Nitrates, a chemical found in vegetables like spinach and lettuce, may be responsible for keeping blood vessels healthy, says a new study.

Past studies have shown that a diet rich in fruits and vegetables can reduce blood pressure.

But it has been difficult to determine exactly which nutrients in fruits and vegetables are responsible for these blood-pressure-lowering effects, according to the online edition of health magazine WebMD.

Scientists at the Swedish School of Sport and Health Sciences examined the effects of short-term nitrate supplementation in a group of 17 healthy, non-smoking young adults.

Each participant rotated between taking a daily dose of nitrate supplement equivalent to the amount normally found in 150 to 250 gm of a nitrate-rich vegetable -- such as spinach, lettuce, or beetroot -- for three days, and taking a placebo for a different three days.

The results showed that average diastolic blood pressure was lower after three days of nitrate supplementation than it was after taking the placebo for three days.

The researchers claimed to have found nitrates in vegetables like spinach and lettuce, which they say may be Mother Nature's way of keeping blood pressure in check.

Since it was a small study the researchers say the chemical's blood-pressure-lowering effects merit further study.

The report has not mentioned the names of the researchers involved in the study.