Why does carbohydrate deficiency lead to incomplete fat oxidation and ketone body formation?

Carbohydrate deficiency causes lipolysis to accelerate as the body tries to use fats for energy, but acetyl CoA from fat oxidation can enter the citric acid cycle only when sufficient carbohydrate intermediates are present. When carbohydrates are deficient, those intermediates are instead converted to glucose to fuel the brain, so fat oxidation is incomplete and acetyl CoA accumulates. The liver then converts the accumulating acetyl CoA into ketone bodies through ketogenesis.

When carbohydrate intake is inadequate, the breakdown of stored fats (lipolysis) speeds up to fill the fuel gap. Normally, acetyl CoA produced from fat oxidation enters the citric acid cycle, but this requires adequate carbohydrate intermediate molecules. In a carbohydrate-deficient state, those intermediates are diverted to glucose production to protect the brain, which relies on glucose. As a result, acetyl CoA cannot be fully oxidized and builds up. The liver handles this excess acetyl CoA by converting it into ketone bodies and releasing them into the blood.