Schematic diagram of fatty acid metabolism in the fasted state. Counter-regulatory hormones such as catecholamines act on adipocytes to increase lipolysis via hormone-sensitive lipase (HSL). Circulating FFA enter the cell and are converted to acyl CoAs, catalyzed by acyl CoA synthase (ACS). Acyl CoA enter the mitochondria via carnitine palmitoyl transferase-I (solid square) and enter the β-oxidation cycle (stippled circle) to produce acetyl CoA that is then available for further metabolism in the TCA cycle, leading to increased ATP and substrates for anaplerosis. In the β-cell, acyl CoA also participate as signalling molecules to promote insulin secretion (see text).