Veganism, once widely seen as an alternative, if not an extreme, lifestyle, is now in the mainstream. Exactly how many people in the UK have eliminated animal products including dairy and honey from their diets is uncertain. One recent survey suggested there could be as many as 3.5 million vegans in the UK. The Vegan Society offers the much lower figure of 600,000 vegans, which still represents a fourfold increase in four years. But either way, and combined with a big increase in vegetarians and occasional meat-eaters (otherwise known as “flexitarians”), a huge shift in eating habits is under way. This week’s annual food and drink report by the supermarket Waitrose says around a third of people have either cut down on or stopped eating meat altogether.

There is more than one explanation for this change in behaviour. More than half of those surveyed by Waitrose said animal welfare was their motivation for changing their diet. The intensive farming and slaughter of livestock has long aroused feelings of regret and distaste in many people who do not object on principle to eating meat. Such feelings, along with concerns around health and sustainability, have led to rising demand for organic and free-range products among those who can afford them. The growing trend towards giving up meat altogether suggests either that the moral objections have spread, that other factors than animal welfare are in play – or, most likely, some combination of both.

These other factors are health and the environment. Expert advice on diet has long focused on a balanced intake of nutrients, and eating more fruit and vegetables and less sugar and fat. There is no simple answer to the question of whether it is healthier to be a carnivore or not. But over the past few years what was once suspected has become official: cured and processed meats can cause bowel cancer, and are classified by the World Health Organization as a group-one carcinogen, alongside tobacco and asbestos. Little wonder, then, that health-conscious people are giving up BLTs.

While the health case for vegetarian and vegan diets is inward-looking, the environmental arguments point urgently outwards and forwards in time. Last month a major international study concluded that huge reductions in meat-eating are essential, especially in the western countries that eat most of it, since food production is a big contributor to global warming as well as deforestation, water shortages and pollution from fertilisers. This was one of the main arguments for veganism made by the freelance journalist Selene Nelson in an article this week, after her exchange of emails with Waitrose Food magazine editor William Sitwell led to his resignation.

Mr Sitwell had responded to her pitch for a vegan series featuring recipes and tips with a reply that joked he might instead commission a series about “killing vegans” to “expose their hypocrisy”. Since Waitrose is currently promoting new vegan lines, it was unimpressed when these comments ended up in the press. What might have been laughed off in a note to a colleague was poorly judged as a response to a freelancer’s pitch for paid work.

Had he thought better of his rudeness and sent a follow-up email, the story might have had a different ending. Vegans, as has been pointed out by Mr Sitwell’s defenders, are not a minority it is a crime to insult. What they are, to those who would rather ignore the peril of rising carbon emissions, is an irritation. To others who share their concerns, they offer a challenge – even an inspiration.