Healthy people used less oxygen while riding an exercise bike after they took a small dose of inorganic nitrate for three days the amount equivalent to that in a plate of spinach or a few red beets, according to a new study.

The results may offer one explanation for the well-known health benefits of fruits and vegetables, and leafy green vegetables in particular, the researchers said. The nitrate improved performance because it increased the efficiency of the mitochondria that power our cells, the researchers said.

"We know that diets rich in fruits and vegetables can help prevent cardiovascular disease and diabetes, but the active nutrients haven't been clear, said study researcher Eddie Weitzberg, of the Karolinska Institute in Sweden. "This shows inorganic nitrate as a candidate to explain those benefits."

Until recently, nitrate wasn't thought to have any nutritional value at all, and some even suggested it might be toxic, the researchers said.

But these researchers showed in an earlier study that nitrate in food, with the help of friendly bacteria found in our mouths, can be converted into nitric oxide. Nitric oxide has long been known to open blood vessels to lower blood pressure .

The new study showed that in addition to these benefits, nitrate improved the efficiency of the mitochondria . Normally, lower levels of some proteins make the cellular powerhouses leaky. "Mitochondria normally aren't fully efficient," Weitzberg said. "No machine is." The nitrates may have made the mitochondria less leaky, the researchers said.

Although the results showed that increased dietary nitrate can have an immediate effect, it's not yet clear what might happen in people who consume higher levels of inorganic nitrate over longer periods of time, the researchers said.

Weitzberg said the researchers will next repeat the experiment in people with conditions linked to mitochondrial dysfunction, including diabetes and cardiovascular disease, to see if they benefit from increased nitrates.

The study also may mean that powerful mouthwashes may have a downside, the researchers said. "We need oral bacteria for the first step in nitrate reduction," Weitzberg said. "You could block the effects of inorganic nitrate if you use a strong mouthwash or spit [instead of swallowing your saliva]. In our view, strong mouthwashes are not good if you want this system to work."

The study was published in the February issue of the journal Cell Metabolism.

Pass it on: Leafy green vegetables may improve cell's use of energy.

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