Why is anyone surprised that a Conservative government has yet again caved in cravenly to industry and produced a shaming non-policy on childhood obesity? That’s what Conservatives do in any clash between business and the environment or general wellbeing, a bias towards profit neatly disguised with a pretended objection to “the nanny state” taking over from personal responsibility.

Labour agonised for far too long over banning smoking in public places. Even Dublin and Glasgow had proved it could be done: that changed cultural attitudes towards smoking overnight. The Tories would never have done it.

The obesity crisis is galloping ahead, exactly as predicted a decade ago, with a third of children leaving primary school overweight. This is far harder to tackle than smoking. Forcing food manufacturers to cut sugar, fat and salt should be the very least the state should do – but it would still be only one step in the right direction. Banning the advertising of junk food during breaks in children’s programming would help, alongside simple labelling, instead of the deliberately baffling small print on kJ and kcal. More sport in school? Good idea, when so many playing fields are sold off – school sport was first to go under Michael Gove’s regime at education, and the Olympic obsession with gold medals leaves no “legacy” for vanishing community pools and sports grounds. Watching Laura Trott from the sofa doesn’t impel us to run to that Zumba class.

Attitudes towards food, our use and abuse of this essential, run deep emotionally, psychologically – and socially. Obesity is no one’s choice, as everyone wants to be thin: young children now worry about body image, and rates of anorexia – obesity’s evil twin – are rising.

‘Watching Laura Trott from the sofa doesn’t impel us to run to that Zumba class.’ Photograph: SWpix.com/Rex/Shutterstock

From Charlie and the Chocolate Factory to Harry Potter, heroes are skinny and lithe, while fat children are mean and selfish. A multibillion-pound industry trades on our wish to be thin, in magazines, slimming products, diet fads and endless advice from pseudo-nutritionists. No public health campaign could begin to compete with the message sent out every day in every way that thin is beautiful, and fat is ugly, undesirable and a sign of moral uselessness. That’s not a nudge, it’s a daily knock on the head with a cudgel. “You can’t be too rich or too thin,” said Dorothy Parker.

What no one says explicitly enough is that fat is a social class issue. Most of the seriously obese are poor. This is tiptoed around, but those with a body-mass index in the red zone, those whose children risk swelling up at a young age, in danger of losing limbs and eyesight to diabetes as they grow up, are the poorest. The hyper-rich are called “fat cats”, but privilege is usually thin and sleek, its body well-exercised by gyms and personal trainers on diets of kale and goji berries. And even then, God knows, to be middle class doesn’t guarantee thinness, as every ageing decade adds to girth. But poverty is a marker for most obesity.

Reports suggest the poor find it harder to afford fresh fruit and vegetables, home cooking, and of course swimming pool and gym fees, ballet and judo lessons for their children. All true, but that’s only part of the story. As some smug middle-class people remind us, well-educated penurious families can feed their children well on lentils, but that’s irrelevant to life at the bottom of the social heap.

To be obese signifies being poor and out of control, because people who feel they have no control over their own lives give up. What is there to struggle for if there is no chance ever of a job that will pay beyond bare subsistence? With no prospects, drinking, smoking and eating the wrong things become small compensations in lives with very little else. Being out of control becomes a mindset ever harder to climb out of. Why defer small gratifications when there is no greater reward on offer?

Most people have social incentives not to give in to temptation – and even then we often fail – but those who have nothing are likely to give up more easily. From every social signal, poor children know from their first day at school that they are low in the pecking order and that gap between them and the rest widens with every school year, as their self-esteem falls away.

Those on the margins, excluded from life’s best things, eat themselves into an early grave. It is inequality and disrespect that make people fat. Look at the historical figures: obesity took off in the 1980s, up more than 400% in the years since inequality exploded. The link between inequality and obesity is stark around the world: among developed nations, America is the most unequal society and the fattest, with Britain and Australia next on both scores. Europe is better and the Scandinavian countries best of all.

There may be social policy reasons, as social democratic governments invest in picking up family problems earliest. But the big picture is this: where the status and income gap in a society is smallest, so are the waistbands. Turn to that great classic of inequality research, The Spirit Level: Why Equality Is Better for Everyone, by Richard Wilkinson and Kate Pickett, and chapter seven documents how wider income gaps mean wider waists.

That dispiriting fact could be an invitation to nothing-can-be-done fatalism, when of course governments should do all they can to stop the food industry making us all needlessly fatter, with the NHS picking up the bill. But yet again, social dysfunctions spring from the unequal state of the whole society.

There is a nasty tendency to fat-shame the poor, a vicious cycle in which people are blamed individually for both their obesity and their poverty.

But the social facts suggest Britain would get thinner if everyone had enough of life’s opportunities to be worth staying thin for. Offer self-esteem, respect, good jobs, decent homes and some social status and the pounds would start to fall away.