U01.01.130 Metabolic fuel use

Written by

in

Learning Objectives

  • Differentiate between anaerobic and aerobic metabolism during exercise.
  • Identify the chronological shift in fuel sources during fasting vs. starvation.
  • Calculate caloric values for carbs, proteins, alcohol, and fats.
  • Explain the metabolic priorities of the brain and RBCs during glucose deprivation.

1. Exercise and Fuel Use

The source of energy depends on the duration and intensity of the activity.

  • Immediate (Seconds): Stored ATP and Creatine Phosphate provide the burst.
  • Short-term (1 min): Anaerobic glycolysis becomes the primary source.
  • Long-term (Hours): Aerobic respiration (oxidative phosphorylation) takes over, utilizing glucose and eventually fatty acids.

Activity

2. Caloric Content (High-Yield Mnemonics)

The energy density of nutrients varies. A simple trick is to count the letters:

Nutrient Energy (kcal/g) Mnemonic
Carbohydrate 4 kcal “Carb” = 4 letters
Protein 4 kcal “Work” (Protein synthesis) = 4 letters
Alcohol 7 kcal “Alcohol” = 7 letters
Fatty Acid 9 kcal “Fatty Acid” = 9 letters

Activity: Caloric Calculation Challenge

3. The Starvation Timeline

The body’s priority is to maintain blood glucose for the Brain and RBCs while preserving muscle protein.

  • Fed State: Glycolysis and aerobic respiration. Insulin promotes storage.
  • Fasting (between meals): Hepatic glycogenolysis is the major source of glucose.
  • Starvation Days 1-3:
    • Glycogen is depleted after 24 hours.
    • Hepatic gluconeogenesis maintains glucose (from lactate, alanine, and glycerol).
    • Adipose tissue releases FFAs. Odd-chain FFAs (propionyl-CoA) are the only lipid components that can contribute to gluconeogenesis.
  • Starvation After Day 3:
    • Adipose stores provide Ketone Bodies, which become the brain’s main energy source.
    • RBCs still require glucose (they have no mitochondria for ketones).
    • Once fat is gone, protein degradation accelerates, leading to organ failure.

Clinical Notes & Corrections:

  • Odd vs. Even Chains: Most fatty acids are even-chained and produce only Acetyl-CoA (cannot make glucose). Only Odd-chain fatty acids produce Propionyl-CoA, which enters the TCA cycle as Succinyl-CoA for gluconeogenesis.
  • Survival Time: This is determined primarily by the amount of adipose (fat) stores. Fat preserves protein.

Activity: Starvation Source Matching

Memory Hook:

Day 1: Done with Glycogen.

Day 3: Ketones are Key.

Activity