When you lose weight, where does it actually go?

By Joe Young | Medically reviewed by Hassan Thwaini

In the world of weight loss, myths and misconceptions are all the rage. If you've been under the impression that fat is converted into muscle or takes a one-way trip down the toilet, it's time for a rethink.

The reality is that most of your lost fat escapes through your lungs. Let's delve into the fascinating biochemical journey that turns fat into thin air.

The surprising facts about fat metabolism

Contrary to popular belief, fat isn’t just burned as energy. It’s transformed into carbon dioxide and water. When you lose 10 kg of fat, approximately 8.4 kg are exhaled as carbon dioxide, and the remaining 1.6 kilograms exit as water through urine, sweat, and other fluids.¹ So your lungs are the primary exit route.

And no, you can’t just breathe out that extra slice of pizza. Exhaling your dinner is the final act in a well-choreographed routine within your body.

So how is fat lost?

When it comes to losing fat, both exercise and diet play key roles:

Exercise: Physical activity increases your muscles' energy demand, drawing from stored triglycerides and carbohydrates. This process promotes fat oxidation, breaking down triglycerides into free fatty acids and glycerol, which are then converted into carbon dioxide and water that you expel through breathing. More intense exercise increases oxygen demand, accelerating this metabolic cycle.¹

Diet: Eating well is crucial. A calorie deficit prompts your body to utilise stored fat for energy. Nutritious foods ensure the body receives essential nutrients for proper metabolic function, thus supporting weight loss effectively.²

But does fat turn into muscle?

A persistent myth is that fat can turn into muscle. In reality, fat and muscle are different tissues with distinct functions. Fat is stored energy, while muscle is made up of fibres that facilitate movement. Exercise can help you lose fat and build muscle concurrently, but one doesn’t transform into the other.³

Rather, through exercise, you can reduce fat tissue while increasing muscle tissue, resulting in improved body composition.³

The numan take

Weight loss is a biochemical journey. Understanding how your body transforms fat reveals the intricate nature of weight loss. By dispelling myths and understanding the pathways of fat loss, you're better equipped to approach weight management with realistic expectations and informed strategies.

References

Meerman, R., & Brown, A. J. (2014). When somebody loses weight, where does the fat go? BMJ, 349, g7257. https://doi.org/10.1136/bmj.g7257
Kim, J. Y. (2020). Optimal diet strategies for weight loss and weight loss maintenance. Journal of Obesity & Metabolic Syndrome, 30(1), 20–31. https://doi.org/10.7570/jomes20065
Pedersen, B. K. (2010). Muscle-to-fat interaction: A two-way street? The Journal of Physiology, 588(Pt 1), 21. https://doi.org/10.1113/jphysiol.2009.184747

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