After nine days in a high-fat diet, rats have less physical endurance and their mental abilities decline, according to a new study.

Researchers at Oxford University found that rats switched to the less-healthy diet could only run half as far on a treadmill.

A news release on the work said that it was known that a diet like this could lead to long-term problems, but less is known about how quickly changes can occur.

Dr. Andrew Murray's team studied 42 rats, testing them on treadmills and sending them through mazes. Half were eventually switched to the high-fat diet, which Murray said simulated a human diet high in junk food.

Five days after switching feed, the high-fat rats ran 30 percent less than those on a standard diet. By the ninth day, they ran half as far.

They also started making mistakes sooner in a maze, which Murray said suggests their cognitive abilities were being affected.

While animal experiments do not always translate directly into people, Murray's team has already started similar experiments in people.