Michael Wolff’s Fire and Fury is a book full of intoxicating detail; a personal favourite is Rupert Murdoch putting down the phone to Donald Trump with a terse and descriptive “fucking idiot”. But so far – if only because the shambles, the directionlessness, the narcissism and the attention deficit were already so well known – the headline news is the way Trump eats.

He has a long-term fear of being poisoned. We’ll come back to that; suffice it to say that the way it is described suggests that this predates his ascension to high office. So it can only have worsened: if you’re worried about a toxic attack when you’re fronting The Apprentice, imagine how much more likely that must seem once you’re president. It’s such a peculiar fear, dated to the point of myth – the kind of threat someone might dream if they had half-listened through one Roman fable and then refracted everything they ever knew through their own ego. But let’s not start intuiting: the picture’s bad enough just with its raw facts.

Obama ate salted caramel and admitted it. Just imagine that being the worst thing you could say about the US president

Trump’s answer to the poisoning threat is to eat McDonald’s, by preference, all the time; the burger chain never knows he’s coming, and it’s all pre-made. This has the distinct ring of post-hoc justification – it would be much more reliable to get a nine-year-old to sit next to him and pre-taste his food. Besides which, even if the staff of McDonald’s don’t in general know who’s coming, it would be unusual for them to feed the president without a heads-up. Much more likely, Trump fears poison and he really likes McDonald’s.

This, too, was an open secret from the time of his trip to the Middle East, when his trailer requests read like the death row meals in a dystopian movie about 100 people all getting electrocuted on the same day. The Twitter account @rogueSNRadvisor also revealed how much weight he had gained in the one year between winning the election and this trip: 100lb (seven stone). I was at a butcher’s counter when I read that, and shared it with the assistant, who said she weighed exactly 100lb. We stood there, lost in wonder, imagining Donald Trump incorporating her mass into himself, using only stolid, mission-based, determined chewing. It would be about the most tenacity and focus the man has ever exhibited.

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The achievement, I’d guess, is historically novel, and he couldn’t have done it without the burger. Yet he is not alone in putting the meat patty at the centre of his presidential self-fashioning.

George W Bush famously claimed that his favourite pizza was the cheeseburger pizza, where you get a pizza, put a burger on it, spread some cheese on top, sprinkle on the desiccated remains of civilisation and bang, you got yourself a 2,500-calorie snack. This revelation said more than “I’m just like you”, it said “See how you are? However bad you think that is, I’m worse.” Bill Clinton, meanwhile, liked his burgers with jalapeños, which as any fool knows is the thing you have to add to a burger to make it go down when you’re a bit too hungover to eat it.

For both these presidents, however, appetite always seemed ancillary to the job, a coincidental thing separate from dignity, expertise and charisma. Now all those things have been stripped away, and only the appetite is left, morphing cancerously to consume not just burgers but also all crisps, all sweets, all fast-food ending in “oh” (Doritos, Cheetos, anything that reminds you of running your tongue round a drainpipe that has been used for the stunt round of It’s a Knock Out).

Obama – nobody forget Obama – walked a desperate tightrope of presenting himself in an honest way while at the same time masking his effortless superiority, which he managed by eating salted caramel and admitting it. Just imagine for one second that being the worst thing you could say about the US president.

• Zoe Williams is a Guardian columnist