Final answer:
Fat is broken down into ketone bodies through the process of Ketogenesis. This process becomes significant when glucose is scarce and involves the liver converting fatty acids to acetyl CoA, which is then used to produce ketone bodies for energy.
Step-by-step explanation:
The process by which fat is broken down into ketone bodies to provide fuel is 3) Ketogenesis.
Ketogenesis is a metabolic process that occurs when body glucose levels are low, such as during fasting, starvation, or low-carbohydrate diets. In this state, the body's primary fuel source transitions from carbohydrates to fats. The liver converts fatty acids into two-carbon acetyl CoA molecules through the process of β-oxidation. When the production of acetyl CoA exceeds the capacity of the Krebs cycle, excess acetyl CoA is then used to synthesize ketone bodies. These ketones can then be used as an alternative energy source by various tissues in the body, including the brain and heart. This mechanism plays a crucial role in maintaining energy metabolism during periods when glucose is scarce.