When someone says they lose weight or that they’ve lost fat, where does it go? All too often, folks from the average gym-bro all the way to medical doctors, respond incompletely that its just converted to heat and/or energy. A study performed a few years ago explains that while yes, as the chemical bonds in fats are broken down they do release energy to fuel the muscles, this answer alone is erroneous as it would violate the Law of Conservation of Mass. In short, within a closed system, the mass of the result of a chemical reaction is equal to the mass of its reactants (its constituent parts undergoing the change). Mass cannot be created or destroyed by physical transformations or chemical reactions.
So, where does that mass go? Well, research showed that as fat is broken down, it’s converted primarily into Carbon Dioxide (80-85%) and Water (15-20%). The former is breathed out and the latter is excreted as you would any other fluid — via sweat, urine, feces, breathing, crying, etc.