Over the last decade or so, research has revealed a clear pattern: People tend to overeat ultraprocessed foods. This could be one reason they’re linked with weight gain and obesity.
What isn’t clear is why we are so prone to overeating them.
Dr. Robert Califf, the former commissioner of the Food and Drug Administration, offered one hypothesis at a Senate hearing in December: “These foods are probably addictive,” he said, adding that they may act on the same brain pathways involved with addiction to opioids and other drugs.
As recently as eight years ago, such a concept was highly controversial, said Ashley Gearhardt, an addiction researcher at the University of Michigan. She described being heckled onstage at a scientific conference in 2017 for suggesting that some ultraprocessed foods may act as addictive substances. Now, she said, more researchers have started coming around to the idea.
But a major question remains: How do you prove it?
A study published Tuesday, the largest of its kind, took a big swing at this conundrum. But its results raised more questions than answers. Here’s what we know — and don’t know — so far.
Can food give you a druglike dopamine hit?
One way researchers study addiction is by looking at the brain levels of the neurotransmitter dopamine — a natural signal that helps you learn to seek what you need to survive. When you eat, your brain releases the chemical, said Dana Small, a cognitive neuroscientist at McGill University in Montreal. If it’s a food you know and like, she said, just thinking about or seeing it can trigger an increase in dopamine, reminding you that it’s a good source of fuel and nudging you to eat more.
Addictive drugs co-opt this survival system by triggering a larger surge in dopamine and driving people to use them again and again, Dr. Small said.
Researchers have wondered if ultraprocessed foods — especially those high in fat and sugar — cause a similarly outsized dopamine response, suggesting they could be addictive in the same way as drugs. Past research in rodents and humans has supported this idea, but the human experiments have been very small.
In the new study, scientists at the National Institutes of Health measured how people’s brains responded to drinking a high-fat ultraprocessed milkshake.
They found that while more than half of the participants had a small dopamine increase after drinking the shake, the rest had a decrease or no change. On average, the researchers concluded, there was no statistical difference in brain dopamine levels before and after drinking the shake.
The authors wrote that this result runs counter to the idea that ultraprocessed foods drive overeating by causing dopamine surges in the brain similar to those of addictive drugs.
But there’s an important caveat: The study measured brain dopamine levels with PET scans, which are commonly used in drug addiction research. These scans can’t measure small dopamine changes very well; it’s likely that the milkshakes did elicit dopamine responses in more participants, and the scans just couldn’t detect them, Kevin Hall and Valerie Darcey, the study’s lead authors who are nutrition and metabolism scientists at the N.I.H., wrote in a statement to The Times.
A few drugs, like cocaine and amphetamines, trigger dramatic surges in dopamine that are obvious on PET scans, but for others, like nicotine or opioids, the dopamine responses are smaller and not always detectable, said Alexandra DiFeliceantonio, a neuroscientist at Virginia Tech who was not involved with the study.
Dopamine ‘responders’ might find ultraprocessed foods especially pleasurable.
Dr. Small was most interested in the new study’s participants who did have small increases in dopamine after drinking the shakes. These “responders,” as the study authors called them, rated the shakes as being more pleasant and said they wanted more of them compared with the other participants.
Several days after the brain scans, the researchers found that the “responders” ate nearly twice as many Chips Ahoy! cookies at a buffet lunch as the other participants.
This tracks with past research on nicotine and opioids, Dr. Gearhardt said. People who have measurable dopamine surges after using the drugs tend to find them more pleasurable and want them more than those who don’t.
Outside researchers praised the new study for its size and rigor. But they and the lead authors said that although the main result seems to suggest that ultraprocessed foods may not be addictive, it’s not the end of the story on that question. “It’s just more complicated than we originally thought,” the study authors wrote.
There’s controversy around the word ‘addictive.’
The questions surrounding food addiction are “a very big minefield,” Dr. Small said.
In a way, she said, food needs to be addictive. The fact that we are drawn to foods high in calories, sugars and fats has ensured our survival as a species. “There’s nothing wrong with that,” she said.
The problem, Dr. Small added, is that this survival mechanism may not serve us well in an environment full of flavorful, heavily marketed and convenient ultraprocessed foods. She hesitates to label the category as addictive, since there are likely many other complex reasons we are prone to overeating them, including that they are often packed with calories and can be eaten more quickly than minimally processed foods.
Dr. DiFeliceantonio, on the other hand, readily says that some ultraprocessed foods can be addictive, citing the ways they stimulate the brain’s “reward” system in an amplified way, similar to addictive drugs. They’re engineered to do this, she said, with enticing flavors and often high levels of fat and sugar that are rapidly absorbed in the gut.
While brain measurements such as the PET scans used in the new study are one way to understand addiction, Dr. DiFeliceantonio added, they can’t definitively prove whether a substance is addictive.
In the past, when scientists determined that substances like nicotine and opioids were addictive, they looked at how those substances affected people’s behavior, Dr. Gearhardt said.
“It wasn’t some magical brain study that convinced people” that cigarettes were addictive, she said. It was that people couldn’t stop smoking even after learning it was harming their health. “That was the nail in the coffin,” Dr. Gearhardt said.
Dr. Gearhardt and her colleagues have developed criteria for assessing if people have symptoms of food addiction (like having cravings or trouble cutting back) that parallel those seen for addictive substances. A large 2021 review of studies that used that metric found that 14 percent of about 19,000 people assessed met the criteria for food addiction.
At the end of the day, Dr. Gearhardt said, we should believe people when they say that they’re addicted to ultraprocessed foods. “The proof is in the pudding,” she said. “People want to stop, but they can’t.”
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