Vegetarian diets on anthropometric, metabolic and blood pressure outcomes in people with overweight and obesity: a systematic review and meta-analysis of randomized controlled trials

Background It is unknown whether vegetarian diets (VDs) may improve outcomes in people with overweight and obesity. Objective To systematically assess the effects of VDs vs. omnivore diets on anthropometric, metabolic, and blood pressure outcomes in people with overweight and obesity. Methods We searched for randomized controlled trials (RCTs) in EMBASE, PubMed, Web of Science, and Scopus until February 2, 2022. Primary outcomes were anthropometric risk factors (weight, body mass index [BMI], waist circumference [WC], hip circumference [HC], and body fat percentage). Secondary outcomes were metabolic risk factors (fasting serum glucose, HbA1c, insulin levels) and blood pressure (systolic blood pressure [SBP], diastolic blood pressure [DBP]). Random-effects meta-analyses were performed and effects were expressed as mean difference (MD) and their 95% confidence intervals (CI). The quality of evidence was assessed using GRADE methods. Results Nine RCTs ( n = 1628) were included. VDs decreased weight (MD −3.60 kg, 95%CI −4.75 to −2.46) and glucose (MD −10.64 mg/dL, 95%CI −15.77 to −5.51), but did not decrease WC (MD −3.00 cm, 95%CI −6.20 to 0.20), BMI (MD −0.87 kg/m2, 95%CI −1.80 to 0.06), or HC (MD: −0.86 cm, 95%CI −3.46 to 1.74). VDs did not decrease HbA1c (MD −0.40%, 95%CI −0.89 to 0.10), insulin (MD −3.83 mU/L, 95%CI −8.06 to 0.40), SBP (MD −0.25 mmHg, 95%CI −2.58 to 2.07), or DBP (MD −1.57 mmHg, 95%CI −3.93 to 0.78). Subgroup analyses by type of VD (four RCTs evaluated lacto-ovo-vegetarian diets and five RCTs vegan diets) showed similar results to the main analyses. QoE was very low for most of the outcomes. Conclusions In comparison to an omnivorous diet, VDs may reduce weight and glucose, but not blood pressure or other metabolic or anthropometric outcomes. However, the QoE was mostly very low. Larger RCTs are still needed to evaluate the effects of VD on anthropometric, metabolic factors, and blood pressure in people with overweight and obesity.