CARTOON character Popeye is right to down a can of spinach when he wants his biceps to bulge, according to a Swedish study showing why the leafy vegetable makes us stronger.

Researchers at the Karolinska Institute in Stockholm said they had conducted a study showing how nitrate, found naturally in spinach and several other vegetables, tones up muscles.

For the study, to be published in the Journal of Physiology, the research team placed nitrate directly in the drinking water of a group of mice for one week and then dissected them and compared their muscle functions to that of a control group.

"The mice that had been on consistent nitrate had much stronger muscles," they said.

The nitrate used "was equivalent to a human's consumption of about 200 to 250 grams of spinach a day, so it's a very easily obtained amount", one of the researchers at the Department of Physiology and Pharmacology, Andres Hernandez, said.

While no effect could be seen in the so-called slow-twitch muscles used for moderate exercise and endurance, the scientists saw a clear change in the fast-twitch muscles used for strength and more high-intensity exercises, Mr Hernandez said.

The tricky question was determining why this happened.

The researchers discovered that the nitrates had prompted an increase in two proteins, found naturally in the muscles, that are used for storing and releasing calcium, which is vital to making muscles contract.

The protein increase in turn led to higher quantities of calcium released in the muscles, Mr Hernandez said, noting: "If you have more calcium released, you have a stronger contraction."

In human terms, consuming nitrates from, for instance, spinach increases the muscle strength available for things like lifting weights or sprinting up a steep hill.

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Popeye's right: spinach makes you stronger