Isotope Tests on Landis

I read a little bit about the tests they are doing to determine whether Landis was using illegal performance enhancing drugs. They can test the ratio of carbon isotopes in the steroids in the urine. I couldn't figure out how that would show whether the steroids were endogenous (made by Landis' own body) or not. It turns out that synthetic testosterone is made from a group of plants that use a particular kind of carbon fixation (from CO2). They are called C3 plants, and a molecule is involved in their kind of carbon fixation that favors the 12C isotope over the 13C isotope. Testosterone synthesized in the body comes from C3 and C4 plants, and so has a different mix of isotopes. Seems like the smart testosterone drug usage move would be to get testosterone synthesized from a similar mix of C3 and C4 plants as we eat.

On a related note that turns me green with envy, it was important that the French lab quickly receive the request that they test the other sample, because "it closes for a two-week vacation this Friday." (source)