Final answer:
Fat cannot be converted into glucose because fatty acids break down into acetyl CoA, which cannot be used in gluconeogenesis. Instead, glycerol from fat breakdown can contribute to gluconeogenesis.
Step-by-step explanation:
Fat cannot be directly used to synthesize glucose because fatty acids metabolize down to acetyl CoA, which enters the Krebs cycle to produce ATP. Acetyl CoA can lead to the production of ketone bodies during states of low glucose levels, but it does not contribute to the process of gluconeogenesis, the synthesis of glucose. This is because acetyl CoA cannot be converted into pyruvate or any of the intermediates of gluconeogenesis. However, glycerol, which is liberated during the breakdown of fats, can serve as a substrate for gluconeogenesis.