A McDonald's restaurant Reuters/Brendan McDermid

Make some passing comment about the dietary intake of the poor and you will invariably be attacked viciously in the press for it. Sometimes unfairly, and sometimes with a little more justification.

Labour shadow foreign secretary Emily Thornberry is the latest to wade into this vituperative minefield, justifying Labour's plan (it has to win an election first of course) to roll out free school meals to all children on the basis that some children are more likely to be overweight than others.

"If you look at poor children now they're not thin, they're overweight," Thornberry told Andrew Marr on Sunday morning.

This, Thornberry added, was down to "poor eating, [and] bad eating habits". Part of a child's education, she said, ought to be about "how to grow a carrot".

It is a testament to the depressing state of political reporting in this country that more time has been spent guffawing about the "growing a carrot" comment than judging whether Thornberry might be making a worthwhile point.

The evidence suggests that the disparity between the dietary prospects of rich and poor children is at least worth talking about. Poor children are more likely to be obese than their more privileged peers. Being a poor child also means you are more likely to grow into a poor adult. According to a recent study, which tracked 20,000 families across Britain, the poorest children were almost three times as likely to be obese as the richest. This gap widened as children got older.

For some, even acknowledging this is to automatically 'patronise' the working class. There is a style of punditry very popular at the moment – elsewhere I've called it the 'prolier-than-thou style' – which finds it profitable to constantly lionise the supposed 'authenticity' of the poor. Everything the latter do, however self-destructive, is intrinsically superior to the prescriptions set down by do-gooders and the 'chattering classes'.

Thus lecturing the poor on 'growing a carrot' nicely lines up a brutal retort about Labour 'looking down on' the people. Furring up your arteries with stodge and grease is supposedly an authentic working class value, along with standing up straight for the Queen or supporting a Premier League football team.

The poorest children are almost three times as likely to be obese as the richest.

Of course, one can sniff out a slightly patronising element in Thornberry's comments without dismissing her broader point. Poor people – both children and adults – are more likely to be overweight. It is not unreasonable for a government – ideally a Labour government – to look into why this might be happening. Obesity increases the likelihood of developing a number of chronic diseases, and unless you accept the increasingly fashionable (but no less self-interested) idea that the poor represent some sort of cognitive lumpenproletariat, then it is worth considering why a lack of money so often equates to an excess of adipocytes.

But as well as cutting through the usual prolier-than-thou tedium, this means taking on those who get it wrong even though they mean well. One of the most striking aspects about contemporary debates around obesity is the extent to which it sidesteps the workplace. An ungenerous interpretation might say that progressives have given up trying to regulate the world of work so they instead throw their energies into regulating what a working person may eat or drink.

One can see something of this in the endless campaigns to slap taxes on everything from tobacco and cheap lager to cans of Pepsi. If only savoy cabbages were cheaper than a Mars bars, or so the thinking goes, you would soon see the queues for greasy breakfasts at branches of Wetherspoon diminishing and Tesco have to put in emergency orders for extra shipments of kale.

I'm exaggerating – but not a great deal. Attempts to wean the poor off junk food with price hikes rest on an only slightly less unpalatable version of the ideology that views poverty as little more than a consequence of 'bad choices'. For the middle class professional, junk food is very often a rational decision representing a sugary pat on the back – a reward carefully slotted into a diet that is otherwise quite healthy.

Attempts to wean the poor off junk food with price hikes rest on an only slightly less unpalatable version of the ideology which views poverty as little more than a consequence of 'bad choices'

For the person subsisting in the burgeoning low pay economy, on the other hand, junk food can be an emotional palliative in the sane way that putting on a sticking plaster might allay the soreness in your feet. I suspect that poor people do not typically 'pig out' because they are weak-willed or uneducated. They know as well as any middle class 'foodie' that a cabbage is good for you and that a packet of chips will lead to the expansion of your waistband. But after working an exhausting day in a dead-end job you don't want to come home and stand in the kitchen for two hours cooking lentils. Who in their right mind would want to do that?

I found this out for myself recently while working for six months in what most people would consider 'low-end' jobs. I had put on about two stone by the end of the process, even though I was eating less frequently than I had been before. The reason I turned from a relatively healthy person into a slightly corpulent one was that regularity of dietary habit is simply incompatible with irregularity of work and income. I still had a head full of learning about healthy eating – I could, if it came to it, survive on £1 a day like one of those people the Daily Mail always manages to dig out who 'prove' that the poor are spendthrifts – but that isn't what you do after a 10-hour shift. Instead you collapse on the bed with a takeaway and feel noticeably better about life once it's inside you.

Overweight adults very often produce overweight children. I am glad Emily Thornberry has at least prompted a conversation about that. But in tackling this scourge, liberals might ask themselves why a wretched and miserable job no longer repulses them quite so much as the behaviour exhibited by the people forced to do such jobs.

James Bloodworth is former editor of Left Foot Forward, one of the UK's top political blogs, and the author of The Myth of Meritocracy.