We know that processed foods are bad for us, and that though they may be tasty, they do not bring us any nutritional benefits. How come we find it so hard to say no to those chips, donuts, and crackers?
Many processed foods — such as potato chips, donuts, crackers, cookies, and fries — have a high content of both (saturated) fat and carbohydrates.
However, they also have little to no nutritional value.
Instead, they are packed with "empty calories," meaning that they can build up our fat levels without providing us with much energy.
If we consistently eat processed foods, or if our diets consist mainly of these, this will gradually increase our risk of metabolic conditions, such as obesity or diabetes, and of other complex diseases, such as cancer.
None of this is new information. Still, though we are aware of the consequences and know which foods are better for our health, many of us still find it difficult to steer clear of these tempting snacks. Why is this?
Researchers from four countries — Germany, Switzerland, the United States, and Canada — have now conducted a series of experiments investigating what happens in the brain when a person is confronted with foods high in carbs, foods that have a high fat content, and foods that are high in both carbs and fats (typically processed foods).
The researchers' findings indicate that, while we may be pretty good at instinctively estimating the nutritional value of foods that are rich either in carbs or in fats, we seem to be worse at evaluating the nutritional value of processed foods, which are high in both.
"The biological process that regulates the association of foods with their nutritional value," says senior author Dana Small, from Yale University's Modern Diet and Physiology Research Center in New Haven, CT, "evolved to carefully define the value of a food so that 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," she explains.
When it comes to processed foods, however, this age-old "cost vs. benefit" mechanism seems to malfunction in humans — so suggests the new study, published in the journal Cell Metabolism.
Processed foods trigger a reward response
The main study involved 206 participants, who were, in the first instance, shown images of snacks whose calories came primarily from either their content of fats, carbs, or a mix of the two.
All of these snacks were then rated by each participant on four counts: liking, familiarity, estimated energy density, and caloric content.
"On a subsequent day," the scientists explain in their paper, "[the participants] arrived [with empty stomachs] to the laboratory and were fed a standard breakfast of 426 [kilocalories] from orange juice, cheddar cheese, whole-wheat toast, white toast, strawberry jam, and butter."
Three hours after this varied breakfast, the participants were all subjected to functional MRI scans as they participated in a bidding game, in which they were shown the pictures of different snacks with different nutritional values. They were asked to say how much they would pay for each.
Based on these experiments, the researchers made several fascinating findings. First, they saw that the fat and carbohydrate combo stimulates the brain's reward system more intensely than foods that are just high in carbs, or only high in fat, do on their own.
Two particular brain regions — the dorsal striatum and the mediodorsal thalamus, which have both been linked to reward mechanisms — have been identified as responsive to the presence of foods with a high fat and carb content.
These brain circuits were, in fact, more active in the presence of fat- and carb-rich foods than in that of an individual's selected favorite food, a sweeter food choice, a more energy-dense snack, or even a more generous portion size.
Then, in the bidding game, it became apparent that, given a limited amount of money to invest, the participants were more likely to pay more for fat- and carb-rich foods than for snacks only rich in carbs, or which only had a high fat content.
"Surprisingly, foods containing fats and carbohydrates appear to signal their potential caloric loads to the brain via distinct mechanisms," says Small.
"Our participants were very accurate at estimating calories from fat and very poor at estimating calories from carbohydrate. [...] [W]hen both nutrients are combined, the brain seems to overestimate the energetic value of the food."
Is this about adaptation to food?
Small and her colleagues hypothesize that this may be due to the fact that our brains have not had enough time to adapt appropriately to the advent of foods that may be pleasurable but do not actually bring us many nutritional benefits.
The researchers explain that our prehistoric ancestors only had access to animal meat and plants, as they were available in nature.
And, Small says, "In nature, foods high in fat and carbohydrate are very rare and tend to have fiber, which slows metabolism. By contrast, it is very common for processed foods to have high fat and high carbohydrate loads."
Processed foods — such as donuts or potato chips — haven't been around very long. In fact, they have only existed for about 150 years, the scientists note.
This, they believe, could mean that unlike with unprocessed or minimally processed foods, we have not yet developed a brain response that allows us to better regulate what we eat and in what quantities.
In any case, the positive signals that our brains circulate when faced with foods that are rich in fats and carbs combined can lead to the reinforcement of unhealthful eating habits, which may lead to obesity.
"[The study's] 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 researchers conclude.