But the preliminary findings don't yet suggest that supplementing your diet with beetroot juice benefits your health, researchers said.
"Our hope is that increasing one's intake of vegetables with a high dietary nitrate content, such as green leafy vegetables or beetroot, might be a lifestyle approach that one could easily employ to improve cardiovascular health," Amrita Ahluwalia, Ph.D., lead author of the study and a professor of vascular pharmacology at The Barts and The London Medical School in London, said.
The beetroot juice contained about 0.2g of dietary nitrate, levels one might find in a large bowl of lettuce or perhaps two beetroots.
In the body the nitrate is converted to a chemical called nitrite and then to nitric oxide in the blood. Nitric oxide is a gas that widens blood vessels and aids blood flow.
"We were surprised by how little nitrate was needed to see such a large effect," Ahluwalia said.
"This study shows that compared to individuals with healthy blood pressure much less nitrate is needed to produce the kinds of decreases in blood pressure that might provide clinical benefits in people who need to lower their blood pressure. However, we are still uncertain as to whether this effect is maintained in the long term," she said.
The study involved eight women and seven men who had a systolic blood pressure between 140 to 159 millimeters of mercury (mm Hg), did not have other medical complications and were not taking blood pressure medication.
The study participants drank 250 mL of beetroot juice or water containing a low amount of nitrate, and had their blood pressure monitored over the next 24 hours.
Blood pressure is typically recorded as two numbers. Systolic blood pressure, which is the top number and the highest, measures the pressure in the arteries when the heart beats. Diastolic blood pressure, the bottom and lower number, measures blood pressure in the arteries between heart beats.
Compared with the placebo group, participants drinking beetroot juice had reduced systolic and diastolic blood pressure - even after nitrite circulating in the blood had returned to their previous levels prior to drinking beetroot. The effect was most pronounced three to six hours after drinking the juice but still present even 24 hours later.
The study is published in the American Heart Association journal Hypertension.