Eating foods like burgers and pies for lunch can ruin your attention span in the afternoon, scientists say.
Volunteers who tucked into treats high in saturated fat saw an almost immediate slump in working memory.
But when they ate healthy unsaturated fats – found in avocados and fish – their concentration levels were maintained.
Previous studies have found regularly eating junk can affect attention.
Eating foods like burgers and pies for lunch can ruin your attention span in the afternoon
But this is believed to be the first showing just one unhealthy snack has an instant impact on brain function.
Scientists from Ohio State University in the US said it is possible harmful fatty acids from junk food get into the bloodstream and cross the brain barrier, damaging mental performance.
They recruited 51 middle-aged women and got them to eat fatty treats, such as burger and chips, before completing a continuous performance test involving clicking on on-screen images.
The women later repeated the experiment, this time having eaten a healthy salad lunch made with sunflower oil.
When people ate a healthy snack rather than junk food they retained their full attention span
The results revealed brain function declined within a couple of hours of eating junk food.
But they retained their full attention span after the healthy choice.
In a report on the findings, researchers said previous studies found cognitive abilities suffer after at least five days of unhealthy eating.
They added: ‘But our research shows a single meal high in saturated fat can impede attention.
‘These findings suggest just one fatty lunch can potentially diminish productivity in the workplace.’