When somebody loses weight, where does the fat go?

Introduction

- A widespread misconceptions about how humans lose weight among general practitioners, dietitians, and personal trainers was found.

- Most people believed that fat is converted to energy or heat, which violates the law of conservation of mass.

- We suspect this misconception is caused by the “energy in/energy out” mantra and the focus on energy production in university biochemistry course

- Other misconceptions were that the metabolites of fat are excreted in the faeces or converted to muscle.

- 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.

  

Novel Calculation

The proportion of a triglyceride molecule’s mass exhaled in CO₂ is the proportion of its molecular weight (daltons) contributed by its 55 carbon atoms plus four of its oxygen atoms:

(661 Da (C₅₅)+64 Da (O₄))/(861 Da (C₅₅H₁₀₄O₆))×100=84%

The proportion of mass that becomes water is:

(105 Da (H₁₀₄)+32 Da (O₂))/(861 Da (C₅₅H₁₀₄O ))×100=16%

- These results show that the lungs are the primary excretory organ for weight loss (fig 2).

- The water formed may be excreted in the urine, faeces, sweat, breath, tears, or other bodily fluids

Lifting the veil on weight loss

- At rest, an average 70 kg person consuming a mixed diet (respiratory quotient 0.8) exhales about 200 ml of CO₂ in 12 breaths per minute

- Each of those breaths therefore excretes 33 mg of CO₂, of which 8.9 mg is carbon.

- In a day spent asleep, at rest, and performing light activities that double the resting metabolic rate, each for 8 hours, this person exhales 0.74 kg of CO₂ so that 203 g of carbon are lost from the body.

- For comparison, 500 g of sucrose (C₁₂H₂₂O₁₁) provides 8400 kJ (2000 kcal) and contains 210 g of carbon.

- Replacing one hour of rest with exercise that raises the metabolic rate to 7 times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g.

- For comparison, a single 100 g muffin represents about 20% of an average person’s total daily energy requirement.

- Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food.

Conclusions

- Our 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.”

Meerman& Brown. BMJ 2014;349:g7257