Craving crispy fried chicken or golden french fries is hard to resist, but have you ever wondered how many times that bubbling oil has been used? Family medicine specialist Dr. Li Sze-yin is warning consumers that repeatedly heating cooking oil can turn it into a toxic “forever oil” loaded with carcinogenic compounds. In a recent Facebook post, Dr. Li highlighted a chilling study: vegetable oil used to fry potatoes 60 times contained nearly 28% polar compounds — exceeding the internationally accepted safety limit of 25% for discarding frying oil.
The Hidden Dangers Inside Overheated Oil
When oil is subjected to repeated high-temperature cooking, it accumulates harmful contaminants, including 3-MCPD and glycidyl esters. Both substances are considered potential carcinogens, and regular consumption may pose a serious long-term health threat.
Dr. Li also pointed to animal research showing alarming health effects. Mice fed oil that had been reheated multiple times suffered from liver damage and even showed precancerous changes. The used oil contained polycyclic aromatic hydrocarbons, cancer-risk compounds that are absent in fresh oil. These contaminants caused oxidative stress in liver cells, reducing antioxidant defenses and triggering early signs of malignant transformation.
Another experiment involved heating oil to 300°C for up to four hours before feeding it to rats for 45 days. The results were striking: longer heating times led to higher levels of “bad” low-density lipoprotein cholesterol and triglycerides, while “good” cholesterol dropped. The animals also developed fatty liver disease and leptin resistance — both key precursors to cardiovascular disease.
Why Does Reused Oil Become So Harmful?
The problem lies in the chemistry of frying. At high temperatures, oil reacts with air and moisture released from food, triggering oxidation, hydrolysis, and polymerization. These reactions break down fatty acids into polar compounds and free radicals. When consumed, these unstable molecules overwhelm the liver’s antioxidant defenses, promoting lipid peroxidation and chronic inflammation throughout the body.
3 Warning Signs Your Oil Is Unsafe
While diners cannot control how often a restaurant changes its oil, Dr. Li suggests watching for these three signs — whether you are inspecting a street stall or cooking at home:
- Dark, cloudy appearance: Fresh oil should look clear and golden. Deep brown or murky oil is a sign of severe oxidation.
- Persistent foaming: Some bubbling during frying is normal, but thick foam that refuses to subside indicates the oil has broken down.
- Strong rancid smell: A sharp, acrid, or stale odor means the oil has reached the end of its usable life and should be discarded immediately.
What You Can Do
For home cooks, the safest approach is to use fresh oil for each deep-frying session, or at least avoid overheating and reuse beyond one or two batches. Consumers can also reduce exposure by choosing eateries that maintain transparent cooking practices — if the oil in the fryer looks dark or smells off, it is best to skip the order.
On a broader level, food businesses can adopt test strips or digital oil quality meters to monitor total polar materials, ensuring frying oil is changed before it becomes a health hazard. As research continues to reveal the long-term impact of dietary toxins, both regulators and restaurants may need to tighten guidelines to protect public health.
For now, a simple rule applies: if the oil looks dirty, smells sour, or foams excessively, it is time to say no to that fry.