'I'm fat," announces my husband.

"That's OK, Toryboy," I say. "Our relationship has never been built on raw physical attraction, but on a kind of sick, intellectual curiosity and an emotional masochism on both our parts that neither of us had hitherto suspected. Your expanding girth will hardly change that."

"But being overweight leads to heart disease, diabetes and untimely death," he says.

"That's OK, too," I say soothingly. "Haven't I always told you that the only good Tory is a dead Tory?"

"But you don't want me to be incapacitated before I go," he says. "Think how nursing me would cut into your Will & Grace repeats-watching time."

The man has a point. I pause Will & Grace to look at him properly. A paunch is definitely present.

"How does one lose weight?" he asks.

"Well, there are two ways," I say. "You can exercise more… "

"No."

"Or you can eat less."

"No."

After prolonged discussion, during which he is persuaded that I am not hiding some secret third way perhaps involving the weight being licked off by kittens, he eventually plumps (as it were) for dietary emendation.

His ignorance – his swirling, fathomless, pitch-black ignorance – of what this entails is only gradually revealed, however. "I'm just having a sandwich for lunch," he reveals proudly the next day. I am impressed – usually it's pasta with a Super Noodles starter – until I see that said "sandwich" is a giant pot of prawn cocktail mix emptied into a bap. I remonstrate.

"But it's brown bread," he says.

"That doesn't make a pint of pink mayonnaise a healthy snack."

"What about the fact that it's cold and not what I really want?"

"That is what should count in our favour on all diets," I say with a sigh. "But never, ever does."

The next day, he wants to eat a pork pie on the grounds that it's "just pork and a bit of pastry". He appears to be under the impression that the meat in his 79p purchase from the local chain bakery is as pure a piece of porcine goodness as was ever carved off an organic Gloucester Old Spot, rather than a dollop of mashed pig lips and fat mechanically recovered from the abattoir floor. And that the pastry is made from fruit.

Starvation sets in over the next few days because he won't eat vegetables. Doesn't like them, won't eat them.

I'm sorry, did I not mention I am married to an eight-year-old boy?

He cannot or will not grasp the difference between calorific value and ability to satiate. In vain do I draw diagrams and construct equations that seek to show how a carrot can be more filling than a cake at less calorific expense.

Eventually, I invent the Salad Deviation Scale.

"You accept – even though you won't eat it, because you 'don't like leaves' – that salad is good for you, yes?" I ask him.

If he doesn't say yes, I promise myself, I will stab him through his engorged and overstrained heart, and put us both out of our misery.

"I am convinced that this is, at least, the prevailing wisdom," he replies.

This is enough – just – to stay my hand.

"OK then," I say. "So, before you put anything on your plate or in your mouth, you must ask yourself the following question: 'How different is this from salad?' So, if it's fruit?"

"Not very different."

"And you may eat. Anything with fat dripping off or scattered in lumps throughout it?"

"Very different. And I must not eat it."

I don't know about him, but I, at least, am a shadow of my former self.