Previous studies have suggested an association between vegetarian diets and lower blood pressure (BP), but the relationship is not well established.
Researchers in Osaka, Japan, conducted a systematic review and meta-analysis of controlled clinical trials and observational studies that have examined the association between vegetarian diets and BP.
The team searched MEDLINE and Web of Science for articles published in English from 1946 to October 2013 and from 1900 to November 2013. Among the 258 studies identified seven controlled trials with 311 participants with an average age of 44.5 years and 32 observational studies with 21,604 participants with an average age of 46.6 years, had been included in this review.
In this review, "vegetarian diets" were defined as excluding or rarely including meat, but including dairy products, eggs and fish.
The results showed consumption of vegetarian diets was associated with a reduction in mean systolic BP (−4.8 mm Hg) and diastolic BP (−2.2 mm Hg) compared with the consumption of omnivorous diets, in the controlled trials.
In the 32 observational studies consumption of vegetarian diets was associated with lower mean systolic BP (−6.9 mm Hg) and diastolic BP (−4.7 mm Hg) compared with the consumption of omnivorous diets.
In their conclusion the team writes “Consumption of vegetarian diets is associated with lower BP. Such diets could be a useful non-pharmacological means for reducing BP.”
The researchers consider a number of reasons why a vegetarian diet may be effective at controlling blood pressure.
One reason is that vegetarians in general have a lower body mass index compared to omnivores. The reasoning is vegetarian diets have higher fiber and lower fat content than omnivorous diets.
Body weight and blood pressure are associated which may partially explain he lowered blood pressure in vegetarians.
Another reason may be due to the fact that vegetarian diets are high in potassium and low in sodium however, some research has disagreed over the impact.
Vegetarian diets are generally proportionally lower than omnivorous diets in saturated fatty acids and higher in polyunsaturated fatty acids which have been linked to lower blood pressure.
The researchers also note that vegetarians normally have lower blood viscosity. Blood viscosity is correlated with all known risk factors for cardiovascular disease, including age, sex, smoking, obesity, inflammation, insulin resistance, high blood pressure, low high-density lipoprotein cholesterol, high low-density lipoprotein cholesterol, and others.
The researchers concluded "Further studies are needed to explore the relationships between specific foods and nutrients and blood pressure. Nevertheless, the results of the meta-analysis of the controlled trials suggest a robust relationship between consumption of vegetarian diets and lower blood pressure."
The team noted all of the studies reviewed took certain factors into account, such as how much people exercised or other lifestyle factors. Also, the components of the vegetarian diet differed from person to person and country to country.
The findings are published in the journal JAMA Internal Medicine.