Body Energy Metabolism: How Food Becomes Cellular Fuel

The body derives energy from macronutrients via catabolic pathways converging on ATP synthesis.

Primary Fuels

• Carbohydrates (4 kcal/g): Glucose → glycolysis → pyruvate → acetyl-CoA → TCA cycle → oxidative phosphorylation. Preferred for brain, RBCs, high-intensity muscle.
• Fats (9 kcal/g): Fatty acids → β-oxidation → acetyl-CoA → TCA/ETC. Primary at rest, low-intensity exercise. Unlimited storage.
• Proteins (4 kcal/g): Amino acids → deamination → gluconeogenesis or TCA intermediates. Not primary fuel; used in starvation, extreme exercise.

Metabolic States

• Fed: Insulin high → glycolysis, lipogenesis, glycogenesis.
• Fasting: Glucagon high → glycogenolysis, gluconeogenesis, lipolysis, ketogenesis.
• Exercise: Intensity/duration dictates fuel mix (RER).

Mitochondrial ATP Yield (Approx.)

• Glucose: 30–32 ATP.
• Palmitate (C16): 106 ATP.

Key Regulation

AMPK (energy sensor), mTOR (growth), PGC-1α (mitochondrial biogenesis), insulin/glucagon ratio.