A fatty acid in which there are one (monounsaturated) or more (polyunsaturated) double bonds in the fatty acid chain. In cellular metabolism hydrogen-carbon bonds (H-C) are broken down (oxidized) to produce energy, thus an unsaturated fatty acid molecule contains less energy (i.e., fewer calories) than a comparable sized saturated fatty acid, which has more hydrogen-carbon bonds. The more unsaturated the fatty acid, the more liquid the fat is.

The greater the degree of “unsaturation” (i.e., the more double bonds) in a fatty acid, the more vulnerable it is to the damaging effects of free radicals. Antioxidants can protect unsaturated fat from oxidative damage (“lipid peroxidation”).