Losing Weight: Where Does It Go?


It seems that one of the universals of human experience is that many of us would like to lose some weight.  Whether it is a “late middle-aged” person like myself whose doctor says I should do it or a first-year student distressed at gaining the legendary “freshman 15” thanks to the plentiful food options in the Mountaineer Dining Room and not enough time for exercise, we might find ourselves above our optimum weight for our height and age and want to do something about that.

We all know that the physiological formula for weight loss is relatively simple: a person must burn more food calories than they consume.  That would seem to involve more or different exercise and less or different eating.  I am not a weight loss professional or a nutritionist, so this blog is not about the complex topic of how we should approach the challenges of actually getting ourselves to do either of those things.  There are real experts who can be more helpful.

Rather, I want to discuss an interesting question of physics and chemistry that lurks here, too.  That question is: if we are succeeding in losing weight, where exactly does that weight go? The somewhat surprising answer is that you breathe it out.

Our bodies make energy from the food we eat, combining it with oxygen from the air we breathe. When the metabolic process is complete and energy has been supplied to our cells and used for all of their complex processes (running, walking, lifting weights, even thinking!) what is left behind are molecules of water and carbon dioxide. If we are normal, healthy individuals, the water component of our weight stays in balance, equal amounts in and out (through various means including breathing out air that is more moist than what we breath in) so permanent weight loss is not explained by the water component.

The carbon dioxide, CO2, is the key.  Each breath in is the same volume as each breath out, a couple of liters for the normal person.  But the constitution of the breath in and the breath out is not the same.  About 20% of the air coming in is oxygen, O2, and of that 20% some is consumed in the above metabolic process and replaced with an equal number of CO2 molecules.  So THAT is where the weight loss actually occurs—it is the replacement of some of the lighter O2 molecules with their weightier cousins.

Of course, it is discouraging that molecules are very light and so each molecular replacement does not involve much weight loss.  Fortunately, there are a lot of them.  A little bit of first-year chemistry establishes that replacing the oxygen in 50 regular breaths with carbon dioxide would result in a loss of 12 grams of mass, which converts to a weight reduction of about 0.03 lbs.  And, on top of that, our bodies are not 100% efficient in replacing all of the oxygen with carbon dioxide in each breath.  It’s only about 20%, typically, so that it actually takes 250 breaths to expel just 0.03 lbs, or about 8000 breaths to lose a single pound, or to compensate for a pound of carbon in the food you ate in your last meal.  I breathe about 20 times per minute when I am just sitting there, so at that rate, it takes me 400 minutes or about 7 hours to breathe out a pound of extra weight.

Still, it is heartening that just sitting there (or even sleeping!) and breathing is doing something.  (That’s why you weigh a bit less in the morning than when you go to bed.)  When we reduce what we are eating sufficiently, the carbon to make the carbon dioxide exhausts what we take in by eating and has to come instead from fat reserves in the body, and that is when we start to see in the mirror the physiological effects of breathing out the carbon.

It is really hard to reduce one’s eating to that point, so the other alternative is to breathe more, to breathe faster and more deeply.  That’s why and how exercise helps.  When we walk briskly or run, we breathe more rapidly—likely twice as fast and 50% more deeply—and the outbreaths have increased carbon dioxide, too.  That way we use up the carbon from eating more quickly and start dipping into the reserves.  So, the key is to do both.  Control the intake of calories and exercise to increase your respiration.President's dog joining a run

With that little lesson in physics and chemistry in mind, I would like to invite everyone to join us for the Run/Walk Club, which meets Tuesdays and Thursdays at 7 a.m. We start at the Seabury Center, and everyone is encouraged to go at their own pace. Walkers go down to Middletown School and back, about two miles, and those who run with me continue on to the Artisan Center, which is about four miles round trip. The Run/Walk Club is a great opportunity for us to encourage each other to stay healthy by increasing the amount “exhaust” we produce.

[I want to thank Dr. A. J. Mortara of the Health and Human Performance department for his capable assistance with the technical details of respiration and exercise in the above.  You should know, too, that Dr. Mortara has equipment for doing detailed metabolic measurements on you while you are exercising, and that he is the campus expert on all of these matters.]