Topic 13

Topic 13 Summary of Energy Metabolism

Readings:

  • Sections 19.1, 19.2 (Insulin, Glucagon, Epinephrine)

  • Section 19.3 (Starvation, Diabetes)

Overview of Energy Pathways

  • Human Energy Metabolism:

    • Pathways involved showcase how various biomolecules contribute to ATP production, which is essential for cellular energy.

    • Major pathways include:

    • Fats:

      • Provide ATP through fatty acid β-oxidation, subsequent citric acid cycle, and oxidative phosphorylation.

    • Similar pathways exist for other lipid classes aside from fatty acids.

    • Carbohydrates:

      • Breakdown via glycogen or starch degradation and glycolysis, followed by the citric acid cycle and oxidative phosphorylation.

    • Proteins:

      • Degraded into amino acids, which serve as intermediates in glycolysis and the citric acid cycle for energy metabolism.

Caloric Content of Food

  • Energy content is typically expressed in Calories (capital C), also known as dietary calories, food calories, kilogram calories, or large calories.

  • Conversions:

    • 1 Calorie = 1000 calories (gram calories or small calories).

    • 1 calorie = Amount of energy to raise 1 gram of water by 1°C at standard pressure.

  • Biochemical Energy in Food:

    • 1 g of fat = approximately 9 Calories.

    • 1 g of carbohydrates or proteins = approximately 4 Calories.

    • Note: These values assume complete absorption and metabolism by the body.

    • Dietary Fiber:

    • A type of carbohydrate that provides little energy, as it is not degraded/absorbed by the body.

Energy Regulation: Fed and Unfed States

  • Movement of substrates through energy metabolism pathways depends on the body’s energy needs.

  • After a Meal (Fed State):

    • Blood glucose levels are elevated, leading to the secretion of insulin by the pancreas.

    • Role of Insulin:

    • Stimulates glucose uptake by tissues (muscle, liver, adipose).

    • Promotes storage of fuel as:

      • Glycogen (in liver and muscle).

      • Triacylglycerols (lipid storage).

      • Encourages protein synthesis and general biosynthetic pathways.

    • Some glucose-6-phosphate is diverted through the pentose phosphate pathway for production of NADPH and ribose 5-phosphate.

  • After Several Hours (Unfed State):

    • Blood glucose and insulin levels decline while glucagon increases.

    • Role of Glucagon:

    • Inhibits glycolysis and stimulates glycogen breakdown in the liver.

    • Glucagon is vital for maintaining blood glucose levels due to glucose being the preferred energy source for the brain.

    • Muscle cells break down their own glycogen and utilize fatty acids from the bloodstream.

    • Stimulates adipose tissue (adipocytes) to release fatty acids.

    • Fatty acid oxidation becomes the main energy source for muscles and liver in the unfed state.

    • Production of ketone bodies increases after several days of unfed status but does not serve as primary energy for the brain in initial fasting days.

    • Glycogen Stores:

    • Liver glycogen stores are typically exhausted within 16 to 24 hours after a meal.

    • Gluconeogenesis:

    • Stimulated by glucagon, signals an important source for glucose when fasting continues.

    • Proteins, mainly from muscle, degrade to release amino acids for gluconeogenesis.

      • For 2 to 3 days without food, glucose from amino acids serves as the brain's primary energy source.

      • Prolonged gluconeogenesis leads to depletion of nitrogen, making indefinite continuation impossible.

Prolonged Fasting and Its Metabolic Consequences

  • After days of fasting, protein degradation decreases as a source of energy.

  • Fatty acid and glycerol release from triacylglycerols continues, maintaining a level of fatty acids in the blood.

    • Increased ketone body production in the liver as an energy source adapts the brain.

    • Some glucose remains necessary for certain cell types (e.g., erythrocytes), while the brain increasingly relies on ketone bodies.

    • Main Substrates for Gluconeogenesis:

    • Glycerol becomes the main substrate, while fatty acid oxidation provides NADH required for the process.

    • Muscle continues to utilize fatty acids for energy, occasionally consuming ketone bodies.

    • Overall metabolism slows down as the body conserves energy.

Final Stages of Starvation

  • Once fat reserves are depleted, the body resorts to protein consumption to supply energy for the brain.

  • Consequences of starvation include:

    • Inability to maintain ion gradients across brain cell membranes leads to potential death.

    • Impaired liver and heart function arises from deficient essential proteins.

    • Survivors may succumb to opportunistic infections, including lung infections due to muscular weakness.

Diabetes Mellitus

  • Definition: Diabetes mellitus is the disease characterized by impaired normal responses to high blood glucose levels.

  • Types of Diabetes:

    • Type 1 Diabetes:

    • Often develops during childhood and involves autoimmune destruction of pancreatic insulin-secreting cells.

    • Patients exhibit symptoms resembling a state of perpetual fasting:

      • Cells do not absorb glucose from the blood.

      • Increased gluconeogenesis and triacylglycerol degradation occur.

      • High ketone body production raises blood acidity.

      • Symptoms include polyuria (excessive urination) and thirst due to glucose in urine drawing water.

      • Hyperglycemia can lead to protein glycosylation and aggregation, damaging small blood vessels, particularly in the eyes, causing sight issues (potential blindness).

      • Nerve damage may occur, leading to sensations of pain, numbness, and weakness.

      • Severe buildup of blood glucose and ketones may lead to unconsciousness and death.

    • Treatment typically involves periodic insulin injections.

    • Type 2 Diabetes:

    • Typically develops later in life characterized by gradually reduced insulin production from the pancreas.

    • Also involves “insulin resistance,” where cells do not respond adequately to insulin levels.

    • Commonly associated with obesity and sedentary lifestyles.

    • Physiological and metabolic symptoms mirror those seen in Type 1 diabetes.

    • Management includes: Weight loss, dietary adjustments (increasing fiber, reducing sugars), pharmacological interventions, and possible insulin therapy.

    • Glucagon-like Peptide-1 Receptor Agonists:

      • Drugs like semaglutide (Ozempic® or Rybelsus®) promote insulin release and inhibit glucagon secretion, encouraging satiety post-meal and slowing gastric digestion, aiding in weight loss.

Review Questions

13-1: Insulin Overdose in Type 1 Diabetes

  • Question: Is it possible for individuals with Type 1 diabetes to overdose on insulin? Why could excessive insulin pose a problem?

  • Answer:

    • Yes. Overdose leads to blood glucose rapidly falling below normal levels (hypoglycemia).

    • Consequences of Hypoglycemia:

    • Insufficient energy supply to the brain resulting in behavioral changes, slurred speech, or severe outcomes like unconsciousness (hypoglycemic coma).

WileyPLUS Questions

  • For Chapter 19: 1, 3, 5, 9, 11, 21, 31, 33, 43, 45, 47, 51, 59a, 59b, 71

  • For Chapter 14: 19

  • For Chapter 18: 69b-e