Abstract
There is increasing evidence that plant based diets are associated with lower cardiovascular risk.
Generated Summary
This meta-analysis of observational studies, published between 1960 and June 2018, investigates the impact of vegan diets on cardio-metabolic risk factors. The study compares the effects of vegan diets to omnivorous diets by analyzing macro-nutrient intake and various cardio-metabolic risk factors. The Newcastle Ottawa Scale (NOS) was used to assess the quality of each study, and the inverse-variance method was employed to pool mean differences. Statistical analyses were conducted using RevMan software version 5.2. The analysis included 40 studies involving 12,619 vegans and 179,630 omnivores, aiming to provide a comprehensive evaluation of how vegan diets affect cardio-metabolic health. The research also sought to identify whether the associations observed were consistent across different populations and over time. The meta-analysis aims to determine the reliability of vegan diet’s effects on diverse cardiometabolic risk factors and identify the consistency of these associations across various populations.
Key Findings & Statistics
- The analysis included 40 studies with 12,619 vegans and 179,630 omnivores.
- Vegans consumed less energy (-11%, 95% confidence interval -14 to -8) and less saturated fat (- 51%, CI -57 to -45) compared to omnivores.
- Compared to controls, vegans had a lower body mass index (-1.72 kg/m², Cl -2.30 to -1.16), waist circumference (-2.35 cm, CI -3.93 to -0.76), low density lipoprotein cholesterol (-0.49 mmol/L CI -0.62 to -0.36), triglycerides (-0.14 mmol/L, CI -0.24 to -0.05), fasting blood glucose (-0.23 mmol/, CI -0.35 to -0.10), and systolic (-2.56 mmHg, CI -4.66 to -0.45) and diastolic blood pressure (-1.33 mmHg, Cl -2.67 to -0.02), p<0.0001 for all.
- For studies with <50 vegans, the difference in BMI was -1.04 (95% CI -1.59, -0.49), p <0.0001.
- For studies with >50 vegans, the difference in BMI was -0.93 (95% CI -1.25, -0.60), p <0.0001.
- In studies from Asia, the difference in BMI was -0.20 (95% CI -1.21, 0.82), p = 0.70.
- In non-Asian studies, the difference in BMI was -1.92 (95% CI -2.52, -1.32), p <0.0001.
- The mean fasting plasma glucose in controls was 5.2 (0.59) mmol/L. The difference in fasting glucose overall was -0.23 (95% CI -0.35 to -0.10) mmol/l, p = 0.0005.
- The mean LDL- cholesterol was 2.85 (4.9) mmol/L for controls. For all studies the difference in LDL-cholesterol between vegans and controls was -0.49mmol/L [95% CI -0.62 to -0.36], p<0.0001.
- The mean triglyceride was 1.24 (0.41) mmol/L for omnivores. Vegans had lower triglyceride levels than controls -0.14 mmol/l [95% CI-0.24 to -0.05], p = 0.004.
- In non-Asians studies systolic blood pressure was -5.87mmHg [95% CI -9.19 to -2.56], p = 0.005) and diastolic blood pressure (-3.19mmHg [-5.90 to -0.48], p = 0.002) were lower in vegans compared to controls.
Other Important Findings
- From food frequency questionnaires in 28 studies, vegans compared to omnivores consumed less energy (-11%, 95% confidence interval -14 to -8) and less saturated fat (- 51%, CI -57 to -45).
- Results were consistent for studies with < and ≥ 50 vegans, and published before and after 2010.
- In several large studies from Taiwan, a vegan diet was not associated with favorable cardio-metabolic risk factors compared to the control diets.
- Total energy intake in vegans was 30% from fat (5.8% saturated), 13% protein and 56% carbohydrate. In controls 33% of total energy was from total fat, 11% saturated fat, 17% protein and 51% from carbohydrate. The nutrient intake was similar across studies including the sole study from Asia (Taiwan) that reported results of FFQ.
- On pre-specified subgroup analysis based on geographic region, there was a statistically significant differences comparing Asian and non-Asian studies for all factors except blood glucose and diastolic blood pressure.
- For geographic regions excluding Asia there was no difference by year of publication or size of study.
- The risk factor with the most evidence is body mass index, and this was consistently lower in vegans compared to controls in diverse geographies outside Asia, in larger and smaller studies, and in studies published over many years.
- Vegans consumed 980 less kilojoules per day which translated into 11% less energy than controls.
- In non-Asian studies LDL cholesterol was 0.6mmol/l lower in vegans compared to controls based on observations in 1014 vegans from 24 studies. This reduction is consistent with effects of reducing saturated fat intake by 51%, and a 26% increase in PUFA intake compared to controls.
- In non-Asian studies, systolic blood pressure was ~ 6mmHg lower in vegans compared to controls.
Limitations Noted in the Document
- It is possible some associations could be influenced by factors other than diet.
- Vegans may choose this diet because of perceived health benefits or religious/cultural reasons, and they may have fewer adverse health behaviours including smoking, drinking alcohol and sedentary lifestyle.
- Individual participant data was not available and this limits the ability to address a number of questions.
- We could not reliably evaluate possible gender differences, or associations between energy or saturated fat intake and cardiometabolic risk factors.
- Food frequency questionnaires are known to be unreliable.
- Most included studies were small, but results were similar in smaller and larger studies and by year of publication.
- Associations with triglyceride need to be interpreted with caution due to the significant scattered distribution seen in the funnel plot.
- Larger cohort studies which evaluate a broad range of risk factors would overcome the limitation of small numbers, and additional studies in diverse populations would provide further information on the effects of a vegan diet compared to other diets.
Conclusion
In most countries a vegan diet has less energy and saturated fat compared to omnivorous control diets, and is associated with a favorable cardiometabolic risk profile, including lower body weight, LDL cholesterol, fasting blood glucose, blood pressure, and triglycerides. These observations support other evidence that plant based diets are likely to lower the risk of cardiovascular disease and diabetes. However the improvement in cardiometabolic risk profile is also likely to depend on the comparison diet, and the difference may be less with some Asian compared to western dietary patterns. “In non-Asian studies LDL cholesterol was 0.6mmol/l lower in vegans compared to controls…” This is consistent with reducing saturated fat intake and avoidance of dairy and meat which may contain trans fatty acids. In non-Asian studies, systolic blood pressure was ~ 6mmHg lower in vegans compared to controls. This would be expected to reduce cardiovascular risk by ~12%. This lower blood pressure is similar to that observed in a meta-analysis of vegetarian diets. It is possible lower body mass index is the principal reason for lower blood pressure, triglycerides and glucose for vegans compared to controls.