The effect of eating asparagus on the eater's urine has long been observed:
"[Asparagus] cause a filthy and disagreeable smell in the urine, as every Body knows." (Treatise of All Sorts of Foods, Louis Lemery, 1702)
"asparagus... affects the urine with a foetid smell (especially if cut when they are white) and therefore have been suspected by some physicians as not friendly to the kidneys; when they are older, and begin to ramify, they lose this quality; but then they are not so agreeable." ("An Essay Concerning the Nature of Aliments," John Arbuthnot, 1735)
"A few Stems of Asparagus eaten, shall give our Urine a disagreable Odour..." ("Letter to the Royal Academy of Brussels," Benjamin Franklin, c. 1781)
Asparagus "...transforms my chamber-pot into a flask of perfume." Marcel Proust (1871–1922) 
There is debate about whether all (or only some) people produce the smell, and whether all (or only some) people identify the smell. It was originally thought this was because some of the population digested asparagus differently from others, so some people excreted odorous urine after eating asparagus, and others did not. In the 1980s three studies from France, China and Israel published results showing that producing odorous urine from asparagus was a common human characteristic. The Israeli study found that from their 307 subjects all of those who could smell 'asparagus urine' could detect it in the urine of anyone who had eaten asparagus, even if the person who produced it could not detect it himself. However, a 2010 study found variations in both production of odorous urine and the ability to detect the odour, but that these were not tightly related. It is believed most people produce the odorous compounds after eating asparagus, but only about 22% of the population have the autosomal genes required to smell them.
In 2010, the company 23andMe published a genome-wide association study on whether participants have "ever noticed a peculiar odor when you pee after eating asparagus?"This study pinpointed a single-nucleotide polymorphism (SNP) in a cluster of olfactory genes associated with the ability to detect the odor. While this SNP did not explain all of the difference in detection between people, it provides support for the theory that there are genetic differences in olfactory receptors that lead people to be unable to smell these odorous compounds.
Asparagus foliage turns bright yellow in autumn
Certain compounds in asparagus are metabolized, giving urine a distinctive smell due to ammonia and various sulfur-containing degradation products, including various thiols and thioesters.
Some of the volatile organic compounds responsible for the smell are:
Subjectively, the first two are the most pungent, while the last two (sulfur-oxidized) give a sweet aroma. A mixture of these compounds form a "reconstituted asparagus urine" odor. This was first investigated in 1891 by Marceli Nencki, who attributed the smell to methanethiol. These compounds originate in the asparagus as asparagusic acid and its derivatives, as these are the only sulfur-containing compounds unique to asparagus. As these are more present in young asparagus, this accords with the observation that the smell is more pronounced after eating young asparagus. The biological mechanism for the production of these compounds is less clear.
The onset of the asparagus urine smell is remarkably rapid. It has been estimated to start 15 to 30 minutes after ingestion.
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