Where Does Lost Fat Go? The Biochemistry of Fat Loss

Fat loss occurs via oxidation of stored triglycerides. The atoms exit the body primarily through the lungs.

Stoichiometry of Fat Oxidation

Typical triglyceride (C₅₅H₉₈O₆) + 78 O₂ → 55 CO₂ + 49 H₂O + energy

• 84% of fat mass leaves as CO₂ via exhalation.
• 16% leaves as H₂O via urine, sweat, tears, breath vapor, feces.

Common Misconceptions

• ❌ Fat converts to energy/heat (violates mass conservation).
• ❌ Fat turns into muscle.
• ❌ Fat exits mainly in feces (only unabsorbed dietary fat does).

Implications

• Breathing more (exercise) increases CO₂ excretion → fat loss.
• Weight loss = carbon dioxide output > carbon intake (food).
• Gastric bypass: rapid weight loss from reduced intake + hormonal changes (GLP-1, PYY, ghrelin) → negative carbon balance.

Every breath carries away a tiny fraction of your lost fat.