It doesn’t really matter. Whatever you choose, your brain wants the oily, starchy fries. Or a doughnut. Or a chocolate-chip biscuit.

There seems to be something unique about these ‘combination’ foods, which mix fat and carbohydrates in a way rarely seen in nature.

They overstimulate our brain, which unconsciously craves them, no matter our conscious food preferences. Possibly, this craving is so strong it may lead to addiction.

“There is just something special about the combination of fat and carbs,” says Professor Robert Boakes.

This is the remarkable conclusion of an important new study from Yale University. It demonstrates for the first time that our brain responds in a very odd manner to ‘combo’ foods.

Our brains have two separate 'systems' for dealing with fatty food and carbohydrate-rich food. The brain uses each system to decide whether it wants to eat a particular food.

But that breaks down when the brain is confronted with modern snack foods that combine fat and sugar. Combo foods are rarely found in nature so our brain has not evolved to deal with them.

"Essentially this research shows that we have separate mechanisms in the brain that can sense fat and carbs. What they show is simultaneous activation of both of them produces a much greater reward effect than either alone,” says Monash University Associate Professor Zane Andrews.

"It’s like - why do we like chips and dip? Or wedges and sour cream? What we’re doing is hijacking fat and carb sensing to give us a huge reward response."

To prove this, Professor Dana Small’s team wired 206 people up to brain scanners, and then showed them images of fatty, sugary and ‘combo’ snacks.

Every time an image of a snack that was both fatty and carby came on the screen, the reward centres in the subjects’ brains lit up like fairy lights on Christmas Eve.

No matter if they said they liked them or not. No matter how many calories they had. The brain craved them.

"Our study shows that when both nutrients are combined, the brain seems to overestimate the energetic value of the food," says Professor Small.

The researchers suspect the brain has separate regions for dealing with carby and fatty foods. When carbs and fats are combined, the two regions activate at the same time, generating a super signal to consume.

Another region, the Striatum, lit up too. This region is linked to habitual behaviour. Addiction.

“What they are showing is there is a distinction between liking something and wanting something – and this comes from the addiction literature,” says Professor Boakes, an obesity scientist at the University of Sydney who was not involved in the study.

“Smoking is a good example. People can crave a cigarette, even if they don’t like it. And this research is saying food craving is particularly strong for the combination of fat and carbohydrate.”

Importantly, the study corrected for how much each adult said they ‘liked’ a particular food. Shown in the study are the raw, unfiltered desires of our unconscious. In some cases, participants said they preferred one food – but their brain told a different story.

Professor Small’s study is published in Cell Metabolism on Friday.

smh.com.au