What synthesizes Acetyl CoA primarily during fasting states in most tissues?

Acetyl CoA is a crucial molecule that plays a significant role in cellular metabolism, particularly in energy production via the Citric Acid Cycle. During fasting states, the body undergoes various metabolic adaptations to maintain energy balance and supply necessary substrates for essential physiological processes. In this context, the breakdown of fatty acids becomes the primary source of Acetyl CoA.

When carbohydrates are scarce during fasting, the body shifts from relying on glucose to utilizing stored fat as an energy source. Fatty acids, which are released from adipose tissue, undergo a process called beta-oxidation within the mitochondria of cells. This process entails the sequential removal of two-carbon units from the fatty acid chain, resulting in the continuous conversion of fatty acids into Acetyl CoA.

Once formed, Acetyl CoA can either enter the Citric Acid Cycle to produce ATP and other energy-rich compounds or be used for ketogenesis, where some of it is converted into ketone bodies for use by other tissues, particularly the brain. Thus, during fasting states, the synthesis of Acetyl CoA primarily occurs through fatty acid breakdown, leveraging stored energy effectively to sustain metabolic demands.