The Delicate Subject of Asparagus Pee

This is not usually the kind of subject to bring up in conversation at a family function, but… what is it with asparagus pee? You know, the strong odor given off by human urine after the person has consumed asparagus. Or perhaps you don’t know, as not everyone can smell it.

Enquiring Gardeners Want to Know

The “asparagus pee” phenomenon has long attracted attention. French scientist Louis Lémery wrote about the strong scent of urine following asparagus consumption as far back as 1702. Benjamin Franklin reported trying to learn how to eliminate it in 1781. (Footnote: he failed!)

Oddly enough, it used to be thought that humans could be divided into two categories: those whose urine stank after eating asparagus and those whose urine didn’t stink. But that was based on people self-reporting their experience with their own urine. Scientists, it turns out, were asking the wrong question.

The truth is, all human urine stinks after eating asparagus, with very, very few exceptions. It wasn’t until the 1980’s that scientists started having people smell the urine of others, not just their own, and they quickly found that those who can smell the asparagus pee odor in their own urine can also smell it in the urine of others. And those who can’t smell their own malodorous urine can’t smell asparagus pee at all, not in anyone’s urine.

The proportion is apparently about 40/60: in a study of 6900 men and women, 40% of were found to be asparagus pee smellers while 60% were “asparagus anosmiacs”. Yes, there is even a scientific term for the inability to smell asparagus pee. Who knew?

So if you’re unable to smell asparagus pee, you’re suffering from asparagus anosmia. (Of perhaps that should be you’re enjoying asparagus anosmia).

Sulfur and Ammonia are the Cause

The foul odor of asparagus pee comes largely from ammonia and various sulfur compounds that are released during digestion. And this happens very quickly: if you take a whizz within 15 minutes of eating asparagus, you’ll notice it… unless you’re an anosmiac, of course.

The Difference is in the Genes

The most recent studies can even trace the ability/inability to smell asparagus pee to our genes. Scientists found more than 800 single-nucleotide polymorphisms in a group of olfactory receptor genes located on chromosome 1 that lead to asparagus anosmia. If you don’t inherit one of these variations, you’ll smell asparagus pee; if you do, you’ll be an asparagus anosmiac.

Why?

The question that remains is… why? What evolutionary purpose could possibly be served by some people being able to smell asparagus pee and others not? Did one or the other make a difference to the survival of the human race over the eons? Does it in fact change anything in our lives?

So far, no reason has been found. But you can bet peeologists (OK, so I made that one up!) all over the world are studying the question!