Cardiovascular disease (CVD) is one of the leading major causes of morbidity and mortality worldwide. It may result from the interactions between genetic and environmental factors including sedentary lifestyle and dietary habits. Vegetable oil is one of the essential nutritional components in daily food consumption. However, the benefits of vegetable oil can deteriorate by repeated heating which leads to lipid oxidation. The practice of using repeatedly heated cooking oil is not uncommon as it will reduce the cost of food preparation. Thermal oxidation yields new functional groups which may be potentially hazardous to cardiovascular health. Prolonged consumption of the repeatedly heated oil has been shown to increase blood pressure and total cholesterol and cause vascular inflammation as well as vascular changes which predispose to atherosclerosis. The harmful effect of heated oils is attributed to products generated from lipid oxidation during the heating process. Given the potential hazard of oxidation products, therefore this review article will provide insight and awareness to the general public on the consumption of repeatedly heated oils which is detrimental to health.
Introduction
According to a health statistic gazetted by the World Health Organisation, non-communicable diseases (NCDs) will be causing more than three-quarters of all deaths in 2030. Of the NCDs, cardiovascular disease (CVD) represents a major risk to worldwide deaths, in the number of mortality from CVD is expected to skyrocket from 17.1 million in 2004 to 23.4 million in 2030.
CVD is collectively defined as a disease that affects the heart and blood vessels. CVD includes coronary heart disease, stroke, rheumatoid heart disease and Chagas disease. While rheumatoid heart disease and Chagas disease are caused by infections, the aetiology of coronary heart disease is closely associated with the development of atherosclerosis. CVD develops slowly due to long-term exposure to several behavioural risk factors such as cigarette smoking, lack of exercise, constant stress, overweight and consuming a diet with high saturated fat contents. Nevertheless, nutritional oil quality is also recognised as an important player in the aetiology of CVD.
In general, the practice of reusing oil which has been repeatedly heated during food preparation is widespread. This practice is not confined only to roadside food stalls, but also to more established food outlets in big cities. It is well-established that heating of dietary oils and fats results in lipid oxidation. Lipid oxidation involves the oxidative destruction of fatty acid chains, consequently leading to the generation of polar compounds and other secondary side products. Furthermore, vitamin E, which is a natural antioxidant, in the oil also deteriorates after repeated heating. Lipid oxidation does not only spoil the taste, smell and colour of oil, but intake of the oil may also impose an unmitigated threat to health. Previous reports have highlighted a direct relationship between CVD risk and the consumption of cooking oil polar compounds.
However, the safety of reusing oil in food preparation is still rather questionable, partly due to limited human studies and some contradictory findings in animals. Because heated oil contains a complex chemical environment, it is thus difficult to identify the respective effects of each oxidation product. Yet, since dietary lipid intake is closely related to the pathogenesis of CVD, it is imperative to understand the effects of repeatedly heated vegetable oil on CVD risk factors. Using comprehensively consulting the published literature, we reviewed the current knowledge concerning some important experimental and clinical effects related to the intake of reheated oil on CVD risk factors including blood pressure, serum lipids and endothelial dysfunction. A better understanding of the mechanisms of reheated oil pathologies helps to serve as a guide to consumers concerning the proper usage of vegetable oils in the preparation of a healthy meal.
Effects of heating on vegetable oils
In deep-fat frying, the oil is usually heated to an extreme temperature of 180 °C and above. At the same time, it is also exposed to moisture and air, resulting in a vast spectrum of chemical reactions collectively known as lipid oxidation. It is a complex phenomenon provoked by oxygen in the presence of initiators such as heat and free radicals.
Hypertension
Epidemiological data has shown that lifestyle factors such as dietary habits are the main driving factors of the high prevalence of hypertension. The quality and quantity of fats and oils in a diet are important in determining the risk of CVD. Various studies in humans and animals have been done to determine the role of reused cooking oil in hypertension.
Conclusions
Based on the information discussed above, consumption of diets containing heated vegetable oils could be harmful to the cardiovascular system. The cardiovascular protective benefits of vegetable oils deteriorate when the oils are repeatedly exposed to extreme temperature, moisture and air during food preparation, particularly in deep-fat frying. Through a complex series of reactions that occurs during deep-fat frying, various oxidation products are formed, affecting the quality of oil and eventually the cardiovascular system.