Question

Sources of Glucose during Starvation The typical human adult uses about \(160 \mathrm{~g}\) of glucose per day. Of this, the brain alone uses \(120 \mathrm{~g}\). The body's available reserve of glucose \((\sim 20 \mathrm{~g}\) of circulating glucose and \(\sim 190 \mathrm{~g}\) of glycogen) is adequate for about one day. After the glucose reserve has been depleted during starvation, how does the body obtain more glucose?

Short answer

The body uses gluconeogenesis to produce glucose from non-carbohydrate sources and relies on ketone bodies as an alternative energy source during starvation.

Step-by-step solution

Understanding Glucose Usage

Recognize that the typical human requires about 160 g of glucose daily, with the brain requiring 120 g of that glucose.

Glucose and Glycogen Reserves

Identify that the glucose reserve consists of about 20 g of circulating glucose and 190 g of glycogen, covering the body's glucose requirement for roughly one day.

Post-Depletion Gluconeogenesis

After the depletion of glucose and glycogen, the body relies on a process called gluconeogenesis. This process synthesizes glucose from non-carbohydrate precursors like amino acids, lactate, and glycerol, primarily in the liver.

Fatty Acid Utilization

During extended starvation, the body uses adipose (fat) tissues by converting fatty acids into ketone bodies. While these ketones primarily serve as an alternative energy source for the brain and other tissues, they spare glucose for critical functions.

Prioritization of Glucose Use

The body prioritizes glucose use for tissues that are obligatory glucose users, such as blood cells and parts of the brain that cannot use ketone bodies effectively.

Key concepts

Glucose Metabolism

In the human body, glucose metabolism is a critical process for energy production. It starts with the breakdown of carbohydrates from the food we eat, leading to the generation of glucose molecules. These molecules enter the bloodstream, providing immediate energy to cells.

This process involves glycolysis, where glucose is converted into pyruvate, yielding energy in the form of ATP. The pyruvate can further undergo oxidative phosphorylation in the mitochondria, significantly enhancing energy production.

• Key Roles: Glucose is the primary energy source for most cells, especially in the brain.
• Energy Output: Conversion to ATP, the energy currency of cells.

Understanding the role of glucose metabolism helps emphasize why maintaining blood glucose levels is vital, especially during periods of fasting or starvation.

Glycogen Reserve

The body maintains a reserved form of glucose called glycogen, primarily stored in the liver and muscles. Glycogen acts as a quick-release store, available when blood glucose levels drop, such as during fasting or intense physical activity.

In average adults, the glycogen reserve consists of approximately 190 g. This supply can sustain immediate glucose needs for about one day without food intake. However, once depleted, the body's reliance shifts to alternative methods of glucose production.

• Liver Glycogen: Mainly used to maintain blood glucose levels.
• Muscle Glycogen: Used as an energy source during muscle contraction.

Effective glycogen storage is crucial for bridging the gap between meals and provides a short-term buffer during starvation.

Ketone Bodies

Ketone bodies are compounds produced by the liver from fatty acids during prolonged fasting or starvation. When glycogen stores are exhausted, the body turns to fat reserves to synthesize these ketones that can substitute glucose for energy.

Ketone bodies become crucial, especially for the brain, which has a limited ability to switch from glucose to fat directly.

• Types of Ketone Bodies: Acetoacetate, β-hydroxybutyrate, and acetone.
• Brain Adaptation: Ketones provide up to 70% of the brain's energy during prolonged starvation.

Understanding ketone bodies is essential for appreciating the body's adaptation abilities during low glucose situations.

Starvation Adaptation

Starvation adaptation involves a series of metabolic changes that allow humans to survive extended periods without food. The body prioritizes vital functions and conserves energy stores for as long as possible.

During starvation, the body activates gluconeogenesis to produce glucose from non-carbohydrate sources like amino acids, lactate, and glycerol. Simultaneously, ketone bodies from fatty acids become the primary fuel for the brain, reducing its glucose needs.

• Phase Shifts: The body shifts from using glycogen to fat stores and eventually proteins in extreme starvation.
• Energy Conservation: Metabolic rate decreases to preserve energy.

Starvation adaptation showcases the human body's resilience and capacity to modify its metabolic pathways in response to nutrient scarcity.