Why does eating asparagus make my pee smell funny?
This is a question that has long bothered both scientists and the curious lay person alike for hundreds of years. The history of research into so-called "asparagus pee" is actually an interesting story that dates back hundreds of years.
One of the earliest pieces of formal scientific research on the topic of "odoriferous urine" was an 1891 study by a Polish scientist named Marceli Nencki. In that study, Nencki successfully identified one of the primary chemical compounds that is still believed today to cause smelly urine. As we'll see, the jury is still out on exactly which metabolite, or combination of metabolites causes the pungent sulfur smell.
If you've never noticed this pungent smelly urine after eating asparagus and have no idea what I'm talking about, you may be a person who either doesn't produce the smelly compounds in your urine or who isn't able to smell those compounds. Despite its crude name, "asparagus pee" is a complex and oddly fascinating area of research.
Since Nencki's time, many other scientists have investigated the curious connection between asparagus and odoriferous urine. Until fairly recently it was believed that some people produce smelly urine and some people don't and that is all there is to the matter. It was only more recently discovered that some people also can't smell whatever this substance is that, in the words of Benjamin Franklin, "give[s] our urine a disagreeable odor."
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So-called "asparagus pee" is a strong sulfur type smell similar to that off cooking cabbage. Not surprisingly, all the metabolites which are suspected of causing this smell do contain sulfur. But more questions remain: Why do other sulfur-containing foods like cabbage, eggs and onion not produce the same smelly urine?
Researchers in a 2011 study from the Monell Chemical Senses Center in Philadelphia, PA delved into this phenomenon. They drew three conclusions about the phenomenon of smelly urine after eating asparagus:
- Some people in the study were able to smell "asparagus pee" and others weren't. This specfic anosmia, or inability to smell a certain odor, was associated with a "single nucleotide polymorphism [SNP] near the olfactory receptor gene OR2M7."
- Some people in the study were unable to produce the metabolites that are believed to cause the "asparagus pee" smell in detectable quantities, and their urine did not smell. The biological mechanism for this difference is unknown.
- The ability to produce the smelly asparagus metabolites was not "not tightly related to the ability to smell them."
In other words, people appear to have differing abilities to smell whatever it is in our urine that causes the pungent smell, and the metabolite(s) responsible haven't been clearly identified yet. We don't know why some people produce the smelly urine and others don't. And finally, there doesn't appear to be much of a connection between the ability to make smelly urine and the ability to smell it!
What causes the smell?
The smelly metabolite originally identified by Nencki in 1891 is called methanetiol, or methyl mercaptan. This sulfur-containing compound is well-known to known to anyone who works with natural gas. Because natural gas is odorless, mercaptan is often added because of its distinctive aroma, which alerts people to the presence of an otherwise undetectable gas leak.
Methanetiol is an organic compound with chemical structure CH3SH and the sulfur molecule is believed to account for the smell. Other sulfur-containing compounds may also contribute to the aroma and it's possible that different people are able to smell different compounds. Different concentrations of these metabolites and differing ability to smell them may account for somewhat inconsistent results in ability to smell the compound in different urine samples.
One other primary odorant chemical is dimethyl sulfide, another sulfur-containing metabolite. In a 2001 study from the Imperial College School of Medicine in London researchers assessed how pungent different substances in "asparagus pee" were and found that methanetiol and dimethyl sulfide were the strongest smelling. Two other substances, dimethyl sulfoxide and dimethyl sulfone, were found to impart a "sweet" aroma. When researches combined all four substances into "reconstituted asparagus urine" they found that the combination smelled subjectively like a "typical asparagus-related bouquet."
Some other metabolites that may cause "asparagus pee" include dimethyl sulfide and some related compounds, hydrogensulfide, and methylpropylsulfide. The 2011 study from Monell Chemical Senses Center cites 29 different compounds that have been identified in research as probable causes of the smell.
How many people produce "asparagus pee?"
The 2011 Monell Chemical Senses Center study, which seems to be the most current and well-performed research available, reported that about 92% of participants did produce the smelly metabolites in sufficient concentrations to be detected by the researcher's methods. This leaves 8% who did not produce a detectable concentration.
The authors qualify their findings by pointing out that "odorant production varies from individual to individual, and people with urine that does not have a detectable odor may produce it, albeit at a low concentration." Some people appear to produce "more odorant" urine than others.
At this point the biochemical mechanism behind varying amounts of odorant production and excretion is unknown.
How many people can smell "asparagus pee?"
Most people, about 94%, in the Monell Chemical Senses Center study were able to smell asparagus pee. About 6% were unable to smell it after ruling out generalized smell loss in study participants.
These findings lead the authors to conclude that a specific anosmia, or inability to smell a certain substance is responsible, noting that "specific anosmias are common for biologically important odors, such as volatile steroid hormones, musk, and sweat, and the smell of human urine in different nutritional states, for example, after asparagus consumption."
In other words, specific anosmias for certain human odors are well established in previous research. These specific anosmias, too, are thought to exist as thresholds along a continuum and not as an all-or-nothing trait.
In addition, the ability to smell asparagus pee may be dynamic and a person's ability to smell a substance can change with repeated exposure. One possible uninvestigated variable is at what frequency had test subjects eaten asparagus leading up to the study. It is possible that frequent exposure to these substances increases one's ability to perceive them in the future.
Why does only asparagus cause smelly urine?
In the research I was able to find, this question has not been addressed. Although many studies describe the smell of "asparagus pee" as being reminiscent of rotting cabbage, none that I found investigated why eating cabbage doesn't also lead to smelly urine.
Could molybdenum reduce asparagus pee?
The mineral molybdenum is available as a nutritional supplement that is used in naturopathic medicine to help people better tolerate wine, which is often high in sulfites. In some people who are sensitive, sulfites tend to cause asthmatic wheezing symptoms. Molybdenum prevents these symptoms by upregulating the sulfite oxidase enzyme that oxidizes sulfite into sulfate, which doesn't cause the same type of wheezing reaction.
It has been suggested that taking supplemental molybdenum may reduce asparagus pee through a similar mechanism of oxidizing sulfite-containing compounds. However, because methanetiol and other metabolites suspected of causing the smell do not contain sulfite groups it is unlikely that molybdenum would help.
Why is this important?
Understanding the biochemical pathway responsible for creating these sulfur-containing compounds in urine may illuminate some currently unknown aspect of kidney function. Many discoveries are made somewhat "by accident" and understanding this pathway may lead to important discoveries down the line.
Humanity has been eating asparagus for 2000 years or more and the phenomenon of "asparagus pee" has been recorded as far back as the early 18th century. Despite centuries of inquiry and more than 125 years of formal scientific investigation, we still don't know a whole lot. Perhaps further study will lead to more conclusive answers. But if history is any guide, the answers won't be simple!
- Nencki, M. "Ueber Das Vorkommen Von Methylmercaptan Im Menschlichen Harn Nach Spargelgenuss." Archiv FÃ¼r Experimentelle Pathologie Und Pharmakologie 28.3-4 (1891): 206-09. Web. 28 Mar. 2017.
- Franklin, Benjamin, and Carl Japikse. Fart Proudly: Writings of Benjamin Franklin You Never Read in School. Berkeley, CA: Frog, 2003. Print.
- Mitchell, S. C. "Food Idiosyncrasies: Beetroot and Asparagus." Drug Metabolism and Disposition. American Society for Pharmacology and Experimental Therapeutics, 01 Apr. 2001. Web. 28 Mar. 2017.
- Dalton, Pamela, Nadine Doolittle, and Paul A.s. Breslin. "Gender-specific Induction of Enhanced Sensitivity to Odors." Nature Neuroscience 5.3 (2002): 199-200. Web. 28 Mar. 2017.