Thanks for all your replies.
What does actually happen when you lose weight? An article based on a Christmas lecture explains this, you cand download it from
https://www.bmj.com/content/349/bmj.g7257. Congrats to
@Pipp who had found it already.
You burn calories when exercising, but this is not where the weight goes, despite 60% of doctors and health professionals getting this wrong as well and claiming the weight is converted into energy and heat, I have attached the plot from the paper here.
Let me try to explain what actually happens in lay terms. Fat is stored as something called “triglyceride” in your fatty parts. When you are breathing your lungs convert oxygen atoms from the air and carbon atoms from trygliceride into carbon dioxide and water and it is through this process that you burn calories. Air contains about 21% oxygen (O2) and with each breath about one fourth to one fifth of the oxygen will be converted in your lungs to carbon dioxide (CO2). The exhaled air contains about 4% to 5% of CO2 by volume.
This is written as a chemical process:
(C55 H104 O6) + 78 O2→55 CO2 + 52 H2O + energy,
but you can ignore this formula. The correct proportions of trygliceride (C55 H104 O6), oxygen (O2), carbon dioxide (CO2) and water (H2O) are however relevant and required if you want to make the following calculation yourself.
The results are the following: To lose 10 kg of fat - which is roughly what I did - I - needed to breath in about 120 kg of oxygen or about 600 kg (half a million litres) of air, which took me a while, each breath is about 0.5 litres and we do 12 of these per minute. Of the oxygen most of it (75% to 80%) is exhaled without interacting, only a total 29 kg of oxygen is used. Of this 84% and 8.4 kg of my body fat was converted in my lungs into 28 kg of carbon dioxide which I breathed out. The remaining 16% of the 29 kg of oxygen and 1.6 kg of fat was converted into 11 kg of water which I excreted as urine, … Such a 10kg weight loss is illustrated in this figure from the paper.
The paper also illustrates how each day you convert about 200 g of carbon into carbon dioxide which in steady state you replace by eating 2000 calories.
Of course you burn calories while exercising, but this has two problems. First it is not very effective. You need to exercise quite strenuously to say burn 400 calories or an additional 40 g of carbon from your body. This corresponds to an increase of 20%. Second, physical activity makes you hungry, and you reward your self easily with food replacing the full 400 calories, e.g. 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.
Summary: during weight loss most of the fat is converted by breathing into carbon dioxide.
Several of your answers mentioned breathing, which confirms that people on this forum are often better informed than healthcare professionals.
Let me know if you have questions.