How do we lose body mass?

People who wish to lose weight while maintaining their fat-free mass are, biochemically speaking, attempting to metabolise the triglycerides stored in their adipocytes. The three most common fatty acids stored in human adipose tissues are oleate (C₁₈H₃₄O₂), palmitate (C₁₆H₃₂O₂), and linoleate (C₁₈H₃₂O₂), which all esterify to form C₅₅H₁₀₄O₆.

The complete oxidation of a single triglyceride molecule involves many enzymes and biochemical steps, but the entire process can be summarised as: C₅₅H₁₀₄O₆+78O₂→55CO₂+52H₂O+Energy.

Stoichiometry shows that complete oxidation of 10 kg of human fat requires 29 kg of inhaled oxygen producing 28 kg of CO₂ and 11 kg of H₂O. This tells us the metabolic fate of fat but remains silent about the proportions of the mass stored in those 10 kg of fat that depart as carbon dioxide or water during weight loss.

To calculate these values, every atom's pathway out of the body was traced using labelled heavy oxygen (O¹⁸).

A triglyceride's six oxygen atoms will therefore be shared by CO₂ and H₂O in the same 2:1 ratio in which oxygen exists in each substance. In other words, four will be exhaled and two will form water. Note that the Da stands for dalton, a unit of mass.

The proportion of oxygen that becomes CO₂ is (661 Da (C₅₅)+64 Da (O₄))/(861 Da (C₅₅H₁₀₄O₆))$\times$100=84%

The proportion of mass that becomes water is (105 Da (H₁₀₄)+32 Da (O₂))/(861 Da (C₅₅H₁₀₄O₆))$\times$100=16%

Calculations show that the lungs are the primary excretory organ for fat. Losing weight requires unlocking the carbon stored in fat cells, thus reinforcing that often heard refrain of “eat less, move more.” The authors recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.