Professor Richard Muller, who teaches the famed "Physics for Presidents" course at UC Berkeley, wrote a column in this month's Technology Review about the physics of gluttony.
Let me address this issue by invoking another physics principle: conservation of mass. More specifically, let me talk about the conservation of carbon atoms. When you digest food, its carbon atoms enter your blood. Unless they are expelled from your body, they add to your weight. But here is the salient observation: the only effective way your body has to get rid of digested carbon is to combine it with oxygen to form carbon dioxide, and then expel it through your lungs. Unless you breathe out the carbon, you gain weight.
Here are some numbers, taken from books on exercise physiology. Fat, protein, and sugar all contain about 0.1 gram of carbon per food calorie consumed. So if you digest 2,000 calories of food (a typical daily diet for adults) then you take in about 200 grams of carbon. At rest, each breath exhales about 0.5 liter of air containing about 1 percent carbon, for about five milligrams per breath. After a day at 12 breaths per minute, you get rid of about 120 grams of carbon. That's less than you ate, so you'll gain weight.
