Study Notes: Metabolism of Proteins, Fats, and Carbohydrates

1. Digestion & Absorption

• Proteins → broken down into amino acids.

• Carbohydrates (sugars) → broken down into glucose.

• Fats (triglycerides) → broken down into glycerol + fatty acids.

Absorption pathways:

• Amino acids + glucose + glycerol + fatty acids → absorbed into portal vein → liver.

• Some triglycerides (as glycerol + fatty acids) → absorbed into lymphatic system → systemic circulation → body → then liver.

2. Storage After a Meal

• Amino acids → stored as proteins.

• Glucose → stored as glycogen (remember: “-ogen” = stored/inactive).

• Glycerol + fatty acids → stored as triglycerides (fat).

3. Energy Production (ATP)

a) From Glucose (primary energy source)

1. Glucose → pyruvate (glycolysis) → small ATP yield.

2. Pyruvate → enters mitochondria → acetyl-CoA.

3. Acetyl-CoA → Krebs cycle (Citric Acid Cycle) → produces CO₂, NADH, FADH₂.

4. Hydrogen (from NADH, FADH₂) → Electron Transport Chain (ETC) with O₂ → ~32–36 ATP via oxidative phosphorylation.

• With oxygen (aerobic): efficient ATP production.

• Without oxygen (anaerobic, e.g., sprint): pyruvate → lactic acid, produces ATP without O₂ but less efficient.

b) From Fats (Triglycerides)

• Glycerol → enters at pyruvate step (can go forward to acetyl-CoA or backward to glucose).

• Fatty acids → enter at acetyl-CoA step.

⚠️ Key rule:

• If substrate enters at pyruvate → it can be converted back to glucose.

• If substrate enters at acetyl-CoA → cannot go back to glucose.

c) From Excess Acetyl-CoA → Ketones

• When glucose is low and fatty acids flood the system:

  • Acetyl-CoA accumulates.

  • Excess converted to ketone bodies:

    • β-hydroxybutyrate

    • Acetoacetate

    • Acetone

• Ketones can be used by tissues (including brain) for ATP.

4. Summary Flow

Proteins → amino acids → liver → energy or protein storage.
Carbohydrates → glucose → liver → glycogen storage or ATP production.
Fats → glycerol (→ pyruvate) + fatty acids (→ acetyl-CoA) → triglyceride storage or ATP production.
Excess acetyl-CoA (low glucose state) → ketone production.

Key Takeaways

• Macronutrients all funnel into ATP production at different points (pyruvate or acetyl-CoA).

• Oxygen availability determines aerobic vs anaerobic pathways.

• Storage vs usage depends on fed (storage) vs fasting (mobilization) state.

• Ketones = backup energy when glucose is limited.