• 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.

• 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 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.

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.

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.

Studies Examined

Data from Studies

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

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).

• 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.

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.