How we lose weight: Oxidation of carbohydrate & fat in the body.

1. Oxidation of Carbohydrate in the body.

Glucose is C6H12O6, or 6(CH2O)

6(CH2O)+ 6(O2) → 6(CO2) + 6(H2O) + energy

Oxygen is inhaled. Carbon Dioxide is exhaled. Water is lost in breath, wee, poo, sweat & other bodily fluids.

As 6 molecules of Oxygen produce 6 molecules of Carbon Dioxide, the Respiratory Exchange Ratio (RER) is 6/6 = 1

Converting molecular weights into their gram equivalents, 180g of Glucose combines with 192g of Oxygen to produce 264g of Carbon Dioxide plus 108g of water plus ~3,012kJ of energy. I'm using kJ rather than kcal, as the human body expends energy as mechanical energy (force x distance) and heat energy.

2. Oxidation of Fat in the body.

Fat is three fatty acids (Stearic Acid, say) attached to a Glycerol backbone. As ~95% of the energy released from a fat is from the three fatty acids, I'm ignoring the Glycerol backbone, to keep the maths as easy as possible.  Stearic Acid is CH3(CH2)16COOH. I'm approximating it to 18(CH2), to keep the maths as easy as possible.

54(CH2) + 81(O2) → 54(CO2) + 54(H2O) + energy

Oxygen is inhaled. Carbon Dioxide is exhaled. Water is lost in breath, wee, poo, sweat & other bodily fluids.

As 81 molecules of Oxygen produce 54 molecules of Carbon Dioxide, the RER is 54/81 = 0.67

Note: The RER for fats is actually 0.7, as the Glycerol backbone is converted into Glucose by the liver. As the RER for  Glucose is 1, this raises the RER of my approximated fat by ~5%.

Converting molecular weights into their gram equivalents, 756g of approximated fat combines with 2,592g of Oxygen to produce 2,376g of Carbon Dioxide plus 972g of water plus ~28,468kJ of energy.

We lose weight by breathing, weeing, pooing, sweating etc. See also Majority of weight loss occurs 'via breathing'.

This doesn't invalidate Energy Balance, as the kcal/kJ values for foods merely represents the amount of chemical energy that can be released by oxidation of the various fuels in the foods. See Why Calories count (where weight change is concerned).

We gain weight by consuming fuels & water.