Question

Ketones are compounds that are derived from _____ and are synthesized primarily in the _____ state. a. glucose, absorptive b. glycogen, absorptive c. fatty acids, postabsorptive d. amino acids, postabsorptive e. triglycerides, absorptive

Short answer

The correct answer is c. fatty acids, postabsorptive.

Step-by-step solution

Understand metabolic states

There are two major metabolic states, the absorptive state and the postabsorptive state. In the absorptive state, the nutrients are being absorbed and used immediately. In the postabsorptive state, the body uses stored nutrients.

Understand ketone bodies formation

Ketone bodies are formed primarily in the liver and are derived from fatty acids. This usually occurs when there is a shortage of glucose or carbohydrates, forcing the body to use its fat stores. Hence, these ketones are synthesized primarily in the postabsorptive state.

Match the options with the analysis

By matching the above information with the given options, it can be determined that the correct answer is 'fatty acids, postabsorptive'.

Key concepts

Metabolic States

The human body operates in different metabolic states to maintain energy balance. Two primary states exist: absorptive and postabsorptive. These states shift based on food intake and energy needs.

• **Absorptive State:** This is usually present during and shortly after meals when nutrients are plentiful. The body primarily utilizes glucose as an energy source, storing excess nutrients as glycogen and fat for future use.
• **Postabsorptive State:** Also known as the fasting state, this occurs when the digestive system is empty, typically several hours after eating. The body relies on energy stores, breaking down glycogen and fat to supply glucose and ketone bodies.

Understanding these states is crucial as they dictate how the body metabolizes carbohydrates, proteins, and fats to maintain energy balance.

Fatty Acids

Fatty acids are crucial components of lipids found in the body. They serve several key functions, one of which is providing energy. When dietary glucose is insufficient, fatty acids become a vital energy source.

• **Role in Energy Production:** Fatty acids undergo beta-oxidation, a process that converts them into acetyl-CoA. This molecule is vital for energy production in the Krebs cycle.
• **Ketone Body Formation:** In the absence of carbohydrates, excess acetyl-CoA is converted into ketone bodies in the liver. These ketone bodies provide an alternative energy source, especially for the brain.

This conversion into ketone bodies primarily occurs during the postabsorptive state, highlighting the importance of fatty acids beyond energy storage.

Postabsorptive State

During the postabsorptive state, the body adapts to a lack of incoming dietary nutrients by using stored energy. This state supports extended periods without food intake by shifting energy sources.

• **Glycogenolysis:** Initially, the liver breaks down glycogen stores to release glucose into the bloodstream.
• **Lipolysis and Ketogenesis:** As glycogen becomes depleted, the body increases the breakdown of fats through lipolysis. Fatty acids are then converted into ketone bodies, a process known as ketogenesis.

This metabolic shift ensures that vital organs, especially the brain and heart, receive a steady supply of energy.

Absorptive State

The absorptive state is marked by high nutrient availability post-meal. At this time, the body makes extensive use of carbohydrates, fats, and proteins from the food to meet immediate energy demands and to create reserves.

• **Glucose Utilization and Storage:** Glucose serves as the primary energy source, with any excess being stored as glycogen in the liver and muscles.
• **Anabolic Pathways:** The body favors anabolic reactions, synthesizing proteins, fats, and glycogen for storage and growth.

This state enables the efficient use and storage of nutrients, paving the way for the postabsorptive state when energy input is halted.