Both sugar and fat activate the reward system in the brain but they do so using different signalling pathways.
Researchers from the Max Planck Institute in Cologne have found that when the nutrients are combined in the same food, however, the reward effect is intensified and even overpowers the feeling of satiety, leading people to overeat.
The study, which was conducted in collaboration with researchers from Yale University in Connecticut, suggests that the modern food environment promotes overeating in this way and facilitating the transition to habitual responding – a trait seen in drug abuse.
The reason behind the intensified reward signal may be because foods high in both fat and carbohydrate rarely exist in nature - one of the rare exceptions is breast milk, which has on average around 3.5% fat and 7% carbohydrate.
Carbohydrates are sugars that come in two main forms - simple and complex - also referred to as simple sugars and starches.
Simple carbohydrates are called simple sugars. Sugars are found in a variety of natural food sources including fruit, vegetables and milk, and give food a sweet taste. But they also raise blood glucose levels quickly.
Sugars can be categorised as single sugars (monosaccharides), which include glucose, fructose and galactose, or double sugars (disaccharides), which include sucrose, lactose and maltose.
Complex carbohydrates, also known as polysaccharides, are starches formed by longer saccharide chains, which means they take longer to break down. Examples include white bread, pasta, cakes and pastries.
Source: The Diabetes Community
"All mammals know breast milk," said lead researcher and head of metabolism research at the Max Planck Institute Marc Tittgemeyer. "Probably we are influenced by breast milk to respond intensively to food rich in carbohydrates and fats and perceive this as particularly rewarding, because this is vital."
Given that the typical processed snack contains closer to 24% fat and 57% carbohydrates, however, the current Western food environment promotes overeating, the authors write.
In order to determine individuals’ preference for foods containing calories from different sources, the researchers recruited 40 volunteers to play a computer game with food rewards.
The foods they could win were high in fat (cheese or salami, for instance), carbohydrates (crackers, bread or gummie-style sweets) or both fat and carbs (chocolate-chip cookies, wafer-based chocolate bars and sugar-coated chocolate sweets).
Subjects ‘outbid’ the computer in order to win them, thus indicating willingness-to-pay. The high-fat and high-carb combination foods were the most popular.
As they played, the scientists measured their brain activity using magnetic resonance tomography, which showed that the fat-carb combo activated the brain’s reward system areas – caudate, putamen and mediodorsal thalamus – much more intensively than the other foods. They were also able to rule out portion size, food liking or energy density as contributing factors.
“The study produced two novel findings that are relevant for understanding food choice,” they write. “First, we demonstrate for the first time that foods containing both fat and carbohydrate are more rewarding, calorie for calorie, than those containing only fat or only carbohydrate [...]."
“Second, we discovered, unexpectedly, that individuals are better able to estimate the energy density of fat compared with carbohydrate and fat + carbohydrate foods, with accurate estimations of energy density depicted in pictures of fatty foods associated with increased coupling of visual sensory areas with the ventromedial prefrontal cortex (vmPFC) and cerebellum."
Sweeteners may impair ability to count calories
A possible reason for this may be the ubiquity of artificial sweeteners in modern diets, they suggest, which degrades the association between carbohydrate-containing foods and energy density, resulting in "an impaired ability to estimate calories".
“These results imply that a potentiated 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."
The study was limited by the fact it did not measure frequency of consumption of the foods in question, the authors write.
Source: Cell Metabolism
“Supra-additive effects of combining fat and carbohydrate on food reward”, 2018.
Available online: 14 June 2018, https://doi.org/10.1016/j.cmet.2018.05.018
Authors: Alexandra G. Di Feliceantonio, Géraldine Coppin, Lionel Rigoux, Sharmili Edwin Thanarajah, Alain Dagher, Marc Tittgemeyer, Dana M. Small