Good for you. Strictly speaking, poop is not your body's output, it's your body's waste, and as such is indeed a good indicator of your body's efficiency in pulling out what it needs from the input. I guess your body's output is energy?
A significant fraction of poop is bacteria. Sources vary a bit, but for example, this study gives the fraction as ~55% of solid matter: https://pubmed.ncbi.nlm.nih.gov/7359576/
To the extent that your body contains a large bioreactor in the gut to maintain a population of beneficial microflora, you could argue poop thus is one of your bodies outputs (and yes, it contains waste products as well).