For far too long we’ve been getting our approach to food and health badly wrong.

We’ve been led to believe that healthy eating is solely a personal problem - a willpower issue. People are told they need to do better, work harder and focus on their individual food choices.

It’s clear this approach hasn’t worked. Obesity rates have doubled since 1990 and today three quarters of over 45s are overweight or living with obesity. This is a big deal for our health - it’s the second biggest preventable cause of cancer and is a major risk factor for diseases like type-2 diabetes.

So what’s driving the problem? It’s not as if our personal willpower has declined since the 90s - over the same period we’ve managed to drink and smoke less.

The truth is it’s the food around us that’s changed. Over time, the food that is readily available and affordable to people has got progressively unhealthier.

But we can change this trajectory.

The thing is we don’t need to make radical changes to see improvements to obesity levels. If we can reduce the average daily calorie intake by just 8.5% for those living with excess weight, then we cut obesity in half.

To do this we need to shift our focus from what individuals can do to other parts of the food system: manufacturers, retailers and regulators. Everyone has a role to play in making it easier and affordable for people to eat more healthily.

That's why today Nesta is publishing new research that proposes mandatory health targets for the UK’s largest supermarket brands.

After analysing the data from 36 million shopping transactions, we calculated the nutritional value of the average kilogram of food sold by 11 of the leading food and grocery retailers. This allowed us to rank each retailer according to the healthiness of their sales, awarding a score based on calorie content, salt, sugar and saturated fats and so on. Averaging all the scores, we rated the sector as a whole as 67 out of 100.

Translating that into our everyday diet, we estimate that if we could raise the supermarkets’ average healthiness score by just two points, it would cut our daily excess calories by around a third. That’s about 80 calories per day - equivalent to a chocolate biscuit.

After three years we estimate that the policy could help 4 million people move to a healthier weight and save around £20 billion every year. That number may seem improbably large. But that’s what happens when you have improvements across such a huge proportion of what is sold nationally. Lots of small changes lead to huge benefits. And small changes are more sustainable in the long term.

We think it’s an achievable goal but only through a coordinated effort. We need every major retailer in the country to pull in the same direction at the same time, which is why we're calling for all major food retailers to have the same mandatory health target to improve the healthiness of the food they sell.

Mostly it would involve retailers making different decisions about what's marketed, stocked and promoted. Many are already working to improve the healthiness of their product ranges. Targets would incentivise large retailers to make different decisions about what they are nudging us to put in our trolleys: changing store and online layouts to avoid pushing us towards unhealthy products, purchasing and stocking a wider range of affordable, healthy food, and using advertising and promotions like buy one get one free for healthier items instead of junk food.

In order to incentivise this shift we must set targets that have teeth. They should be set by government with penalties for retailers who don’t meet the goal.

In making these changes it’s vital that healthy food is affordable, especially during a cost of living crisis. An independent economic analysis found that these improvements are possible without increasing the average spend per basket. By setting a target, but not mandating how retailers hit it, we allow the market flexibility and freedom to find their own solutions - without hurting their business or their customers’ pockets.

This policy would also help move the focus from the individual to the food environment that surrounds us.

It would be an important change to our food system - directly targeting the largest source of what we eat. The core idea, to incentivise many small improvements, could be applied to manufacturers, restaurants, takeaways and school food. Most importantly it would quantify what good looks like, so the different parts of the system are all using the same goal for improving public health.