Reheated Oil Causes Liver and Brain Damage in Rats

Reheated Oil: Rat Study Raises Alarms

A group of female rats consumed a diet supplemented with 0.1 ml daily of reheated sesame or sunflower oil for 30 days. At the end of the experiment, they exhibited liver inflammation, colon damage, and signs of neurodegeneration that were also present in their offspring. The study was not conducted in Spain, did not use olive oil, and has not undergone human clinical trials. Nevertheless, the warning has reached a country where frying is a daily habit and where oil prices have become a major domestic issue.

The research was carried out by investigators from Central University of Tamil Nadu, in Thiruvarur (India), together with the University of Illinois at Chicago, and was presented at Discover BMB, the annual meeting of the American Society for Biochemistry and Molecular Biology held in San Antonio. The animals were divided into five groups: standard unaltered food; standard food with unheated sesame oil; the same with unheated sunflower oil; reheated sesame oil; and reheated sunflower oil.

The last two groups showed increased oxidative stress and liver inflammation, significant colon damage with changes in endotoxins and lipopolysaccharides, altered fat metabolism in the liver, and a notable reduction in the transport of the omega-3 fatty acid DHA to the brain. The outcome was neurodegeneration in the animals and also in their pups. "We are the first to report that long-term supplementation with fried oil increases neurodegeneration in first-generation offspring," summarizes Kathiresan Shanmugan, the lead author.

Why Damage Travels from Liver to Brain

The explanation proposed by the authors involves the gut-liver-brain axis, the bidirectional communication network connecting these three organs, which has already been linked to neurological disorders in previous studies. According to their hypothesis, oil subjected to high temperatures repeatedly triggers a cascade: liver inflammation, deterioration of the intestinal barrier, and consequently, altered transport of nutrients and signals to the brain.

The publication itself details what is generated when oil is reheated: acrylamides, trans fats, peroxides, and polar compounds, in addition to the loss of natural antioxidants in vegetable fats. In the experiment, neurotoxicity was tested by inducing damage with monosodium glutamate: pups that had consumed reheated oil showed more neuronal injury than the control group.

As possible mitigating factors, researchers point to supplementation with omega-3 fatty acids and nutraceuticals such as curcumin and oryzanol, which would reduce liver inflammation and associated neurodegeneration. None of this has been proven in humans, and the authors themselves call for clinical trials. Their next step is to study the effect of frying oil on Alzheimer's, Parkinson's, anxiety, depression, and neuroinflammation.

How Many Times Can Frying Oil Be Reused?

On paper, and admitting all cautions, the previously considered reuse margin was generous: up to three times in restaurants, always filtering between uses, and up to 25 times in private homes. This figure surprises anyone who has ever thrown away a pan. Some argue that in the hospitality sector, oil should be changed every two or three weeks, based on twelve hours of daily use, something that does not always happen given current oil prices and tight profit margins.

The Indian study does not measure home fryers or bar snacks: it measures reheated oil under experimental conditions. The difference matters. It also matters that the quality of the starting oil determines how long it lasts before degrading, and that cheaper oils are precisely those most reused for household economy, as acknowledged in the article itself.

Sesame Oil Nobody Fries With and Sunflower Oil That Is Used

The detail that stands out in Spain is the choice of fats. Sesame oil is expensive, used in drops for stir-frying or dressing in Asian cuisine, and practically no one puts it in a deep fryer. Its composition, however, is rich in monounsaturated fats and similar to that of olive oil, according to one of the participants. "The sesame oil used in the study is very expensive, only a few drops are used in Asian cooking for stir-frying or dressing, nobody uses it for frying or reheating," notes one of the most repeated objections.

Sunflower oil is another story: it is what many Spanish households put in the fryer when economics dictate, and they do so more than ever with olive oil prices skyrocketing. Here enters the perennial debate in hospitality and home cooking: extra virgin olive oil, pomace, sunflower, or animal fat. An olive oil expert left a phrase for the record: frying with EVOO is a waste, while pomace oil is cheap and healthy.

From Air Fryers to Bacon Grease

The practical reaction has been the usual one: look for the cheapest alternative. The air fryer has entered kitchens as a substitute for oil baths, with the nuance that it works better with fresh foods than with pre-cooked items. Others resort to lard, and some directly utilize the grease released by bacon on the grill: "From a half-kilo tray of bacon, you get a medium cup of grease," details an amateur cook who reuses it for meats, fish, and eggs.

Lard can also be found for about €3.50 for a 400-gram tub, as noted in the discussion, although there is no agreement on whether this makes it cheaper or more expensive than olive oil. What none of these alternatives resolves is the fundamental question: how much reheated oil has already passed through each person's body. And whether the warning applies only to Indian rats or also to the home fryer. With available data, the honest answer is that we don't know.