French fries and onion rings could contain chemicals that increase the risk of dementia: Scientists warn deep fat frying does more than just add pounds...

Eating onion rings and french fries could raise your risk of dementia, a new study on the harms of deep-fat frying suggests.

Researchers looked at the effect of food made using large amounts of cooking oil that's reused — which is often the case in restaurants.

They found that old oil can contain brain-damaging impurities, which ultimately end up in the food.

Rats fed a diet high in reused oils showed more signs of bodily inflammation, said to damage blood vessels and speed up cognitive decline.

Researchers also detected more signs of brain damage in rats fed diets high in reheated oils, compared to a control group.

The scientists, from the University of Chicago, said: 'Deep-frying at high temperatures has been linked with several metabolic disorders.

Researchers warned foods fried in reused cooking oil could cause more neural decline 

'To our knowledge, we are the first to report that long-term deep fried oil supplementation increases neurodegeneration.'

The results were presented at the annual meeting of the American Society for Biochemistry and Molecular Biology in San Antonio, Texas.

Restaurants and homes regularly reuse cooking oil in deep-fat friers to make a variety of foods, also including vegetable fritters, donuts and shrimp tempuras.

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Deep-fat fryer owners are told to change the oil in friers every two to eight uses, although in some cases people may keep the oil in for longer.

Experts have previously warned that  reused cooking oil can raise the risk of exposure to impurities such as peroxides, which can damage the brain, and acrylamide, a chemical compound made in deep fat fryers that has been linked to cancer.

For the experiment, the scientists split rats into five groups.

One group, the control, ate only a standard chow diet for 30 days.

But the other four consumed a diet with oils mixed in for the same period.

These received a standard chow diet with 0.1 ml of either unheated sesame oil, unheated sunflower oil, reheated sesame oil or reheated sunflower oil.

Results showed that, compared to other groups, the rats who consumed reheated sesame or sunflower oil had increased inflammation in the liver.

They also showed significant damage in the colon, as well as the proliferation of bacteria that release harmful toxins and damage cells.

Researchers also looked at the rats' brains under a microscope, and found that the brains of those who ate the reheated oils showed a pattern of damage indicative of dementia.

The researchers said the tests would need to be carried out in humans to confirm the findings, but that the results were concerning.

They are planning a follow-up study to investigate the effects of deep-frying oil on the risk of neurodegenerative diseases such as Alzheimer's and Parkinson's, as well as anxiety, depression and neurodegeneration.