Final answer:
The body uses ketones as its primary energy source when there is an insufficient supply of carbohydrates. Ketone bodies become the major source of fuel for the heart and other organs after several days of starvation. Survival is dependent on the amount of fat and protein stored in the body.
Step-by-step explanation:
When there is an insufficient supply of carbohydrates available for energy, the body will instead use ketones as its primary energy source. After several days of starvation, ketone bodies become the major source of fuel for the heart and other organs.
During starvation or when carbohydrate intake is low, the body adapts by using fat reserves for energy. Initially, the liver breaks down fatty acids into acetyl CoA. If this supply exceeds the capacity of the Krebs cycle, acetyl CoA is diverted to create ketone bodies, which become the major fuel source especially for the brain and heart.
Over time, if fat reserves are depleted, the body may resort to breaking down proteins from muscle tissue to generate glucose, a process that can lead to muscle wasting and overall weakness.
As starvation continues, fatty acids and triglyceride stores are used to create ketones for the body. Once these stores are fully depleted, proteins from muscles are released and broken down for glucose synthesis. Overall survival is dependent on the amount of fat and protein stored in the body.
Therefore answer is b) fats.