| The majority of fatty acids required by man are supplied in the diet; however, pathways for their de novo synthesis (lipogenesis) from 2-carbon compounds are present in many tissues such as liver, brain, kidney, mammary gland, and adipose tissue. In general, the pathway of de novo synthesis is used in conditions of excess energy, and in particular, carbohydrate intake. In such conditions carbohydrates are converted to fatty acids in the liver and stored as triacylglycerols (TAG; also known as triglycerides) in the adipose tissue. In man, adipose tissue is not an important site of synthesis of fatty acids; the main organ of lipogenic activity is the liver. Lipogenesis does not appear to be a critical requirement in humans, and no life-threatening illnesses associated with its malfunction have been identified. It does, however, have an important bearing on the development of obesity, and is inhibited in type 1 diabetes mellitus.
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| The pathway for lipogenesis is not simply the reverse of oxidation of fatty acids (see Chapter 14). Lipogenesis requires a completely different set of enzymes from lipolysis. Also, it is located in a different cellular compartment, the cytosol, and uses nicotinamide dinucleotide phosphate (NADP+) as a source of reductive power, as opposed to the nicotinamide dinucleotide (NAD+) that is required for β-oxidation of fatty acids.
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