Metabolism of Fats and Proteins

All fuels converge. A key insight: carbohydrates, fats, and proteins all feed into the same central energy pathways (especially the citric acid cycle and oxidative phosphorylation). They mostly enter by being converted into intermediates like acetyl-CoA or citric acid cycle molecules. So the machinery you learned for glucose also powers fat and protein metabolism.

Fat metabolism (beta-oxidation). Fats (triglycerides) are a huge energy reserve. To use them:

• Triglycerides are broken into glycerol and fatty acids (lipolysis).
• Fatty acids undergo beta-oxidation in the mitochondria — they are chopped, two carbons at a time, into many acetyl-CoA molecules, which enter the citric acid cycle. Beta-oxidation also produces NADH and FADH₂.
• Because fatty acids are long and carbon-rich, they yield large amounts of ATP — more per gram than carbohydrates — which is why fat is the body's most concentrated energy store.

Ketone bodies. When carbohydrates are scarce (fasting, starvation, very low-carb diets, or uncontrolled diabetes), the liver converts excess acetyl-CoA from fat breakdown into ketone bodies (ketones). These are released into the blood as an alternative fuel, especially important for the brain (which normally prefers glucose) during prolonged fasting. Mild ketosis is a normal adaptation. However, excessive ketone production (notably in uncontrolled type 1 diabetes) can cause diabetic ketoacidosis (DKA) — dangerously acidic blood — a medical emergency (detailed in Pathophysiology).

Protein/amino acid metabolism. Proteins are primarily for building and function, but amino acids can be used for energy when needed (e.g., excess protein intake, or during starvation when other fuels are low). To use amino acids for energy:

• The amino group (–NH₂) is removed (deamination), producing ammonia, which the liver converts to urea for excretion by the kidneys.
• The remaining carbon skeletons enter energy pathways (as pyruvate, acetyl-CoA, or citric acid cycle intermediates).

Using protein for fuel means breaking down body tissue, so it's generally a last resort (prominent in starvation).

How it works

Fat and protein metabolism:

All fuels converge → acetyl-CoA / citric acid cycle → oxidative phosphorylation
FATS: triglyceride → glycerol + fatty acids → BETA-OXIDATION (2 carbons at a time) → many acetyl-CoA (+ NADH/FADH2) → lots of ATP
KETONES: low carbs (fasting/diabetes) → liver makes ketone bodies (alt fuel, esp. brain)
   excess (uncontrolled diabetes) → diabetic ketoacidosis (DKA) — emergency
PROTEINS/amino acids (when needed): remove amino group (deamination) → ammonia → liver → UREA → kidneys excrete
   carbon skeleton → energy pathways; uses body tissue = last resort (starvation)

Comparisons

Common confusions

• All fuels converge on acetyl-CoA / the citric acid cycle — carbs, fats, and proteins share the machinery.
• Beta-oxidation breaks fatty acids into acetyl-CoA (not a separate energy system).
• Ketones are a normal alternative fuel, but excess → diabetic ketoacidosis (emergency).
• Using protein for energy requires removing the amino group (→ ammonia → urea) and breaks down tissue (last resort).

Memory aids

• "All roads lead to acetyl-CoA."
• "Beta-oxidation = chop fatty acids 2 carbons at a time."
• "Low carbs → ketones; too many ketones (diabetes) → ketoacidosis."
• "Protein for fuel → amino group off → urea out (kidneys)."

Quick review

• Carbohydrates, fats, and proteins all converge on acetyl-CoA / the citric acid cycle and oxidative phosphorylation.
• Fats are broken down by beta-oxidation (fatty acids → many acetyl-CoA), yielding large amounts of ATP.
• Ketone bodies form when carbs are scarce (fasting, low-carb, uncontrolled diabetes) as alternative fuel; excess causes diabetic ketoacidosis (DKA).
• Amino acids can be used for energy by removing the amino group (deamination → ammonia → urea, excreted by kidneys); using protein for fuel breaks down tissue and is a last resort.