Are fatty foods making you stupid?

Okay, so we know fatty foods aren't good for us. They can make you lazy, irritable and generally affect your physical health.

But according to new research, it's not just our body that's suffering. That chocolatey mid-morning treat could be slowing you down in more ways than one.According to an Oxford University study, not can a high-fat diet reduce your physical endurance, it can slow down your ability to think clearly within just days.

Funded by the British Heart Foundation, the research revealed that after just nine days of being fed a high-fat diet, rats could manage only 50 per cent of the distance they had previously run on a treadmill.

And when it came to the maze task, they began making mistakes much sooner.

Professor Kieran Clarke, head of the research team, told the Daily Mail: "It shows that high-fat feeding even over short periods of time can markedly affect mind and body."

As well as affecting the efficiency of the heart, high-fat diets are known to lead to a decline in cognitive ability over time, but the researchers were "startled" by the speed of the decline in the rats.

Dr Alan Maryon-David, president of the UK Faculty of Public Health, says it could be down to our ancient hunter-gatherer predecessors.

"A high-fat diet sends a message to the brain that it's a time of plenty, so the neurotransmitters – brain chemicals – tell us that we don't have to be quite so frenetic about finding food. Our huntergatherer instinct is switched off, or at least put on hold," he said.

So just how much junk will slow us down? Professor Clarke explained: "The rat intake translates to a diet of about 3,000 calories a day. Something like going to McDonald's every day for lunch would probably do it.

"If you have a heavy weekend of excess, it's probably a good idea to make up for it with a healthier few days."

That might explain why the Monday workload seems so much heavier.