Trans fatty acids (TFAs) create deleterious effects; thus their existence in humans is a great health concern. TFAs can be obtained through diet, or they can be formed endogenously by radical-induced cis to trans isomerization. The mechanism of isomerization of fatty acid catalyzed by radicals including nitrogen dioxide (NO2 center dot), thiyl (RS center dot), and peroxide (ROO center dot) radicals were investigated using density functional theory. With linoleic acid, a fatty acid consisting of two homoconjugated C=C bonds, we found that the radical addition mechanism is more favorable than the hydrogen abstraction mechanism. For all investigated radicals, the isomerization catalyzed by RS center dot radical involves the smallest reaction barrier. We found that NO2 center dot reactions through the N-terminus are more favorable than reactions through the O-terminus. The reaction barriers for NO2 center dot catalyzed isomerizations were found to be lowered to a larger extent in polar solvent. beta-carotene and lycopene were shown to protect fatty acids from isomerization by intercepting the isomerization-causing radicals.