Why does asparagus make your pee smell funny?

Dear Cecil: My faith in you remains unshaken and my respect for the sagacity of your judgment undiminished. However, I am afraid that, as Mitzi Malone might say, your interlocutors have gotten pretty weenie. What you need are some good questions, such as the following: Why does my urine smell funny after I’ve eaten asparagus? F.K., Dallas, Texas

Cecil replies:

I’ve probably been asked this question a dozen times since I started this job. The first ten times I dealt with it as you might expect. I burned the letter, had the room fumigated, and prayed for the writer’s immortal soul. This week, however, I got the question twice in the same mail, leading me to think this is something the Teeming Millions truly yearn to know, God help us. So I give up. Here’s the story:

Many have noted the peculiar effect of certain products on human urine. For example, Benjamin Franklin, in a wide-ranging discussion of bodily discharges, once noted, “a few stems of asparagus eaten shall give our urine a disagreeable odor; and a pill of turpentine no bigger than a pea shall bestow upon it the pleasing smell of violets.” It’s said that in a venerable British men’s club there is a sign reading “DURING THE ASPARAGUS SEASON MEMBERS ARE REQUESTED NOT TO RELIEVE THEMSELVES IN THE HATSTAND.”

Serious scientific research in this field dates back to 1891, when M. Nencki tentatively identified a compound known as methanethiol as the culprit. The odor appears within an hour after eating just a few spears of the offending vegetable. According to Allison and McWhirter (1956), the ability to produce the odor is controlled by a single autosomal (i.e., non-sex-related) dominant gene. In a sample of 115 persons, 46 were rendered fragrant by asparagus and 63 weren’t. (This leaves 6 unaccounted for. Urology evidently is an inexact science.)

In 1975 Robert H. White, then with the chemistry department at the University of California at San Diego, found that the odor-causing chemical wasn’t methanethiol after all. Instead, using gas chromatography-mass spectrometry — maybe it’s just pee, one imagines Bob thinking, but by God we’re going to get to the bottom of this — he found that the aroma was in fact caused by several S-methyl thioesters, specifically S-methyl thioacrylate and S-methyl 3-(methylthio)thiopropionate. (Thioesters are compounds that result from the reaction of an acid with a sulfur-containing alcohol. They tend to be smelly.) I know you’re very interested and are following me closely, F., so I’m going to give you the exact chemical formulation for these chemicals, in case somebody asks you at some fancy social soiree. The thioacrylate recipe is:

CH2=CHC(=O)SCH3

The thiopropionate is:

CH3SCH2CH2C(=O)SCH3

We’re faking the above somewhat given the limitations of the ASCII character set, but I know you’re grateful for whatever information we can provide.

Anyway, says Bob, the “metabolic origin [of the compounds — i.e., how and why they end up in the urine] remains an open question.” I can’t exactly say research is continuing, but if anything develops I’ll let you know.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.