A short holiday from eating healthily may seem worth the risks, however new research shows that this can impact how your brain reacts to food…after your holiday is over. In fact, only 5 days of a bad diet changes your brain!
A recent study (March 2025) from the University Hospital of Tübingen and Helmholtz Munich, published in Nature Metabolism [1], reveals that just five days of eating ultra-processed, high-calorie snacks can disrupt how the brain responds to insulin—even in healthy, lean people.
Insulin normally acts in the brain to suppress appetite, helping to regulate eating behaviour. But when insulin sensitivity in the brain drops (a condition known as central insulin resistance), the brain no longer gets the signal to stop eating, potentially triggering chronic overeating and fat storage. This mechanism is now being recognised as a root driver of type 2 diabetes and obesity—even before any visible weight gain occurs.
In the study, one group of healthy men consumed an extra 1500 calories per day from processed snacks. After just five days:
- Liver fat significantly increased (even without alcohol)
- Insulin sensitivity in the brain dropped (reducing tolerance to sugar and carbs)
- These changes persisted even after returning to a normal diet for a week (even in metabolically healthy men*)
These findings suggest that short-term dietary choices can quickly rewire metabolic function—especially in the brain—setting the stage for long-term disease.
This research highlights why treating the root causes of chronic illness means going beyond “calories in, calories out.”
Supporting brain health, insulin regulation, and inflammation through nutrition and lifestyle choices is essential for preventing obesity, diabetes, and neurodegenerative disease.
Does this mean you can never eat processed foods?
Absolutely not—we’re all human, and the occasional indulgence likely won’t have a lasting impact. However, knowing that is only takes 5 days of a bad diet to change your brain (or likely even less) is a reminder not to let your habits unravel completely over a holiday, thinking it will be easy to bounce back. In reality, getting your body back to where it was pre-holiday can take twice as long—or more—than the time you spent off track.
My advice?
Enjoy yourself on holiday—just don’t let a relaxed mindset turn into a full day (or days) of overindulgence. Try starting with a nourishing breakfast or lunch, so you can comfortably enjoy a special dinner or some pre-dinner treats. Or flip it—fill most of your plate with whole, nutrient-dense foods and leave room for a few chips or a shared dessert. Wellness isn’t about being rigid; it’s about making conscious choices that support your goals while still allowing joy and flexibility. Everything you do adds up—and yes, you can feel great, maintain your progress, and have a fantastic time.
*I know…it would have been nice to have some women in the study!
Study Reference:
Kullmann, S., Wagner, L., Hauffe, R. et al. A short-term, high-caloric diet has prolonged effects on brain insulin action in men. Nat Metab 7, 469–477 (2025). https://doi.org/10.1038/s42255-025-01226-9