The rising popularity of vegetarian and vegan diets in Western countries warrants investigation into their impact on health. These diets are characterized by higher consumption of fruits, vegetables, legumes, and whole grains, and lower intake of saturated fat and cholesterol. A key area of interest is the potential influence on inflammation, as chronic low-grade inflammation is linked to numerous chronic diseases like type 2 diabetes, cardiovascular disease, and certain cancers. Elevated inflammatory markers such as hs-CRP, IL-6, and TNF-α are associated with these diseases, while adiponectin levels show an inverse relationship. Previous research suggests plant-based diets may modulate inflammatory biomarkers, reducing markers like CRP, IL-6, and sICAM-1. However, meta-analyses on vegetarian diets and inflammatory markers have been limited, and the impact of vegan diets is even less studied. This systematic review and meta-analysis aimed to comprehensively investigate the effects of vegetarian and vegan diets on a wide range of inflammatory biomarkers (CRP, IL-6, IL-18, IL-1RA, TNF-α, E-selectin, ICAM-1, MCP-1, adiponectin, omentin-1, and resistin) in both healthy and diseased populations.

Existing literature suggests a link between vegetarian diets and lower CRP levels. However, the impact of exclusively vegan diets on inflammatory biomarkers has been sparsely investigated. Studies have shown associations between plant-based diets and a reduction in inflammation markers; however, more comprehensive research, particularly meta-analyses encompassing a wide spectrum of inflammatory biomarkers, is needed to solidify these findings and clarify the effects of vegan diets.

A systematic literature search was conducted in PubMed and EMBASE databases until April 15, 2020, using relevant keywords for vegan/vegetarian diets and various inflammatory biomarkers. Initially, 1073 publications were identified; after removing duplicates and screening titles and abstracts, 60 articles proceeded to full-text assessment. Finally, 21 cross-sectional studies met the inclusion criteria and were included in the systematic review and meta-analysis. The inclusion criteria specified studies that compared vegetarian or vegan diets to an omnivorous control group in adults (≥18 years), with specific definitions for each dietary pattern. Various study designs were considered, but only English and German language studies were included. Data extraction included study characteristics (author, year, country, design, sample size, age, gender, diet duration, biomarker values) and study quality assessment using the Newcastle-Ottawa Quality Assessment Scale (NOS). Meta-analyses were performed using random-effects models when at least three studies were available for a specific biomarker. Heterogeneity was assessed using the I² index, and publication bias was evaluated using funnel plots, Egger's regression, Begg's rank correlation, funnel plot regression, and trim-and-fill tests (for meta-analyses with ≥10 studies). Sensitivity analyses were performed stratifying by study quality, continent, and duration of vegetarian diet.

The meta-analysis revealed a significant association between vegan diets and lower CRP levels compared to omnivorous diets (mean difference -0.54 mg/l, 95% CI: -0.79 to -0.28, p<0.0001). This effect was less pronounced in vegetarians (-0.25 mg/l, 95% CI: -0.49 to 0.00, p=0.05). However, in patients with impaired kidney function, the association between vegetarian diets and lower CRP levels was considerably stronger (-3.91 mg/l, 95% CI: -5.23 to -2.60, p<0.0001). No substantial effects were observed for the other inflammatory biomarkers investigated, though many had data from only a single study. Sensitivity analysis showed no significant effect modification by study quality, continent, or diet duration (though a stronger effect was seen for vegetarian diets lasting ≥10 years). Meta-regression analysis revealed no effect modification by BMI.

This study provides substantial evidence that both vegan and vegetarian diets are associated with lower CRP levels compared to omnivore diets, particularly in those with impaired kidney function. These findings support the idea that plant-based diets could mitigate inflammatory processes and potentially reduce the risk of chronic diseases linked to inflammation. The stronger association in individuals with impaired kidney function highlights a potential benefit in pre-diseased populations. The mechanisms behind these effects may involve differences in gut microbiota composition and inflammasome activity, though more research is necessary. The observed association was stronger with longer durations of vegetarian diets.

This comprehensive systematic review and meta-analysis suggests that vegan and vegetarian diets are associated with lower CRP levels, a key marker of inflammation. The association was particularly strong in vegetarians with impaired kidney function. While the evidence for other inflammatory biomarkers is limited due to a lack of studies, further research is needed to fully understand the impact of plant-based diets on inflammatory pathways and health outcomes. Future studies should include more diverse and larger sample sizes and evaluate a wider range of inflammatory biomarkers.

The study's reliance on cross-sectional studies prevents causal inference. The majority of included studies had relatively small sample sizes, and substantial heterogeneity was observed in the vegetarian diet group. The analysis is limited by the available data, including variations in assay quality and biomarker selection across studies. Lack of sufficient data for many biomarkers prevented definitive conclusions beyond the findings on CRP.