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Researchers said for some people, eating a vegetarian diet could be a good way to treat high blood pressure without medication.
Vegetarian diets exclude meat, but may include dairy products, eggs and fish in some cases. They emphasise foods of plant origin, particularly vegetables, grains, legumes and fruits.
High blood pressure contributes to a person's risk of heart disease, stroke, kidney disorders and other health problems. For many people, the only treatment has been medication, but that means costs and possible side effects, lead author Yoko Yokoyama told Reuters Health in an e-mail.
Giving hope to many people
"If a diet change can prevent blood pressure problems or can reduce blood pressure, it would give hope to many people," Yokoyama said. She is a researcher at the National Cerebral and Cardiovascular Centre in Osaka, Japan.
Read: Tips to reduce your blood pressure
"However, in order to make healthful food choices, people need guidance from scientific studies," she said. "Our analysis found that vegetarian diets lower blood pressure very effectively, and the evidence for this is now quite conclusive.
"According to the American Heart Association, blood pressure readings under 120 mm Hg systolic and 80 mm Hg diastolic (120/80) are considered normal.
Observational studies
High blood pressure starts at 140/90.The new review, published in JAMA Internal Medicine, combined results from 39 previous studies, including 32 observational studies and seven controlled trials.
"Observational studies show what happens when people have chosen their own diets and stuck with them, often for years," Yokoyama said.
"Controlled trials are different – a diet is given to people who had not tried it before, and that will show the effect of beginning a new way of eating."Together the studies included close to 22 000 people.
The researchers found that in the observational studies, people who had been eating a vegetarian diet had an average systolic blood pressure that was about 7 mm Hg lower than among meat-eaters and a diastolic blood pressure that was 5 mm Hg lower.
No cost involved
Participants in the clinical trials who were given vegetarian diets to follow had, on average, a systolic blood pressure that was 5 mm Hg lower and a diastolic blood pressure that was 2 mm Hg lower than participants in control groups who were not on vegetarian diets.
"Unlike drugs, there is no cost to a diet adjustment of this type, and all the 'side effects' of a plant-based diet are desirable: weight loss, lower cholesterol, and better blood sugar control, among others," Yokoyama said.
Read: Cocoa could reduce blood pressure slightly
Low fat and high fibre
She said a plant-based diet is typically low in fat and high in fibre, so it helps people lose weight, which, in turn, causes a healthy drop in blood pressure."But there is more," Yokoyama said.
"Plant-based foods are often low in sodium and are rich in potassium, and potassium lowers blood pressure." The same foods are also very low in saturated fat – the type of fat in meat and cheese – and eating less saturated fat means blood can circulate more easily, she explained.
"I would encourage physicians to prescribe plant-based diets as a matter of routine, and to rely on medications only when diet changes do not do the job," Yokoyama said. "And I would encourage everyone to try a plant-based diet, and especially to introduce plant-based diets to their children – they could prevent many health problems.
Read: Whey supplements lower blood pressure
Kick-start programme
Yokoyama said doctors who would like to prescribe diet changes need tools. "We have developed a free program, called the 21-Day Kick-start programme, which introduces a plant-based diet through daily emails that provide menus, recipes, cooking videos, and a discussion board for questions.
It is available at no charge in English, Spanish, Mandarin, and Japanese, along with a special English-language programme for India," Yokoyama said. The programme is affiliated with the Physicians Committee for Responsible Medicine, an organisation that promotes plant-based diets.
Read more:
Diet tips for pregnant vegetarians
Is my vegetarian diet balanced?
Vegetarian diet a good alternative
