Dairy-free diets are now in vogue. Soya yoghurts, almond milk, coconut ice cream, and tofu are widely sold in supermarkets. Vegan cafes and restaurants are popping up everywhere. And there's plenty going on in the media too: as celebrities ditch the white stuff, scientists debate the impact of veganism on climate change.

It wasn't always like this. Back in the 1970s and 1980s, eating a plant-only diet was seen as a far more radical and counter-culture choice than it is today. At that time, even lacto-vegetarians--those who eat dairy and honey but no meat or fish--were on the fringes...