Kale is not king.

Despite its rapid rise up the ranks of so-called superfoods in recent years, kale does not even make the top 10 "powerhouse fruits and vegetables", according to a new study.

Rather, watercress was the unlikely winner, packing a peppery punch with its cross-section of critical nutrients.

Cress, said by Hippocrates to be the "cure of cures" more than 2400 years ago, was followed by its close cousins including Chinese cabbage, collard and mustard greens.