Ruben Meerman and Professor Andrew Brown from the School of Biotechnology and Biomolecular Sciences, University of New South Wales published a paper in 2014 When somebody loses weight, where does the fat go? 
Professor Brown specialises in the study of lipids (fats) including cholesterol. Meerman has a physics degree and is known as the surfing scientist and has appeared in a number of television shows including ABC’s Catalyst program. He is the author of Big Fat Myths, a book that expands on the previous paper. The original paper and later book explains how every molecule of fat escapes the human body during weight loss.
According to Meerman,
Excess carbohydrates are NOT converted to fat, except in forced circumstances. Excess sugars and carbohydrates are stored as glycogen in the liver and muscle cells. This is why athletes perform “carbohydrate loading” to build up there store of glycogen before endurance events. “Hitting the wall’ occurs when these stores are depleted.
Professor Stewart Truswell, professor of Human Nutrition at University of Sydney, explains: “In some animal species, carbohydrates in excess of requirements are converted to fat via the pathway of lipogenesis. […]. Other than in the experimental situation of gross carbohydrate overfeeding, conversion of carbohydrate to stored lipids does not occur to any appreciable extent in humans.”
Marc Hallerstein, professor of Nutritional Science at Berkeley: “Fat cannot be converted to carbohydrate in animals because animals lack the enzymes of the glyoxylate pathway, and carbohydrate is not converted to fat because of a functional block of uncertain cause.”
He also states, “Under most dietary conditions, the two major macronutrient energy sources (CHO and fat) are therefore not interconvertible currencies; CHO and fat have independent, though interacting, economies and independent regulation.”
A research team at University of Lausanne: “These findings challenge the common perception that conversion of CHO to fat is an important pathway for the retention of dietary energy and for the accumulation of body fat.” 
According to Meerman and Brown:
The body has complex feedback mechanisms which allow us to maintain homeostasis – a dynamic static of equilibrium that ensures a multitude of variables remain within a functional and viable range. This includes body temperature, electrolyte balance and body weight. The minerals in our blood need to be maintained within a very narrow range which is normally achieved despite a wide variation in their consumption. The idea that our “weight-loss strategy is easily foiled by relatively small quantities of excess food” just does not apply in the real-world.
One well-known study showed that ADDING 12 slices of white bread (at 70 calories a slice) or high-fiber bread (at 50 calories a slice) to existing diet of overweight participants (a diet that resulted them in being overweight in the first place), resulted in an average weight loss of 9 kg over a period of 8 weeks. There was no change in their physical activity or exercise.
Meerman and Brown states that:
The Lifson paper was studying the source of oxygen in respiratory carbon dioxide after oxidation of glucose. Oxidation of fats and proteins were not considered in this study.
Meerman and Brown seem to ignore the fact that carbon dioxide that we exhale is also created during oxidisation of glucose – which is why blood sugar and glycogen stores are lowest early morning. They make the fundamental mistake of assuming that the carbon dioxide we exhale is produced only from the metabolism of fats.
The conclusion of the paper states:
This advice simply does not work. Not surprisingly, what you eat has an enormous impact on your health and weight – far behind the simple (simplistic) advice of “eat less, move more.”
For example, “An overwhelming amount of evidence shows that the ratio of fat to carbohydrate in the diet is the primary factor in the macronutrient composition of the diet that easily causes passive over-consumption of energy and thus leads to weight gain. In contrast, high-carbohydrate diets seem relatively benign, regardless of the type of carbohydrate.” 
- Fats are more readily absorbed from the intestine than carbohydrates.
- Fats are much more energy dense – the same volume of fats contain 2¼ times more energy than carbohydrates. Also, because they take less volume, they are not as filling so we eat more.
- The energy costs of digesting carbohydrates are much greater than fats – we expend much more energy to digest carbohydrates than we do to digest fats.
- People on a predominately plant-based diet have a higher body temperature than those on a animal-based diet. People on a ketogenic diet, have a lower body temperature. Raising the body temperature expends more energy.
The National Weight Control Registry (NWCR) was established in 1994. It is the largest study into long-term weight loss programs. The principal researchers are Rena Wing (Brown Medical School, Rhode Island) and James Hill (University of Colorado). There are now over 10,000 participants enrolled in the study. 
To be eligible to enroll in the study participants must be at least 18 years old, have lost at least 13.6 kg (30 lbs) of weight and kept it off for at least one year.
According to the researchers “almost nobody is on a low-carbohydrate diet” and they have looked “very hard to locate them.
The suggestions from the NWCR to successfully maintain a healthy weight-control program are:
- engaging in high levels of physical activity
- eating a diet that is low in calories and fat
- eating breakfast
- self-monitoring weight on a regular basis
- maintaining a consistent eating pattern; and
- catching “slips” early
- Meerman, R. & Brown, A. J. (2014) When somebody loses weight, where does the fat go? BMJ. [Online] 349 (dec16 13), g7257–g7257.
- Meerman, R. (n.d.) Ruben Meerman [online]. Available from: https://rubenmeerman.com/ (Accessed 7 September 2018).
- Mann, J. & Truswell, A. S. (eds.) (2017) Essentials of Human Nutrition. Fifth Edition. London: Oxford University Press.
- Hellerstein, M. K. (2001) No common energy currency: de novo lipogenesis as the road less traveled. The American Journal of Clinical Nutrition. 74 (6), 707–708.
- Hellerstein, M. K. (1999) De novo lipogenesis in humans: metabolic and regulatory aspects. European Journal of Clinical Nutrition. 53 (1), s53–s65.
- Acheson, K. et al. (1982) Glycogen synthesis versus lipogenesis after a 500 gram carbohydrate meal in man. Metabolism. 31 (12), 1234–1240.
- Mickelsen, O. et al. (1979) Effects of a high fiber bread diet on weight loss in college-age males. American Journal of Clinical Nutrition. 32 (8), 1703–1709.
- Lifson, N. et al. (1949) The Fate of Utilized Molecular Oxygen and the Source of the Oxygen of Respiratory Carbon Dioxide with the Aid of Heavy Oxygen. Journal of Biological Chemistry. 180 (2)
- Saris, W. H. (2003) Sugars, energy metabolism, and body weight control. American Journal of Clinical Nutrition. 78 (4), 850S-857S.
- Bell, E. A. et al. (1998) Energy density of foods affects energy intake in normal-weight women. The American Journal of Clinical Nutrition. 67 (3), 412–420.
- Astrup, A. (2001) The role of dietary fat in the prevention and treatment of obesity. Efficacy and safety of low-fat diets. International Journal of Obesity. 25 (S1), S46–S50.
- The National Weight Control Registry (n.d.) National Control Registry [online]. Available from: www.nwcr.ws/ (Accessed 26 September 2017).
- Wing, R. R. & Phelan, S. (2005) Long-term weight loss maintenance. The American Journal of Clinical Nutrition. 82 (3), 222–225.