Potential health implications of the consumption of repeated and overheated cooking oils

Abdulrahman Olagunju FASLN & Micheal Chukwudi FASLN

Nigerians are known to be lovers of tasty meals and globally, cooking oil continues to play a crucial role in the preparation of such meals.


Cooking oils are integral part of a human diet as they are used in almost all types of food preparations including frying, baking, and extrusion cooking. However, they are either consumed fresh or thermally oxidized- i.e when the fresh form of cooking oil is heated at high temperatures during various food preparations to increase palatability. This is a usual domestic practice in Nigeria, and is not only limited to various homes; but also, a regular practice in restaurants and the commercial food industry.


Understanding the Science of Cooking-oils

Cooking oils are usually extracted from seeds, kernels, or mesocarp (middle layer) of the fruit of plants. The initial crude forms of the oils are usually refined, bleached, and deodorized before being marketed as a golden yellow liquid product—the color preferred by most consumers.


However, fat molecules known as triglycerides(TGs) form the bulk (95%–99%) of cooking oils, with micro components (such as carotenoids, vitamin E, and other antioxidants) forming the remaining 1%–5%.


"Cooking oils are of different types. There are saturated oils (which are usually solid at room temperature) and unsaturated oil which you see in the liquid form. However, the unsaturated oils are healthier than the saturated ones,"says Dr Atta ur Rehman Madni, a clinician nutritionist.


"[Moreso], there are different types of unsaturated oils - the polyunsaturated and monounsaturated oils. But the best type of oils are the ones rich in monounsaturated fatty acids,"he added.


On a nutritional viewpoint, cooking oil can contribute up to two-thirds of the total fat in our diet and therefore form the major dietary fat as well as an important source of energy. Most importantly, cooking oils and other edible fats provide us with the essential fatty acids that our body cannot synthesise.


Oil and Heat: A complicated relationship

The major use of cooking oil is in frying, where it functions as a heat transfer medium and contributes flavour and texture to foods.


"All fats, including the oils you typically use in the kitchen, have a smoke point, the point at which they will begin to produce smoke. Specifically, it’s when anything in the oil — fats, proteins, sugars, and other organic material — starts to interact with oxygen and burn," Joseph Provost, a chemistry and biochemistry professor at the University of San Diego who co-wrote The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking highlighted in The Washington Post.


Smoke point refers to the temperature at which an oil starts to burn and smoke. "At the smoke point, the oil gives off smoke, fat molecules begin to break down, evaporate and settle down on kitchen walls and utensils. Besides adversely affecting the taste and flavor of the cooked product, its chemical composition changed, thereby making it toxic," he added.


"Any type of cooking that requires repeating or reusing cooking oils multiple times, makes the properties of the oil become deteriorated. The metabolic change and other chemical reaction that takes place makes the quality of the oil poor." Madni said


Health Implications of thermally oxidized cooking oils

Studies have shown that continuous heating and reheating of oil aids its transformation into trans fats, which not only raises the bad cholesterol - low density lipoprotein (LDL) - levels in the body, but also lowers the good cholesterol - high density lipoprotein (HDL) levels.


"The chemical changes that take place in reheating of cooking oils will increase the percentage of trans fats - a form of fat which is harmful to the body- thereby leading to an increase in the bad cholesterol profile known as low density lipoprotein (LDL),"madni said.


The cholesterol usually formed from this fat travel to different parts of the body system through the blood, alongside the transportation of vitamins and minerals. However, once there's an increase in the bad cholesterol (LDL), it builds up on the walls of the blood vessels, thereby forming what is known as plaque,which overtime narrows the blood vessels leading to different health risks especially, heart related diseases.


"The practice of reheating oils, which changed the chemical composition of the oil into trans fats, leads to increased bad cholesterol, decrease of good cholesterol (HDL), blockage of blood vessels as well as higher risks of dietary related diseases such as diabetes, stroke, heart attack, high blood pressure amongst others,"says Dr Folu Olatona, a nutrition and wellness expert, cum a consultant public health physician at the college of medicine, University of Lagos, Nigeria.

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Alternative ways of using of cooking oils

"There are lots of alternatives to using cooking oils, but it still boils down to how you're using the oils. You can't be reheating these oils and expect that it will still be good for your body," Yetunde Adeseluka-Oladejo, a public health professional and Nutritionist explained.


"Oils such as Avocado, butter as compared to margarine oils, olive oil, mustard oil, canola oil, coconut oil and palm oil- which is usually free of trans fat- are best alternatives to cooking oils,"she added.


Furthermore, "for optimal use of cooking oils, it is necessary to distinguish between different frying conditions. The most important parameters to be monitored are duration of use and nature of the foods to be fried. If food fats enter the frying oil, food components could destabilize the oil and the water content of the material could influence the frying operation. Whether the use is continuous or intermittent is relevant, continuous use provides a protective water vapour blanket that protects against oxidation," according to the report of a joint expert consultation organized by the FAO and WHO.


Moreover, "the quantity of oil to be used when cooking should be kept as low as possible because they are rich in energy, which makes it prone to the risk of "metabolic syndrome"- a cluster condition that increases the risk of heart disease, stroke and diabetes,"says Dr Madni.


"Alternatively, oils can also be added to food when it's almost done in order to avoid overheating and excess oil on food,"Dr Olatona added.


Moving from Advocacy to Action.

Several studies, and reports had suggested that repeated and/ overheating of cooking oils at high temperatures during cooking brings about changes in the physiochemical characteristics of the oil and also involves various chemical reactions which makes them harmful for human consumption.


Nevertheless, this has become a common practice due to low levels of awareness among the public about the bad effect of this practice. "We need to continually educate the public on the need to understand the use of cooking oils. We need nutrition education on a continual basis which can be done through jingles on radio and television programs. This will help the public to change their attitude and perception on the use of these oils,"Adeseluka-Oladejo noted.


Furthermore, it's of urgent need that government agencies and research institutes related to the development and certification of foods related substances take necessary action in order to curb the menace of putting people's life at risk due to ignorance and negligence.


"There are government agencies such as NAFDAC with policies on ground, however, the level of implementation of these policies is very low,"Adeseluka-Oladejo bemoaned. "These policies should be implemented and enacted on people especially food vendors, restaurants and hotels, such that it targets nutrition education in order to improve their awareness and attitude,"she added.


Heat, Light and Air - The Final Piece of Advice

Experts identified heat, light and air as the three villainsthat are working against the health benefits of cooking oils. They explained that:


Sunlight degrades the quality of oil hence keeping it in a dark-coloured bottle. Excessive heat or prolonged exposure to light will speed up deterioration of flavour, and temperature around 30ºC is ideal.


Also, if the oil has constant access to fresh air, the process of oxidation starts and it deteriorates faster. The bottle should have a tight-fitting cap to avoid this.

Finally, keep the oil in clean and dry containers because exposure to moisture will subject it to oxidation, eventually leading to rancidity.

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