Circle of fatty acid metabolism. Schematic pathway of conversion of 18 carbon essential fatty acids to 20 and 22 carbon polyunsaturated fatty acids via chain elongation and desaturation and finally beta-oxidation. The pathway is drawn in a circle to highlight the fact that there is only a single delta-6 desaturase in mammalian systems that is utilised twice. This results in up to six fatty acids completing for access to the delta-6 desaturase at any one time depending on dietary fat intake.