When somebody loses weight, where does the fat go?

Abstract

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.

Serious analytical inconsistencies challenge the validity of the energy balance theory

“…Therefore, body mass decreases as we excrete CO 2 , water, urea and SO 3 but not as consequence of the heat content in the EE [ 40 ]. The amount of daily mass excretion is, nonetheless, modulated by neural, gastric and endocrine signaling systems that direct body weight regulation [ 39 ].…”

Fructose metabolism as a common evolutionary pathway of survival associated with climate change, food shortage and droughts

“…While fat and glycogen are used as fuel (ATP) sources during food shortage, they are also a major source of water in settings where water is less available, as fat generates about 1.1 g of water for every g of fat oxidized, while for glycogen the ratio is about 3–4 g of water per g of glycogen oxidation . Being able to produce water from stored fat and glycogen is critical for any animal that has a lack of water availability, but especially for mammals living in the desert and salt water oceans and for animals that do not drink during prolonged periods of hibernation or long‐distance migration.…”

Nutrition Knowledge Translation Performance in Health Professionals: Findings from the 2017 Unified Forces Preventive Nutrition Conference (UFPN)

“…The survey included one knowledge question on weight loss physiology, which was translated from a survey among family doctors, dietitians and athletic trainers in the UK [13].…”