Science: Asparagus Revisited (With Bonus Urine!)

In my previous recipe, I mentioned my love of asparagus. I was talking to a friend online as I cooked, and he told me that although he loved asparagus, he hated how it made his urine smell. I've heard people mention in the past that asparagus gives their urine a distinctive sulfurous stench, but I personally have always been unaffected. So, why does asparagus make your pee smell? And why does it happen to some people and not others? The answer, it turns out, is more interesting than you might think.

Unfortunately, part of the answer is that we just don't know. We have, on the other hand, some pretty good ideas and some legitimate science pointing us in the right direction.

The intuitive answer is a simple one, an it isn't far off from the truth: some people process asparagus in a certain way that leads to a stinky molecule being excreted in the urine. Back when I was younger, I remember hearing that there was a dominant gene found in about 50% of the population that causes this. Such a gene hasn't been proven to exist. If it does exist, it alone doesn't provide a complete answer. In a recent NIH study, evidence suggested that only very few people (with their testing methods, 8%) produce unscented urine after eating asparagus. However, the strength of the smell seems to vary widely from person to person. If the ability to break asparagus down into sulfur-like compounds is genetic, it probably relies on multiple genes. For now, scientists aren't able to do chemical analysis on the ability to produce the scent - surprisingly, the specific molecule or molecules responsible have never been pinpointed.

To me, the interesting part of this issue comes from the fact that there's more to it than scent production. It seems that certain individuals have a diminished ability to smell the asparagus stink, regardless of whether or not they excrete it. Like excretion, detection also appears to be on a continuum - it seems that this isn't an all-or-nothing ability. However, data suggest that a diminished ability to smell asparagus is associated with a very specific, very small mutation! The mutation occurs near a gene (called OR2M7) that allows us to smell geranital and cintrone, which smell like rose and citrus respectively. It's unknown whether the mutation affects this gene directly or another one nearby; OR2M7 is found in a cluster of genes that are all active in olfaction.

So it turns out that there are a few possible reasons why my trips to the bathroom are so carefree after I eat asparagus. Perhaps I legitimately don't create the smell. Perhaps I create it only at a very small concentration - a concentration too low for my nose to actually detect. I know that I don't have the most sensitive of sniffers - people around me always seem to notice smells a second or two before I do. My completely unsceintific guess is that I do indeed have stinky urine, but that my defective nose can't tell.

Fun fact: your nose has a shockingly small number of receptors for different odorous compounds. Only four hundred or so different genes (link) code for odor receptors. Based on which receptors are being triggered and to what extent, the brain interprets what manner of compound is present.