Here is a review of some of the studies going into the isotopic chemistry of living organisms: https://www.omicsonline.org/open-access/biogenic-chemical-elemen

Here is a review of some of the studies going into the isotopic chemistry of living organisms: https://www.omicsonline.org/open-access/biogenic-chemical-elements-isotope-ratios-in-living-organism-as-a-newpotential-indicator-of-physiological-state-2161-0398-1000218.php?aid=74925 https://www.omicsonline.org/open-access/biogenic-chemical-el...