When somebody loses weight, where does the fat go? (2024)

1. Ruben Meerman, researcher,
2. Andrew J Brown, professor

1. ¹School of Biotechnology and Biomolecular Sciences, University of New South Wales, Sydney, 2052, Australia

1. Correspondence to: R Meerman rubenmeerman{at}me.com

• Accepted 14 November 2014

Abstract

Ruben Meerman and Andrew Brown explain why the answer might not be what you expect

Weight we want to “lose”

Excess carbohydrate or protein in the diet is converted to triglyceride and stored in the lipid droplets of adipocytes. Excess dietary fat needs no conversion other than lipolysis and re-esterification. 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 chemical formula for an average triglyceride molecule can be deduced from fatty acid composition studies. In 1960, Hirsch and colleagues published data that yield an “average fatty acid” with the formula C_17.4H_33.1O₂.1 This 50 year old result is in remarkable agreement with more recent data.2 Three “average fatty acids” esterified to the glycerol backbone (+3C, +6H) give an “average triglyceride” with the formula C_54.8H_104.4O₆. The three most common fatty acids stored in human adipose tissues are oleate (C₁₈H₃₄O₂), palmitate (C₁₆H₃₂O₂), and linoleate (C₁₈H₃₂O₂),1 2 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, we traced every atom’s pathway out of the body. The carbon and hydrogen atoms obviously depart as CO₂ and H₂O, respectively. The fate of a triglyceride molecule’s six oxygen atoms is a conundrum solved in 1949 by Lifson and colleagues.3 They used labelled heavy oxygen (O¹⁸) to show that the oxygen atoms of body water and respiratory carbon dioxide are rapidly exchanged through the formation of carbonic acid (H₂CO₃). 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.

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 (C₅₅H₁₀₄O₆))×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.4 Each of those breaths therefore excretes 33 mg of CO_2, 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 seven 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.

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.” We recommend these concepts be included in secondary school science curriculums and university biochemistry courses to correct widespread misconceptions about weight loss.

Notes

Cite this as: BMJ 2014;349:g7257

Footnotes

• Competing interests: I have read and understood BMJ policy on declaration of interests and have no relevant interests to declare

• Provenance and peer review: Not commissioned; externally peer reviewed.

References

1. ↵

   Hirsch J, Farquhar JW, Ahrens EH, Jr, Peterson ML, Stoffel W. Studies of adipose tissue in man. A microtechnic for sampling and analysis. Am J Clin Nutr1960;8:499-511.

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2. ↵

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3. ↵

   Lifson N, Gordon GB, Visscher MB, Nier AO. The fate of utilized molecular oxygen and the source of the oxygen of respiratory carbon dioxide, studied with the aid of heavy oxygen. J Biol Chem1949;180:803-11.

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   Ainsworth BE, Haskell WL, Herrmann SD, Meckes N, Bassett DR, Jr, Tudor-Locke C, et al. 2011 Compendium of physical activities: a second update of codes and MET values. Med Sci Sports Exerc2011;43:1575-81.

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