Just five days of overeating could prime the brain for future bad food habits, a European study has found.

German researchers recruited 29 healthy men in their 20s and made them eat ultra-processed foods to understand what happens to the brain on a high-calorie diet.

One group was given ultra-processed foods like chocolate bars, brownies and chips, which they ate over five days to boost their calorie intake.

They were compared to a control group that ate a regular diet.

Short-term overeating was found to increase liver fat and disrupt insulin action in brain pathways involved in motivation and decision-making.

The research suggests that five days of overeating could prime the brain for long-term bad eating habits. 

“We do need to reduce our consumption of ultra-processed foods,” dietitian and nutritionist Jemma O’Hanlon said.

“Currently, the Australian diet is 42 per cent ultra-processed foods, which is far too high.

“These ultra-processed foods, we’re seeing research to demonstrate, they can be addictive.”

Experts say you should only eat ultra-processed food occasionally and in small amounts.