How we use cooking oil can significantly affect our health.

13/10/2025 (Borneo Bulletin) - According to toxicologist at Universiti Kebangsaan Malaysia Associate Professor Dr Razinah Sharif, repeatedly reheating oil can make it unsafe to consume.

Each time oil is exposed to heat, its fat molecules begin to break down and become unstable, producing harmful substances like aldehydes and free radicals.

These by-products can lead to oxidative stress in the body, which damages healthy cells and tissues.

Prolonged oxidative stress has been associated with chronic diseases, including cancer.

Interestingly, different oils react differently when exposed to high temperatures.

“If you compare different oils at the same temperature, palm oil actually produces the lowest amount of aldehydes,” said Dr Razinah.

“People often assume palm oil is the worst option, but for deep frying at high temperatures, it generates fewer aldehydes than oils such as olive or canola.”

The two biggest concerns are aldehydes and trans fats.

Aldehydes are strongly associated with cancer risk, while trans fats are known to raise the risk of heart disease.

While most people link oil consumption only to heart health, she stressed that its effects extend far beyond that, disrupting many other processes in the body.

HOW FAST IT BREAKS DOWN

Just as different chemicals break down in their own ways, oils also degrade differently when exposed to heat.

“Different oils have different chemical bonds,” she explained.

“For example, canola oil, olive oil, palm oil and coconut oil each react differently depending on their structure.”

This is why fats are classified as saturated, monounsaturated and polyunsaturated.

Their stability depends on the bond type, cooking temperature and how often the oil is reused.

Even within palm oil, there are notable differences.

Dr Razinah said: “The cheaper versions – usually sold in plastic packets – go through heavier refining and are less stable, meaning they should only be used once or twice.

“While premium bottled versions are more stable and can withstand higher heat without breaking down as quickly.”

Lower-grade oils also become rancid more easily. Instead of staying golden yellow, they quickly turn brown, or even black, after heating, especially when reused.

“Restaurants often reuse oil continuously to cut costs, which can be seen in fried snacks such as ayam gunting or churros, where the oil foams, turns dark and leaves the food looking almost burnt.”

She added “This discolouration is a sign of oxidation, which isn’t something you’d want to eat.”

Palm oil, like all oils, does degrade, but it is more stable for deep frying than olive or coconut oil, making it a practical choice for high-heat cooking. Olive and canola oil are good choices for stir-frying, but they are pricier in general than palm oil.

Still, the real issue is not the oil itself, but how much of it we use.

HOW MUCH YOU USE

According to Dr Razinah, there is a guiding principle in toxicology: “The dose makes the poison.”

In other words, anything – even vegetables or vitamins – can be harmful in excess.

Steaming or air frying are healthier alternatives, but if frying is your choice, then high-quality palm oil remains a sensible option, provided you avoid cheap versions that have been reused repeatedly.

Asked which oil is the “healthiest”, the toxicologist emphasised that it depends on the cooking method.

For stir-frying, olive oil, canola oil, coconut oil and even red palm oil, are good choices.

Palm oil has its benefits and is among the best for high-heat cooking, but the problem lies in cultural habits, where repeatedly using the oil for deep frying is common.

HOW TO TELL THE OIL’S OFF

How do you know then when you have used your cooking oil too much?

“The only way to know if oil has gone bad is to look for signs,” pointed out Dr Razinah.

These include a rancid smell, a shift in colour from golden yellow to dark brown or black, or foaming and bubbling when heated.

As a rule of thumb, cheap packet palm oil should only be reused once or twice, while bottled oil may last three to four uses, although this also depends on what is being fried.

Anchovies or foods that leave burnt bits can spoil oil after just one use, while lighter items like crackers may allow a little more reuse.

Once oil darkens, foams or smells stale, it should be discarded.

Straining debris does not make degraded oil safe again.

Reusing oil too many times, whether palm, vegetable, canola or coconut, poses health risks. Families can reduce these risks by limiting frying, using air fryers, or choosing stable oils like palm oil for high-heat cooking.

At home, it’s easier to manage.

But in eateries, deep fryers often run for hours without oil changes.

Continuous high heat quickly degrades oil, sometimes within a single day.

“At roadside stalls, you may notice oil that looks cloudy, dark or gives off a strong smell, but in most restaurants, it remains hidden in the kitchen.

Adding to the problem, there may not be any regulations requiring food operators to change frying oil.

“Some stalls may replace it several times daily, while others keep the same batch for weeks, unaware of the risks.

“For consumers, the safest approach is simple: avoid food cooked in oil that looks blackened, tastes unpleasant or smells off.” – Emalin Zalani

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