Food preferences aren't always something we're born with. A study published Wednesday in the journal Cell Metabolism suggests that eating fatty or sugary snacks alters our brain activity and creates lasting preferences for these less healthy items.
For the study, researchers at Yale University and the Max Planck Institute for Metabolism Research in Germany gave one group of participants a high-fat, high-sugar yogurt twice daily for eight weeks, while another got a low-fat, low-sugar version. Aside from that, both groups continued their normal eating habits.
At the end, the groups rated puddings with varying fat concentrations and apple juices with a range of sugar levels. The group that ate the high-fat, high-sugar yogurt said they did not like low-fat pudding and did not want low-sugar apple juice as much as they had at the start.
Next, the participants underwent MRI scans while drinking milkshakes. The scans showed that the treat increased brain activity in the group that had eaten the high-fat, high-sugar yogurt, but not in the other.
The researchers concluded that fatty, sugary snacks activate the brain’s dopamine system, which gives people a feeling of motivation or reward.
“Let’s say a new bakery opens up next to your work and you start stopping in and having a scone every morning. That alone can rewire your basic fundamental dopamine learning circuits,” said Dana Small, the study’s senior author and director of Yale University School of Medicine’s Modern Diet and Physiology Research Center.
It's an intuitive idea for anyone who's ever gotten into the habit of eating dessert frequently — say, around the holidays — then found the pattern hard to break.
Small said diet has such a strong effect on brain activity that dopamine signals can fire even when someone anticipates eating fatty or sugary food, like when they pass by a bakery or smell a pastry.
“It just tells us how sensitive we are to the food environment, and how the food environment can actually change our behavior,” she said.
Sugary and fatty foods alter brain activity
The new study was small: It included just 49 people, all of whom were healthy, didn’t smoke or take medication, and were not overweight or obese. Overall, the participants did not gain a significant amount of weight over the eight weeks.
Small said the study is the first to demonstrate in humans that even small dietary changes can rewire brain circuits and increase the long-term risk of overeating or weight gain.
Previous research has shown that obesity can alter people’s brain activity, and that people have an innate aversion to bitter foods and a proclivity for things that taste sweet.
Experiments in rodents, meanwhile, have showed that high-fat, high-sugar foods can rewire dopamine neurons and lead to overeating. But scientists knew less about how human eating habits influence food preferences.
“There’s enough evidence now to be pretty confident that this happens, and happens in multiple species,” Small said.
Susan Swithers, a behavioral neuroscientist at Purdue University who was not involved in the research, said it's possible that people might start to prefer foods that they eat regularly, then gravitate toward them.
"People think that we eat what we like, but we actually like what we eat," Swithers said.
There may even be biological reasons why people prefer fatty, sugary foods, according to Garret Stuber, a neuroscience professor at the University of Washington who wasn’t involved in the study. Early humans likely sought energy-dense foods high in carbohydrates and fat, he said, so people today could instinctively share those preferences.
"Thousands of years ago, those things were very sparse and not so widely abundant, but the fact that they’re everywhere now in pretty much everything we eat is sort of working against biology," Stuber said.
How much can food preferences change over time?
One question left to answer, Small said, is whether people can change their preferences after they've gotten accustomed to a high-fat, high-sugar diet.
"Perhaps it is the case that if you decrease gradually to more acceptable levels of fat, that eventually you can change your preferences in a more sustainable way. But I don’t think we know that," she said.
A 2012 study showed that after being routinely exposed to soup without added salt, people eventually liked those soups as much as saltier versions. Small said it's possible that such a process could work for fat and sugar, too.
But Stuber said it's hard for people to forget that fatty, sugary foods taste good.
"If you just stop presenting something to people, something that’s rewarding, that memory doesn’t go away," he said.
When it comes to disliking certain foods, he added, those preferences can last a lifetime.
"Think about food poisoning, for example — you can eat one food and get sick from it, and you’ll have an enduring and long-lasting aversion to that food," Stuber said.