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Lost Weight? Where Did the Fat Go?

Screen Shot 2016-04-28 at 12.00.21 PMWhen someone loses weight, where does the fat actually go?

The answer may surprise you.

It’s a simple enough question but there is surprising amount of misinformation regarding possible answers.

You would be wrong if you thought any of the following:

× Energy/ heat

× Faeces

× Becomes Muscle

× Sweat/ Urine

The correct answer is in fact:

√ Carbon dioxide (CO2)!

Most of us believe that fat is converted to energy or heat when we exercise.

The main problem with this is that it violates the law of conservation of mass!

This law states that “matter can be changed from one form into another, mixtures can be separated or made, and pure substances can be decomposed, but the total amount of mass remains constant”.

We have well and truely fallen for the energy in/ energy out mantra regarding weight loss and management.


What Really Happens

Excess carbohydrate or protein in the diet is converted to triglyceride and stored in the lipid droplets of adipocytes. Adipocytes are specialised cells that store fat and are found in connective tissue.

Any excess dietary fat needs minimal conversion.

Those of us that wish to lose weight while maintaining a fat-free mass are essentially attempting to metabolise the triglycerides stored in the adipocytes.

For example when somebody loses 10 kg of fat (triglyceride) (believe it or not) 8.4 kg is exhaled as CO2.

Lungs are therefore a primary excretory organ for fat and weight loss.

Losing weight requires unlocking the carbon stored in fat cells; so a more accurate mantra would be “Eat less, Move more.”

As well as CO2, fat may also be excreted as ketone bodies under particular conditions or minor amounts of lean body mass, the nitrogen in which may be excreted as urea.

At rest, an average 70 kg person consuming a mixed diet exhales about 200 ml of CO2 in 12 breaths per minute. Each of those breaths excretes 33 mg of CO2, of which 8.9 mg is carbon.

In a day spent asleep, at rest, and performing light activities which doubles the resting metabolic rate, then for each 8 hour block, this person exhales 0.74 kg of CO2 so that 203 g of carbon are lost from the body.

Replacing one hour of rest with exercise which raises the metabolic rate to seven times that of resting by, for example, jogging, removes an additional 39 g of carbon from the body, raising the total by about 20% to 240 g.

In comparison, 500 g of sucrose provides 8400 kJ (2000 kcal) and contains 210 g of carbon.

A single 100 g muffin represents about 20% of an average person’s total daily energy requirement. Physical activity as a weight loss strategy is, therefore, easily foiled by relatively small quantities of excess food.



Final Word

Biochemically the mantra we should be reinforcing is “Eat Less, Move More”.

To lose weight, exercising more efficiently would increase the metabolic rate, increasing the rate of CO2  exhaled.

This, in combination with a diet rich in vegetables, good quality protein and fats will assist in long term, healthy weight maintenance.


Further Reading



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