The smell from eating asparagus

Question

I am wondering about one thing. I have read different accounts of this (prior to reading it on Wikipedia), so I am wondering.

Here are the two versions I have read:

1. asparagus leads in a certain percentage of people to a smell that has to do with a reaction of asparagusic acid. That is: not every human body creates the reactions leading to the "smelly" thiol (sulphuric) compounds.
2. asparagus leads to the processes of the aforementioned asparagusic acid in everybody but only a part of the population is able to smell those compounds.

Which one is true? Or is perhaps some other version I haven't heard about.

btw: The Wikipedia article on asparagus references both versions, too. So I am hoping for a definite answer with references to the respective scientific papers/studies.

Answer 1

Apparently, both are true.

...Individual differences exist in both odorant production and odor perception. The biological basis for the inability to produce the metabolite in detectable quantities is unknown, but the inability to smell the odor is associated with a single nucleotide polymorphism (rs4481887) within a 50-gene cluster of olfactory receptors. Pelchet et al. 2010

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It's an open access paper (yay!), but to cut a long story short:

• Pelchet et al. asked subjects to provide 'before-eating-asparagus' and 'after-eating-asparagus' urine.
• The subjects then acted as urine-smellers for their own and others samples, and were asked to choose between two unlabeled flasks which contained the 'before' urine and which contained the 'asparagus' urine.
• Most subjects produced 'asparagus' urine in which an odor was perceived significantly more times than expected by chance, and
• Most subjects perceived 'asparagus' urine correctly significantly more times than expected by chance, however
• ~8% of subjects were non-producers, and
• ~6% of subjects were non-smellers.
• Finally, looking at the actual amount of successes in perception: for Caucasian subjects, one third of the variation in the amount of correct odor perception was related to the A allele of rs4481887 (having AA or AG rather than GG). There was no connection between this allele and production.

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Pelchat, ML, C Bykowski, FF Duke, and DR Reed. 2010. Excretion and perception of a characteristic odor in urine after asparagus ingestion; A psychophysical and genetic study. Chemical Senses doi:10.1093/chemse/bjq081