There may be no elusive diet secret but in 2014 Sarah Vine realised that weight loss is much easier for some people than others

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This was going to be The Year. Mid-December, and I was going to be jaw-droppingly svelte. Shimmying about the place in something fabulously small and soignée. Smiling gracefully as the congratulations rolled in - "Oh my, I hardly recognised you!" "How much weight did you say you had lost?" "Two stones? Really? That's fantastic!"

It wasn't for want of trying. James Duigan, Mr Bodyism himself, was funny, charming, endlessly encouraging. Then there was the lovely Ollie, who chased me around the park with kettle bells and elastic bands  and all manner of strange fitness contraptions. Handsome Luke at Ten Pilates, ever patient, ever tolerant.

Amelia Freer , she did her level best. Sent me daily deliveries of her delicious food : delicate poached salmon fillets, lovingly soaked seeds, little notes of encouragement. The nice people at Victoria Health gave me some raspberry ketones. I spent a heavenly week at Grayshott , eating tantalising healthy titbits and nodding sagely as I learnt all about good fats and bad fats and the evils of sugar.

And yet here I sit, lardacious as ever. Well, not quite. Ladies and gentlemen, in 2014, under the tutelage of some of the most effective and experienced experts in the business, I have lost precisely three and a half kilograms.

I have no one to blame for this appalling failure but myself. I am lazy, ungrateful and very possibly criminally irresponsible. I am the sort of person who shows up in government statistics about fat people costing the NHS a fortune.

And I'm a hypocrite too: I am the co-founder of a health and wellness website, for heaven's sake. Surely on that basis alone I should be a size 10.

The thing is, I have a funny suspicion that, bad as I am, I am not alone. That my behaviour is not as abnormal as, on paper, it would appear.

In fact, the more I think about it, the more convinced I become that being a healthy weight is as much to do with the mind as it is to do with the body.

Not a day goes by when I don't wish I could be two stone lighter. I flick through my wardrobe: nope, too fat for that; can't wear those trousers; that shirt gapes at the bust; that dress pulls on the back. I have to resist the temptation not to spend my days in giant black sacks.

And yet if I desire slenderness so much, why then does it continue to elude me?

Is it because secretly I don't really care? Am I perhaps just pathologically lazy? Or is my inability to slip into a pair of size 12 Joseph leggings simply a reaction to the relentless fat-shaming that seems to dominate today?

If there is one thing I do know about the psychology of weight it's that the more you nag a fat person about their size, the hungrier they become. That is because very often being overweight - especially in young girls - is not a sign of indolence or greed; it is a symptom of unhappiness.

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I understand comfort eating  because I do a lot of it myself. Food can ease anxiety in me like nothing else. I would far rather tuck into a piece of hot buttered toast with Marmite than smoke a cigarette or have a vodka and tonic. It is the best emotional anaesthetic I know.

If food is your principle source of solace, then you have a real problem. For while it is possible to give up cigarettes and alcohol and enjoy nothing but positive effects, humans cannot give up food and remain alive.

That is why people who suffer from anorexia also tend to be binge eaters. They fear food in the way an alcoholic fears a shot of vodka. They're terrified that if they have just one, they won't be able to control themselves. In the end they feel the only way to overcome their compulsions is to stop altogether. Which of course is not possible.

Anorexia is really just an extreme form of food phobia; obesity, meanwhile, is at the other end of the scale: a food addiction. Both are equally damaging; and in both cases the symptoms may be physical, but the disease is mental.

That is why cajoling the overweight into exercise programmes and lecturing them about dietary regimes is about as effective as force-feeding an anorexic.

Their conscious mind understands that their behaviour is abnormal; but their subconscious can't reform. Most people with weight issues fall somewhere in between the extremes, myself included. And every year, around now, we tell ourselves that this time it's going to work. That we will embark on the 5:2 , or the High Intensity Training , or whatever it happens to be - and we will stick to it. But deep down, we're lying. We know that, come February, we'll have fallen off the food wagon - again. And what little we've lost will start creeping back on again.

Those people I know who do succeed in staying slim well into their 30s and beyond all have one thing in common: they have absolutely no emotional engagement to food. For them it is simply a fuel, their stomach a tank, to be replenished when empty, like that of their car.

They are the ones that interest me. I want to know which switches in which part of my brain I need to flick in order to be able to think like them.

Meanwhile, I wish you a happy Christmas. Enjoy those mince pies.