Radio-labeled isotopes like C14 and tritium have been used for over 50 years to measure metabolite turnover, providing information on production and clearance.

“for more than 50 years we've been able to buy radio labeled isotopes and put a in c14 which is radioactive low dose radiation or tritium which is a form of hydrogen and then give it to a person animal and do blood sampling and actually measure then turnover of that metabolite so that's telling that's very important information many many important studies have used this and sedates we still use this to track production and clearance of whatever we're labeling”