Can the Reward Circuit Be Rewritten in Just 5 Days? The Precarious Relationship Between Junk Food and the Brain

A 2025 study published in Nature Metabolism demonstrated that when healthy men consumed ultra-processed snacks for just five days, their brain insulin response was disrupted, and reward learning became biased even before any weight change occurred. Additionally, liver fat increased. Immediately after the intervention, there was heightened activity in the "reward system" areas such as the insular cortex and midbrain. A week later, decreased activity was observed in the hippocampus and fusiform gyrus, suggesting that the "aftereffects of overeating" linger in the brain. While caution is needed in generalizing these findings due to the sample being limited to young men, the study highlights the potential for short-term binge eating to trigger a vicious cycle of eating behavior and metabolism. Practical measures include avoiding "extra 1,000 kcal per day" scenarios during holidays and long weekends, resetting taste preferences with a high-protein, high-fiber diet after overeating, and ensuring adequate sleep and physical activity.