The marketing of vegetable oils as heart-healthy alternatives to saturated fat was based on their lower saturated fat content and higher polyunsaturated fat content. This logic was sound in isolation but overlooked the distinction between omega-3 and omega-6 polyunsaturated fats, and it overlooked the stability of these oils under heat and the absence of the polyphenols that give olive oil its specific health effects. Decades later, the evidence clearly distinguishes between these oil categories.
Oxidative Stability and Heat
Polyunsaturated fatty acids are chemically unstable under heat. When refined seed oils high in linoleic acid are used for cooking at high temperatures, the double bonds in the fatty acid chains oxidise to produce aldehydes including 4-hydroxynonenal and malondialdehyde, both of which are cytotoxic and mutagenic. Extra-virgin olive oil, which is predominantly oleic acid (a monounsaturated fat with only one double bond), is significantly more heat-stable and produces substantially fewer oxidation products when used for frying or roasting.
Polyphenol Content
Extra-virgin olive oil is unique among cooking oils in retaining significant polyphenol content: oleocanthal, oleuropein, and hydroxytyrosol at concentrations that produce measurable biological effects. These polyphenols are responsible for the peppery sensation at the back of the throat from high-quality extra-virgin olive oil, they inhibit inflammatory enzymes, they reduce LDL oxidation, and they have anti-cancer effects in cell culture. Refined seed oils contain none of these compounds.