In a cross-sectional study of 1,226 people (aged 18-65 years) living in Pizarra, Spain, the presence of excess concentrations of polar compounds (defined as > 20%) in home cooking oil was associated with an increased risk of hypertension (p < 0.01). Of the two main oils available in Spain for cooking, the use of sunflower oil was associated with an increased risk of hypertension, whereas the use of olive oil was associated with a decreased risk. These associations persisted after adjustment for age, gender, obesity, and the presence of abnormal carbohydrate metabolism. The presence of high concentrations of polar compounds in cooking oil was presumably due in large part to the reuse of these oils one or more times for cooking.

Comment: During the process of frying with oils, new compounds are formed as a result of oxidation, polymerization, and hydrolysis. Polyunsaturated fatty acids, which are present in relatively large concentrations in sunflower, corn, and soybean oil, are much more susceptible to thermal degradation than are monounsaturated or saturated fatty acids, which are present in higher concentrations in olive oil and peanut oil, respectively. The results of the present study suggest that repeated heating of vegetable oils that are high in polyunsaturated fatty acids results in the formation of toxic compounds that increase the risk of hypertension. By implication, it is probably not a good idea to use these oils at all for frying. Olive oil and peanut oil appear to be safer oils with which to fry. While butter and lard do not contain high amounts of unstable polyunsaturated fatty acids, they do contain cholesterol, which can be converted to blood vessel-damaging cholesterol oxides during high-temperature cooking.

Soriguer F, et al. Hypertension is related to the degradation of dietary frying oils. Am J Clin Nutr 2003;78:1092-1097.