (cheese, margarine, mixed dishes and vegetables)

The results were similar with all three models. The data from Model Two was shown in the table below.

Multivariate-Adjusted Relative Risk of Death at 30 Years According to Baseline Fish Consumption*

Cause of Death
Total Deaths
Fish Consumption
Relative risk (95% confidence interval)
0g/day
1-17g/day
18-34g/day
>35g/day
P for trend**
Overall
MI
293
1.00
0.88
(0.60-0.28)
0.76
(0.52-1.12)
0.56
(0.33-0.93)
0.017
All CHD
430
1.00
0.88
(0.63-1.22)
0.84
(0.61-1.17)
0.62
(0.40-0.94)
0.040
All CVD
573
1.00
0.94
(0.70-1.25)
0.89
(0.67-1.19)
0.74
(0.52-1.06)
0.010
All causes
1042
1.00
1.02
(0.82-1.27)
0.98
(0.79-1.22)
0.85
(0.64-1.10)
0.175
MI Non-sudden death
94
1.00
1.04
(0.54-2.02)
0.67
(0.35-1.33)
0.33
(0.12-0.91)
0.007
Non-sudden death (>12hours)
70
1.00
1.31
(0.59-2.90)
0.77
(0.34-1.76)
0.51
 
 

Author Conclusion:
  • In conclusion, data from the Chicago Western Electric Study show a significant, graded, independent inverse association between baseline fish consumption and the 30-year risk of fatal myocardial infarction, particularly non-sudden death from myocardial infarction.  This relationship accounted for the lower rates of death from all coronary causes, all cardiovascular causes and all causes in association with higher fish consumption, which persisted throughout the 30 years of the study and in analyses adjusted for potentially confounding demographic, biomedical and dietary factors.
  • Further studies (observational and interventional) are needed to determine whether regular ingestion of moderate amounts of fish provides substantial protection against myocardial infarction.
Funding Source:
Government: NHLBI
University/Hospital: St. Luke's Hospital
Not-for-profit
0
Foundation associated with industry:
Reviewer Comments:
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) N/A
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) N/A
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? No
2. Was the selection of study subjects/patients free from bias? ???
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? N/A
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) N/A
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? N/A
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) N/A
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? Yes
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? Yes
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) Yes
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? No
  4.4. Were reasons for withdrawals similar across groups? N/A
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? N/A
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? N/A
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) N/A
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? N/A
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? N/A
  6.6. Were extra or unplanned treatments described? ???
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? N/A
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? No
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes