Somewhat OT, but kidney donation is surprisingly low risk in the long term.
"Compared to the general public, most kidney donors have equivalent (or better) survival, excellent quality of life, and no increase in end-stage kidney disease"
I would suspect that there is a selection bias here, as a whole lot of the low end of the general health distribution would prevent you from donating a kidney, or at least make it less likely. So you'd expect them to do better than average if donation had no effect.
To know if there is an effect, you need to not compare to the general public, but to a subset similar in factors which are associated with the outcome measures to kidney donors.
To know if there is an effect, you need to not compare to the general public, but to a subset similar in factors which are associated with the outcome measures to kidney donors.