The health effects of dietary fats are complex and not fully understood. While most dietary fats are neutral in terms of their health effects, there are a small number of fats that can be classified as detrimental to human health. Of these, highly processed deodorised supermarket cooking oils are one category of fats that have been consistently shown to have adverse affects on health. Supermarket oil contain a number of compounds that are chemically altered from their natural form, such as trans fatty acids, which are implicated in the development of cancer and cardiovascular disease. Using these oils to cook further increases the chemical changes because the heat applied to the oil can create both volatile and non-volatile derivatives. The non-volatile derivatives remain in the oil or can be absorbed by food, and if consumed can have serious implications for health.
The non-volatile compounds found in cooking oil are products of thermo-oxidation and can be classified as polymers and polar compounds. Re-use of cooking oil can drastically increase their concentration within the oil. Researchers1 used a cross-sectional designed study to investigate the effects of cooking oils on various anthropometric measurements. Cooking oils were taken from the kitchen of 1226 individuals aged between 18 and 65 and analysed for polar compounds and polymers that were used as markers for the deterioration of the oil chemically. The researchers found that hypertension was associated with the use of sunflower oil and the presence of polar compounds in the cooking oil. In contrast, use of monounsaturated fat was inversely associated with hypertension. Both these associations remained after the researchers controlled for age gender, obesity and the presence of carbohydrate metabolism disorder.
Of the oil analysed, >20% contained polar compounds, which suggested excessive re-use of the oil. However, the polar compounds varied between different cooking oils suggesting that some were more resistant to chemical breakdown that others. Olive oil appeared to be more resistance to the development of polar compounds, with just 6.2% of samples containing significant amounts of polar compounds. In contrast 11.9% of sunflower oil and 16.2% of mixed oils contained polar compounds. The monounsaturated fatty acids in olive oil are more resistant to chemical alteration by heat and light because they contain fewer double bonds in their structure than polyunsaturated fatty acids present in sunflower oil. These results support previous findings that suggest that processing and heating unsaturated oils produces chemical compounds that are detrimental to health. The use of vegetable oil for cooking, especially sunflower, in an independent risk factor for hypertension.
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