Restricting carbohydrate and increasing lipid produces ketone bodies

Emily "Doctor, but why does carbohydrate have to be restricted for the body to produce ketone bodies?"

Dr. Lee "Good question. The answer is easiest to understand by looking at the TCA cycle in biochemistry. As you can see in the figure, in the TCA cycle the four-carbon molecule oxaloacetate reacts with acetyl-CoA — taking two carbons from acetyl-CoA — to form the six-carbon molecule citrate. Citrate then continues through the cycle, releasing two carbons as CO₂ and generating energy, and converting back to four-carbon oxaloacetate. For this cycle to keep turning, oxaloacetate must continually be supplied — which is done by carbohydrate and protein. But when oxaloacetate is not supplied, acetyl-CoA cannot react and accumulates. Once the concentration is high enough, acetyl-CoA is converted into ketone bodies."

Emily "Ah — so it's like two parts that need to be assembled. If one is missing, the other has to be transformed into something else."

Dr. Lee "Exactly. Acetyl-CoA is part A, and oxaloacetate is part B. They need to combine. If oxaloacetate isn't supplied, acetyl-CoA is converted into ketone bodies. And oxaloacetate is supplied by carbohydrate and protein."