We already know high-fat diets lead to obesity, heart disease and diabetes as well as a decline in cognitive ability. But the surprise is just how quickly fatty foods may wreak havoc on our health, according to a study by Oxford University researchers as reported by HealthNews.com.

Researchers conducted experiments on 32 rats to test the short-term effects of a high-fat diet.

Initially, scientists fed rodents a low-fat diet and trained them to complete a maze and treadmill run. Once rodents learned to negotiate the maze, researchers switched half of them to a high-fat diet.

Researchers found that the rats on the high-fat diet ran 30 percent less on the treadmill, took longer to complete the maze and made more mistakes while learning. A possible reason for the rodents’ cognitive decline? Fatty foods. They trigger insulin resistance, which prompts the body to inefficiently use glucose. (Glucose is essential to brain function.)

In addition, scientists found that the fatty-food-eating rats showed increased levels of a specific protein—uncoupling protein 3—in their muscle cells. This protein hinders the cells’ ability to use oxygen to create the energy needed for exercise. Because of this, the these rats’ hearts had to work harder and become enlarged. Both results possibly explain why these rodents’ treadmill performance suffered.

“We expected to see changes, but maybe not so dramatic and not in such a short space of time,” said the study’s lead author, Andrew Murray, a lecturer in physiology at Cambridge University.

Researchers also performed similar studies on young men who ate high-fat diets and completed exercise and cognitive tests. Doctors concluded that the short-term effects of a fatty diet on humans are similar to results indicated in the rat experiment.

Check out tips on how to avoid junk food here.