Scientists tested the effect on healthy volunteers of consuming 200 grams, or two cupfuls, of blueberries for a month.

Over the course of the study participants' systolic blood pressure was lowered by an average 5mmHg (millimetres of mercury).

Systolic pressure is the pressure that builds up in blood vessels with each pump of the heart. It reduces when blood vessels relax and dilate and increases when they contract.

The level was similar to that seen in patients taking blood pressure-reducing drugs.

For the study, 40 volunteers aged 18 to 70 were randomly given either a drink containing 200 grams of blueberries or a similar tasting drink containing no blueberries.

Effects on blood vessel function were seen in as little as two hours after consuming the blueberry drink.

The "control" drink, which was varied to be neutral or to contain fibre or vitamins and minerals, had no significant effect.

The scientists traced the blood pressure benefits of blueberries to anthocyanins, plant pigments responsible for the blue, red and purple colour of some fruits and vegetables.

Drinks containing purified anthocyanins led to improvement in the function of endothelial cells that control blood vessel relaxation and contraction.

Lead researcher Dr Ana Rodriguez-Mateos, from King's College London, said: "Although it is best to eat the whole blueberry to get the full benefit, our study finds that the majority of the effects can be explained by anthocyanins.

"If the changes we saw in blood vessel function after eating blueberries every day could be sustained for a person's whole life, it could reduce their risk of developing cardiovascular disease by up to 20 per cent."

The research is published in the Journal of Gerontology Series A.

Australian Associated Press