A few weeks ago I found myself being filmed in a New Zealand supermarket searching for a healthy meal for four. The Kiwi equivalent of The One Show wanted me to help them demonstrate how much more expensive it is to cook a nutritious meal than to eat out in a well-known hamburger chain. In fact, I was able to buy all the ingredients for a good quality, chicken stir-fry for half the price it would have cost to buy four basic fast food meals. This seemed to come as a shock to the show’s presenters and, I would guess, to a fair number of viewers since it is widely believed that burgers and chips are a cheaper alternative to wholesome, home-cooked dinners.

Comparing the price of a fast food meal to the ingredients of a homemade dish is, to be perfectly candid, a bit of a cheat. What about the opportunity cost of preparing the meal? What about the financial cost of heating the oven or taking the car to the supermarket? Even if we had taken those costs into account, I suspect that the well-known hamburger chain would still have lost the battle of the bargains, but it must be conceded that the experiment was not entirely fair.

Comparing the price of ‘healthy’ and ‘unhealthy’ uncooked food in a supermarket, on the other hand, is extremely fair. Again, it is widely believed that the healthy stuff is more expensive and this perception is strengthened by headlines such as ‘Healthy diet costs three times that of junk food’. Upon closer inspection, such claims are only tenable if you measure the cost of food in a rather peculiar way and if you ignore the word ‘diet’.

The study that inspired this headline was published in PLoS One this week. It didn’t look at the cost of a healthy meal, let alone a healthy diet. It looked at how much you would have to spend to get 1,000 calories out of various food products. By this method, lettuce—one of which I bought yesterday for 49p—comes in at a whopping £16.45 whereas energy dense foods like doughnuts and jam ‘cost’ less than a pound. Voila! Healthy food is more expensive than unhealthy food.

To see the problem here, we need to ask how foods come to be defined as ‘unhealthy’ in the first place. The study in question relies on a Food Standards Agency test which is used to decide what can and cannot be advertised on children’s television. One of the key criteria is calorie content. The unhealthy foods have lots of energy in them and this is measured by how many calories there are in 100 grammes of the product. The more calories, the more likely it is to be placed on the naughty list.

Calories are not unhealthy per se, of course, but the FSA’s concern is that eating energy-dense food is more likely to lead to obesity than eating low-calorie food. That’s all well and good, but it doesn’t give the ‘healthy’ food much chance of ever being cheap. Once you’ve decided that the price of food is dictated by how much of a calorific bang you get for your buck, low calorie food becomes expensive by definition. It is a system that guarantees that a cauliflower will always be more expensive than a pizza. A lettuce would have to be sold in the shops for less than 2p for it to become cheaper than a sponge cake by this measure.

Defining high-energy food as unhealthy and then defining cheapness as being ‘food with lots of energy in it’ is tautologous. You might as well declare heavy objects to be cheaper than light objects on the basis that they cost less per ounce. It would be true, but not very informative.

Low calorie foods are obviously not the best choice if you are in need of energy. Complaining that people would have to buy lots of them to get a thousand calories is to turn their virtue into a vice. It would undoubtedly be expensive to meet your recommended daily calorie allowance by eating lettuce, but it would also be horrible and probably impossible. It is precisely because lettuce is low in calories that we buy the awful, tasteless stuff. We then put it on a plate with other food to create a ‘meal’ and it is the cost of the meal that matters.

Food prices have certainly risen in recent years—by 8.6 per cent in real terms since 2007, according to DEFRA—but they are still lower than at any time before 2002, and the price of fruit and veg has actually fallen in the last year. To claim, as the Independent has, that ‘Eating well is increasingly become the preserve of the rich’ borders on the histrionic. Your local supermarket will still sell you a cauliflower for 89p, a bag of potatoes for £1.50 and a kilo of rice for 40p. If people are not eating as healthily as the Food Standards Agency would like, it is not because we feel unable to get our day’s calories from the vegetable aisle. As the obesity figures show, getting enough calories is the least of Britain’s problems.