Meta-analysesEffect of vegetarian diets on the presentation of metabolic syndrome or its components: A systematic review and meta-analysis
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.
Section snippets
Data sources and search strategy
The systematic review was conducted according to the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analysis) guidelines [17]. Cochrane Library, EMBASE PubMed, Web of Science, and Scopus were searched from database inception through 24 May 2017. The bibliographies of all identified systematic reviews were also searched for additional relevant studies. The database searches were carried out independently by one author (VP). The search strategies are available as Supplementary
Study selection
Our search strategy identified 3222 publications. After removal of duplicates, 2660 unique articles remained. Upon screening of studies by title and abstract review, 2466 articles were excluded. After review of 194 full-text articles, 123 studies were excluded: 10 were duplicates, 9 reported more than one intervention diet aside VD, 16 reported no control group, 12 reported insufficient outcome data, 9 were published in other languages (Czech, Russian and Slovak); other reasons for exclusion
Discussion
Our meta-analysis found that the consumption of VD was not associated with MetS when compared with consumption of omnivorous diets. Likewise, meta-analysis of RCTs and cohort studies indicated that consumption of VD were not associated with MetS components when compared with consumption of omnivorous diets. However, meta-analyses of cross-sectional studies showed that the consumption of VD was associated with significant lower levels of systolic and diastolic blood pressure, fasting glucose,
Statement of authorship
The author's responsibilities were as follows – MCP, JAL, JMR, and AVH participated in the conception and design of the project; MCP, JAL, JMR and VP contributed to the study selection, data extraction and risk of bias assessment; all authors conducted the statistical analyses; MCP, JAL and JMR contributed in the drafting of manuscript; all authors took part in the writing and final review of the manuscript; and AVH led study supervision.
Conflicts of interest
The authors have no conflicts of interest to disclose.
Funding sources
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.
Acknowledgments
We thank Dr. Krithiga Shridhar for sharing her data from the study “The Association between a Vegetarian Diet and Cardiovascular Disease Risk Factors in India: The Indian Migration Study.”
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