Frying is one of the most widely used cooking methods around the world. From restaurants and fast-food establishments to home kitchens, millions of people consume foods prepared in oils exposed to high temperatures every day. However, behind this seemingly harmless practice lies a biochemical reality that has attracted growing attention from researchers, oncologists, and public health experts: what happens when cooking oil is heated over and over again?

The reuse of cooking oils is a common practice largely driven by economic considerations. Nevertheless, numerous studies have demonstrated that each heating cycle profoundly alters the chemical structure of oil, promoting the formation of potentially toxic compounds capable of affecting human health. Over the past two decades, scientific evidence has suggested that prolonged exposure to these degradation products may be associated with chronic inflammation, cellular damage, genetic alterations, and even an increased risk of developing certain types of cancer.

 

What happens when oil is repeatedly heated?

Vegetable oils are composed primarily of triglycerides and various types of fatty acids. Under normal conditions, many of these compounds provide important nutritional benefits, supplying energy and participating in essential cellular functions.

However, when oil is repeatedly exposed to temperatures exceeding 180°C (356°F), a series of complex chemical reactions is triggered. Oxidation, hydrolysis, and polymerization progressively alter the composition of the oil, leading to the formation of free radicals and reactive molecules that were not originally present. As exposure to heat increases, so does the concentration of aldehydes, ketones, lipid peroxides, and other compounds generated through fat degradation.

These changes do not only affect the taste, smell, and appearance of the oil. They also modify its biological properties, transforming an initially safe ingredient into a potential source of harmful substances for the body.

Among the compounds generated during repeated heating are polycyclic aromatic hydrocarbons (PAHs), a family of contaminants extensively studied by the International Agency for Research on Cancer (IARC).

PAHs are formed when fats degrade under high temperatures and can be found both in reused cooking oils and in the fumes released during frying. Some of these compounds, such as benzo[a]pyrene, have been classified as human carcinogens due to their ability to induce genetic damage and promote tumor development.

Several studies have demonstrated that PAH levels increase significantly as oil is reused multiple times. In fact, some investigations have reported concentrations up to six times higher in reheated oils compared with fresh oils. This progressive accumulation is particularly relevant in food establishments where the same oil may be used continuously over several days.

 

From oil to DNA damage

One of the most important findings in modern research is that the byproducts generated from degraded oils do not remain confined to the digestive tract. Once ingested or inhaled, many of these compounds can be absorbed and distributed throughout different tissues in the body.

Certain PAHs are metabolized by cellular enzymes and converted into highly reactive molecules capable of binding directly to DNA. This process results in the formation of structures known as DNA adducts, which many researchers consider one of the earliest steps in carcinogenesis.

When DNA becomes damaged and cellular repair mechanisms fail to fully correct these alterations, permanent mutations may occur. If these mutations affect genes responsible for controlling cell growth, apoptosis, or genomic stability, the likelihood of developing cells with tumor-like behavior increases substantially.

This mechanism has been extensively documented in experimental studies and represents one of the main reasons why repeatedly heated oils have become a growing concern within the scientific community.

 

Chronic inflammation and oxidative stress

Cancer rarely arises as the result of a single factor. Rather, it is typically the consequence of multiple biological processes interacting over many years.

Among these processes, chronic inflammation and oxidative stress play particularly important roles. Oils exposed to high temperatures generate large quantities of reactive oxygen species (ROS), commonly known as free radicals. These molecules can damage proteins, cellular membranes, and genetic material.

Continuous exposure to these compounds promotes a persistent inflammatory state that alters the cellular microenvironment. Today, chronic inflammation is recognized as a key contributor to the initiation and progression of numerous cancers, including colorectal, liver, pancreatic, and prostate cancers.

For this reason, many researchers believe that the impact of reused cooking oils should not be evaluated solely based on their direct mutagenic effects, but also on their contribution to a biological environment that favors the development of chronic diseases.

Figure 1. Thermal degradation of cooking oils generates harmful compounds that can be inhaled or ingested, leading to oxidative stress, DNA damage, and inflammation, which may contribute to cancer development.

 

Which types of cancer have been associated with this exposure?

The scientific review published in Critical Reviews in Food Science and Nutrition identified associations between exposure to repeatedly heated oils and several types of cancer, although the authors emphasized that the strength of the evidence varies depending on the tumor type and the study design.

 

Lung cancer

This is probably one of the most extensively studied associations. Several investigations conducted in Asian populations have found that individuals frequently exposed to frying fumes, particularly in poorly ventilated kitchens, may face an increased risk of lung cancer.

This phenomenon is especially relevant for professional cooks, restaurant workers, and individuals who spend many hours cooking each day. Frying fumes contain ultrafine particles, aldehydes, and PAHs that can reach deep regions of the respiratory tract and cause cumulative damage to lung tissue.

 

Colorectal cancer

Foods fried at high temperatures may contain additional mutagenic compounds, such as heterocyclic amines and acrylamide. These substances have been linked to alterations in the intestinal lining and an increased risk of colorectal cancer in several epidemiological studies.

 

Breast cancer

Research on breast cancer has explored the possible relationship between diets rich in fats exposed to high temperatures and increased tumor risk. Although findings are not entirely consistent, some observational studies suggest that frequent consumption of foods prepared with degraded oils may contribute to greater exposure to carcinogenic compounds.

 

Prostate cancer

Evidence has also pointed to associations between regular consumption of fried foods and an elevated risk of prostate cancer. Proposed mechanisms include the formation of advanced glycation end products (AGEs), oxidative stress, and low-grade systemic inflammation.

 

A balanced perspective: what we know and what remains uncertain

It is important to interpret these findings with caution. Most studies investigating the carcinogenic effects of reused cooking oils have been conducted in animal models, laboratory experiments, or observational populations. This means that while there is a strong biological basis for concern, long-term clinical and epidemiological studies are still needed to more accurately determine the magnitude of the risk in humans.

Modern science rarely provides absolute answers. It cannot be stated that occasionally reusing cooking oil directly causes cancer. However, the accumulation of evidence consistently points toward increased exposure to potentially carcinogenic substances when oils are repeatedly subjected to high temperatures.

Fortunately, several simple measures can significantly reduce exposure to these harmful compounds:

• Avoid reusing cooking oil multiple times.
• Discard oil when it becomes dark, develops an unpleasant odor, or produces excessive smoke.
• Maintain appropriate frying temperatures and avoid overheating.
• Use efficient kitchen ventilation systems.
• Prioritize cooking methods such as baking, steaming, light sautéing, or air frying.
• Incorporate a diet rich in fruits, vegetables, and natural antioxidants that help counteract oxidative stress.

The available scientific evidence suggests that repeated heating of cooking oils generates a complex mixture of chemical compounds with genotoxic, inflammatory, and carcinogenic potential. Although further human studies are needed to precisely define the associated risk, current findings support limiting the consumption of foods prepared with reused oils and minimizing exposure to frying fumes.

Cancer prevention does not depend on a single decision but rather on the accumulation of multiple daily habits. In this context, paying attention to the quality of the cooking oil we use may seem like a minor detail, yet it represents a simple and accessible opportunity to promote safer eating habits and protect long-term health.

 

Main reference:

Ganesan, K., Sukalingam, K., & Xu, B. (2019). Impact of consumption of repeatedly heated cooking oils on the incidence of various cancers- A critical review. Critical reviews in food science and nutrition, 59(3), 488–505. https://doi.org/10.1080/10408398.2017.1379470 

 

Other references:

Ganesan, K., & Xu, B. (2020). Deep frying cooking oils promote the high risk of metastases in the breast-A critical review. Food and chemical toxicology : an international journal published for the British Industrial Biological Research Association, 144, 111648. https://doi.org/10.1016/j.fct.2020.111648 

Yao, Z., Li, J., Wu, B., Hao, X., Yin, Y., & Jiang, X. (2015). Characteristics of PAHs from deep-frying and frying cooking fumes. Environmental science and pollution research international, 22(20), 16110–16120. https://doi.org/10.1007/s11356-015-4837-4 

Srivastava, S., Singh, M., George, J., Bhui, K., & Shukla, Y. (2010). Genotoxic and carcinogenic risks associated with the consumption of repeatedly boiled sunflower oil. Journal of agricultural and food chemistry, 58(20), 11179–11186. https://doi.org/10.1021/jf102651n