Fatty acids containing one or more carbon-carbon double bonds. Those with a single double bond are termed monounsaturated (e.g., oleic acid); those with two or more are polyunsaturated (e.g., linoleic and alpha-linolenic acids), including the essential fatty acids humans cannot synthesize. Unsaturated fatty acids are abundant in vegetable oils, nuts, seeds, and oily fish, and replacing saturated fats with them improves blood lipid profiles. Their kinked chains keep membranes fluid and oils liquid, but the double bonds are oxidation sites: lipid peroxidation generates rancid off-flavours and potentially harmful compounds, so processing and storage must limit heat, light, and oxygen exposure.