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Reheating cooking oil may increase neurodegeneration, according to an animal study. Image credit: Guillermo de la Torre/Stocksy.
  • Consuming deep-fried oils has been linked to oxidative stress and inflammation, which are risk factors for neurodegenerative diseases and other chronic conditions.
  • A new study in rats suggests a potential connection between the long-term consumption of reheated cooking oils and increased neurodegeneration.
  • The gut-brain-liver axis appears crucial in maintaining neurological health, and consuming reheated oils may disrupt this balance.
  • Experts recommend diets rich in antioxidants and omega-3 fatty acids, cautioning against the frequent consumption of fried foods.

According to the Alzheimer’s Association 2024 Alzheimer’s Disease Facts and Figures report, Alzheimer’s now affects nearly 7 million adults age 65 and older in the United States.

Amid this rising neurodegenerative health crisis, a recent rat study has highlighted a potential link between long-term consumption of reused deep-fried oil and increased neurodegeneration.

The study abstract was presented at Discover BMB 2024, the American Society for Biochemistry and Molecular Biology’s annual meeting, and will be published in a virtual supplement to the Journal of Biological Chemistry.

The new study found that rats fed diets with reheated cooking oils exhibited significantly higher levels of neurodegeneration compared to rats consuming a standard diet.

The research suggests that reheated oil may increase neurodegeneration by disrupting the liver-gut-brain axis, which is crucial for maintaining physiological balance and has been linked to neurological disorders.

Deep frying is a prevalent cooking method globally, often used in fast-food restaurants, street vendors, and home cooking.

Studies have associated deep-fried food consumption with cardiometabolic conditions and certain cancers. However, few have examined the long-term effects of consuming reheated cooking oils on polyunsaturated fatty acids (PUFAs) metabolism and disease development.

Dr. Kathiresan Shanmugam, PhD, an associate professor at the Central University of Tamil Nadu in Thiruvarur, India, led the research team to explore this issue.

The study team organized female rats into five groups, fed a normal diet (control group) or a normal diet supplemented with unheated sesame oil, unheated sunflower oil, reheated sesame oil, or reheated sunflower oil daily for a period of 30 days.

This approach was designed to mimic the conditions of consuming reused deep frying oil.

Compared with their counterparts on different diet regimens, rats fed diets with reheated oils exhibited heightened oxidative stress and inflammation in liver tissues.

Additionally, significant colonic damage was observed in these rats, which led to altered levels of endotoxins and lipopolysaccharides, indicating the presence of toxins produced by specific bacterial strains.

In secondary experiments, monosodium glutamate (MSG) was used to promote neurotoxicity in offspring. The offspring fed diets including reheated oils showed greater susceptibility to neuronal damage than the control group fed no oils or diets with unheated oils.

In a press release, Shanmugam explained:

“As a result, liver lipid metabolism was significantly altered, and the transport of the important brain omega-3 fatty acid DHA was decreased. 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.”

The study revealed that diets inclusive of reheated oils led to escalated levels of cholesterol, LDL cholesterol, triglycerides, AST and ALT, and inflammatory markers, alongside considerable damage to liver and colon structures, pointing to potential cardiometabolic and organ harm.

Consumption of reheated oils also resulted in specific brain damage, especially in areas crucial for regeneration, highlighting the neurological risk of reheated oil consumption.

In contrast, rats fed unheated oils showed better markers for brain health compared to rats in reheated oil groups.

Heating oils to high temperatures significantly alters their natural chemical structure, reducing their beneficial antioxidants and forming harmful compounds such as trans fats, acrylamide, and aldehydes.

Reheating oils, especially for deep-frying, further exacerbates this process as the oil becomes increasingly unstable, losing health benefits and generating more toxins with each use.

Alyssa Simpson, registered dietitian and owner of Nutrition Resolution, who was not involved with the study, explained that repeated heating causes oils to break down, “resulting in changes in fatty acid composition, and increased levels of lipid oxidation products such as reactive oxygen species (ROS).”

An imbalance in ROS and biological antioxidants can trigger oxidative stress in the brain, which, according to Simpson, potentially damages neurons and heightens the risk of neurodegenerative diseases.

Additionally, Simpson emphasized the presence of oxidized fats and advanced glycation end products (AGEs) in reheated frying oils, which are linked to chronic diseases, including neurodegenerative conditions.

Dr. Alexandra Filingeri, a registered dietitian and doctor of clinical nutrition who was not involved with the study, agreed, and clarified the negative effects of reheating cooking oil on its nutritional content.

“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,” she shared with Medical News Today.

While more research is needed, reheated oils have been linked to increased levels of cholesterol and inflammation, which are risk factors for cardiometabolic diseases and neurodegenerative disorders.

The liver, which filters and detoxifies substances in our bodies, may be particularly vulnerable to damage from reheated oils.

According to Simpson, “[r]epeated intake of oils subjected to high temperatures could disrupt liver lipid metabolism,” potentially leading to nonalcoholic fatty liver disease (NAFLD) and accelerating liver diseases due to ROS-induced oxidative stress.

Additionally, Simpson and Filingeri voiced concerns about oxidized fats affecting gut microbiota and intestinal permeability, potentially leading to dysbiosis, inflammation, and gut barrier dysfunction.

Filingeri highlighted that maintaining a healthy gut microbiome is essential for liver health, as an imbalance could result in harmful bacteria penetrating the liver via increased intestinal permeability, thus causing oxidative stress and inflammation.

This disruption affects the liver-gut-brain axis, critical for neurological health through its metabolic, immune, and hormonal communication pathways, potentially leading to neuroinflammatory conditions and neurological disorders, according to Simpson.

Furthermore, Simpson elaborated that disturbances in the metabolism of specific lipids could disrupt brain cell communication and function, potentially damaging neurons.

To mitigate the harmful effects of reheated oil consumption, Simpson recommends diets rich in omega-3 fatty acids and nutraceuticals like curcumin and vitamin E, which can be found in turmeric, almonds, and other foods.

According to Simpson, incorporating antioxidants, fiber, and polyphenols from fruits, berries, vegetables, nuts, and green tea can reduce oxidative stress and inflammation, thus protecting the brain.

Additionally, Filingeri emphasized the importance of probiotics from foods like kefir and kimchi for gut and liver health.

Following overall healthy dietary patterns like the Mediterranean or MIND diet may help prevent neurodegeneration.

On the other hand, “[f]requent intake of fried foods may contribute to weight gain, insulin resistance, and metabolic syndrome, further elevating the risk of chronic diseases,” Simpson cautioned.

When cooking with oils, “research suggests quality of oils before heat exposure matters,” explained Filingeri.

“When choosing cooking oils consumers should choose oils high in polyunsaturated fats and choose preparation methods that require low heat,” she stated. “Oils should not be reused for repetitive cooking sessions and oils should not be heated to high [temperatures].”

Simpson recommended rotating cooking oils and monitoring oil temperature in establishments that use deep frying to prevent overheating and excessive oxidation.

To further avoid unhealthy oils, the experts suggested consumers ask about cooking oils used in restaurants and choose dishes prepared through healthier methods like grilling or baking.

This new research suggests that regular consumption of reheated oils may disrupt liver function and increase oxidative stress, leading to a heightened risk of neurodegenerative diseases.

Although this study was conducted on rats, its results highlight the possible health hazards linked to the consumption of reheated oils and advocate for mindful dietary choices.

Regarding research directions, Simpson concluded:

“Future research should prioritize understanding how reheated oils impact liver lipid metabolism, gut health, brain health, and neurodegeneration, focusing on elucidating specific mechanisms and potential therapeutic interventions.”