Horatio Wood’s ‘Treatise on therapeutics’ (1879) says that “Barley-water is used as a nutritious, demulcent drink in fevers.” It is still in use. You can buy it flavored with citrus in bottles, beside the lemonade, in British grocery stores. I can just remember drinking barley water as a child – it was also popular in Asian cultures. I have no idea why; as I recall it was completely flavorless. It looks like pasta water, or the fluid you pour off after you rinse the rice before cooking it. I am certain there’s nothing in it but starch — you will note from the recipe below that you strain it out so that there’s no fiber or indeed anything solid left. Click here for a link to the Chinese recipe. And here is an English recipe from 1891:
Take 1 oz barley and having washed it put it into a clean saucepan with a quart of water. Let it just come to the boiling point. Pour away the water and add again to it another quart of water. Allow it this time to boil for nearly an hour before straining it. Strain the juice of one or two lemons, and cut the rind of one very thin and add to the barley water with sugar to taste.
Barley water was also popular with teetotalers – in THE WORSHIPFUL LLUCIA, one of the Mapp and Lucia novels by E. F. Benson Lucia takes care to have barley water on offer so that her guests have no excuse for inebriation. It was also held to be less stimulating (i.e. with less caffeine) than tea or coffee, and so at one point in MARIAN HALCOMBE Walter advises Marian to take it and calm down.