It is estimated that today’s obesity epidemic costs the global economy about $2tn (£1.6tn) or some 3% of GDP. For individuals, deciding what to eat is a jealously guarded privilege, but for economists obesity is not really about people exercising free-market choice. Instead it is a market failure.

The causes of the epidemic are complex, spanning the social sciences to biology and technology. Consider, for example, the shift towards urbanisation and car transport. By reducing many people’s daily physical activity, these are estimated together to reduce individuals’ need for food by 300 calories a day. So how much less food should the car driver eat to compensate? About one biscuit less a day – a trivial change that only goes to illustrate that few of us really understand the energy needs of our bodies.

In market terms, making a rational choice at the dining table requires people to know how much energy they need and how much they are getting – yet neither of these is known. As any one who has ever tried to lose weight knows, in these matters talk is cheap and advice is unreliable. At various times sugar, protein, fat, starch, fast food, home cooking and snacks have all been held to be responsible for the obesity epidemic.

Back in the 1950s, the dominant theory was that eating fat was responsible for making you obese – and by sating appetite, sugar could help to reduce weight gain.

However, food companies prefer to deflect rhetoric about poor diet being the primary cause of obesity, and instead promote messages focused on exercise and other factors – a phenomenon termed “leanwashing”.

Nonetheless, the political spotlight is now on sugar, with “environmental” changes proposed to reduce the appeal of sugary foods, such as warning labels and nutritional information panels. There’s serious talk of taxing them, in the way that tobacco has been. None of this is supported by any real evidence, but certainly new taxes are always popular with governments. (It’s pretty obvious, after all, that there are plenty of thin children who enjoy sugary foods, and plenty of overweight people whose tastes lie elsewhere.)

Prof Kevin Fenton, national director for health and wellbeing at Public Health England, says there are “practical solutions”, by which he means a de-sugaring of foods such as cakes, biscuits and puddings. But these foods are not sugary by accident – they are sugary because that’s what we like about them. It’s like salted crisps having salt removed … but yes, the health industry has pushed that one too.

Solutions based on labelling of foods by calorific content to improve consumer choices belie both human psychology and the complexity of the body’s digestive mechanisms. As a result, it is futile leaving a solution to the food and drink industry, or on relying on “education” to correct consumer behaviour.

However, consumer choices are being skewed by government actions, such as massive programmes of agricultural support that actually favour fast foods over healthy eating. Billions of dollars go towards subsidising junk foods, through farm subsidies for producing ingredients such as soya and high-glucose corn syrup.

Obesity affects poor households far more than their richer neighbours – and the cost of eating healthily is a very practical reason why.

And, as the US economist Richard McKenzie has pointed out, much of the rise of obesity is precisely a consequence of free-market economics. For example, fast food has become cheaper, in part because of mechanisation and in part because the workers producing it are paid less and less. At the same time, the economic forces that propelled 1960s women away from the kitchen and into jobs also propelled families towards processed foodstuffs and eating out … or just snacking.

When markets go wrong, governments need to step in. Agricultural support should be switched away from junk foods and towards producing healthy ones. The low wages and poor training that fast-food outlets rely on should be made a thing of the past, and tax incentives (such as zero VAT) should support the labour-intensive production of “real food” in cafes and restaurants.

Market corrections, rather than punitive moves aimed at individuals, are the only way to tackle what has become, in economic as well as in social terms, a very real crisis.

Martin Cohen is editor of the Philosopher. His recent book, Paradigm Shift (Imprint 2015), discusses ‘how expert opinions keep changing’