How a junk-food splurge can change your brain activity

Study participants were given customized snack packs of their favourite ultra-processed foods but still struggled to consume the requisite 1,500 extra calories per day. Credit: Creative Touch Imaging Ltd./NurPhoto/Getty

Five days of indulging in chocolate bars, crisps and other junk foods can lead to lingering changes in brain activity, a study shows1. The resulting brain patterns are similar to those seen in people who have obesity.

A junk-food splurge shifted brain patterns in healthy young men despite their body weight and composition remaining unchanged, according to the study, published on 21 February in Nature Metabolism.

“I didn’t expect the effect to be so clear in a healthy population,” says neuroscientist Stephanie Kullmann at the University of Tübingen in Germany, who led the study.

But another scientist says that there are limitations to the study, which relied on a nasal spray to deliver the digestive hormone insulin. “The authors give very large doses of insulin,” some of which enters the bloodstream, potentially confounding the results, says physiologist Christoph Buettner at Rutgers Robert Wood Johnson Medical School in New Brunswick, New Jersey, who was not involved in the study. Kullmann responds that her team studied and controlled for the effect of insulin entering the bloodstream.

Chocolate and chips

After a bite of food, the pancreas releases insulin to aid metabolism. Some of that insulin enters the brain, where it works to dampen appetite. But the brain’s response to the hormone is weakened in some people with obesity2. This ‘brain insulin resistance’ affects how the body processes food.

To learn more about the effects of insulin in the brain, Kullmann and her colleagues recruited 29 healthy male volunteers (Kullmann has a forthcoming study in women). Eighteen went on a high-calorie diet for five days. A nutritionist made customized 1,500-calorie packs of high-fat, high-sugar snacks for each participant to eat on each of the five days — but the men succeeded in increasing their caloric intake by an average of only 1,200 calories per day. “At the beginning, they were kind of excited,” says Kullmann, but by day 4, eating the extra processed food was a slog. Control participants ate their normal diet during the same period.

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The researchers imaged blood flow in the participants’ brains as a proxy for brain activity. Imaging was performed before the five-day period, immediately after it and one week later. Just before each imaging session, participants used an insulin nasal spray to boost brain levels of the hormone.

At the end of the five days, the junk-food group had higher activity than the control group in three brain regions involved in responses to dietary changes and rewards. This brain-activity pattern is similar to that seen in people with obesity or insulin resistance, which can lead to type 2 diabetes.

Seven days after their splurge had ended, the junk-food group had lower brain activity in two regions associated with memory and response to visual food cues. In an earlier study2 of people with obesity, those whose brains were sensitive to insulin lost more weight after lifestyle changes than did those with insulin-resistant brains, says Kullmann.

Although the amount of high-calorie food in this study might sound extreme, “I think it’s quite similar to what we experience at the holidays”, Kullmann says. “We’re all constantly confronted with all kinds of very processed foods.”