Britain is getting more and more overweight. According to TV foodie Hugh Fearnley-Whittingstall, 25 per cent of us are obese already, and by 2050, if things don't change, that will rise to 50 per cent. If it does, he says, 'the country will be buckling under an epidemic of type 2 diabetes, cancers, amputations, strokes and heart attacks'.

Well he may be right, and he has some ideas about how to fix that. But one of the interesting things about obesity is this: there's often the unspoken thought that people are obese because they are greedy and eat too much. It isn't necessarily true, of course. Some people are just built like that. Others can't exercise for medical reasons. Others don't know how to or can't afford to eat well. There are lots of reasons.

But moralising about weight cuts both ways: if this person is 'bad' because they're fat, I must be 'good' because I'm thin. And this is just as wrong and foolish – but if anything, it's an even more spiritually dangerous delusion. It lets me think I'm OK, when I might not be.

Because, as CS Lewis point out, there is more than one kind of greed. In his wonderful book The Screwtape Letters – instructions from an old devil to a younger one on the best way of tempting his 'patient' – he talks about the 'gluttony of delicacy'. Gluttony is not so much about excess, he says (the book was written in 1942, during wartime rationing); it has become a different kind of vice altogether. Of the mother of young Wormwood's 'patient', Screwtape says: 'She is always turning from what has been offered her to say with a demure little sigh and a smile. "Oh please, please ... all I want is a cup of tea, weak but not too weak, and the teeniest weeniest bit of really crisp toast." You see? Because what she wants is smaller and less costly than what has been set before her, she never recognises as gluttony her determination to get what she wants, however troublesome it may be to others.'

And, Screwtape says, 'She would be astonished – one day, I hope, will be – to learn that her whole life is enslaved to this kind of sensuality, which is quite concealed from her by the fact that the quantities involved are small.'

Today, there's a whole industry devoted to this kind of 'gluttony of delicacy'. The celebrity chefs, the endless cookery programmes on television; the Sunday supplements with their expensive recipe sections; the supermarket magazines with their insistence on just this particular brand of olive oil, the steak from a particular part of a rare breed cow (double points if you can tell your guests its name) and vegetables with zero food miles: Screwtape is rubbing his hands with glee. If we are so choosy about what we eat that a perfectly good cheese from Morrisons turns to ashes in our mouths because it isn't Waitrose's, the chances are that we are succumbing to the sin of gluttony.

Lewis, of course, wrote before TV had really come into its own. All those Bake Offs and Masterchefs had yet to appear. But in Mere Christianity he wrote about sex, and specifically stripteases, saying: 'Suppose you came to a country where you could fill a theater by simply bringing a covered plate on to the stage and then slowly lifting the cover so as to let every one see, just before the lights went out, that it contained a mutton chop or a bit of bacon, would you not think that in that country something had gone wrong with the appetite for food?'

Oddly enough, that's exactly the country we live in. And yes, perhaps something has gone wrong with our appetite for food.

I remember a good many years ago an old man in my congregation, not given to outrage, being genuinely offended at the trend for celebrity recipes that was emerging. He had grown up in the pre-war years, in the countryside, in poverty. 'When I was young, we ate because we were hungry,' he said.

There's every reason why we'd want to try to tackle obesity as a nation. As a health issue, it's a bad thing. But those who want to moralise it and take pride in their own thinness should beware: they are probably gluttons, guilty of precisely the sin they condemn in others, just a bit more fashionable with it.

Mark Woods is the author of Does the Bible really say that? Challenging our assumptions in the light of Scripture (Lion, £8.99). Follow him on Twitter: @RevMarkWoods