There is a huge problem with obesity in Britain. It affects many aspects of our lives, and it is crippling the NHS as more and more people are diagnosed with type 2 diabetes and other preventable dietary-related issues.

Among poorer communities, those struggling to meet high rents, living in substandard accommodation, or working zero-hours contracts and living from one pay day to the next, the problem is even worse.

And it’s not because of those in poverty “think in a different gear”, as Jamie Oliver has been quoted as saying. While Oliver is trying to move beyond patronising attitudes, such a suggestion can still lead to wider misconceptions. The popular narrative surrounding those in poverty, and what they eat, seems to come from a place that is “other”. Us and them. The educated and the stupid. This is how poverty is seen and reported, and it is unfair.

The majority of those struggling and living in poverty are in work; they are not avoiding healthy food because they prefer junk; they are not avoiding broccoli because they’re too stupid to cook it. This problem has very little to do with a fabled “working-class mentality”. Poor people are not obese because they are lazy, or simply because they eat too much (although some do), rather because the price of good wholesome food is beyond their financial reach.

Weekly, I have to feed six to seven people three meals a day. I do it on about £50 a week, sometimes £65, sometimes less than £30. It all depends what I have left in the bank after I have paid everything I need to pay.

I can’t afford to cook as I wish to do. It’s a big difference, and one we need to address

Some weeks our plates will be bright and colourful, filled with “good things”. Sometimes I get lucky and can find marked-down fresh produce, often near its end-use date, that I can put to good use. A reduced butternut squash has often saved our plates from tedium. Freezing food is good – great, in fact – but I only have a small freezer and I can’t stock up for more than a few days. Our meals are simple, often plain, but they fill us up. They are, in the main, healthy, but I can’t pretend vegetables don’t cost more to put on the plate. There are many times they get left off entirely; they are an addition – a luxury item – we have to forgo if we are going to eat at all.

The only thing stopping me from cooking wonderful meals packed full of vegetables and healthy things is cost. I can cook, so can all of my children. I can’t afford to cook as I wish to do. It’s a big difference, and one we need to address, rather than assuming the “poor” occupy another planet. We live in the same world and we don’t think in a different gear, but we do face an unsettling reality of not having a spare 60p to buy a cabbage – an addition to a plate which, while good for us, will not fill a stomach like potatoes or rice.

“What you see is parents who aren’t even thinking about five fruit and veg a day: they’re thinking about enough food for the day,” Jamie Oliver said this week. This is something I have addressed, writing for the Health Foundation: “For families on the breadline, eating healthy food is secondary to eating at all.”

Value brands are the order of the day – not fast food (have you seen the price of a pizza or a kebab? I can’t afford that!) – but things that cost little in energy usage, and can be cooked in an oven in 25 minutes. Stuff I know will be eaten. Risk-free food I know won’t end up in the bin. When I have just £2 in my pocket, it’s not the time to experiment and find out whether my 13-year-old would appreciate a quinoa-and-aubergine bake.

I can’t afford to take risks with food, because there is literally nothing else to offer if the new food is disliked. Children in poverty aren’t fighting obesity because of a lack of willpower (although it can be a factor, just as it is with those who are middle class), but because the only foods their families can afford are often filled with bad things.

Good food, and being able to afford to cook it, is the domain of the wealthy. To say families should cook chickpea stews or slow-cook over a long period, which will cost more, is patronising in the extreme. It discounts the very real factors that prevent healthy diets. From energy costs to a lack of cooking facilities, from education to simple risks that cannot be taken, there’s more at play than the poor not having the same logic as the perceived middle class.

It’s not a class problem. It’s a financial problem. We all want the best for our children. We just need to be able to afford to put the building blocks in place.

• Kathleen Kerridge is an author of LGBT fantasy fiction