Scientists aren't exactly sure which chemical or chemicals are responsible for creating the odor, but it's probably due to some of the sulfur-containing compounds found in asparagus. Methanethiol was the first one to be blamed in 1891, but since then, many other compounds have been proposed as the possible stinkers:
- Carbon disulfinde
- Carbon oxide sulfide
- Dimethyl sulfide
- Dimethyl disulfide
- Dimethyl trisulfide
- Hydrogen sulfide
- Methanesulfonic anhydride
Asparagus has been around for over two thousand years, but the odor of asparagus pee may be a newer phenomenon, or at least it didn't appear in any literature until 1731 when John Arbuthnot wrote about it in An Essay Concerning the Nature of Aliments. Benjamin Franklin wrote about the aromatic benefit of eating asparagus as an example of how the various things that go into the body can affect the odors that come out of it in a historical document urging scientists of the day to create a drug that might change the offensive odor of expelled gas:
"Certain it is also that we have the Power of changing by slight Means the Smell of another Discharge, that of our Water. A few Stems of Asparagus eaten, shall give our Urine a disagreable Odour; and a Pill of Turpentine no bigger than a Pea, shall bestow on it the pleasing Smell of Violets. And why should it be thought more impossible in Nature, to find Means of making a Perfume of our Wind than of our Water?"It's interesting to note that sulfur-containing fertilizers were first used to improve the flavor of asparagus late in the 17th century and descriptions of asparagus pee began to appear shortly thereafter.
Some people don't notice anything different about their urine after eating asparagus. Scientists who study these things have proposed a couple different reasons for the lack of asparagus pee in these individuals. One hypothesis is that genetic differences prevent the metabolism of the odor-producing compounds from occurring. The other possibility is that those people may produce the odors but have a genetic defect that prevents them from actually being able to detect them so they don't think their urine smells any different.
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Pelchat ML, Bykowski C, Duke FF, Reed DR. "Excretion and perception of a characteristic odor in urine after asparagus ingestion: a psychophysical and genetic study." Chem Senses. 2011 Jan;36(1):9-17.