Five days on calorie-laden snacking: scientists find out how it affects your brain and metabolism

A team of researchers from the Institute for Diabetes and Metabolic Disease Research at the Helmholtz Centre (Munich) at the University of Tübingen, together with the German Centre for Diabetes Research, found that just five days of increased consumption of high-calorie ultra-processed foods can lead to a marked decrease in the brain's sensitivity to insulin and increase the amount of fat in the liver - even in healthy young men with normal body weight. What's more, some changes in brain function remain after returning to a normal diet.

Briefly about the study

• Study subject: 29 healthy men, 19-27 years old, with a body mass index (BMI) of 19-25 kg/m².
• Methodology
  1. HCD (High-Caloric Diet) group: 18 men who were asked to consume an additional approximately 1500 kcal daily on top of their normal diet. The main sources of calories were ultra-processed and high-calorie snacks.
  2. Control group: 11 people who maintained their usual diet.
  3. Physical activity: All participants had reduced mobility (no more than 4000 steps per day).
  4. Observation period: 5 days of high-calorie diet, then return to normal diet for at least 1 week.
• Methods of measurement: functional magnetic resonance imaging (fMRI) with intranasal administration of insulin, MR spectroscopy to assess liver fat, oral glucose tolerance test and psychophysiological tests for reward and punishment.

Main results

1. Liver fat growth

   • Participants in the HCD group had an increase in liver fat percentage from approximately 1.55% to 2.54% over 5 days.
   • No changes were recorded in the control group.

2. Brain response to insulin

   • Measured by activity of brain areas (right insula, left rolandic operculum, right midbrain/varolian bridge). After 5 days of overeating, an increase in local insulin sensitivity was observed.
   • However, one week after returning to a normal diet, insulin sensitivity was conversely decreased in several sections (right hippocampus, bilateral spindle gyrus area).
   • The researchers attributed these changes to increased fat in the liver and changes in fat intake.

3. Disruption of the reward system

   • Participants were offered a "go/no-go" reinforcement learning (reward learning) task. It measures how sensitive people are to positive and negative cues.
   • The HCD group showed a decreased response to rewards and increased sensitivity to punishments. After a week of normal diet, there was some reversal, but not full recovery to baseline.

4. Insulin resistance in the brain

   • Previously, similar patterns have been observed predominantly in obese and prediabetic people.
   • That just five days of a high-calorie diet can induce similar changes emphasises the delicate balance in the appetite and metabolic regulatory system.

Conclusions and significance of the study

• Short-term overeating has long-term consequences
  Despite no significant weight gain or impairment of peripheral insulin sensitivity, young adults developed changes resembling the pathophysiological processes in obesity.
• Relationship to "back-feeding" behaviour
  The reward system responsible for shaping eating habits may 'remember' these changes and subsequently promote overeating, increasing the risk of gaining excess weight.
• Health risks
  Increased fat in the liver, as well as shifts in central appetite regulation, are potentially threatening factors in the development of obesity and related diseases (insulin resistance, metabolic syndrome).

Practical recommendations

1. Avoid "crash" diets
   Even a few days of ultra-processed and very calorific food can create an unfavourable background in the body.
2. Try to maintain physical activity
   Limiting steps to 4000 per day contributed to the fact that extra calories were stored rather than burned.
3. Monitor liver health
   Regular increases in liver fat have been linked to non-alcoholic fatty liver disease (NAFLD).
4. Monitor eating behaviour
   If short-term 'breakdowns' become recurring, this may alter not only metabolism but also the brain's internal 'settings' regarding food.

Thus, the study clearly shows how quickly the brain and metabolic processes respond to increased calorie intake from "fast food". These changes may lay the groundwork for long-term weight and health problems in general.

Details: 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

Julian Nowogrodzki, How a junk-food splurge can change your brain activity, Nature (2025). DOI: 10.1038/d41586-025-00549-7