U01.01.121 Metabolic fuel use*

Understanding how the body utilizes different fuel sources under various conditions (exercise, fasting, and starvation) is key for the USMLE Step 1. This guide outlines how energy is derived from various macronutrients and the metabolic adaptations during fasting and starvation.

Energy Use During Exercise

The body shifts between aerobic and anaerobic metabolism depending on the duration and intensity of the exercise. The following table summarizes the maximal energy contribution by each source:

Duration of Exercise	Energy Source	% Maximal Energy Contribution
2 sec	Stored ATP	100%
10 sec	Creatine phosphate	Major contributor
1 min	Anaerobic metabolism	High contribution
2 hr	Aerobic metabolism	Major contributor

Key Points:

Stored ATP: Provides immediate energy for high-intensity, short-duration activities.
Creatine Phosphate: Used for quick energy but depleted within 10 seconds.
Anaerobic Metabolism: Kicks in for intense activities lasting up to 1 minute.
Aerobic Metabolism: Dominates after 1 minute, especially for prolonged endurance activities.

Energy Yield from Macronutrients

The body uses various macronutrients for energy, and the following table compares the caloric yield from each:

Macronutrient	Energy Yield
Carbohydrate	4 kcal/g
Protein	4 kcal/g
Alcohol	7 kcal/g
Fatty Acid	9 kcal/g

Points to Remember:

1g of carbohydrate or protein = 4 kcal.
1g of alcohol = 7 kcal.
1g of fatty acid = 9 kcal.

Metabolic States: Fed, Fasting, and Starvation

Metabolic processes vary depending on the nutritional state of the body. Here’s an overview of key processes during different states:

1. Fed State (Post-meal):

Metabolic Process: Glycolysis and aerobic respiration.
Hormonal Regulation: Insulin stimulates storage of lipids, proteins, and glycogen.

2. Fasting State (Between meals):

Metabolic Process: Hepatic glycogenolysis (major), gluconeogenesis (minor), and adipose release of free fatty acids (FFAs).
Hormonal Regulation: Glucagon and epinephrine stimulate the use of fuel reserves.

3. Starvation (Days 1-3):

Metabolic Process:
- Hepatic glycogenolysis to maintain blood glucose.
- Adipose release of FFAs.
- Muscle and liver shift fuel use from glucose to FFAs.
- Hepatic gluconeogenesis from lactate, alanine, glycerol, and propionyl-CoA.
Key Adaptation: Glycogen reserves are depleted after the first day.
RBCs: Unable to use ketones due to lack of mitochondria.

4. Starvation After Day 3:

Metabolic Process: Adipose stores (ketone bodies) become the main energy source for the brain.
Key Adaptation: Vital protein degradation accelerates after adipose stores are depleted, leading to organ failure and death.
Survival Time: The amount of stored fat determines how long the body can survive without food.

Starvation and Its Phases

The following graph summarizes the energy sources used during starvation:

Time	Energy Source	Fuel Breakdown
Day 1	Hepatic glycogenolysis	Blood glucose maintained
Day 2-3	Adipose release of FFA	Shift from glucose to FFA
Week 1-3	Ketone bodies	Main energy source for brain
After Week 3	Protein degradation accelerates	Organ failure leads to death

Points to Remember:

Days 1-3: Blood glucose maintained by glycogenolysis, FFA release, and gluconeogenesis.
After Day 3: Ketone bodies replace glucose for brain energy.
Prolonged Starvation: After adipose stores are depleted, protein breakdown accelerates, leading to organ failure and death.

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USMLE Step 1 Review Series

Curriculum

U01.01.121 Metabolic fuel use*

Energy Use During Exercise

Key Points:

Energy Yield from Macronutrients

Points to Remember:

Metabolic States: Fed, Fasting, and Starvation

1. Fed State (Post-meal):

2. Fasting State (Between meals):

3. Starvation (Days 1-3):

4. Starvation After Day 3:

Starvation and Its Phases

Points to Remember:

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