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Watercress scored the highest of eighty-seven foods on the US nutrient-density index.

The peppery taste comes from glucosinolates, the same family of compounds found in mustard and rocket.

Buy cultivated watercress. Wild cress from a stream can carry liver fluke.

Shāhi grows in running water, which is where both its name and its problems come from. Its mustard bite wakes up a mouthful of bread and cheese. Gram for gram, it carries more vitamins and minerals per calorie than almost any other food measured. In Iran, it sits on the sabzi khordan plate beside tarragon, basil and radish, and people eat it by the fistful rather than the leaf.

Known to botanists as Nasturtium officinale.