Effect of vegetarian diets on the presentation of metabolic syndrome or its components: A systematic review and meta-analysis

Summary

Background & aims

Several studies have examined the effect of vegetarian diets (VD) on metabolic syndrome (MetS) or its components, but findings have been inconsistent. The aim of this study was to perform a systematic review and meta-analysis of randomized controlled trials (RCTs) and observational studies to assess the association between VD and MetS or its components (systolic blood pressure [SBP], diastolic blood pressure [DBP], fasting glucose triglycerides, waist circumference [WC], HDL-cholesterol (HDL-C)) in adults.

Methods

The Cochrane Library, EMBASE, PubMed, Web of Science, and Scopus were searched. RCTs, cohort studies and cross-sectional studies evaluating the effects of VD on MetS or its components in adults, with omnivore diet as control group, were included. Random effects meta-analyses stratified by study design were employed to calculate pooled estimates.

Results

A total of 71 studies (n = 103 008) met the inclusion criteria (6 RCTs, 2 cohorts, 63 cross-sectional). VD were not associated with MetS in comparison to omnivorous diet (OR 0.96, 95% CI 0.50–1.85, p = 0.9) according to meta-analysis of five cross-sectional studies. Likewise, meta-analysis of RCTs and cohort studies indicated that consumption of VD were not associated with MetS components. Meta-analysis of cross-sectional studies demonstrated that VD were significantly associated with lower levels of SBP (mean difference [MD] −4.18 mmHg, 95%CI −5.57 to −2.80, p < 0.00001), DBP (MD −3.03 mmHg, 95% CI −4.93 to −1.13, p = 0.002), fasting glucose (MD −0.26 mmol/L, 95% CI −0.35to −0.17, p < 0.00001), WC (MD −1.63 cm, 95% CI −3.13 to −0.13, p = 0.03), and HDL-C (MD −0.05 mmol/L, 95% CI −0.07 to −0.03, p < 0.0001) in comparison to omnivorous diet. Heterogeneity of effects among cross-sectional studies was high. About, one-half of the included studies had high risk of bias.

Conclusions

VD in comparison with omnivorous diet is not associated with a lower risk of MetS based on results of meta-analysis of cross-sectional studies. The association between VD and lower levels of SBP, DBP, HDL-C, and fasting glucose is uncertain due to high heterogeneity across the cross-sectional studies. Larger and controlled studies are needed to evaluate the association between VD and MetS and its components.

Introduction

Based on the various definitions established by the International Diabetes Federation (IDF) and the Adult Treatment Panel III (ATP III), the worldwide prevalence of metabolic syndrome (MetS) varies between 34% and 39% [1] in developed countries; a lower prevalence of 16–34% has been reported in developing countries [2]. An increasing worldwide prevalence of MetS is associated with an increase in obesity and physical inactivity [3], [4]. Patients with MetS are twice as likely to develop cardiovascular disease, and are five times more likely to suffer from type 2 diabetes mellitus compared to an individual without MetS [3]. Globally, MetS and its potential health consequences are now considered as a public health concern [3].

Vegetarian diets (VD) are dietary patterns that exclude meats from the diet. These diets emphasize the consumption of plant foods like legumes, vegetable, grains and fruits. However, some of them include animal derived foods such as milk, eggs, fish and seafood [5]. Among the best-known VD, are the semi-vegetarian diet, which rarely includes meat and poultry, and the vegan diet, which excludes all types of animal products. A few VD are characterized by dietary exceptions of specific food groups such as fish-vegetarian diet, lacto-vegetarian diet, and lacto-ovo-vegetarian diet [6]. VD are usually adopted based on religion and cultural practices, health benefits, and concern towards animals and environmental sustainability [7]. In some cases they are used as dietary strategies in the treatment and prevention of MetS. Randomized controlled trials (RCTs) in adults consuming VD have shown an improvement in weight control [8], glycemic control [9] and other cardiovascular risk factors [10].

Several studies have evaluated the association between VD and a decreased risk of MetS and/or its components. In a study linking VD and health benefits, the prevalence of MetS and its components, including serum triglycerides, fasting glucose, blood pressure and waist circumference (WC), were significantly lower in the vegetarian population when compared with a non-vegetarian population [11]. Lower blood pressure levels have been associated with VD in comparison with omnivores [12]. Also, lower levels of fasting glucose [13], [14], WC [13] and triglycerides [14], have been reported in vegetarians. These findings suggest that vegetarian individuals have a lower risk of MetS in comparison to non-vegetarians [13]. In contrast to the beneficial effects of VD, few studies have reported the opposite [13], [14], emphasizing lower levels of HDL-C and higher prevalence of MetS [16] in the vegetarian population [15]. In a meta-analysis of RCTs and observational studies assessing the effect of vegan, semi-vegetarian and lacto-ovo-vegetarian on blood pressure, a component of MetS, a significant decrease in systolic blood pressure and diastolic blood pressure was shown in the vegetarian population; nevertheless, this study presented several methodological limitations that should be considered when interpreting the results [5]. The inconsistent reports and the uncertainty of the association between VD and blood pressure and other MetS components, warrants further studies. The objective of this study was to determine the association between VD and the MetS and/or its components in adults.