Researchers believe they've discovered the reason why hot fried chips or a doughnut are hard to resist for many people.
Foods containing both fat and carbohydrate overstimulate the reward centre of the brain, making them appear more rewarding than foods containing only fat or carbohydrate, according to a study published in journal Cell Metabolism.
The Yale University researchers say how the brain biologically reacts to the combination of fat and carbs could help explain how the obesity epidemic took hold.
"In nature, foods high in fat and carbohydrate are very rare and tend to have fibre, which slows metabolism," said senior author, Professor Dana Small, director of Yale University's Modern Diet and Physiology Research Center.
"By contrast, it is very common for processed foods to have high-fat and high-carbohydrate loads," she added.
For the study, 206 adults underwent brain scans while being shown images of foods that were carbohydrate-rich, fatty, or both.
Participants were then asked to estimate each food's energy density/calorie content and say how appealing each was; they also stated how much they'd be willing to pay for each.
Surprisingly, the scans showed people's brains reacted differently to images of foods containing both fats and carbohydrates to those showing only fat or only carbohydrate.
Prof Small said participants were very accurate at estimating calories from fat and very poor at estimating calories from carbohydrate.
"Our study shows that when both nutrients are combined, the brain seems to overestimate the energetic value of the food," Prof Small said.
The authors conclude that this reward signal generated by foods high in both fat and carbohydrate "may be one mechanism by which a food environment rife with processed foods high in fat and carbohydrate leads to overeating".