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How Does Your Body Actually Burn Fat?


Sweat is dripping down your forehead. Your muscles are warm and burning. Pacing around the gym, you work to catch your breath. It was a good workout.

The stair climber says you burned 200 calories. Your cardio boxing instructor states his class torches 1,000 calories in an hour. The smart watch on your wrist buzzes to congratulate you on meeting your calorie goal for the day. Another victory against fat.

But what actually happens to the fat you burned? Where does it go?

Fat cells primarily exist to store energy. This energy is in the form of fatty acid molecules. When we go from a caloric surplus (consuming more calories than we burn on a daily basis) to a caloric deficit (consuming fewer calories than we burn on a daily basis), our body both burns fat cells by depleting their stores of fatty acids and also ceases to create new ones.

But where does the “burnt” fat go?

There are a number of theories and misconceptions as to where the fat goes as we work out. Some would say it mostly leaves the body through sweat. Or that it goes away as some ambiguous “energy.” Or that when we flush the toilet, those stubborn fat cells leave along with last night’s dinner. Or that the fat is converted to muscle.

The real answer is a simple chemistry equation. From an article published on the topic in the British Medical Journal:

In simpler terms, (fat) + (inhaled oxygen) → (carbon dioxide) + (water). For our purposes, there’s no reason to make the fat-burning process more complicated than that.

Bottom line: We mostly breathe the fat away! 84% of fat is exhaled as carbon dioxide. The other 16% of fat is excreted as water via bodily fluids like urine, sweat, tears, etc.

For example, say someone lost 10 kilograms of fat over a period of time. 8.4 kilograms of those pounds were actually exhaled away as carbon dioxide. The remaining 1.6 kilograms left their body as water through sweat, urine, feces, etc.

A diagram from the aforementioned study showing how fat is “lost”

But simply breathing more does not guarantee greater fat loss. The key is putting yourself in a consistent caloric deficit. Otherwise, your body has little incentive to burn fat. Ultimately, your diet is going to be the most important factor in achieving a caloric deficit. Food makes up 100% of the calories that go into your body. Exercise accounts only for about 10-25% of the calories that leave it. So while doing exercise that elevates your heart rate and has you “breathing hard” is certainly helpful, it’s not the golden ticket to weight loss. This is why you should be weary of any one workout, food or supplement that guarantees fat burning yet doesn’t account for the other aspects of your lifestyle.

It is simple science and mathematics. Want to lose weight? Eat less. Move more. Breathe.

Interested in learning more about the simple science behind fat loss? Check out “The Mathematics of Weight Loss,” a TEDxQUT talk by Ruben Meerman: