Abstract
Approximately 20 %–25 % of adults worldwide have metabolic syndrome. Vegetarian and vegan diets have demonstrated effectiveness in improving body weight, glycemic control, and cardiovascular risk factors, as compared with conventional therapeutic approaches, and are potentially useful in the prevention of metabolic syndrome. This article consists of two steps: (1) a review of the literature on studies examining vegetarian and vegan diets and metabolic syndrome and (2) a review of foods and nutrients that are protective against or associated with metabolic syndromes that may help to explain the beneficial effects of plant-based dietary approaches for metabolic syndrome. The present review found eight observational research studies, and no intervention studies, examining the association of plant-based dietary approaches with metabolic syndrome. These studies, conducted mostly in Asian populations, yielded varying results. The majority, however, found better metabolic risk factors and lowered risk of metabolic syndrome among individuals following plant-based diets, as compared with omnivores. Some dietary components that are lower in the diets of vegetarians, such as energy intake, saturated fat, heme iron, and red and processed meat, may influence metabolic syndrome risk. In addition, plant-based diets are higher in fruits, vegetables, and fiber, which are protective against the development of metabolic syndrome.
Generated Summary
This article reviews existing literature on the relationship between vegetarian and vegan diets and metabolic syndrome. The study involved a literature review of studies examining vegetarian and vegan diets and metabolic syndrome, as well as a review of foods and nutrients that are protective against or associated with metabolic syndromes to explain the beneficial effects of plant-based dietary approaches. The review found eight observational research studies. The majority found better metabolic risk factors and lowered risk of metabolic syndrome among individuals following plant-based diets, as compared with omnivores. The article also discusses key dietary components of vegetarian and vegan diets, which may be beneficial for the prevention and treatment of metabolic syndrome. The study conducted a search of the Medline database using the terms “vegan” or “vegetarian” and “metabolic syndrome” for articles published from 1992 to February 2014. A three-step process was used: examining the title, the abstract, and then the full text. The research also used synonymous search terms suggested by the National Library of Science-Medical Subject Headings (MeSH) to increase the range of data collected.
Key Findings & Statistics
- Approximately 20%–25% of adults worldwide have metabolic syndrome.
- Metabolic syndrome doubles the risk for having a heart attack and triples stroke risk [2].
- In the U.S., cheese is one of the largest contributors to saturated fat intake.
- The U.S. Dietary Reference Intakes do not specify a recommended level of dietary saturated fat but state that intake should be “as low as possible while consuming a nutritionally adequate diet.”
- Current intakes in the U.S. exceed this level (mean of 11% energy from saturated fat) [52].
- In the U.S., average fiber intake falls well below the recommended levels for adults of 25 g/day for women and 38 g/day for men [56].
- Mean fiber intake ranges from 15 to 17 g/day in U.S. adults [57].
- One cup of cooked legumes generally provides a ratio of 1 g of fiber to 1 g of protein (e.g., 1 cup of cooked black beans contains 15 g of fiber and 15 g of protein) [59].
- Most studies do not demonstrate that serum ferritin levels of vegetarians are at suboptimal levels [5].
- In a 74-week randomized trial comparing adherence to a low-fat vegan diet or the dietary approach recommended by the American Diabetes Association (ADA), most participants in the ADA group did not meet ADA diet criteria, due to exceeding saturated fat recommendations [53]. Those following the vegan diet had mean percentage of energy from saturated fat of 5.0±0.5 [53].
Other Important Findings
- Vegetarian and vegan diets have demonstrated effectiveness in improving body weight, glycemic control, and cardiovascular risk factors, as compared with conventional therapeutic approaches.
- Studies have demonstrated that well-planned vegan and vegetarian diets can provide adequate nutrition and may have health benefits for disease prevention and treatment.
- People following vegan and vegetarian diets have lower body mass indices (BMIs), as compared with nonvegetarians, as well as a lower prevalence of type 2 diabetes.
- Clinical trials using vegetarian and vegan diets have demonstrated significant improvements in body weight, glycemic control, and cardiovascular risk factors, as compared with conventional therapeutic approaches (e.g., reduced fat and/or energy diets).
- Vegan and vegetarian diets have also been used effectively for weight loss and maintenance.
- Key dietary components of vegetarian and vegan diets, which may be beneficial in the prevention and treatment of metabolic syndrome are discussed: Energy intake and body weight, Dietary saturated fat, Dietary fiber, and Fruits and vegetables.
- Plant-based diets are higher in fruits, vegetables, and fiber, which are protective against the development of metabolic syndrome.
- Three studies found significantly lower waist circumferences in vegetarians and vegans, as compared with other diets, and two found better triglyceride levels.
- No studies found significantly better HDL cholesterol levels among vegetarians or vegans, as compared with omnivorous or other diets.
Limitations Noted in the Document
- The literature search revealed no intervention studies with adequate details about metabolic syndrome or at least three of its components as an outcome.
- A change in a single nutrient (such as decreasing dietary fat or increasing fiber) may not be sufficient enough to promote significant changes in metabolic syndrome risk.
- There may be several nutrients and dietary components that are working together to have a protective effect.
- The heterogeneity is present when all vegetarian diets are collapsed into one group (vs. examining differing patterns of vegetarian diets, such as vegan and vegetarian) or the variability in patterns of food intake by geographic location.
- Differences in the risk factors for metabolic syndrome are more pronounced in studies conducted in Western populations, which may be due to dietary factors being more similar between vegetarians and nonvegetarians in Asian populations.
- The observational nature of the research.
- The lack of randomized controlled trials using plant-based dietary approaches with metabolic syndrome as a primary outcome.
Conclusion
The review of studies indicated that vegan and vegetarian diets are associated with a reduced risk of metabolic syndrome. The article highlights that the review has provided an overview of potential dietary components that may play a role in the protective effect of plant-based diets. These include lower energy intake, saturated fat, heme iron, and an absence of red and processed meat intake among vegetarian and vegans, as compared with other dietary patterns. In addition, higher intakes of fiber and fruits and vegetables among those following vegan or vegetarian diets may be protective against metabolic syndrome. The study indicates that future research should test these dietary approaches in a more controlled manner, which would allow for a better understanding of how transitioning to these type of dietary approaches impacts risk factors of metabolic syndrome. The review of studies included in this report found that vegan and vegetarian diets had the most impact on blood pressure and fasting glucose. A randomized design could assess whether vegetarian and vegan diets also have the greatest impact on these parameters of metabolic syndrome when used for treatment or whether other parameters are impacted as well. Conducting a randomized controlled study examining the impact of plant-based dietary approaches on metabolic syndrome, combined with the epidemiological evidence, could help strengthen the evidence base for potential dietary strategies for the prevention and treatment of metabolic syndrome.