During fasting, what is the main regulation of the citric acid cycle?

During fasting, the primary regulation of the citric acid cycle is influenced significantly by the concentration of Acetyl CoA. During fasting states, the body shifts its metabolism to utilize fatty acids and ketone bodies as primary energy sources. The breakdown of fatty acids through beta-oxidation produces Acetyl CoA, which enters the citric acid cycle.

An increase in Acetyl CoA concentration signals the need for energy production via the citric acid cycle, especially because glucose availability is reduced during fasting. This high concentration enables the cycle to operate efficiently and meet the energy demands of the body. Additionally, Acetyl CoA also influences the regulation of enzymes in the cycle, particularly its entry point.

In contrast, while the concentrations of NADH, FADH2, and citrate may play roles in the cycle's overall function and regulation, they are generally more stable or secondary factors when compared to the immediate influence of Acetyl CoA during fasting conditions. Thus, the concentration of Acetyl CoA is the main regulatory factor driving the citric acid cycle in a fasting state.