Abstract
Background: Beneficial effects of vegetarian and vegan diets on health outcomes have been supposed in previous studies. Objectives: Aim of this study was to clarify the association between vegetarian, vegan diets, risk factors for chronic diseases, risk of all-cause mortality, incidence, and mortality from cardio-cerebrovascular diseases, total cancer and specific type of cancer (colorectal, breast, prostate and lung), through meta-analysis. Methods: A comprehensive search of Medline, EMBASE, Scopus, The Cochrane Library, and Google Scholar was conducted. Results: Eighty-six cross-sectional and 10 cohort prospective studies were included. The overall analysis among cross-sectional studies reported significant reduced levels of body mass index, total cholesterol, LDL-cholesterol, and glucose levels in vegetarians and vegans versus omnivores. With regard to prospective cohort studies, the analysis showed a significant reduced risk of incidence and/or mortality from ischemic heart disease (RR 0.75; 95% CI, 0.68 to 0.82) and incidence of total cancer (RR 0.92; 95% CI 0.87 to 0.98) but not of total cardiovascular and cerebrovascular diseases, all-cause mortality and mortality from cancer. No significant association was evidenced when specific types of cancer were analyzed. The analysis conducted among vegans reported significant association with the risk of incidence from total cancer (RR 0.85; 95% CI, 0.75 to 0.95), despite obtained only in a limited number of studies. Conclusions: This comprehensive meta-analysis reports a significant protective effect of a vegetarian diet versus the incidence and/or mortality from ischemic heart disease (-25%) and incidence from total cancer (-8%). Vegan diet conferred a significant reduced risk (-15%) of incidence from total cancer.
Generated Summary
This study presents a comprehensive systematic review with meta-analysis of observational studies examining the association between vegetarian and vegan diets and multiple health outcomes. The review included a total of 98 cross-sectional studies and 10 cohort prospective studies. The primary aim was to clarify the relationship between vegetarian and vegan diets, risk factors for chronic diseases, all-cause mortality, incidence and mortality from cardio-cerebrovascular diseases, total cancer, and specific types of cancer, through meta-analysis. The research employed the PICOT format, searching databases like Medline, Embase, Scopus, The Cochrane Library, and Google Scholar. The methodological quality of the trials was assessed using the Newcastle-Ottawa Scale (NOS). Statistical analyses involved the generic inverse variance method with random-effects weighting. Weighted mean differences (WMD) were calculated for cross-sectional studies, while relative risks (RRs) were reported for prospective studies. Heterogeneity was evaluated using the I² statistic, and sensitivity analyses were conducted to investigate heterogeneity across studies. The goal of the review was to offer insights into the impact of plant-based diets on health outcomes and provide guidance for individuals considering such dietary patterns.
Key Findings & Statistics
- The study included a total of 56,461 vegetarians and 8,421 vegans compared with 184,167 omnivorous participants.
- Cross-sectional studies showed that vegetarian diets were significantly associated with lower BMI (-1.49), total cholesterol (-28.16 mg/dL), LDL-cholesterol (-21.27 mg/dL), HDL-cholesterol (-2.72 mg/dL), triglycerides (-11.39 mg/dL), and blood glucose levels (-5.08 mg/dL) compared to omnivores.
- Vegan diets were also significantly associated with lower BMI (-1.72), serum total cholesterol (-31.02 mg/dL), LDL-cholesterol (-22.87 mg/dL), and blood glucose levels (-6.38 mg/dL).
- In prospective cohort studies, vegetarians showed a reduced risk of incidence and/or mortality from ischemic heart disease (RR 0.75, 95% CI 0.68 to 0.82).
- The analysis of cohort studies revealed a significant reduction in the risk of total cancer incidence among vegetarians (RR 0.92, 95% CI 0.87 to 0.98) and vegans (RR 0.85, 95% CI 0.75 to 0.95).
- For all-cause mortality, the RR was 0.94 (95% CI 0.86 to 1.04) for vegetarians, with heterogeneity (I² = 83%; P < 0.001).
- Vegan diet showed a RR of 0.88 (95% CI 0.75 to 1.02; P = 0.42) for all-cause mortality.
- There was no significant association found for vegetarians regarding incidence and/or mortality from cerebrovascular diseases (RR 0.93, 95% CI 0.78 to 1.10).
- Vegetarians showed a nonsignificant reduced risk of breast cancer (RR 0.94, 95% CI 0.84 to 1.06) and mortality from colorectal (RR 0.90, 95% CI 0.76 to 1.05), breast (RR 0.94, 95% CI 0.56 to 1.58), prostate (RR 0.90, 95% CI 0.63 to 1.29) and lung (RR 0.86, 95% CI 0.62 to 1.19) cancer.
Other Important Findings
- Sensitivity analyses revealed that the association between vegetarian diet and all-cause mortality and breast cancer mortality was significant only among studies conducted in U.S. Adventist cohorts.
- No evidence of publication bias was found in the study.
Limitations Noted in the Document
- The cross-sectional studies included were susceptible to bias, with the majority showing a moderate-to-high risk of bias.
- The study’s analysis could not account for the duration of adherence to vegetarian or vegan diets in different cohorts, which is a relevant factor.
- The definition of the control group (omnivorous diet) was not always consistent, leading to potential variability.
- The accuracy of the assessment of vegetarian and vegan status might vary between studies.
Conclusion
The meta-analysis suggests that vegetarian diets are associated with a significant protective effect against ischemic heart disease and cancer incidence. Vegan diets also showed a lower rate of cancer incidence. However, these findings, especially those from cross-sectional studies, should be interpreted with caution due to the risk of bias and heterogeneity. The study highlights that plant-based diets could be beneficial for reducing the risk of certain diseases, especially in terms of cancer and heart health. The authors suggest that the adoption of these dietary patterns could offer significant health benefits, but these conclusions are primarily based on the analysis of observational studies. Further research is needed to confirm the specific impacts and generalizability of these findings. It’s crucial to consider the study’s limitations, like the moderate-to-high risk of bias in the cross-sectional studies and the variable definitions of dietary patterns, when applying these results in the context of dietary guidelines or individual health decisions. The authors’ findings are interesting for helping to give correct information to subjects who want to adopt such dietary patterns.
IFFS Team Summary
- 86 cross sectional studies and 10 prospective cohort based studies
- Vegetarians and vegans have decreased total cholesterol, LDL, glucose levels, and BMI
- Combined Vegetarians and Vegans have 25% less incidence of cardiovascular disease (incidence and mortality combined)
- and 8% less cancer incidence (though no decreased mortality from cancer)
- and 8% less cancer incidence (though no decreased mortality from cancer)
- Vegans have 15% less cancer incidence than general population (but no decreased mortality in this study)