Dirty Medicine: Metabolic Fuel Utilization

Introduction

• Metabolic fuel utilization involves understanding which macronutrient (fat, glucose, protein) is used for energy during starvation.
• Often poorly taught in medical school, but crucial for exams like USMLE and COMLEX.
• Aim: Simplify the concepts using graphs and mnemonics.

Overview of Starvation Fuel Utilization

• Graph Setup:
  • Axis 1: Stored energy (glycogen, fatty acids, protein)
  • Axis 2: Time (days/weeks)
• Goals:
  • Explain physiology during starvation.
  • Use charts and mnemonics for better recall.

Fuel Sources by Time Frame

Up to 1 Day

• Fuel Source: Glycogen
• Process: Glycogenolysis (glycogen breakdown)
• Mnemonic: "Like One Gin" (Glycogen for 1 day)

Between 1 and 3 Days

• Fuel Source: Hepatic Gluconeogenesis
• Driven by: Alanine and Lactate
• Mnemonic: "BLAH I'm so hungry" (B=Brain, L=Lactate, A=Alanine)

After 3 Days

• Fuel Source: Free Fatty Acids
• Mnemonic: "Three Fatty Acids" (Primary source from 3 days to 1 week)

After 1 Week

• Fuel Source: Protein
• Mnemonic: "Weak without protein" (Week 1 = protein breakdown)

Biochemical Drivers

• Up to 1 Day: Glucagon and epinephrine drive glycogenolysis.
• 3 to 7 Days: Ketone body degradation supports brain function.
• After 1 Week: Absence of lipid stores leads to protein breakdown.

Additional High-Yield Points

Red Blood Cells

• Cannot use ketones.
• Lack mitochondria.
• Mnemonic: "Really Basic Cells" (RBCs)

Insulin

• Drives the fed state.
• Mnemonic: "Insulin insulates the body with fuels."

Priority Targets

• Brain and RBCs are priority during starvation to maintain function and oxygen transport.

Energy Content of Macronutrients

• Carbohydrates: 4 kcal ("Carb" has 4 letters)
• Protein: 7 kcal ("Protein" has 7 letters)
• Fatty Acid: 9 kcal ("Fatty Acid" has 9 letters)

Conclusion

• Use mnemonics and charts to remember fuel utilization.
• Focus on high-yield concepts to excel in exams related to metabolic processes.
• Be familiar with the summarized graph and chart for effective study.