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Chapter 23: Problem 14

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

Expert verified
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

01

Understanding Glucose Usage

Recognize that the typical human requires about 160 g of glucose daily, with the brain requiring 120 g of that glucose.
02

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.
03

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.
04

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.
05

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.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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, 尾-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.

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Most popular questions from this chapter

Potency of Hormones Under normal conditions, the human adrenal medulla secretes epinephrine \(\left(\mathrm{C}_{9} \mathrm{H}_{13} \mathrm{NO}_{3}\right)\) at a rate sufficient to maintain a concentration of \(10^{-10} \mathrm{M}\) in circulating blood. To appreciate what that concentration means, calculate the volume of water that you would need to dissolve \(1.0 \mathrm{~g}\) (about 1 teaspoon) of epinephrine to a concentration equal to that in blood.

Decisions on Drug Safety The drug rosiglitazone (Avandia) is effective in lowering blood glucose levels in patients with type 2 diabetes, but a few years after rosiglitazone came into widespread use, it seemed that using the drug came with an increased risk of heart attack. In response, the U.S. Food and Drug Administration (FDA) severely restricted the conditions under which it could be prescribed. Two years later, after additional studies had been completed, the FDA lifted the restrictions, and today rosiglitazone is available by prescription in the United States, with no special limitations. Many other countries ban it completely. If it were your responsibility to decide whether this drug should remain on the market (labeled with suitable warnings about its side effects) or should be withdrawn from the market altogether, what factors would you weigh in making your decision?

Function of Prohormones What are the possible advantages of synthesizing hormones as prohormones?

Type 2 Diabetes Medication The drugs acarbose (Precose) and miglitol (Glyset), used in the treatment of type 2 diabetes mellitus, inhibit \(a\)-glucosidases in the brush border of the small intestine. These enzymes degrade oligosaccharides derived from glycogen or starch to monosaccharides. Suggest a possible mechanism for the salutary effect of these drugs for individuals with diabetes. What side effects, if any, would you expect from these drugs? Why? (Hint: Review lactose intolerance, p. \(523 .\) )

Water-Soluble versus Lipid-Soluble Hormones On the basis of their physical properties, hormones fall into one of two categories: those that are very soluble in water but relatively insoluble in lipids (e.g., epinephrine) and those that are relatively insoluble in water but highly soluble in lipids (e.g., steroid hormones). In their role as regulators of cellular activity, most water-soluble hormones do not enter their target cells. The lipid-soluble hormones, by contrast, do enter their target cells and ultimately act in the nucleus. What is the relationship between solubility, the location of receptors, and the mode of action of these two classes of hormones?
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