“I don’t smell my pee,” a friend said to me a little while ago. It might be a bizarre thing to say, but it was an honest answer to my question: “Have you noticed that when you eat asparagus your pee smells funny?” On the other hand, it was a little offensive of her. I didn’t mean to suggest that I had intentionally put my head near the toilet bowl to take a whiff. Rather, I had discovered a very pungent smell coming from the bowl when I ate some asparagus, and thought it was interesting. After my friend’s mildly uncomfortable response I decided not to bring the topic up again. And it wasn’t until last week, when two friends asked me on separate occasions why their pee smelt funny when they ate asparagus that I took a deep breath and decided to broach the topic once more.
Even though asparagus has been in the human diet for over 2000 years, it wasn’t until the early 18th century that people started reporting ‘asparagus pee’ (they didn’t call it asparagus pee in the 18th century, but it’s a convenient name). And it wasn’t until two hundred years later, in the 1950s, that scientists started seriously trying to explain the phenomenon.
Originally, scientists thought there were two kinds of people in this world: those who made asparagus pee, and those who didn’t. (And because it was such a taboo, everyone just assumed that everyone else was like them.) In the late 1970’s and throughout the 80’s a new idea took hold: maybe we would all make asparagus pee, but some people just couldn’t smell it.
In the 80’s three studies from France, China and Israel published results showing that, just like laughter and tears, making smelly urine after eating asparagus was a universal human characteristic. The Israeli study asked 307 subjects to eat asparagus. They found that 90% of the subjects in the Israeli study couldn’t smell asparagus pee, but the 10% who could smell asparagus pee, could also smell it in the urine of everyone else.
The culprit in asparagus that makes pee smelly is called asparagusic acid. This acid is broken down in our gastrointestinal tract, and when this happens it releases sulphur, which is thought to make our urine smell. Asparagusic acid is the only thing with sulphur in it that is unique to asparagus, which is why it’s our primary candidate for causing asparagus pee. Further support that asparagusic acid is to blame for asparagus pee are studies showing that young asparagus plants, which have the most amount of asparagusic acid when compared with older plants, are notorious for producing the smelliest pee.
In a review of asparagus pee published in 2001, the author, S.C. Mitchell suggested that a small percentage of the population might not be able to absorb asparagusic acid in their gastrointestinal tract, meaning that, in these people, the acid would never get broken down, and smelly sulphur filled pee would never get excreted. So, after all his research, Mitchell seemed to think that there were still some people in this world that couldn’t make asparagus pee.
Putting all these results together it seems that there are three kinds of people in the world: those who can make asparagus pee and smell it (me!), those who can make asparagus pee and can’t smell it (quite a lot of people according to the Israeli study), and those who can’t make asparagus pee at all (probably a pretty a small group of people). So what I really should have told my friend a little while ago was “Yeah, I do smell my pee, and I can smell yours too”. That would have really shown her.
