The conversion of saturated fatty acids to unsaturated fatty acids in the body. Desaturation occurs in the endoplasmic reticulum and in the microsomes. The enzymes that catalyze desaturation are the Δ9, Δ6, or Δ3 desaturases. They are sometimes called mixed function oxidases because two substrates (fatty acid and nicotinamide adenine dinucleotide phosphate [NADPH]) are oxidized simultaneously. Fatty acid desaturation can be followed by elongation and can be repeated such that a variety of mono- and polyunsaturated fatty acids are formed. The body converts the dietary saturated fatty acids (S) to unsaturated fatty acids (P) in order to maintain an optimal P:S ratio in the tissues. There is one desaturation reaction that cannot occur in mammals: the conversion of 18:1 to 18:2. Further, felines cannot elongate and desaturate 18:2 to 20:4, arachidonic acid. The activity of the desaturases can be increased through feeding high-saturated-fat and/or high-sugar diets. Both dietary maneuvers increase the need to synthesize unsaturated fatty acids. Desaturase activity is stimulated by insulin, triiodothyronine, and glucocorticoid. Desaturase activity is decreased when high polyunsaturated fats are fed.