Most people know processed food irritates the gut. But it can also irritate the brain. Researchers found a toxic effect of fried foods beyond weight gain and increased risk of chronic disease. More specifically, food cooked in reheated oil may also contribute to neurodegeneration. This can lead to diseases like Alzheimer’s Disease, Parkinson’s Disease, and Amyotrophic Lateral Sclerosis (ALS).
Reheated oil in processed food
Deep frying is a common cooking method, especially in fast food. It involves dipping foods into a large pot of boiling oil. Most restaurants reuse oil to save money, cut down on waste, and streamline the daily preparation process. Despite being famously unhealthy, fried and processed foods are a staple of many people’s diets. But research has mostly focused on fried food in general, not on how many times the oil has been reheated.
For that reason, the research presented at Discover BMB 2024 was a breakthrough. “Deep-frying at high temperatures has been linked with several metabolic disorders, but there have been no long-term investigations on the influence of deep-fried oil consumption and its detrimental effects on health,” said Kathiresan Shanmugam to ASMBM Today. Shanmugam is the associate professor at Central University of Tamil Nadu in Thiruvarur, India, and research team lead. “To our knowledge, we are first to report long-term deep-fried oil supplementation increases neurodegeneration in the first-generation offspring.”
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The study and its results
During the 2024 study, Shanmugam and his team divided a group of female rats into five groups. One was fed only standard feed every day. The second and third received the same daily feed with 0.1 ml of unheated sesame oil or unheated sunflower respectively. The fourth and fifth had the feed with 0.1 ml of reheated sesame oil or sunflower oil daily.
After the 30 days, the rats who ate the reheated sunflower or sesame oil had higher levels of liver inflammation and oxidative stress, as well as colon damage that made changes in endotoxins and lipopolysaccharides. “As a result, liver lipid metabolism was significantly altered, and the transport of the important brain omega-3 fatty acid DHA was decreased,” Shanmugam continued. “This, in turn, resulted in neurodegeneration, which was seen in the brain histology of the rats consuming the reheated oil as well as their offspring.”
This is just the beginning, says the team, who are planning further investigation into this topic. For instance, they want to examine the effect of deep-frying oil on neurodegenerative diseases, such as Alzheimer’s and Parkinson’s, as well as anxiety, depression, and neuroinflammation. They also plan to explore the link between the brain and gut. This may help “identify potential new ways to prevent or treat neurodegeneration and neuroinflammation.”
Why is reheated oil so unhealthy?
Heating oils to high temperatures changes their chemical structure. This lowers the amount of beneficial antioxidants, and forms harmful compounds like aldehydes, acrylamide, and trans fats. (Bear in mind, the U.S. has banned trans fats in processed foods.) Deep frying requires high heat, forcing oils to become unstable, less healthful, and filled with more toxins each time.
Alyssa Simpson, registered dietitian and owner of Nutrition Resolution, explains that reheating makes oils break down, which results “in changes in fatty acid composition, and increased levels of lipid oxidation products such as reactive oxygen species (ROS).” This can trigger oxidative stress in the brain, which may increase the risk of neuron damage and neurodegenerative diseases.
“Repeat exposure of heat to cooking oil negatively impacts fatty acid composition decreasing health promoting polyunsaturated fats and increasing trans isomers and saturated fatty acids,” said Dr. Alexandra Filingeri, a registered dietitian and doctor of clinical nutrition, to Medical News Today. Therefore, it may also lead to increased risk factors, like high cholesterol and inflammation, for chronic diseases.
How to consume oil safely
Start by being picky about quality while grocery shopping. “When choosing cooking oils consumers should choose oils high in polyunsaturated fats and choose preparation methods that require low heat,” stated Filingeri. “Oils should not be reused for repetitive cooking sessions and oils should not be heated to high [temperatures].” And when you eat out, look for non-fried options, such as grilled or roasted food.
Aside from skipping fried processed food altogether, the study authors recommend omega-3 fatty acid supplements and foods with vitamin E and curcumin (like almonds and turmeric). These items may help mitigate the inflammation and neurodegeneration caused by reheated oil. Furthermore, Filingeri recommends strengthening gut and liver health with probiotics from fermented foods like kimchi and sauerkraut. The researchers conclude the study by encouraging others to conduct identical studies with humans to further understand the effect of oils on the body and all of its systems.
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