A friend and I have a disagreement on where the mass lost during weight loss goes.  Short of a nuclear reaction, mass is always conserved.  Burning calories does not make fat mass disappear, so it must exit the body in some manner.

My friend contends that the majority of mass is lost via physical excretions (defecation, urine, skin secretions, hair & nail growth, cellular death & replacement, etc).

I've contend that most of the mass is lost via respiration -- O2 comes in, CO2 goes out, CO2 being an end waste product from the breakdown of glucose.

Her argument goes that the mass lost via breathing is far less than the routes she argues for (it's a gas! It can't weigh that much!).  I argue that the bowel is a one-way system that only removes unabsorbed food material.  And since weight loss will (rapidly) occur if you stop eating, the bowel must not be significantly involved with weight loss.

Some mass is of course lost in urine since it's not pure water and if I remember my biology correctly is the eventual destination of the materials collected by the lymphatic system (dead cells, etc).  Of course oily skin excretions are a direct loss of mass but I argue that while there is an increase in such excretions during exercise but we aren't shedding pounds and pounds of oil when we run on our treadmills.

So which one of us is right?  Is the majority of mass lost as a result of weight loss lost via respiration or via other pathways?