Ruben Meerman and Andrew Brown explain why the answer might not be what you expectConsidering the soaring overweight and obesity rates and strong interest in this topic, there is surprising ignorance and confusion about the metabolic process of weight loss among the general public and health professionals alike. We encountered widespread misconceptions about how humans lose weight among general practitioners, dietitians, and personal trainers ( fig 1⇓). 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 courses. Other misconceptions were that the metabolites of fat are excreted in the faeces or converted to muscle. We present a novel calculation to show how we "lose weight." 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 17.4 H 33.1 O 2 .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 54.8 H 104.4 O 6 . The three most common fatty acids stored in human adipose tissues are oleate (C 18 To calculate these values, we traced every atom's pathway out of the body. The carbon and hydrogen atoms obviously depart as CO 2 and H 2 O, 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 (O 18 ) to show that the oxygen atoms of body water and respiratory carbon dioxide are rapidly exchanged through the formation of carbonic acid (H 2 CO 3 ). A triglyceride's six oxygen atoms will therefore be shared by CO 2 and H 2 O 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 calculationThe proportion of a triglyceride molecule's mass exhaled in CO 2 is the proportion of its molecular weight (daltons) contributed by its 55 carbon atoms plus four of its oxygen atoms: 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.

No abstract