Dietary lipids have the capacity to regulate many aspects of metabolism in a manner depend on chain length and the number, and position, of double bonds in the fatty acids they contain. For example, while long chain saturated fatty acids increase plasma cholesterol, polyunsaturated fatty acids (PUFA) have the opposite effect. PUFA also have a major impact on the activity of enzymes associated with carbohydrate metabolism and lipid biosynthesis and oxidation. The role of fatty acids as substrates in these pathways, and their conversion to eicosanoids, with different levels of activity depending on the parent molecule, represents two mechanisms whereby they exert such effects. However, more recently it has been established that fatty acids and/or their derivatives also have direct effects on the expression of genes for proteins regulating carbohydrate and lipid metabolism.