‘You vote at least three times a day with your fork,” says a farmer in this sombre, painful, relevant documentary about agribusiness from film-maker Christopher Quinn, based on the 2009 non-fiction bestseller by Jonathan Safran Foer, co-written with him and narrated by Natalie Portman, who was so energised by the book that it reportedly turned her into a vegan activist.

It’s a film that tells you what you already knew – and then made a conscious or unconscious decision to forget, if you are a meat-eater. Factory farming is a horrible business. The economics demand ever greater productivity under conditions of ever greater squalor and cruelty for the animals themselves: the normalisation of what is grimly known as “confinement agriculture”. Chicken nuggets are the product of something pretty horrendous. Small farmers who are contracted by the big corporations to provide meat product are themselves treated as serfs, the upper homo sapiens layer of degradation.

Whistleblowers pay a terrible price for informing the public about what is going on, in terms of harassment and unemployment; journalists and reporters are harassed, too. Yet the film ends, as so many documentaries of this sort end, on a note of optimism, a series of relatively upbeat bulletins in white-on-black intertitles: campaigners are having sporadic successes; plant-based protein companies and small organic farmers aren’t doing so very badly.

But to what conclusion is the film’s logic pointing us? It can only be to stop eating meat and dairy products – like quitting cigarettes – although the film is chary about saying this too clearly or too hectoringly, maybe for fear of alienating its own customer base. The film is about what William Burroughs called the “frozen moment when everyone sees what is at the end of every fork”.