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How do we lose weight? Yes, I know, diet and exercise. But where does the weight actually go? This was a question asked in a short article in 2014 in the BMJ, which you can read here. The authors point out that many people have misconceptions about this question, believing perhaps that fat is “converted to muscle,” or burnt-up in heat (energy), or perhaps defecated out. I suspect most physicians cannot correctly answer this simple question; but the answer is very important for clinical practice.

So where does it go? We breathe it out, in the form of CO2 and water. The authors provide the stoichiometric calculations and show how fat is broken down and exhaled. The average 70 kg person, who doesn’t exercise, exhales about 0.74 kg of CO2 per day, or about 203 g of carbon. Adding an hour of cardiovascular exercise, like jogging, will remove an additional 39 g of carbon, or a gain of about 20% for the day (though this is equal to only about one muffin).

If you are going to lose 10 kg, about 8.4 kg will be lost as exhaled CO2 while 1.6 kg will be lost as water.

So what does this mean clinically? If you aren’t breathing fast, your exercise isn’t going to help you lose weight. Your lungs have to exchange gases, and the more you breathe the more CO2 you are going to breathe out. Walking as an exercise to lose weight is probably not valuable, unless a person is so de-conditioned that walking causes her to breath excessively fast. Pedometers and other activities monitors encourage people to feel self-satisfied that they are “exercising” by counting steps, but users may not be gaining any weight loss advantage if they are not breathing faster while doing it. Oh, in case you were wondering, breathing fast by itself won’t help; that’s called hyperventilation. The breathing must match the metabolic rate.

The second point is that you have to eat less to lose weight. A whole hour of jogging is eliminated by eating one muffin (a large one, 100 g, or about 320 calories). Your body can live on just five muffins per day, or six if you jog for an hour. Most obese patients severely underestimate just how little they should be eating to lose weight.

So, yes, eat less and breathe faster; it’s really simple science. No medicine will boost “metabolism” and no superfood will cut fat. It really is just a simple chemistry equation.