The human brain is wired to prefer foods that are packed with fat and sugar, according to new research.
Its reward centre values them higher than those containing just one or other – but not both.
The finding sheds light on how the processed food industry cashes in on the human craving for unhealthy diets.
Senior author Prof Dana Small, director of the Modern Diet and Physiology Research Centre at Yale University in the US, said: “Surprisingly, foods containing fats and carbohydrates appear to signal their potential caloric loads to the brain via distinct mechanisms.”
The study of more than 200 adults with a normal BMI (body mass index) found they were willing to pay more for foods that combined fat and sugar.
What’s more, the fat-carb combo lit up neural circuits in the striatum in the prefrontal coretx of the brain more than a favourite, sweeter or more energy dense food – or a bigger portion size.
Prof Small said: “Our 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.”
The findings, published in the journal Cell Metabolism, support the idea these kinds of foods hijack our body’s inborn signals governing consumption.
Prof Small said: “The biological process that regulates the association of foods with their nutritional value evolved to carefully define the value of a food so organisms can make adaptive decisions.
“For example, a mouse should not risk running into the open and exposing itself to a predator if a food provides little energy.”
It could help explain brain-body mechanisms underlying the genetic predisposition for obesity, eating in the absence of hunger and difficulty losing or keeping off excess pounds.
The participants, all in their 20s, underwent brain scans while being shown photographs of familiar snacks containing mostly fat, sugar or a combination of them. They were given a limited amount of money to bid for them.
Our hunter-gatherer ancestors ate mostly woody plants and animal meat, the researchers said.
Prof Small said: “In nature, foods high in fat and carbohydrate are very rare and tend to have fibre, which slows metabolism. By contrast, it is very common for processed foods to have high fat and high carbohydrate loads.”
After the domestication of plants and animals and the development of grain and dairy production around 12,000 years ago, opportunities to consume fat and carbohydrates together increased.
But processed foods such as doughnuts, which could contain 11 grams of fat and 17 grams of carbohydrate, have only been around for 150 years, not long enough for us to evolve a new brain response to them.
Scientists believe our past experience with the nutritive properties of carbohydrates releases the feelgood chemical dopamine in the brain through an as-yet-unknown metabolic signal.
These kinds of signals seem to help regulate what and how much we eat. The researchers believe the simultaneous activation of fat and carbohydrate signaling pathways launches an effect the human body has not evolved to handle.
Consistent with this suggestion, mice given access to fat or sugar alone regulate their total daily caloric intake and body weight. But given unrestricted access, they quickly gain weight.
Prof Small said: “These results provide the first demonstration that foods high in fat and carbohydrate are, calorie for calorie, valued more than foods containing only fat
or carbohydrate and that this effect is associated with greater recruitment of central reward circuits.”
She added that it “may be one mechanism by which a food environment rife with processed foods high in fat and carbohydrate leads to overeating.”