Wednesday, May 14, 2008

The Chemistry of Weight Loss

One of my favorite questions to ask my students is, "When you exercise and 'lose' weight, where does the weight go?"

My favorite responses are their inital ones: "Into my sweat," they often say, or "Into the sewage system."

But when I push my students to think about fat loss, they often propose that the fat is converted into energy. At this point, I stop and remind them that unless their bodies are nuclear reactors, there will never be measurable conversion of mass to energy because of the very famous and universally obeyed Law of Conservation of Matter which definitively states that matter (mass) can never be destroyed.

Anyhow, I guess the real answer to this question may be most obvious to a swimmer. As I have been preparing for my first triathlon, I have been listening to the steady bubble...bubble of my now rhythmic exhaling as I swim, day after day, stroke after stroke.

Yes, isn't it strange...when you lose weight, you lose it by breathing it out - fat gets converted into carbon dioxide and thus we breathe away our extra poundage. What a creative idea.

Which brings us to the corollary of the first question, "When you look at a seed and then at a tree, where did all of the extra mass of the tree come from?" Typically my students begin to answer, "Water and nutrients from the ground."

Certainly some measurable mass of the tree came from the ground. But the bulk of a tree is made up of carbon.

Its origin? Ahh, yes, that useless carbon dioxide that we breathed out constitutes (through the miracle of photosynthesis) the mass of the tree.

An amazing design, if I am allowed to be judge. The mass we lose is the same mass that the plant gains is the same mass that we eat again is the same mass that we lose again... Pretty nifty.