Vegetarians seem to have lower blood pressure, according to a new analysis. Could adopting a vegetarian diet be a useful strategy for lowering blood pressure?
It is well known that high blood pressure is a major risk factor for heart disease. Controlling blood pressure through lifestyle factors, such as diet and exercise, is key for avoiding heart problems.
But medical opinion has been split over whether a vegetarian diet is effective for reducing blood pressure. Different studies on how vegetarian diets influence blood pressure have reported conflicting results.
To clarify the issue, researchers in Osaka, Japan, have performed a meta-analysis of existing studies looking at the relationship between vegetarian diets and blood pressure.
Their findings are published in the journal JAMA Internal Medicine.
The researchers analyzed the findings of seven clinical trials (looking at 311 participants in total) and 32 observational studies (looking at 21,604 participants in total).
In this review, "vegetarian diets" were defined as excluding or rarely including meat, but including dairy products, eggs and fish.
The meta-analysis found that vegetarian diets are associated with lower blood pressure, compared with omnivorous diets.
The researchers measured the difference in millimeters of mercury (mm Hg) - the unit blood pressure is measured in - between participants who followed a vegetarian diet and participants who followed an omnivorous diet.
In terms of systolic blood pressure (the pressure in the arteries when the heart beats), the researchers found that the blood pressure of vegetarians was 4.8 mm Hg lower overall than omnivores in clinical trials and 6.9 mm Hg lower in observational studies.
In terms of diastolic blood pressure (the pressure in the arteries between heartbeats), the researchers found that the blood pressure of those following a vegetarian diet overall was 2.2 mm Hg lower in clinical trials and 4.7 mm Hg lower in observational studies.
This reduction, the researchers say, is similar to the health benefits of a low-sodium diet or a weight reduction of 5 kg. Reducing systolic blood pressure by 5 mm Hg is also associated with a 9% lower risk of death from coronary heart disease and a 14% lower risk of death from stroke.
Why do vegetarians have lower blood pressure?
The researchers consider a number of reasons why a vegetarian diet may be effective at controlling blood pressure.
Vegetarians generally have lower BMIs and a lower risk of obesity than omnivores, probably because vegetarian diets have higher fiber and lower fat content than omnivorous diets.
Because body weight and blood pressure are linked, this might partially explain the lowered blood pressure in vegetarians. However, other studies have found that a vegetarian diet lowers blood pressure regardless of body weight.
Another suggestion is that vegetarian diets are high in potassium and low in sodium, but again, some studies have disagreed over the impact this might have.
Some studies have also found that alcohol consumption is lower in vegetarians, compared with the general population. Alcohol intake can influence blood pressure, but five of the seven clinical trials analyzed in this study were limited to participants who drank no more than moderate amounts of alcohol. Therefore, the results of this analysis were probably not influenced by alcohol intake.
Vegetarian diets are usually proportionally lower than omnivorous diets in saturated fatty acids and higher in polyunsaturated fatty acids - characteristics that are associated with lower blood pressure.
And, finally, vegetarians usually have lower blood viscosity. This could affect their blood pressure.
There are some things the meta-analysis could not be sure about. For example, not 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 researchers conclude:
"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."
In 2013, Medical News Today reported on a study that found mindfulness training may help to control blood pressure.