The Surprising Effects of a 5-Day High-Calorie Diet | Brain Insulin Response and Health Risks Explained

Do you ever indulge for a few days and think, “It’ll be fine if I cut back later”?
Recent research suggests that even five days of a high-calorie diet may have lasting effects on how the brain responds to insulin.

Study Overview: A Five-Day “Mini Overeating” Experiment

Researchers divided 29 healthy young men into two groups. One group consumed about 1,500 kcal per day of ultra-processed snacks, in addition to their regular meals, for five days (high-calorie diet intervention). The other group maintained their usual diet.

The high-calorie group experienced a significant increase in daily caloric intake and also showed an increase in liver fat content. However, there were no prominent changes in body weight, blood glucose, or other general health markers. In other words, their body weight did not increase within such a short period, but excess fat still accumulated in the liver.

Notable Changes in the Brain’s Insulin Sensitivity

Even in just five days, researchers observed clear alterations in the brain’s insulin response. While certain brain regions initially showed a heightened reaction to insulin right after the intervention, this effect switched to reduced sensitivity one week after the participants stopped the high-calorie diet—particularly in the hippocampus, which is linked to memory.

The brain’s reward system was also affected. Participants became less sensitive to rewards but more sensitive to negative stimuli. Furthermore, even returning to a normal diet did not completely reverse these changes, suggesting a lingering impact of short-term overeating on the brain.

Health Risks: Could This Lead to Obesity or Diabetes?

When the brain becomes less responsive to insulin signals—referred to as “brain insulin resistance”—it’s linked to obesity, type 2 diabetes, and even cognitive decline. A five-day overeating period may trigger these changes and potentially raise long-term metabolic risks.

Practical Takeaways: How Can You Improve Your Eating Habits?

Even over a short time, it’s wise to avoid extreme high-calorie, high-sugar, or high-fat meals. A balanced diet, such as one rich in vegetables and fish (like the Mediterranean diet), can have positive effects on both the brain and the body. On the other hand, consuming foods loaded with sugar and additives can rapidly spike insulin levels and disrupt normal brain function.

If you do overindulge during holidays or vacations, try to return to a balanced eating pattern as soon as possible. Reducing the frequency of overeating can be critical in lowering long-term health risks.

References

1. Kullmann et al. (2025). “A Short-term, High-Caloric Diet Has Prolonged Effects on Brain Insulin Action in Men.” Nature Metabolism.
2. “Ultra-processed foods? Just say no.” New research suggests that ultra-processed foods (UPFs) raise the risk of cognitive impairment and strokes. June 17, 2024. By Andrew E. Budson, MD, Contributor; Editorial Advisory Board Member, Harvard Health Publishing.