We’ve all had that week when you let your diet slide—grabbing fast food, indulging in sweets, and telling yourself you’ll eat better next time. As it turns out, we should pay more attention to lapses like this because even a short junk food binge can have lasting effects.

A new study published in Nature Metabolism highlights how quickly a junk food spree can disturb critical metabolic signals, potentially setting the stage for weight gain, diabetes, or other metabolic diseases down the road.

Hidden effects

Junk foods are ultra-processed or highly refined foods that are dense in calories, sugar, salt, and saturated or trans fats. They’re typically poor in nutrients but rich in things that aren’t good for you. These items—such as candy bars, chips, sugary drinks, and fast-food burgers—are engineered for taste, texture, and long shelf lives. They’re also very addictive and cheap, which is why they’ve been linked to the ongoing obesity crisis.

In the experiment, 18 lean male volunteers (19–27 years old) were asked to consume about an extra 1,500 calories per day from ultra-processed snacks on top of their normal diet for five days​. Another 11 men maintained their regular eating habits as a control group. After five days, the researchers documented the changes in the participants’ bodies and brains.

The participants’ weight remained largely unchanged. So a person having a week-long junk food binge could be fooled into thinking nothing’s changed. But behind the scenes, other body metrics showed different results.

The first clue came from liver fat content, which jumped from 1.5% at baseline to around 2.5% in all participants. This is a significant increase but no reason for panic. A more impactful difference came from insulin response. The brain’s responsiveness to insulin was impaired, indicating that the men had developed insulin resistance in the brain within days​. Insulin resistance can lead to higher blood sugar levels and an increased risk of diabetes and metabolic disorders.

Researchers also gave participants cognitive tests. In these tests, the junk food participants became less sensitive to rewards and more sensitive to punishments, hinting that the junk food spree changed how their brains process pleasure and rewards.

In other words, serious metabolic disturbances were unfolding under the radar, without obvious outward signs. The men didn’t suddenly look or feel different in that short time, but their liver was storing extra fat and their brains were already starting to “tune out” insulin’s signals. Even one week after returning to a normal diet, the participants still showed lingering insulin resistance in certain brain regions involved in memory and cognition.

Why insulin response matters so much

If the brain stops responding to insulin, you’re much more likely to gain weight.

Insulin isn’t just for blood sugar control in the body — it also acts in the brain to help regulate appetite and metabolism. In a healthy state, insulin signals the brain to reduce appetite and modulate how the body uses and stores nutrients​. Essentially, when you’ve eaten, insulin in the brain contributes to the feeling of fullness and tells organs like the liver to efficiently manage blood glucose and fat. However, in an insulin-resistant state, the brain no longer responds properly to insulin signals.

This breakdown can lead to increased hunger, overeating, and mismanagement of blood sugar and fats. Over time, that can pave the way for weight gain and metabolic disorders. Indeed, insulin resistance in the brain is strongly associated with obesity and type 2 diabetes, and even linked to cognitive problems.

What’s striking about the new study is how quickly brain insulin resistance set in — within days — and that it appeared before any weight gain. “Brain insulin responsiveness is linked to long-term weight gain,” the study authors note.

A few days of junk food can rewire your brain

Beyond the insulin, the study also found that the junk food blitz disrupted the brain’s reward system. The participants performed a go/no-go learning task designed to gauge how their brains respond to rewards and punishments. After five days of indulging, they were less motivated by positive rewards and became more sensitive to negative outcomes. Essentially, the junk food binge seemed to dull the pleasure they normally would get from a rewarding experience, while making them more responsive to feeling “punished.”

This is a key finding because humans aren’t robots. Our eating behavior is strongly driven by the brain’s reward circuitry. If that circuitry becomes shifted, a person might be more or less motivated to eat healthily. It’s a vicious cycle: a brief flood of ultra-processed, high-sugar, high-fat foods could condition your brain to desire more of the same, creating a vicious cycle of craving and indulgence.

This idea is backed up by other recent research. In 2023, a team of scientists reported that giving people a daily high-sugar, high-fat snack for eight weeks effectively trained their brains to prefer those junky foods​. The study’s volunteers ate a pudding rich in fat and sugar every day, and by the end, their brain’s dopamine-rich reward centers lit up more vigorously whenever they saw or tasted high-calorie snacks.

Many of us assume that only a long-term poor diet leads to serious health issues and that short splurges (like a weekend of junk food) are relatively harmless as long as we “get back on track” afterward. But mounting evidence suggests that our bodies and brains have a rapid response to even brief periods of unhealthy eating.

So what does this all mean?

From a health perspective, the research adds even more weight to reducing ultra-processed foods in our diets. This doesn’t mean that a slice of cake or a burger will doom your brain, but it does mean that our everyday choices, and the occasional “cheats” can significantly affect our metabolic health. Each short-term binge leaves a mark and even though the scale may not show it, your brain knows.

The flip side is that healthy choices can also leave marks. Just as the brain’s insulin response was impaired by five days of junk, perhaps it can be improved by a stretch of healthy eating (research suggests weight loss and exercise can enhance brain insulin sensitivity in the long run).

Journal Reference: Stephanie Kullmann et al, A short-term, high-caloric diet has prolonged effects on brain insulin action in men, Nature Metabolism (2025). DOI: 10.1038/s42255-025-01226-9