“THENCE to the whey-house and drank a great deal of whey,” wrote Samuel Pepys in 1663. He had been at the theatre, but today’s whey-drinkers tend to come straight from the gym. After a lull in popularity—in the 19th century it was fed to pigs—this by-product of cheese-making is resurgent. Sales of sports-nutrition products, most of them based on whey protein, have doubled since 2007 to reach £260m ($396m) last year.

The growth was driven by the decision in 2005 of Maximuscle, the leading supplier, to break the tight link between protein supplements and extreme bodybuilding, says Lucas Brown of Total Media, the agency which managed the account. It employed rugby stars as “brand ambassadors” and advertised in lifestyle magazines to reach mainstream consumers. By 2012 one in ten men was using sports supplements at least once a week, according to YouGov, a research firm. Eager consumers muscled sceptics aside like nine-stone weaklings. (Richard Miller of the British Dietetic Association says that for most gym users, a pint of milk provides sufficient protein for muscle recovery.)

Gym denizens are unlikely to drop their protein shakes for milk cartons. But bulkier opponents are appearing. Since December food marketing claims have been pre-approved by the European Food Safety Authority, a quango with a reputation for rigour. Its rulings mean that protein supplements can no longer be flogged as shortcuts to an Olympian torso. More troubling still, in October sports-nutrition products became subject to value-added tax. GlaxoSmithKline, which bought Maximuscle in 2010, says that as a result, “overall category growth has declined significantly”.

Salvation may come from the seven-eighths of Britons who are not gym members. Volac, Europe’s largest whey-protein manufacturer, has just launched a protein-packed dairy drink which will sit alongside fruit smoothies on supermarket shelves. Government surveys show average protein intake is currently “well above” the recommended level, but producers are confident that demand will rise anyway. They may be right: body image is a strange thing.