Brain scans were conducted on 206 adult test subjects while being shown photographs of snacks containing mostly fats, mostly sugars, or a combination of the two. Subjects were allocated a limited amount of money to bid on first choice foods, it was observed that subjects were willing to pay more for foods that were carbohydrate and fat combinations. Neural circuits in the reward center of the brain were observed to be lit up more than when seeing a favorite food, larger portion size, or a sweeter more energy dense food when seeing the food that were combinations of carbohydrates and fats.
Past experience with nutritive properties of carbohydrates releases dopamine in the brain through an unknown metabolic signal, signals of this sort help to regulate how much and what we eat. Simultaneous activation of carbohydrate and fat signaling pathways are theorized to trigger an effect that human physiology has not yet evolved to handle. Rodents given access to carbohydrates or fats alone regulate their total daily intake and body, yet given access to the combination they quickly gain weight, which supports the theory.
Ancestors were hunter and gatherer types, the carb and fat combinations were very rare and tended to have fiber which slows metabolism; contrasting that today common processed foods fit in that combination. Domestication of plants and animals and development of dairy and grain production 12,000 years ago made opportunities for that combination more possible with items such as donuts that have only been around for 150 years which is not long enough for the brain to evolve a new response for them.
