The immune system can respond to a fast food diet in much the same way it does to a bacterial infection, according to a 2018 study on mice, raising new questions about just how damaging regular trips to burger and pizza chains could be to our health.

Mice fed the equivalent of a "Western diet" high in saturated fats, sugar, and salt for a month, with nothing in the way of fresh fruit, vegetables, or fibre, were shown to increase the number of immune cells in their blood, just as they would if they'd been hit by a microbial infection.

What's more, this aggressive state of alarm that fast food triggers could stick around for the long term, said the international team of researchers – that's based on recent research into the way our immune systems can remember aspects of past battles they've fought.

"The unhealthy diet led to an unexpected increase in the number of certain immune cells in the blood of the mice, especially granulocytes and monocytes," said one of the team, Anette Christ from the University of Bonn in Germany back in 2018.

Those white blood cells pointed the scientists towards certain genes that were activated by the mouse diets, genes containing progenitor cells – the types of cells responsible for raising up an immune cell army.

That genetic breadcrumb trail matters, because it's these progenitor cells that have previously been found to have a kind of memory in dealing with biological attack.

In other words, once the body has started to react to a fast food diet, returning to a healthy eating regime may not be enough to completely undo the changes, and that's got some implications for our overall health.

Indeed, when the mice went back to their regular cereal diet after a month, the inflammation disappeared – but the genetic reprogramming that kept the mice more sensitive to a future attack stuck around.

"It has only recently been discovered that the innate immune system has a form of memory," said one of the researchers, Eicke Latz from the University of Bonn. "After an infection, the body's defences remain in a kind of alarm state, so that they can respond more quickly to a new attack."

It's normally an infection that produces such a response, but here it was the equivalent of a fast food diet for mice. It means inflammation – and the problems associated with it, like type II diabetes – could be more easily triggered in the future.

For now we've only got evidence of this in mice, even if the animals are chosen for their genetic similarity to human beings.

If the same sort of reaction is happening inside our bodies then it's more evidence for the link between an unhealthy diet and health problems such as type II diabetes, obesity, and issues with the heart, say the researchers.

The scientists also identified a "fast food sensor" inside the immune cells, based on examinations of 120 mice. A signalling system called the NLRP3 inflammasome seems to be the one on the look-out for this kind of diet change, though we don't yet understand how it works.

It's another warning sign that even as the rate of deadly infections and viruses drops, we're balancing out these health improvements with bad diets and a lack of exercise, say the researchers.

"These findings therefore have important societal relevance," said Latz. "The foundations of a healthy diet need to become a much more prominent part of education than they are at present."

"Children have a choice of what they eat every day. We should enable them to make conscious decisions regarding their dietary habits."

The research was published in Cell.

A version of this article was first published in January 2018.