Physics can sometimes cut through the mess of complex problems with a simple conservation law. A year ago, in my column "The Physics Diet," I applied conservation of energy to the problem of obesity. I argued that exercise burns so few calories that it cannot be a major way of losing weight.

But many people I have spoken to believe there is another benefit to exercise: it changes your metabolism. When that happens, you burn more calories naturally, and so your food doesnt turn into fat.

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. Thats less than you ate, so youll gain weight.

But few of us spend the whole day resting. Walking increases your respiration by a factor of two to three. Running at eight kilometers per hour (five miles per hour) increases it by a factor of eight to 10. Put together a nice combination, and youll lose all the carbon you consumed, and your weight will be stable. Walking, running, and being active does increase your metabolism--the rate at which you burn calories--and it increases your breathing rate too.

The mistake people make is to think that an hour of moderate exercise will change their body chemistry enough so that theyll "burn" away the calories even when they are inactive. But unless you breathe more rapidly, the carbon will stay in your body. If you want to lose weight, eat less or breathe more. And the only effective way to breathe more is through increased activity. There is no such thing as stimulating your body into a higher resting metabolic rate.

But what about all those people who seem to eat more than we do, but stay thin? Dont they contradict my conclusion? I wondered about that too, so I started asking these people three questions: How often do you snack? Do you always finish your meals? How often do you have dessert after a meal?