The key to a healthy heart?

A team led by Imperial College London, which studied 4,680 people aged 40-59, said it was not clear why eating more vegetable protein had such an effect.

But amino acids - the building blocks of protein - or vegetable components, like magnesium, may be key, they said.

However, they found no strong evidence that high meat consumption is linked to high blood pressure. The study features in Archives of Internal Medicine.

Understanding more about these vegetarian proteins could help guide us in how to prevent or treat heart and circulatory disease

Belinda Linden

British Heart Foundation

Some types of amino acid have been shown to influence blood pressure, for instance arginine has been shown to dilate the blood vessels.

Different proportions of amino acids are found in meat and vegetarian matter.

Healthy lifestyle

Writing in the journal, the researchers, led by Professor Paul Elliott, said more research was needed to pin down exactly why vegetables helped lower blood pressure.

High blood pressure is associated with an increased risk of heart disease and stroke.

But they stressed: "Our results are consistent with current recommendations that a diet high in vegetable products be part of a healthy lifestyle for prevention of high blood pressure and related chronic diseases."

Belinda Linden, medical spokesperson for the British Heart Foundation, said previous research had also shown that blood pressure was better controlled with a vegetarian diet.

She said: "Vegetables contain less salt and more antioxidant vitamins, are low in calories, high in fibre and also help to control diabetes.

"Having proteins found in vegetables rather than those derived from animals may offer a number of natural benefits for people who are at a high risk of coronary heart disease, stroke or diabetes.

"Understanding more about these vegetarian proteins could help guide us in how to prevent or treat heart and circulatory disease and allow us to lead an appropriate healthy lifestyle."

During the study, volunteers had their blood pressure read eight times.

Each participant wrote down everything they had eaten and drunk over the previous 24 hours before each check-up, including dietary supplements.