"My version: my husband is undergoing an infinitesimal alteration of his eating habits. His version: he is being starved to death.

For our purposes, let us split the difference and agree that he is on a diet. This is because a few weeks ago, he came across our wedding photos and noticed that, four years ago, he was thinner. Which meant that he was now fatter. ‘And this,’ I said, eyeing the gently rounded belly that now strains at the shirts he has been wearing since he was a student, ‘is news to you how?’

‘It’s ridiculous!’ he shouted.

‘It’s not so ridiculous,’ I pointed out. ‘You think mixing vegetable supernoodles with chunks of salami is a balanced meal. You drink beer. You’re nearly 40. And you haven’t taken any exercise since 1991. And that was chasing a chip van.’ He said he wanted to lose weight. I agreed to help him. The weeks since then have been an education for both of us.

My husband is an intelligent man. There’s nothing he doesn’t know about history, politics, current events. He reads voraciously and doesn’t just retain all the information he comes across but synthesises it and makes it an organic whole in his mind, instead of a mere series of facts. In the world of food, diets and nutrition, however, he is dumber than the toast he can no longer have while standing in front of the fridge deciding what he actually wants to eat for lunch.

I, on the other hand, can’t pick any politician (except spambot Cameron) or monarch (except Henry VIII, if he had his six wives standing next to him) out of a line-up, find Syria on a map or retain any fact I’ve read since my brain calcified overnight at 18. And yet, it turns out, I can knock out a bespoke weight-loss programme in seconds, using a reserve of information my husband swears he has never in his life heard the smallest mention of.

I’ve explained vitamins, the value of fibre, the need for stable blood sugar levels and not skipping meals. I’ve tried to explain the need to play the fat, fibre and glucose contents of each meal off against each other, why fizzy drinks are bad even if they have a sporty logo, and just why, mouthful for mouthful, salad is a better dietary tool than sausage.

Lucy Mangan: "Men just can't handle a diet"