Abstract
To summarize and evaluate the evidence on the health impact of a vegan diet, we conducted an umbrella review of systematic reviews and meta-analyses. PubMed, Cochrane Library, Web of Science and Epistemonikos were searched up to September 2021. Meta-analyses were recalculated by using a random effects model. The certainty of evidence (CoE) was evaluated by the GRADE approach. For the general healthy population, a vegan diet was effective for reducing body weight [MD (95% CI): −2.52 kg (−3.06, −1.98), n=8 RCTs; moderate CoE] and was associated with further health benefits (with low CoE), including a lower risk of cancer incidence [SRR (95% CI): 0.84 (0.75, 0.95), n=2] and a trend for lower risk of all-cause mortality [SRR (95% CI): 0.87 (0.75, 1.01), n=2], as well as lower ApoB levels [MD (95% CI): −0.19 µmol/L (−0.23, -0.15), n=7 RCTs). The findings suggested adverse associations for a vegan diet with risk of fractures [SRR (95% CI): 1.46 (1.03, 2.07), n = 3; low CoE]. For persons with diabetes or at high CVD risk, a vegan diet reduced measures of adiposity, total cholesterol, LDL and improved glycemic control (CoE moderate to low). A vegan diet may have the potential for the prevention of cardiometabolic health, but it may also impair bone health. More well-conducted primary studies are warranted.
Generated Summary
This umbrella review examines the health impact of vegan diets, focusing on meta-analyses of observational and clinical studies. The review aimed to synthesize existing evidence, evaluate the certainty of the findings, and identify gaps in knowledge. The methodology involved searching PubMed, Cochrane Library, Web of Science, and Epistemonikos for relevant systematic reviews and meta-analyses up to September 2021. The certainty of evidence (CoE) was evaluated using the GRADE approach. The review included meta-analyses based on adults from the general healthy population, children, adolescents, pregnant women, as well as persons with underlying chronic diseases or at high risk. The primary outcomes included health outcomes, nutritional status, and nutrient intakes. The study sought to provide evidence-based guidance on the health effects and safety of vegan diets, which could be translated into public health and primary healthcare sectors. The results are presented in relation to general health outcomes, outcomes among persons with underlying diseases, and dietary intake.
Key Findings & Statistics
- For the general healthy population, a vegan diet was effective for reducing body weight [MD (95% CI): -2.52 kg (-3.06, -1.98), n=8 RCTs; moderate CoE]
- A vegan diet was associated with a lower risk of cancer incidence [SRR (95% CI): 0.84 (0.75, 0.95), n=2] and a trend for lower risk of all-cause mortality [SRR (95% CI): 0.87 (0.75, 1.01), n=2], as well as lower ApoB levels [MD (95% CI): −0.19 µmol/L (−0.23, -0.15), n=7 RCTs]
- For persons with diabetes, a vegan diet was effective in reducing body weight [MD (95% CI): −2.51 kg (−3.37, −1.65); I2: 0%; n = 4 RCTs; moderate CoE], BMI [MD (95% CI): −0.67 kg/m2 (- 1.07, −0.28); I2: 64%; n = 4 RCTs; moderate CoE] and waist circumference [MD (95% CI): −2.32 cm (−3.52, −1.12); I2: 0%; n = 2 RCTs; low CoE].
- There was low CoE for a higher risk of fractures for a vegan diet compared to an omnivore diet [SRR (95% CI): 1.46 (1.03, 2.07), n = 3; low CoE].
- Evidence from cohort studies showed an inverse association between a vegan diet and the incidence of cancer (SRR: 0.84; 95% CI: 0.75, 0.95; I2: 0%; n = 2 studies) and a trend for all-cause mortality (SRR: 0.87; 95% CI: 0.75, 1.01; I2: 0%; n = 2 studies), both with low CoE (Figure 1).
- Based on the data from RCTs, a vegan diet was effective in reducing body weight [MD (95% CI): −2.52 kg (−3.06, −1.98); I2: 3%; n = 8 RCTs; moderate CoE].
- HDL- and ApoB-levels were reduced by −0.10 mmol/L (95% CI: −0.20, −0.00; I2:31%; n = 9 RCTs; low CoE) and −0.19 µmol/L (95% CI: −0.23, −0.15; I2:61%; n = 7 RCTs; low CoE), respectively (Figure 2).
- For persons with diabetes or at high CVD risk, triglyceride levels were higher for a vegan intervention compared to the control group [MD (95% CI) for person with diabetes: 0.21 mmol/L (0.02, 0.40); I2: 38%; n = 5 RCTs; moderate CoE]
- For persons with diabetes, the intervention was effective in improving glycemic control [MD for HbA1c (95% CI): -0.27% (-0.50, -0.04); I2: 44%; n = 5 RCTs; moderate CoE].
- A vegan diet was associated with a lower risk of cancer incidence (SRR: 0.84; 95% CI: 0.75, 0.95) and a trend towards lower all-cause mortality (SRR: 0.87; 95% CI: 0.75, 1.01), but these results are based on a limited number of studies and have low CoE.
Other Important Findings
- The study identified 17 published systematic reviews with meta-analyses, including 79 estimates for a vegan diet and 38 different outcomes.
- In the general healthy population, vegan diets were associated with reduced body weight and a trend towards lower risk of all-cause mortality and lower cancer incidence.
- There was no clear effect of a vegan diet on SBP and DBP compared to a non-vegan diet.
- Vegan diets were effective in reducing body weight, BMI and waist circumference, and improving glycemic control and lipid profiles in those with diabetes or at high CVD risk.
- The study noted that a vegan diet was effective in improving cardiometabolic markers, but the effectiveness was reduced when compared to other “healthy” dietary interventions.
- The study found potential adverse effects of a vegan diet related to bone health (higher risk of fractures) and adverse effects on HDL-cholesterol and triglyceride levels.
Limitations Noted in the Document
- The meta-analyses were based on a limited number of primary studies for some outcomes, especially those related to clinical endpoints.
- Some meta-analyses included specific populations, such as Buddhist nuns or Seventh-day Adventists, which may limit generalizability.
- The definitions of a vegan diet varied across studies, impacting the consistency of results.
- Some meta-analyses relied on cross-sectional studies, limiting the ability to establish causal relationships.
- The ROBIS tool, used to assess the risk of bias, has a limitation in that it only categorizes risk as high or low, which affects the robustness of the assessments.
Conclusion
The umbrella review underscores the potential benefits of vegan diets, particularly for weight management, cancer incidence, and cardiometabolic health in specific populations. The authors state, “In patients with diabetes, a vegan diet was effective in reducing body weight, waist circumference, LDL-cholesterol, fasting glucose, and HbA1c, but also decreased HDL-cholesterol.” However, the review also highlights some potential adverse effects, mainly regarding bone health, evidenced by a higher risk of fractures. The authors state, “There was moderate CoE that a vegan diet might improve some (reduction of body weight, BMI, waist circumference, LDL-cholesterol and HbA1c) but not all (TG, HDL-cholesterol) cardiometabolic health indices, especially in persons with diabetes or at high CVD risk.” The authors also note, “A vegan diet was related to a lower risk of cancer incidence and tends to be associated with lower all-cause mortality.” The review emphasizes that more robust, well-conducted prospective studies are needed to strengthen the evidence base and expand the understanding of the long-term health effects of vegan diets. Furthermore, the review also emphasizes the need for studies that differentiate between the general population, patient groups, and also the lifestyle of those with vegan diets.