I’ve noticed for some time now that thin people, genuinely slim ones, have a secret loathing of fatties. Kindly though they may otherwise be, the sight of rolls and overhangs, jowls and bulges, makes them angry. One extremely thin woman I know finds it hard, she told me, even to have fat friends. Another shivers with horror if she catches some poor podge in the act of wolfing a treat. It’s not an aesthetic affront, she says, so much as a moral one. Where’s their willpower, where’s their grit?

It’s hard to argue with a censorious thinny. We all know, these days, that there’s no excuse for being a lardarse. Faulty glands, slow metabolism — all tripe. The brutal truth is that if you’re fat it’s because you eat too much. It’s a simple calculation. A very sad calculation for those of us fond of cake.

And something must be done about the extraordinary rise of fatties. In less than a decade’s time, according to a study in the Lancet, a fifth of all adults on the planet will be obese. A fifth! And rising. As a child I had a book, Fattypuffs and Thinifers, written in the 1930s by a Frenchman called Maurois. The book was set in a world inhabited by two warring tribes, the benign, obese Fattypuffs and the mean and angular Thinifers. I increasingly see this not as fantasy but prophecy — especially for the UK.

We may be sliding down the inter-national academic league tables, but in the Fatty-puff stakes we really shine. By 2025, a third of British adults will be Fattypuffs; the most obese nation in all of Europe, in or out of the EU.

I feel for the Fattypuffs. I’m a Fattypuff by disposition. I say yes to seconds. My heart sinks at the thought of salad and sings at the prospect of profiteroles. When my lean and glamorous friends lord it over the poor Puffs, when they assume that it’s not just their figures but their wills that are superior, I long to puncture their self-belief.

So it was exceptionally pleasing to find out that science has finally granted Fattypuffs a boon — a genuine excuse to throw at the censorious Thinifers — and that in this discovery there may well be the beginnings of a solution to the problem of the world’s expanding girth.

As it turns out, there is a hardwired reason that some of us run to fat, though it’s nothing to do with metabolic rate. There’s a gene, a variant of the FTO gene, which is highly correlated with obesity. We’ve known that for a while — but what’s new is that we have an idea of how it works. Scientists at UCL, the Medical Research Council and King’s College London have watched images, in real time, of the brains of people with the fat gene. They’ve watched as the Fattypuffs eat, and they’ve discovered that not only are they naturally hungrier than those without the gene, but that they must eat much more to feel full.

The fat gene means a higher level of the ‘hunger hormone’ ghrelin in the blood, which means Fattypuffs feel peckish again sooner. What’s more, fruit and veg just don’t cut it for the fat-gene gang. Their brains only signal real satisfaction if there’s something calorific in the offing. Just a photo of a bacon double cheeseburger makes their greedy brains begin to glow with excitement.

Now I know what those judgmental Thinifers are thinking. They assume the porkers have made themselves that way; that they’ve eaten so much that their brains crave burgers. Not so. That’s not the way it works. Even a slim man or women with this variant of the FTO gene will feel the siren call of cheesecake. The gene evolved to help its hosts survive tough times by gorging during periods of plenty. It virtually forces junk food into its human host.

Oh Fattypuffs, can you see how delightful this discovery is? It stops the size 6 sneerers in their shallow tracks. No more lectures on grit and moral fibre, no more crowing. They just don’t feel the same temptation, so they can’t take credit for their own restraint.

It might also help clear up several mysteries, the first of which is the great and total failure of all diets anywhere to work. They don’t. It’s true. None of them. It was reported, once again in the trusty Lancet, though no paper picked up on it. I suppose it’s the definition of a story no one wants to hear.

‘Although low-carb diets work better than low-fat diets,’ said the report, ‘no diet works particularly well.’ This was a serious study, one of the most extensive ever done into dieting. It was a meta-analysis of 53 separate long-term research projects involving 68,000 people. Consider this alongside the fat gene discovery and it suddenly makes sense. If even your genes are crying out for carbs, then food will find a way. The numbers add up, too. Roughly one in five of us worldwide have the troublesome variant of the FTO gene — and almost half of all west and central Europeans. Doesn’t that fit beautifully with those projected obesity numbers?

Very satisfying but very worrying, too. We can’t all just morph into the world of Maurois’s book; divide completely into tribes of different sizes. So what to do? Thinifers are all for public health initiatives, for fizzy pop taxes and world courgette day, a bust of Jamie Oliver in every school. But that diet study was conclusive. No government ‘task force’ will ever have better data or more assiduous research, and if diets don’t work for people actually motivated enough to try them, what chance does any campaign have? You can lead a Fattypuff to water, but you can’t make him drink it.

The only answer is to face the gene. There are drugs being designed that suppress the hungry hormone, ghrelin — and those would be a better use of public money than any poster campaign. Perhaps Fattypuffs themselves should be taught to understand their hungry selves. Only when Fattypuffs understand how they differ from Thinifers will they be able to buck their DNA.

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