Why your brain really craves french fries and donuts in particular

Obviously, you're going to crave french fries and donuts over healthier food options. 

But why is it that we sometimes really, really yearn for these two foods in particular over others that aren't so good for us, like candy or cheese? Well, you can probably blame your brain.

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A study published in Cell Metabolism found that foods combining both fat and carbohydrates are valued higher by our brain's reward system, as opposed to foods containing one energy source.

A group of 206 adults were studied, undergoing brain scans as they were shown photos of snacks, which either contained fat, sugar, or a combination of the both.

The subjects were then given a limited amount of money to bid on their preferred foods, and were found to be prepared to spend more on those which combined fat and carbohydrates.

Researchers say our brains seem to estimate how many calories there are in foods with just fat or carbs, helping to regulate how much we eat. But it's when the two combine where things get tricky.

"Our study shows that when both nutrients are combined, the brain seems to overestimate the energetic value of the food," Dana Small, from Yale University, explained online.

Our tendency toward these foods could also be linked to the facts that fat and carb combined foods rarely exist in nature. The exception is breast milk, which is handy as babies need to learn how to suckle to survive, Small said.

"In the modern food environment that is rife with processed foods high in fat and carbohydrate like donuts, French fries, chocolate bars, and potato chips, this reward potentiation may backfire to promote overeating and obesity," she added.

Our ancestors primarily ate mostly woody plants and animal meat, with processed foods only appearing in the last few centuries. 

"In nature, foods high in fat and carbohydrate are very rare and tend to have fiber, which slows metabolism," Small said. "By contrast, it is very common for processed foods to have high fat and high carbohydrate loads."

Researchers think our brains haven't yet evolved to figure out that we shouldn't be eating these kinds of foods all the time. Maybe we'll get there soon enough.

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