Harriet Walker: You can eat the pasty, but you know full well that no good can come of it

I joined a gym this month – it's the first I've been to in almost 10 years, and pretty much the first exercise I've done in just as long, apart from the odd dance-floor workout or trot for the bus. And since I joined, I've found the way I approach food has become like that of a hangman contemplating death.

For most of us, most of the time, death is just something that happens at the end of our lives, regardless of the fact it informs almost everything we do and the way our lives are structured. For the hangman, though, it is his bookmark, his morning call; he knows its difficulties, its ins and outs, its ubiquity.

For those of us who go to gyms (I've been three times – and once was just swimming), food is no longer something you eat when you're hungry, or bored, or socialising, or just standing absent-mindedly by the sink with some cheese in your hand.

Those of us who go to the gym – we who count each calorie sweated out on the bomb-like monitor that can have little to no understanding of how many calories you're burning because it has never met you before and has no eyes to see if you're an incredibly tall woman or an incredibly fat man – will never again be able to snaffle a slice of salami or corner of bread without a stabbing feeling of inevitability, pointlessness and despair.

Once you have burnt off the equivalent of a Mars Bar, counted every calorie as it passes and realised it takes about five times as long and 10 times the exertion as eating it did, you can never again fully enjoy the experience of tasting the goo, swallowing the clag and smacking your lips.

Like Eve eating the apple and then being booted out of paradise, once such knowledge is acquired, it can never be forgotten. You can eat the cheese-and-onion pasty, but you know full well before, during and after that no good can possibly come of that five minutes in which you felt truly alive.

The relationship the gym bears to food is one – like work does to a holiday – which proves that God exists, and is trying to make life as hard as possible for us.

You begin to feel like the Hegelian tragic hero, pulled by opposing forces of equal and vital import in completely different directions. Namely, towards the nearest fast-food outlet and simultaneously towards the treadmill. One, so much easier to give into; the other, utterly unappealing, but for the consequences it promises. The consequences of the former, meanwhile, include becoming housebound and being able to wear only kaftans.

I didn't even want to eat fried chicken at any time of day or night before I started going to the gym and saw the glitch in the matrix. Now I can't stop thinking about it. Sometimes, I think about it while I'm actually eating it.

Naturally, my attendance at the gym has declined in the way church-going has, if you consider last week as the 19th century and tomorrow Armageddon. This much I had predicted. But the knowledge hasn't faded.

"I remember when I realised it," a friend sympathised (we had fried chicken together last weekend). "It was horrid. I realised I was going to be on a diet forever. And that I hated the gym. Then I went straight to McDonald's."

"Dreadful," nodded another. "Awful. It made me rail with fury that I hadn't been born a French woman.

"They're born with that knowledge," she continued. "It isn't something they acquire. They just know from Day One, so it's built into their psyche. They don't even feel the weight of giving up unhealthy food – or food altogether – because they've never eaten it."

She gesticulated with a fork. "You think your need to eat unhealthy food because you love it is equal to your need not to get fat. It isn't. You think you deserve to eat the food you want and also to be thin. You don't. French women know that."

A couple of months ago, I had lunch in one of the coolest restaurants in Paris. I had a steak; every other woman in there had a salad. At the time, I thought it was just because they felt like it.

I will never be able to eat like a French woman, I realise, nor exercise like a Californian one. I grew up in Yorkshire and my habits reflect that. So I've decided to take that hangman as my role model, savouring things as if they might not be around again – be that fast food, French food or even the treadmill.

(Full disclosure: I ate fried chicken three times last week. I haven't been back to the gym since.)