Shedding Light on Weight Loss: Where Does It All Go?

Embarking on a weight loss journey often raises a curious question: Where does the lost weight physically go? 

Common misconceptions suggest that fat simply “burns off” or vanishes into thin air. The reality is much more scientifically complex and fascinating. Weight loss involves a biochemical transformation, wherein stored body fat is converted into energy, carbon dioxide and water. 

This transformation is the cornerstone of understanding how and where weight is lost. Contrary to popular belief, a significant portion of weight loss occurs through the respiratory process. A study published in 2014 clarified this by illustrating that when fat is metabolised, about 84% of the fat mass is exhaled as carbon dioxide through the lungs. The remaining 16% converted into water, which exits the body through urine, sweat, and other bodily fluids. This insight not only dispels common myths about weight loss but also highlights the physiological journey of fat reduction.

The Biochemical Pathway of Fat Reduction

The initial step in the weight loss journey is the metabolic breakdown of stored fat within the body’s cells. This process is known scientifically as lipolysis. Lipolysis triggers the conversion of triglycerides, the main form of body fat storage, into glycerol and free fatty acids. 

This conversion is pivotal, as it makes these substances available for the body to use as energy. The free fatty acids released during lipolysis can travel through the bloodstream to muscles and other organs, where they undergo further oxidation. Oxidation is a process that breaks them down in the presence of oxygen to produce energy, water, and carbon dioxide. This stage of metabolism is critical for weight loss. It is the main route through which fat stores are mobilised and utilised for energy production. 

Hydration and Fat Metabolism: The Role of Water in Weight Loss

While carbon dioxide represents the primary exit route for metabolised fat, the role of water in the weight loss process is equally significant. The remaining 16% of the by-products from fat metabolism, not expelled as carbon dioxide, is converted into water. This water may be excreted from the body through various means. Other forms of excretion include urine, sweat, and other bodily fluids, facilitating the physical elimination of weight. 

Carbon dioxide is a key component of weight loss, but also the production and excretion of water is an essential process in managing and reducing body weight. This dual pathway highlights the importance of staying hydrated during weight loss efforts. Adequate hydration is crucial for optimising metabolic processes, including facilitating the transport of nutrients and the excretion of waste products. 

Moreover, the generation of water as a by-product of fat metabolism signifies the body’s intricate balance in energy use and waste elimination. Proper hydration supports the body’s natural processes, including thermoregulation and physical performance, which are vital for effective weight loss and overall health. Therefore, understanding the interconnected roles of carbon dioxide and water in the weight loss journey provides a holistic view of the physiological changes occurring during fat reduction efforts.

Maximising Weight Loss Through Oxygen Consumption

The role of aerobic exercise in enhancing the weight loss process cannot be overstated. Aerobic exercise increases the body’s demand for oxygen. In turn, this accelerates the rate of fat metabolism. This leads to an increased production of carbon dioxide and water — the two primary by-products of fat breakdown. This enhanced metabolic rate not only facilitates the burning of stored fat but also significantly contributes to the volume of carbon dioxide exhaled, reinforcing the pivotal role of respiration in weight loss. 

Another studied highlighted the impact of exercise intensity on fat oxidation rates. This demonstrates that moderate-intensity aerobic exercise optimises the body’s fat-burning capacity. The efficiency of aerobic exercise in promoting weight loss underscores the importance of integrating regular physical activity into any weight management strategy. By engaging in activities that boost oxygen consumption, individuals can effectively increase the removal of fat through the respiratory process, making aerobic exercise effective for weight loss and overall health maintenance. This aligns with the scientific understanding that weight loss is not just about diet but also about how the body metabolises fat, with oxygen playing a critical role in this complex biochemical equation.

Integrating Insights for Holistic Weight Management

In synthesising the insights from our exploration into the weight loss process, it becomes clear that losing weight is a complex phenomenon based on biochemistry and physiology principles. The journey of fat moving from stored energy to exiting as carbon dioxide and water through our breath and bodily fluids highlights the intricate processes our bodies perform to maintain balance and health.

Aerobic exercise, by boosting oxygen intake, plays a crucial role in this metabolic pathway. It enhances the body’s ability to convert fat into expendable energy efficiently. This revelation not only empowers individuals with the knowledge to approach weight loss with scientifically backed strategies but also emphasises the importance of viewing weight management through a holistic lens. It prompts a balanced consideration of diet, physical activity, and hydration. As we navigate our health and fitness journeys, let us remember the profound complexity and capability of our bodies, guided by the science that reveals the path to achieving sustainable weight management and overall well-being.


  1. Meerman, R., & Brown, A. (2014, December 16). When somebody loses weight, where does the fat go? The BMJ.
  2. Frayn, K. N. (2002, July 24). Adipose tissue as a buffer for daily lipid flux. Diabetologia.
  3. Romijn, J. A., Coyle, E. F., Sidossis, L. S., Gastaldelli, A., Horowitz, J. F., Endert, E., & Wolfe, R. R. (1993, September 1). Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration. American Journal of Physiology-endocrinology and Metabolism.

Share via

Also worth reading

People also read:

Asian woman touch thyroid gland graves' disease
Graves’ Disease: A Cause Of Hyperthyroidism

Graves’ disease is an autoimmune disorder which causes hyperthyroidism, the overproduction of thyroid hormones. The disease usually affects young and middle-aged women who are genetically predisposed.  What causes hyperthyroidism? Graves’

Read More »
insomnia man trying to sleep in bed
Insomnia – is Sleep for the Weak?

Insomnia is a sleep disorder that is characterised by difficulty initiating and/or maintaining sleep. Affecting approximately 35% of the world’s population, insomnia has a predilection for women, the elderly, and

Read More »