Researchers have discovered that eating a bowl of spinach a day makes your muscles "profoundly" more efficient.
They found that eating 300g of the vegetable reduced the amount of oxygen needed to power muscles by as much as five per cent when exercising.
The effect is so powerful it works after just three days.
The secret is not iron but nitrates which are abundant in the vegetable.
These chemicals make the mitochondria – the "engine rooms" of every cell – more efficient, they found.
"It is like a fuel additive for your muscles – it makes them run much more smoothly and efficiently," said the lead author Dr Eddie Weitzberg of the Karolinska Institutet in Sweden.
Dr Wietzberg, who reported his findings in the journal Cell Metabolism, fed people pure nitrate supplements – the equivalent to the amount in a plate of spinach – every day for three days.
At the beginning and end of the experiment they were made to pedal strenuously on an exercise bike while their oxygen intake was measured via a tube to the mouth
It was found that the difference in energy intake was between three and five per cent – a significant figure.
Spinach is well known as the superfood that gave Popeye the Sailor Man his bulging muscles.