Prolonged starvation, the liver will release glycogen in response to the glucagon secreted from the pancreas as a result of low blood glucose level. Glucagon synthesis is inhibited by glycogen by triggering cyclic AMP. In the meantime, glucose is synthesised by the hydrolysis of glucose 6-phosphate derived from glycogen. The glucose is released from the liver into the blood to increase its level of glucose.

The lipids are mobilised from adipose tissues as free fatty acids to supply energy for the muscle tissues. This involves the preparation of malonyl CoA from acetyl CoA and Co2 by acetyl CoA carboxylase. Carbons are then added in pairs to a growing fatty acyl chain in a four-step sequence by fatty acid synthase. The acetyl CoA formed can then be shunted to ketone bodies in the liver for energy production. 2 acetyl CoA are joined to release 1 CoA and 1 acetoacetyl CoA. The acetoacetyl CoA is then converted into acetoacetate where it can futher decompose into acetone or reduced into beta-hydroxybutyrate. Both acetone and beta-hydroxybutyrate are ketones that can be exported to muscle tissues to provide them with energy.

Fatty acid synthesis is effectively the reverse of degradation process.

The final synthesized product is a polymer, and hence obviously the process is initiated with the collecting of monomers together.

Considering the case of an activated acyl group and malonyl units, the malonyl unit is condensed with the acetyl unit and results in the formation of a four-carbon fragment. The carbonyl is reduced in order to produce the required hydrocarbon chain. The process will proceed exactly opposite to that of degradation process.

[Activated acyl group (monomer) + Activated malonyl group (monomer) ]

↓

Condensation

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Reduction

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Dehydration

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Reduction

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Activated acyl group (polymer with additional two carbon atoms)

The resulted four-carbon fragment will be reduced, dehydrated, and reduced again for carrying the carbonyl group to the level of a methylene group along with butyryl CoA formation.

Also, another activated malonyl group condenses with the butyryl unit and the process will continue until a C16 fatty acid is synthesized.