Vegetarian Nutrition

VN: Cardiovascular Disease (2008)

Citation:

Key TJ, Fraser GE, Thorogood M, Appleby PN, Beral V, Reeves G, Burr ML, Chang-Claude J, Frentzel-Beyme R, Kuzma JW, Mann J, McPherson K. Mortality in vegetarians and nonvegetarians: detailed findings from a collaborative analysis of 5 prospective studies. Am J Clin Nutr. 1999 Sep; 70 (3 Suppl): 5,16S-5,24S. PMID: 10479225.

PubMed ID: 10479225
 
Study Design:
Prospective Cohort Study
Class:
M - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
  • Data from five prospective cohort studies combined to compare the death rates from common diseases of vegetarians with those of non-vegetarians with similar lifestyles
  • To test the hypothesis that the vegetarians would have lower mortality rates than the non-vegetarians from ischemic heart disease and from colorectal, breast and prostate cancers.
Inclusion Criteria:
  • Selection of studies: No protocol given for inclusion of studies in the review.
  • Five prospective cohorts:
    • The Adventist Mortality Study
    • The Health Food Shoppers Study
    • The Adventist Health Study
    • The Heidelberg Study
    • The Oxford Vegetarian Study
  • Aged 16 years to 89 years at time of recruitment
  • Not diagnosed with cancer prior to recruitment
  • Needed to provide sufficient information to classify diet type and smoking category.
Exclusion Criteria:
  • Exclusion of studies: No rationale for excluding other studies
  • If any subjects were in more than one cohort, they were only included one time in this comparison.
Description of Study Protocol:

Recruitment

Selection of studies not described.

Of the studies selected:

  • The Adventist Mortality Study recruited members of the Seventh-Day Adventist church from 198 congregations in California with follow-up for mortality by record linkage and personal contact
  • The Health Food Shoppers Study recruited people in the United Kingdom via health food shops, vegetarian societies and magazines. Follow-up was performed by linking records with the National Health Service Central Register.
  • The Adventis Health Study recruited Seventh-Day Adventists throughout California. Follow-up was performed by linking records with the California death certificate file, the National Death Index and church records.
  • The Heidelberg Study recruited through vegetarian magazines in the former Federal Republic of Germany. Follow-up was conducted through the registrar's office of the last place of residence.
  • The Oxford Vegetarian Study recruited through the Vegetarian Society of the United Kingdom and the news media. Non-vegetarians were recruited from friends and relatives of vegetarians. Follow-up was similar to the Health Food Shoppers Study.

Design

Review study: Data combined from the five above studies where possible.

Blinding Used

NA.

Intervention

NA.

Statistical Analysis

Death rate ratios at ages 16 years to 89 years were calculated by Poisson regression and all results were adjusted for age, sex and smoking status. A random-effects model was used to calculate pooled estimates of effect for all studies combined.

Data Collection Summary:

Timing of Measurements

  • At time of recruitment
  • Outcomes measured at follow-up (varied depending upon the cohort study; five to 21 years).

Dependent Variables

  • Death rate; Diseases examined:
    • Stomach cancer
    • Colorectal cancer
    • Lung cancer
    • Female breast cancer
    • Prostate cancer
    • Ischemic heart disease
    • Cerebrovascular disease
    • All other causes
    • All causes combined.

Independent Variables

Diet group.

Control Variables

  • Gender
  • Lifestyle (exercise, smoking)
  • Alcohol
  • BMI
  • Level of education.
Description of Actual Data Sample:

Studies Examined

Study
Location
Median Year of Recruitment
Number of Subjects
End of Follow-Up
Mean Length of Follow-Up
Adventist Mortality
California
1960
24,538
December 1965
5.6
Health Food Shoppers
United Kingdom
1974
9,878
December 1995
18.4
Adventist Health
California
1976
28,952
December, 1988
11.1
Heidelberg
Germany
1978
1,757
May 1989
9.9
Oxford Vegetarian
United Kingdom
1981
11,047
December 1995
13.7

Data from Studies

Study and Reference
Median Age
Current Smokers (%)
Mean BMI
Current Alcohol Users (%)
High Education Level (%)
High Exercise Level (%)
Men Adventist Mortality
Non-Vegetarian (N=5023)
49
7.2
25.7
2.0
54.4
77.9
Vegetarian (N=3971)
51
2.9
24.6
0.7
63.3
81.2
Health Food Shoppers
Non-Vegetarian (N=2462)
46
30.6
NA
NA
NA
NA
Vegetarian (N=1519)
41
20.1
NA
NA
NA
NA
Adventist Health
Non-Vegetarian (N=9045)
52
8.5
25.4
14.0
78.7
65.8
Vegetarian (N=3169)
51
0.2
23.8
0.4
86.5
71.3
Heidelberg
Non-Vegetarian (N=304)
45
9.9
22.1
29.4
62.8
34.8
Vegetarian (N=480)
43
4.2
21.3
14.1
56.4
40.3
Oxford Vegetarian
Non-Vegetarian (N=2,572)
34
29.1
23.0
86.0
67.9
62.6
Vegetarian (N=1,603)
33
17.5
22.0
63.2
64.8
67.4
Women Adventist Mortality
Non-Vegetarian (N=9,257)
50
1.2
25.1
1.6
51.6
NA
Vegetarian (N=6,287)
54
0.1
24.0
0.7
59.7
NA
Health Food Shoppers
Non-Vegetarian (N=3,626)
45
18.0
NA
NA
NA
NA
Vegetarian (N=2,271)
47
12.3
NA
NA
NA
NA
Adventist Health
Non-Vegetarian (N=11,904)
52
2.7
24.8
4.8
78.4
51.9
Vegetarian (N=4,834)
54
0.2
23.0
0.2
85.4
54.9
Heidelberg
Non-Vegetarian (N=370)
49
3.2
21.3
24.9
46.6
32.8
Vegetarian (N=603)
53
2.2
20.9
7.3
43.7
37.0
Oxford Vegetarian
Non-Vegetarian (N=3,801)
34
18.5
22.1
76.3
59.9
58.4
Vegetarian (N=3,071)
32
13.4
21.3
56.5
60.1
65.4

Summary of Results:

Death Rates for Different Diseases

When confounders were controlled for, the only death rate ratios comparing vegetarians to non-vegetarians that were significantly different from 1.0 were for ischemic heart disease (P< 0.001 for men and P<0.05 for women). This finding is stronger in younger age groups (the bottom line of Table Five).

Death Rate Ratios of Vegetarians Compared with Non-Vegetarians and 95% CIs

All Studies
Stomach cancer
Colorectal Cancer
Lung Cancer
Breast Cancer
Prostate Cancer
Ischemic Heart Disease
Cerebro-vascular
Disease
Other
All Causes
Death Rate Ratio
1.02 (0.64,1.62)
0.99 (0.77,1.27)
0.84
(0.59,1.18)
0.95 (0.55,1.63)
0.91 (0.60,1.39)
0.76 (0.62,0.94)
0.93 (0.74,1.17)
1.06 (0.90,1.24)
0.95 (0.82,1.11)
Number of Deaths
107
278
203
210
137
2,264
909
4,222
8,330

Death Rates and Duration of Time as Vegetarian

Comparison of Mortality in Diet Groups

Compared with the regular meat-eaters, death rate ratios were significantly lower for:

  • Lung cancer in the vegetarians (P<0.05)
  • Ischemic heart disease in the occasional meat-eaters, fish-eaters (both P<0.01) and vegetarians (P<0.001)
  • Other causes of death in the occasional meat-eaters (P<0.01)
  • All causes of death in the occasional meat-eaters (P<0.001), fish eaters and vegetarians (both P<0.05).

Mortality from Ischemic Heart Disease

The ischemic heart disease death rate ratio for vegetarians compared with non-vegetarians (adjusted for age, sex, and smoking):

  • In general: 0.80 (95% CI 0.70, 0.92)
  • In participants with a history of cardiovascular disease or diabetes: 0.76 (95% CI 0.59, 0.97)
  • In participants without a history of cardiovascular disease or diabetes: (x2 1 for heterogeneity = 0.17; P>0.1).
Author Conclusion:
  • Data presented represents most of the information available comparing western vegetarians with western non-vegetarians with similar lifestyles
  • The highly significant association observed, the consistency of the association among various sub-groups and the absence of evidence of confounding suggest that the lower mortality observed from ischemic heart disease was due to the dietary differences between vegetarians and non-vegetarians
  • Vegetarians had a 24% lower mortality from ischemic heart disease than non-vegetarians, but no associations of a vegetarian diet with other major causes of death were established.
Funding Source:
Not-for-profit
0
Foundation associated with industry:
Reviewer Comments:

Strengths

  • Large combined data set
  • Vegetarians and non-vegetarians had similar lifestyles.

Weaknesses

  • No protocol described for study inclusion and no discussion of the relative strengths or weaknesses of each study
  • There was significant heterogeneity between studies (P<0.01)
  • Diet based largely on food frequency questionnaire at time of recruitment, therefore difficult to definitively ascertain that diet remained the same throughout duration of study.
Quality Criteria Checklist: Review Articles
Relevance Questions
  1. Will the answer if true, have a direct bearing on the health of patients? Yes
  1. Will the answer if true, have a direct bearing on the health of patients? Yes
  2. Is the outcome or topic something that patients/clients/population groups would care about? Yes
  2. Is the outcome or topic something that patients/clients/population groups would care about? Yes
  3. Is the problem addressed in the review one that is relevant to dietetics practice? Yes
  3. Is the problem addressed in the review one that is relevant to dietetics practice? Yes
  4. Will the information, if true, require a change in practice? Yes
  4. Will the information, if true, require a change in practice? Yes
 
Validity Questions
  1. Was the question for the review clearly focused and appropriate? Yes
  1. Was the question for the review clearly focused and appropriate? Yes
  2. Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described? No
  2. Was the search strategy used to locate relevant studies comprehensive? Were the databases searched and the search termsused described? No
  3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? No
  3. Were explicit methods used to select studies to include in the review? Were inclusion/exclusion criteria specified andappropriate? Wereselectionmethods unbiased? No
  4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? No
  4. Was there an appraisal of the quality and validity of studies included in the review? Were appraisal methodsspecified,appropriate, andreproducible? No
  5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? N/A
  5. Were specific treatments/interventions/exposures described? Were treatments similar enough to be combined? N/A
  6. Was the outcome of interest clearly indicated? Were other potential harms and benefits considered? Yes
  6. Was the outcome of interest clearly indicated? Were other potential harms and benefits considered? Yes
  7. Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described? Yes
  7. Were processes for data abstraction, synthesis, and analysis described? Were they applied consistently acrossstudies and groups? Was thereappropriate use of qualitative and/or quantitative synthesis? Was variation in findings among studies analyzed? Were heterogeneity issued considered? If data from studies were aggregated for meta-analysis, was the procedure described? Yes
  8. Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included? Yes
  8. Are the results clearly presented in narrative and/or quantitative terms? If summary statistics are used, are levels ofsignificance and/or confidence intervals included? Yes
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? Yes
  9. Are conclusions supported by results with biases and limitations taken into consideration? Are limitations ofthe review identified anddiscussed? Yes
  10. Was bias due to the review's funding or sponsorship unlikely? Yes
  10. Was bias due to the review's funding or sponsorship unlikely? Yes