Sending good vibes to all of my trans comrades 
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Generally as you loss mass more than you gain (from eating) you lose it by breathing out CO2. Always wild to think about, but that's where it goes.
Reverse for a tree, a tiny lil seed gets all the mass it needs to make more tree not by the soil but by CO2 in the air.
As you lose muscle it gets converted to calories and the essential proteins find somewhere else to be and the electrolytes just get excreted in urine. The calories do their thing in mitochondria, convert a bunch of ADP to ATP in the presence of O2, the CO2 that's produced as a byproduct enters your blood as carbonic acid, your brain detects the decrease in pH and you breath it out.
What you poop out is basically just bilirubin from dead red blood cells and also a whole lot of your internal microbiome (dead bacteria/their waste/excess bacteria). And whatever else you couldn't digest, like fiber.