After eating asparagus, some people may detect a strange odor, while others may not notice anything. What’s happening?
The astringent properties of asparagus have been known for a long time. As early as 1731, the Scottish mathematician and physician John Arbuthnot wrote that asparagus imparts a «foetid odor» to urine.
However, this smell is not always unpleasant for everyone: Marcel Proust maintained that it had the effect of transforming his «humble chamber into a room full of aromatic perfumes».
If you conduct a short poll at dinner (assuming you feel the time is right to raise these types of issues), you’ll find that your guests typically fall into certain groups.
Some people have never detected anything unusual in their urine after eating asparagus. Others assume that because their urine has a strong odor after consuming them, then so will everyone else’s.
Someone can even complicate matters further by revealing that they have smelled it after their partner has been in the bathroom, but are unable to detect it in their own urine.
Based on other observations, it would appear that some people produce urine containing the astringent indices of asparagus, and people are able to detect them, although the two do not go hand in hand.
But the plural of the anecdote is not the data, so more evidence is needed.
There is no point in asking people whether or not they can smell the smell of asparagus in their own urine. If someone says yes, then that person is both a producer and a detector, but if he says no, we don’t know if he simply wasn’t able to detect the scent.
Therefore, we need laboratory experiments. Sniffing other people’s urine may not be a fun idea for everyone, but several researchers have managed to recruit the necessary volunteers to do it.
In 1956, a team of British researchers found that less than half of people produce the odor in their urine, which is apparently related to the influence of a single gene.
Another British study from 1987, which produced a larger sample (800 people), found a similar proportion. On the other hand, different studies have found a higher percentage of producers. An American study carried out in 1985 gave the figure of 79% and another from 2010 gave almost 92%. This raises the possibility of ethnic differences.
Perhaps the clue is in the chemicals that produce the characteristic odor. The prime suspect, appearing in several studies, is a sulfur compound called methanethiol.
However, in 1956, researchers found methanethiol in the urine of some asparagus consumers, though not all consumers.
But in this study the researchers looked for these compounds only in the urine itself, which does not mean that the odor could be detected.
For that, you have to examine the vapor released by the urine. Analysis of the vapor by gas chromatography revealed the existence of four compounds. Stronger-smelling are methanethiol and dimethyl sulfide, which smell like old cabbage.
Likewise, there are two compounds that could also contribute to the sweet tinge of the odor.
These compounds are unlikely to be found in asparagus as it is eaten as they are small, labile molecules that can be destroyed by cooking treatments.
So really, what we need to find is a substance found only in asparagus that stays intact during cooking but breaks down in the body producing the little odor compounds.
In fact, there is a substance unique to asparagus called asparagusic acid. Could this be the source of the smell?
So far no one has been able to prove that this acid is responsible for the characteristic odor, but no other chemical has been found that matches what was described above.
And to smell it?
That’s all we know about the producers, but what about the detectors?
Some studies seem to confirm the suspicion that not everyone is capable of detecting the unpleasant smell. When different urine samples were sniffed in Israel, only 10% were able to detect traces of asparagus. On the other hand, only 24% were able to do so in a sample in China.
The problem with some studies, such as the Israeli one, is that they compared urine containing asparagus with water, instead of comparing it with urine without asparagus, and this could cause the appearance and detection of other notes in the urine, instead of the hints of asparagus in particular.
At the Monell Center for the Chemical Senses in Philadelphia, biological psychologist Marcia Pelchat set out to get to the bottom of the mystery.
One day he asked his volunteers to drink a bottle of water and eat asparagus that had been cooked for eight minutes in a little olive oil with a pinch of salt.
After two hours, nature took its course and the subsequent urine was kept in the freezer.
The next day, the same people received the same size bottle of water along with Italian bread that contained the same amount of oil and salt as the asparagus.
The rest of the procedure is then repeated. It was found that, of all the volunteers who sniffed both their own urine and the urine of others, only 8% were producers and 6% detectors.
In theory, it is possible that certain people are missing the enzyme that prevents them from producing and detecting a particular odor in their urine. But so far there isn’t much evidence to prove it: the study showed that only one person did not produce or detect traces of asparagus.
From his results, Pelchat found evidence that the ability to detect odor is linked to a single gene, but found no association with odor generation.
So we still don’t know why some people seem not to produce this smell.
Could it be that they have not absorbed it, that the body has not processed it or that they have not excreted it?
Or maybe we all excrete it, but some excrete it in such minute amounts that they can’t eat several bunches of asparagus so the smell can be detected by others.
So we still can’t answer the simple question of what makes urine smell strange.
And even if we do find the answer, we may never know why Arbuthnot thought the smell unpleasant while others, like Proust, found it delicious.
Sometimes, simply, there is nothing written about tastes.
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