Where does excess weight go when we lose it?

Everyone has their own unique way of losing weight. Between 5:2 and detox diets, cross fit programs and yoga, there are countless ways to shed excess kilos and placate the scales.

While each individual usually mashes together their own program for slimming down and toning up, the physiological processes in shedding fat are not so particular – each body breaks down fat the same way, through a process called oxidation.

But all this begs the question. Where does the excess weight actually go? How does it leave the body?

Is it excreted as waste? Transformed into muscle? Converted to heat energy?

None of these are the case, according to new research from the University of New South Wales (UNSW) in Australia. Instead, it’s all in the breath.

A new study, by Prof. Andrew Brown and Ruben Meerman at UNSW, has shown that when weight is lost, the majority of it is breathed out as carbon dioxide.

Did you say breath?

Once in the body, excess carbohydrates and protein are converted to a type of fat called triglycerides. These molecules are made up of carbon, hydrogen and water. When we lose weight, triglycerides are broken down and the atoms are separated in a process called oxidation.

The UNSW researchers set out to track these atoms, and follow the path they take when leaving the body. They found that in 10kg of oxidized fat, 8.4kg was converted and excreted as carbon dioxide (CO2) via the lungs, and 1.6kg became water (H2O).

The findings suggest the lungs are the main excretory organ for weight loss – deep breathing exercises any one? – and the remaining H2O departs the body in urine, feces, breath and sweat.

Quick stats

The researchers calculated that 29 kg of oxygen must be inhaled for 10kg of human fat to be oxidized. This oxidation then produces a total of 28kg of CO2 and 11kg of H2O.

“Keeping the weight off simply requires that you put less back in by eating than you’ve exhaled by breathing,” state the authors.

Sleep and exercise

The researchers found that a person weighing 70 kg will exhale at least 200g of carbon each day, with around a third of this weight loss occurring during eight hours of sleep.

As well as this, by substituting one hour of rest for one hour of moderate exercise, the daily total of CO2 excretion increases by 20% to around 240g/day.