Re: Why does ones urine have a distinctive aroma after eating aspargus?

For years, I had been taught that the telltale odor produced in the urine shortly after eating asparagus was due to high levels of the amino acid Asparagine in the plant. It is true that asparagus contains high levels of asparagine (that's where the amino acid got its name), but purified asparagine does not have the noxious odor associated with asparagus urine. Lynn Bry sent me to a web site at Urban Legends which gives a detailed discussion of the causes of this ailment. Two modified, sulphur-containing amino acids found in asparagus, S-methylmethionine and asparagusic acid, are probably the precursors of several noisome substances which have been identified in urine after consuming asparagus. The following excerpt from a 1975 Science Article describes the breakdown products of these compounds that are responsible for the odor.

Gas chromatography-mass spectrometry was used to determine the odor-causing agent (or agents) present in the urines of humans after they have eaten asparagus. S-Methyl thioacrylate and S-methyl-3-(methylthio)thiopropionate were identified from methylene chloride extracts of such urines and appear to be the odor-causing compounds.

White RH. (1975) "Occurrence of S-methyl thioesters in urines of humans after they have eaten asparagus." Science 189(4205):810-11.